• Sterilise the tap outlet with a flame or an alcohol-soaked cotton-wool swab.
• Do not touch or otherwise contaminate the inside of the sample bottle or cap.
• When collecting bacteria samples, consider wearing surgical gloves to minimise potential contamination.
• Ensure that your hands are clean.
• Run the water for several minutes to flush what was lying in the storage tank or pipes.
• Rinse the bottle 2-3 times with the sample water.
• Use a clean, sterile plastic or glass bottle (available from Labserve).
• Don’t use dirty containers, or milk, mayonnaise, chemical etc. as these will affect the results.
• The size of the container is important to ensure you have enough sample to run the analysis required.
• Place the samples into a cooler with ice for immediate delivery or shipment to the laboratory.
• Bacterial samples for drinking water must be received within 30 hours of being collected.
• Label samples correctly. Record information such as the location, date and time collected, and specify the analysis you require.

Regular testing of water identifies problems, ensures that water is suitable for the intended use, especially if used for drinking by humans and animals. You can track changes over time and determine the effectiveness of your treatment system.

Labserve offers the following standard water analyses for the agricultural sector, municipalities, and the hospitality industry. We can also do custom analysis based on your specific needs:

• Potable (Drinking) Water
• Irrigation Water
• Bottling Water
• Effluent Water
• E.coli and Total Coliforms
• GlobalG.A.P. Water
• Farming for the Future Water
• Fish Water Analysis
• Pesticide Levels in Water

Drinking water quality can vary dramatically in winter compared to summer, therefor we advise that you test your water in both seasons to get a more accurate idea of your water quality.

Labserve Analytical Services is a SANAS, ISO 17025:2017 accredited reputable, independent, laboratory group. Please feel free to contact our friendly staff for bulk sample discounts, sampling guidelines, or additional information on 086 137 0290 or view our price list and parameters.

1. Purpose
   To ensure that representative fruit samples are collected in the orchard for quality evaluation/testing, as well as for fruit and peel analysis so that accurate prediction of bitter pit and quality sorting can be conducted.
2. Scope
   Entire orchards earmarked for export.
3. Responsibilities
   Management representative ~ communication with pack house, farm manager, laboratory Farm manager ~ communication with fruit sampling personnel.
4. Responsibilities
   Management representative ~ communication with pack house, farm manager, laboratory Farm manager ~ communication with fruit sampling personnel.
5. Procedures
   Selection of representative trees: Select at least five trees in an orchard. The selected trees must be evenly spread out over the total area occupied by the orchard (see Figure 1) and represent the general/overall condition of the orchard’s trees. Trees at the edge of the block and at the end of rows should not be sampled – they may be coated with soil/dust particles and/or differ in their nutrient status and irrigation regime from the rest of the block. Mark the trees. When follow-up/consecutive sampling is done, sample a healthy tree immediately to the west, then to the north, then the east and then the south of the marked trees.

Figure 1. At least five representative trees must be selected for sampling of fruit – these trees (e.g. the red dots) should be evenly spread throughout the orchard. In orchards that have areas where trees differ, e.g. are weaker or diseased, instructions provided by the farm manager must be followed, e.g., either a separate sample must be taken or trees in that area must not be sampled at all.

Selection of representative fruit: From each of the representative/marked trees eight fruits must be picked as illustrated below.

E.g. Four sides of the tree x (inside + outside) = eight fruit per tree.
Do not sample diseased, insect damaged, or inferior (e.g. poorly coloured or sun damaged) fruit that would not be considered for packing/export.
Packaging and transport of sampled fruit: Place the fruit in a clean, properly marked, plastic bag on which the following is indicated: • Farm / company • Block / orchard name • Cultivar • Sampling date
Keep the sample cool – do not expose it for longer than 20 minutes to direct sunlight, and do not store it for longer than two hours at temperatures exceeding 25oC. Within 48 hours from the time that sampling was done, the sample must be sent to the laboratory for analysis. Ensure that the samples reach the laboratory within 60 hours since sampling.

In order to correctly manage the amount of fertiliser applied to a crop for an optimal yield, an accurate analysis of the amount of nutrients already available in the soil is required. Sampling error is often the most significant source of error that affects the accuracy of analysis results. Although apparently daunting, accurate soil sampling is not a mystical and virtually impossible art for a farmer who knows their farmland. Representation is vital: a sample must accurately represent a particular sampling area. The following are possible sources of variation and compensating for these will reduce sampling error, thus improving accuracy of results:

• Irrigation system differences such as pressures, emitter type and rate
  • Plan sampling areas per irrigation block especially if the irrigation system was designed on differences in soil type.
  • Plan sampling areas on similar water pressures and type of emitter as this affects nutrient retention, uptake and organic matter percentage in soil.
• Organic material
  • Plan sampling areas on similar levels of soil carbon or organic material as the more humus present, the more nutrients will be retained.
• Soil texture
  • Plan sampling areas on similar textures as the finer the texture, the more the soil will tend to hold nutrients.
• Slope and altitude
  • Plan sampling areas on similar slope and altitude as this affects drainage which affects nutrient retention, organic matter levels and soil redox conditions.
• Row orientation or aspect
  • Plan sampling areas on similar row orientation as this can affect the amount of sun on the soil surface and the rate of breakdown of organic material.
  • Aspect can also affect drainage on areas of the same slope.
• Colour
  • Plan sampling areas on soil colour where relevant.
  • For example waterlogged soils can be grey in colour, soils with signs of wetness can have mottles, well-drained soil can tend to be red and soil high in organic material can darker in colour.
• Number of subsamples and sample area size
  • Take a sufficient number of subsamples per area to minimize local variation. About 10 – 20 subsamples per area is common.
  • Demarcate adequately sized sampling areas which usually span about 4-6ha depending on the variation present.
• Management history, cultivar
  • Designate sampling areas based on management history such as pruning practices etc. as this can influence soil nutrient levels.
  • Designate sample areas based on different cultivars as this can influence nutritional requirements.

Other factors influencing accuracy of results include the time of the year, fertiliser application, sampling method and laboratory extraction method. Take soil samples at the same time of year and from the same places as the previous year to minimize variation. Take soil samples simultaneously with leaf samples to save time and labour. Do not take soil samples soon after fertilisation as this will increase the concentrations of nutrients detected. Extraction method can have an influence results depending on the chemistry involved. Keep to a specific extraction where possible or reasonable.

Soil should be sampled as follows:

• Use a soil auger or a shovel to take subsamples in a zigzag, grid or transect pattern or at specific marker trees or plants within the sampling area. Normal samples are to 30cm depth with the top 10cm of soil discarded.
• Combine the subsamples into a composite sample in a plastic bucket (not metal), remove debris, mix thoroughly and place about 500g into a labeled plastic bag.
• Do not sample soil with fertiliser granules present and do not use old fertiliser bags as sampling bags.
• Do not expose samples to moist or hot conditions and submit to the lab in a timely manner.

The UN Agency’s Intergovernmental Technical Group on Soils (ITPS) defined healthy soil as “the ability to sustain productivity, diversity, and environmental services of terrestrial ecosystems” (Isonio, 2021). If, in an orchard context, soil health constitutes the ability of soil to ensure the sustained successful production of a crop, then it is something that producers must seek and maintain.

A populistic view is that soil health is associated with the preservation and enhancement of so-called useful microbes in the soil and that conditions must be created to promote the proliferation of these microbes. However, the chemical and physical characteristics of the soil (rather than its organic matter and biology per se) mostly determine its ability to ensure successful production of a crop.

In pursuit of soil health as defined above, the focus should firstly be on proper soil preparation that focuses on ensuring and maintaining a balanced soil chemical status and good physical attributes, i.e., a chemically homogenous, well aerated soil that is free from physical restrictions. Any soil characteristic that reduces aeration, e.g., compaction or wetness, will reduce the soil biological activity, and more so, the plant root activity. Healthy, adequately fertilised, properly managed trees in well-aerated (not over-irrigated) soil will have healthy roots that create and maintain their own rhizosphere microbial populations. Active growing roots exude organic compounds (exudates) into the soil. These compounds are a food source for micro-organisms, raising their number up to 500 times higher than in the rest of the soil (McLaughlin et al., 2000). The characteristics of the exudates changes according to the age of the tree, season, chemical inputs, water availability and tree nutritional status. Therefore, the rhizosphere is a constantly changing environment in which the relative number of each member group adjusts according to the nature of the exudates and the environmental conditions. For example, when a plant wilts due to water stress, more amino acids are released by the roots to the rhizosphere. Likewise, more organic acids are released into the rhizosphere in response to iron (Fe) or P deficiency, which then solubilise Fe, P, Zn and other metals, thereby enhancing their bioavailability (McLaughlin et al., 2000). Thus, the addition of external soil microbiological systems has little direct effect on root and plant health or plant nutrition. We can then, instead of referring to “healthy soil”, rather pursue healthy roots through well-managed soil.

In established orchards, maintenance of an adequate amount of organic carbon (C) in the soil, ensures that microbial activity and diversity in the soil outside the rhizosphere is sustained. The simplest and most logical way to increase the soil biological activity and diversity beyond the root zone is therefore simply to increase the soil organic C (preferably Walkley-Black analysed) – it was proven that there is a direct correlation between soil C content and microbial biomass and activity (Witter et al., 1993; Leita et al., 1999; Bhogal et al., 2009), as shown in Figure 1. In addition, soil organic material also increases the water holding capacity and nutrient retention ability of soil. An increase in soil C from a very low level, and subsequent increase in soil biological activity, will also enhance overall soil physical conditions, i.e., reduce susceptibility to compaction and increase soil aeration. This benefits root health and growth, and consequently the rhizosphere microbial population, which fosters tree health. The maintenance of an appropriate (not necessarily high) soil organic C level is, therefore, good common sense.

Soils that are higher in clay content generally retain more organic C than sandy soils. As a result, well-managed clayey soils have higher biological activity than sandy soils. But structure, water holding capacity, nutrient retention and biological activity of sandy soils can be increased when inputs of organic material are greater than losses, but the converse is also true. In situ plant biomass production is a viable input, whereas amendments added to soils are more costly. And since losses occur when organic matter decomposes, the cost and effort to retain a certain level of soil organic C needs to be weighed up against the benefits. It often does not justify the cost and effort.

Producers who want to establish the soil C levels must note that laboratory analysis must be interpreted correctly. Total soil carbon (TC) cannot be equated with soil organic carbon that is oxidisable and therefore, utilised by the soil micro-organisms (i.e., Walkley-Black analysed carbon). Any TC analysis also includes other carbon sources, like the carbon in lime (CaCO3), biochar and even plastic or coal (ash) residues. All last-mentioned forms of C are inert and do not originate from biological organisms. Therefore, total carbon is not a certain indicator of the soil’s microbiologically available carbon.

It is suggested that producers who want to monitor and manage their soil carbon, distinguish between the different types of carbon found in the soil, as illustrated in Figure 2. Many soils contain free lime, and sometimes residues of biochar or wood ash. Hence, a Leco analysed carbon value (total combustion) is not always helpful. Rather, it’s advisable that either the total C (TC) as well as the inorganic C (TIC) are analysed to obtain the total organic C (i.e., TC-TIC = TOC), or that the Walkley-Black carbon (i.e., oxidisable C) is quantified. For reference, appropriate soil oxidisable organic carbon levels (as analysed by the Walkley-Black method) for South African conditions are indicated in Table 1.

Producers who are concerned about soil health, should focus on responsible soil management practices, i.e., correcting and maintaining the soil chemical and nutritional status, limit soil compaction and avoid over-irrigation. Where physical and chemical conditions are optimal for the intended crop, balanced and beneficial soil biology is established. Maintenance of soil C is usually also a given in well-managed orchards where the soil is not cultivated regularly, and the produced plant biomass (e.g., dropped leaves, prunings and inter-row vegetation) is not removed. A well-managed soil is a “healthy soil”.

References

Bhogal, A., F.A. Nicholson, and B.J. Chambers. 2009. “Organic carbon additions: effects on soil bio-physical and physicochemical properties.” European Journal of Soil Science 60, 276–286.

Isonio, E. 2021. What is healthy soil? FAO coins the official term. RE Soil Foundation. https://resoilfoundation.org/en/ environment/healthy-soil-official-definition/

Leita, L., M. De Nobili, C. Mondini, G. Muhlbachova, L. Marchiol, G. Bragato, and Contin. 1999. “Influence of inorganic and organic fertilization on soil microbial biomass, metabolic quotient and heavy metal bioavailability.” Biol. Fertil. Soils 28, 371-376.

McLaughlin, M.J., R.E. Hamon, R.G. McLaren, T.W. Speir, and S.L. Rogers. “Review: A bioavailability-based rationale for controlling metal and metalloid contamination of agricultural land in Australia and New Zealand.” Aust. J. Soil Res. 38, 1037-1086.

Schütte, S., R. Schulze, and G. Paterson. “Identification and mapping of soils rich in organic carbon in South Africa as a climate change mitigation option (2019).” Department of Environmental Affairs, Pretoria, South Africa.

Witter, E., A.M. Martensson, and F.V. Garcia. “Size of the soil microbial biomass in a long-term field experiment as affected by different N-fertilizers and manures.” Soil Biol. Biochem. 25(6), 659-669.

In addition to the negative publicity that the agricultural sector faces regarding the detrimental impact of unnecessary fertilisation on the environment, the cost of fertilisers is forcing producers to pursue ways to reduce fertilisation inputs. A proper balance between unreasonably conservative fertilisation and unwarranted application of nutrients is therefore required, where the emphasis should be to maintain tree health and sustain an orchard’s ability to produce sufficient yields of good quality. A producer who wants to manage his fertilisation inputs accordingly needs to comprehend certain principles and take them to heart.

Soil and leaf analysis is cheaper than unnecessary fertilisation. The first habit to develop is a routine of taking proper, representative soil and leaf samples. Soil samples do not have to be taken annually, but a wise decision regarding fertilisation requirements cannot be made without proper knowledge of the orchard’s soil chemistry and nutrient content. The foliar analysis provides you with insight regarding the extent to which nutrients in the soil are taken up – it might point you to a need to address root health and functioning. Leaf analysis also provides insight regarding the need to apply nutrients like P, Ca, and Mg that one wants to avoid due to their added cost.

The trees can perform well with less fertilisation than you reckon. Be conservative in your fertilisation approach – the trees do not perform better when supplied with more than the required nutritional rates. Although prolonged undersupply of necessary nutrients will lead to diminishing tree performance, it is not affected overnight. Furthermore, trees with healthy roots can utilise the pool of available P, K, Ca, and Mg effectively and for a long time – making fertilisation redundant or reducing its need. It is also worthwhile calculating the extent of the available pool of these nutrients before they are included in a fertiliser programme.

Over-irrigation is a waste of fertilisers. The sustained performance of many orchards during dry seasons, when less irrigation is applied, can be ascribed to improved tree nutrition. The healthier roots and less leaching from the root zone improve nutrient uptake. Avoid over-irrigation at all costs.

Trees with large root systems require less fertilisation. The larger the volume of soil occupied by the roots, the longer it takes to deplete the nutrients to a deficient level. It has been shown that by applying as low as 40% of crop removal of P and K one is able to keep the soil P and K near optimal levels, while not reducing crop yield and quality in the short term. Active, protected roots close to the soil surface can potentially utilise much N from organic material being mineralised. However, roots affected by nematodes or phytophthora (mostly due to over-irrigation) will not function optimally, and the efficiency of nutrient uptake will be reduced – producers need to make sure their roots are healthy and take steps to rectify any issues in this regard.

The law of diminishing returns.

Tree performance will improve linearly as nutrients are supplied from extremely deficient levels. However, as the optimal nutritional status of the tree is progressively being reached, the additional benefit of increased fertiliser reduces (see the figure below). Eventually, any further inputs have no advantage and can even be harmful to the trees or crop.

With these principles in mind, the following practical, common sense, guidelines might be useful to consider. The authors are of the opinion that if producers implement these guidelines, they can save up to 30% on their fertilisation costs without having any impact on tree performance – given that they do not over-irrigate, which is a most unproductive, wasteful practice.

Firstly, in micro-irrigated orchards, no P must be applied if the concentration in the soil is above the minimum norm. The same applies to K on soil with a clay content > 10%. Furthermore, take more detailed soil samples to represent different performing sections. If the soil P concentrations in these different areas vary, apply differentiated amounts of granulated fertiliser according to the requirement – use a variable spreader for this. The same applies to K on heavy soil, while areas with sandy soils are marked for maintenance fertilisation.

Drip-irrigated blocks require special attention – sampling of soil for analysis needs to be done meticulously and more regularly. Avoid depletion of all nutrients in the wetted zone and accept that nutrient requirements will have to be met in the limited volume of roots. Optimal nutrient uptake should be sought by applying each nutrient intermittently, and then at the specific concentration that each is most effectively taken up. Where soil pHKCl is below 6.0, the fertiliser mixture should also be buffered to a pH between 6 and 7.

In sandy soil over-irrigation, and consequently, leaching of nutrients, must be avoided through exactly managing irrigation cycle lengths – the leaching loss of N and K can be especially high in drip-irrigated blocks. Ensure you know the depth of the root zone and make a point to monitor the depth of water movement after an irrigation cycle – either by making a profile hole, installing wetting front detectors just below the root zone, or using a probe. Although the cost of organically enriched fertilisers might not always justify the benefits, or actually reduce losses, the theory behind it that justifies experimentation.

Responsible soil management can assist tree nutrition and reduce the overall fertiliser requirement, or improve fertiliser use efficiency. For example, in a biologically active soil (i.e., that has a fair amount of organic material) more nitrogen becomes plant-available through the predation and excretion of plant-available nitrogen by soil organisms. Through biological nitrogen fixation, done primarily by bacteria in association with legume plants, additional amounts of N can be obtained. The practice of establishing or maintaining legumes as an inter-row cover crop should be considered.

The history of each orchard must also be considered – especially foliar nutrient concentrations. If an orchard traditionally has high concentrations of a specific nutrient, the applied amounts of that nutrient should be reduced accordingly. It is also very important to note that, when interpreting foliar analysis, any level above the minimum norm should be regarded as optimal. The amount of fertiliser required to obtain a shift from a low level to a higher level within the traditionally acceptable ranges can be excessive, and without any benefit (remember the law of diminishing returns). In cases where leaf analysis indicates deficient nutrient levels, scrupulous application of foliar nutrition might be a more cost-effective short-term method to address deficiencies than soil application – this applies specifically to micro-nutrients, and even more so, where high soil pH reduces micro-nutrient availability. Furthermore, “scrupulous” foliar application implies that producers ensure application protocol that will be most effective, i.e., the correct concentration, suitable application volume, spray mixture that is properly buffered, application when trees have young leaves, etc.

Choice of fertilisers is also a consideration. It would generally be recommended to avoid using mixtures since the cost per kg nutrient is often higher, and if you do not need a specific nutrient, it is not wise to apply it – even more so if it carries a cost. The use of organic materials, e.g., manure or compost, and enriched organic fertilisers, should be considered carefully. If the cost justifies it, then it can be used. But if the composition does not correspond to what is required to maintain a balanced, optimal soil nutrient content, then it must not be used for more than a season or two.

Lastly, prioritisation of orchards is required in times when the exorbitant fertiliser process threatens financial viability. Avoid under-fertilising all orchards – the potentially detrimental effect of deficient nutrition should be limited to unprofitable, poor-yielding blocks. Nurture the healthy, good performing, productive orchards.

In the light of decreasing availability and quality of water used for irrigation of citrus in many regions of South Africa, producers are compelled to investigate the possibility of using new water sources, i.e., groundwater or wastewater. However, the quality of these sources often raises concern, making citrus producers unsure whether they can use them to supplement limited irrigation water supplies or for new developments. In this article, the most common factors that determine whether water is suitable for irrigation and possible mitigation options are discussed.

Water salinity

The first quality parameter that must be considered is total dissolved solids (TDS), which measures the total quantity of various inorganic salts dissolved in water. The TDS is assessed by measuring the electrical conductivity (EC) of the water. The principle of EC measurement is based on the fact that salts in water increase the rate at which electricity is conducted in the water – the higher the salt content, the higher the conductance. Therefore, the TDS is typically calculated mathematically by laboratories from the EC by using a factor that will differ according to the unit in which the EC is expressed. The relevant factors are provided in Table 1 below, with the TDS being expressed as milligrams per litre (mg/L), which is equal to parts per million (ppm).

Interpretation of the total salt content of water is made using a scale of EC values – the higher the EC, the less suitable the water becomes for citrus production. Different EC ranges are used to classify the water quality – the different classes that are internationally used as an indication of the suitability of the water for citrus production are provided in Table 2. The EC, however, does not indicate the type of salts that are present in the water.

The sodium content of the water

Even though the water’s total salinity places it in salinity Class C1, C2 or C3, it can still be unsuitable for irrigation due to disproportionately high sodium (Na) in the water. In addition to being a salt that can damage plant tissues in extremely high concentrations, Na is harmful to soils since it displaces Ca and Mg on clay particles resulting in clay dispersion and breakdown of soil structure. A decreased water infiltration and/or drainage, with an exponential increase in the soil salinification rate, is the end result.

To evaluate the risk that Na poses to the soil, the ratio of Na in relation to calcium (Ca) and magnesium (Mg) in the water is used, which is expressed as a factor called the sodium adsorption ratio (SAR). The SAR is a value that is mathematically calculated by the laboratory from the water Na, Ca, and Mg analysis results. An interpretation of the SAR values is provided in Table 3.

Irrigation water analysis reports will therefore often contain a so-called irrigation class, which is expressed with C and S symbols denoting the salinity (C = conductivity) and sodium status (S = sodium). For example, highly saline water (EC > 2.25) with a low SAR (<5) will be classified as C4S1 water. From the tables above, these classifications can therefore be interpreted.

Crust formation that typically occurs on the surface of soil that is irrigated with water that has a high residual sodium carbonate index or an excessive SAR – the result is poor water infiltration, i.e., excessive run-off

Residual sodium carbonate index

When the pH of the irrigation water exceeds 8.0, a useful alternative measure of the risk that sodium poses is the residual sodium carbonate (RSC) index. In this index, the bicarbonate (HCO3-) and carbonate (CO32-) concentrations in the water are brought into consideration. The logic is that precipitation of soil Ca and Mg with HCO32- and CO32- in the irrigation water subsequently leads to an increase in the relative proportions of sodium to the other cations in the soil solution. This situation, in turn, will increase the sodium hazard of the soil-water to a level greater than indicated by the SAR value of the irrigation water. Therefore, a high RSC of irrigation water results in an increase in the soil’s exchangeable sodium percentage (ESP), with the consequent dispersion of clay, as referred to above.

On their own, high HCO3- and/or CO32- levels in irrigation water also increase the water pH, cause clogging of irrigation systems due to calcite or lime deposition, and a whitish deposition forms on leaves and fruit from the droplets of irrigation water.

In arid and semi-arid regions, underground water often has a high pH and RSC. The higher the pH of the water, the higher the HCO32- and CO32- concentration will be, with a progressive shift towards CO32- as the pH increases. The clogging of irrigation systems increases as the HCO32- and CO32- concentration and water pH increase. The guidelines in Table 4 can be used to assess the risk of drip irrigation systems becoming blocked due to calcite/lime deposition.

To assess the risk of sodification of soil, a calculation of the RSC should be done if the HCO32- and CO32- concentration of the water exceeds 120 mg/L and 15 mg/L respectively, and the SAR > 3. The RSC can be calculated from normal irrigation water analysis results, using the following formula:

RSC index = {(HCO3/61) + (CO3/30)} – {(Ca/20) + (Mg/12)},

Where the concentrations are in mg/L, and the resulting RSC index value is expressed in milliequivalents per liter (meq/L).

The following indicates the suitability of water for irrigation on account of the RSC index (meq/L):

Management and mitigation possibilities

From the above tables, it is implicit that while it is possible to use a wide variety of water quality types for irrigation, management procedures become more demanding as the water quality decreases – the suitable range of soils and citrus also become more restrictive. The possible management options to prevent the build-up of salt in the soil, and ensure long-term tree performance, are:

Soil drainage: To avoid the build-up of salts, the soil must be permeable – permeable soil can be leached to wash out the salts. In this regard, sand is much more forgiving than heavy soil.

Leaching: Salt build-up in the soil is avoided through leaching. Through regular monitoring of salt levels in the soil (measuring the soil’s EC or resistance), the need for leaching (or its effectiveness when practiced) can be determined. Generally, the higher the salt level of the water, the more arid the climate, the greater the leaching requirement will be.

Irrigation frequency and duration: Irrigation scheduling has a significant impact on salt accumulation or reducing levels in the soil. Short, regular cycles will more likely result in the salt build-up in the topsoil because of evaporation. On the other hand, long cycles leading to deep water penetration mitigate the build-up of salts in the root zone.

Soil and water amelioration: Irrigation water can be modified with gypsum to improve its SAR. The quantity of gypsum needed for adding to irrigation water depends upon the quality of water (the SAR and RSC) and the annual quantity of water required for irrigation of the trees. The addition of gypsum increases water salinity, so this is only an option when the water’s salinity is not too high, and when the soil is well-drained. Soils can also be modified using gypsum to ensure proper drainage (it helps to maintain and improve the soil’s structure), but this should only be used when the soil’s exchangeable sodium percentage exceeds 10% – otherwise, the soil is actually further salinified.

In this article, the three most important water quality parameters were discussed, i.e., total salinity, SAR, and RSC. These are the most common restricting factors that must be considered. Water quality problems can also be associated with the presence of other constituents, like excessive levels of iron (Fe) or manganese (Mn), that can result in drip irrigation being blocked.

Drip irrigation lines become blocked when irrigation water is used with high bicarbonate and carbonate concentrations

Literature cited

Prasad, A., Kumar, D., Singh, D.V., 2001. Effect of residual sodium carbonate in irrigation water on the soil sodication and yield of palmarosa (Cymbopogon martinni) and lemongrass (Cymbopogon flexuosus). Agricultural Water Management 50, 161-172.

Wilcox, L.V., 1955. Classification and the use of irrigation waters. Circular No. 969, USDA, Washington.

Zaman, M., Shahid, S.A. & Heng, L., 2018. Guideline for salinity assessment, mitigation, and adaptation using nuclear and related techniques. IAEA, Vienna. https://doi.org/10.1007/978-3-319-96190-3.

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In citrus new vegetative shoots provide the bearing positions from which flowers develop in spring. These new vegetative shoots are strong sinks for carbohydrate supply from both mature leaves and reserve carbohydrates from the previous season. One could expect that citrus trees should be able to meet all the carbohydrate requirements since it is an evergreen plant that has ample time for photosynthesis, and relatively large starch reserves are usually present in various tree organs. However, it is evident that citrus trees are often “source-limited” and that a lack of availability of photosynthate then restricts vegetative growth, flower development, and fruit set.

Soil and leaf analysis is cheaper than unnecessary fertilisation. The first habit to develop is a routine of taking proper, representative soil and leaf samples. Soil samples do not have to be taken annually, but a wise decision regarding fertilisation requirements cannot be made without proper knowledge of the orchard’s soil chemistry and nutrient content. The foliar analysis provides you with insight regarding the extent to which nutrients in the soil are taken up – it might point you to a need to address root health and functioning. Leaf analysis also provides insight regarding the need to apply nutrients like P, Ca, and Mg that one wants to avoid due to their added cost.

Fruit is a major carbohydrate sink and can disturb the balance between spring vegetative shoot development, root growth, and flower development – all demanding large amounts of photosynthate. The persistence and health of the previous year’s foliage in citrus, therefore, plays a critical role in the provision of photosynthate during the emergence of the spring flush, at least prior to full expansion of the new leaves. In this regard, nutrient deficiencies, particularly in autumn, will contribute to a reduction in carbohydrate availability – producers should therefore ensure that their trees’ nutrient status should be adequate at the onset of flower induction (April – June). The elements (especially N and K) that are not within the norm during the leave sampling period (Feb – May) should be brought back to adequate levels through a combination of soil and leaf application. Furthermore, the application of LB Urea (eight weeks prior to budbreak) to ensure a sufficient N-status of the leaves can be used to enhance the leaves’ ability to produce sufficient carbohydrates (sucrose) required for budbreak, spring shoot growth, and flower development.

In circa 1500 Leonardo da Vinci said: ”We know more about the movement of celestial bodies than about the soil underfoot”. Although our knowledge of both these fields of science has developed immensely over the last five centuries, this statement undoubtedly remains true. Soil life, and especially the soil microbial system, is very dynamic and sensitive, and the potential impacts on its composition are so vast in number that it is very difficult to predict its characteristics under all possible circumstances.

Since the living part of soil consists of a huge number of different species that live in a dynamic ecosystem, where they prey on one another and other times live symbiotically together, the science of soil microbiology is complicated. As a result, very few scientists will claim to be specialists in more than one or two components of soil microbiology. Because of the interest in this topic that has developed among grape producers in recent years, this article attempts to apply relevant elements of this biological science to grape production systems and to address some of the factors that impact vineyard soil- ecosystems.

Soil Organic Matter

Organic matter is a critical component in the microbial ecosystem of soil since it is the energy source of soil micro-organisms. With a reduction in soil organic matter content, microbial activity decreases. The presence of soil organic matter therefore ensures microbial activity and affects the characteristics of the soil in various ways. Whilst attempting to manage his vineyard soil responsibly, a viticulturalist will benefit from having some knowledge regarding the properties of soil organic matter.

Organic matter is defined as all carbon compounds formed by living organisms. In soil it consists of three components, namely dead organic matter, living parts of plants (eg. roots) and living microbes and animals (Figure 1). Of these three, the first is the largest component, comprising between 70% and 90% of the soil organic matter (Gregorich et al., 1997). Recently died plant material, soil microbes and insects/animals are in a very dynamic state in that it quickly develops into humus. As a result, humus is the largest component of the dead organic matter fraction.

The dead organic matter (humus and fresh soil organic matter) consists of various chemical compounds that individually plays a role in the ecosystem, breaks down at different rates, and gives each soil its own unique character. The most important compounds, that make up the bulk of the material, are: (1) cellulose, (2) lignin, (3) chitin, (4) lipids, (5) protein, and (6) simple soluble molecules.

Cellulose is a carbohydrate that composes of glucose that is bound together in long chains (Figure 2). Carbon (C), oxygen (O) and hydrogen (H) are the main elements of this compound. It is prominent in woody substances, straw, stubble and leaves. Through hydrolysis fungi and bacteria convert it to carbon dioxide (CO2) or cell carbon. Cellulose is relatively resistant to microbial attack and the chemical process of hydrolysis. The initial pace of cellulose hydrolysis is therefore slow, often making it the rate limiting reaction in microbial breakdown of soil organic material.

Lignin is a complex long-chain compound (polymer) that is made up of thousands of rings of C atoms joined together in a long chain. The way in which they are linked up varies along the chain. It also contains only C, H and O, but is aromatic compared to cellulose in that it contains phenolic groups. Lignin binds to cellulose fibers to harden and strengthen cell walls of plants. It is plentiful in older higher woody plants, especially in the branches and trunks (Table 1). The most important characteristic of lignin is its high resistance to enzymatic breakdown. The result is that, with the exception of a few specialized fungi and actinomycetes (unicellular filamentous microorganisms that possess properties intermediate between the fungi and the bacteria), soil organisms cannot digest wood. Decomposed soil organic matter will therefore often end up having high lignin contents but little cellulose.

Chitin is also a polymer, analogous in chemical structure to cellulose, but consists of units of a nitrogenous derivative of glucose. It is, therefore a polysaccharide, with amino sugars as its basic structural unit. Like cellulose, chitin contributes strength and protection to organisms. Soil chitin originates from the remains of insect cells and fungi, as well as some actinomycetes and yeasts. It is mainly broken down by actinomycetes, which proliferate in its presence, especially those that excrete anti-fungal substances.

Lipids are substances produced by organisms that are insoluble in water. It includes a lot of compounds like pigments, fats, waxes, phospholipids, sterols, di- & triglycerides, etc. They generally are hydrophobic. Lipids are broken down by specific groups of bacteria that secrete an enzyme (lipase) which hydrolyze it to break down to fatty acids. Last mentioned are then broken down further through oxidation.

Protein is a group of complex organic compounds, mainly one or more chains of amino acids that are linked together by peptide bonds. It therefore contains C, H, O, sulphur (S) and very characteristically, nitrogen (N). They are broken down very easily through hydrolyses of the peptide bonds by enzymes that are excreted by an innumerable amount of heterotrophic soil organisms (organisms which require carbon in organic form). Thereby the amino acids are released which then serve as sources of C and N to the soil organisms. Because of its high N content, protein contributes to the N in soil organic matter and therefore increases the C:N ratio of the soil.

Simple soluble molecules are usually the product of microbial breakdown of organic matter. Examples are, as mentioned above, amino acids, also acids, sugars, alcohols and aromatic groups. These are readily available for uptake or utilization by the soil micro-organisms.

As dead organic matter breaks down, it forms into the most important part of dead soil organic matter, i.e. humus. Last mentioned can be defined as a mixture of colloidal degradation products that accumulates in the soil because it breaks down slower than the products it was formed from. Humus is therefore relatively resistant to microbial breakdown. One of the major substances of humus is residual lignin. Despite its resistance to microbial breakdown, it oxidizes to give humus its black colour. Furthermore, the phenolic groups also contribute to the characteristic smell of humus. The slow process of humus decay happens in two ways, namely oxidation in hot dry climates and microbial breakdown in warm wet climates. A flux in humus decay usually occurs after a dry spell. This is due to the oxidation that takes place in the dry period whereby C is lost from the humus as CO2 and the chemical composition of the resistant compounds in the humus is altered. Afterwards, when the soil is moistened, an explosion in the population of soil organisms occurs due to the sudden increase in digestible compounds and the favourable climatic conditions.

Breakdown of Soil Organic Matter

By now it must be clear that the process of soil organic matter breakdown is complex. It involves different stages of breakdown taking place simultaneously, with different compounds breaking down at different rates and due to the activity of different organisms (Figure 3). The stages of breakdown can be separated into (1) the colonization and physical fragmentation of fresh organic matter, (2) the chemical alteration of the organic matter and, finally, (3) the release of mineral nutrients.

Leaf- and root surfaces are colonised by micro-organisms even before they die. Once the dead organic matter lands on the soil, they immediately excrete enzymes that oxidize the organic matter to obtain energy and C. In the process C is lost through the release of CO2. The result is a decrease in the C:N ratio of the organic matter. Simultaneously, earthworms and other larger soil animals such as mites and ants, start fragmenting the fresh organic matter. The surface area is thereby increased allowing more micro-organisms to colonise and decompose it. Soil animals, especially earthworms, assist with the colonisation process by incorporating organic matter into the soil (where conditions for microbial colonization are more favourable) and mixing it with intestinal microbes as they digest the material. The fungi, bacteria and actinomycetes secrete enzymes to break up the organic compounds which all are, as mentioned above, long and complex chains. Simpler molecules are then formed from which, with further decomposition, are mineral nutrients finally released. This process is called mineralization, i.e. the biological process by which organic compounds are chemically converted to simpler organic compounds and finally to mineral nutrients (Figure 4).

Because of the large amount of N that can be released from incorporated soil organic matter for plant nutrition, N-mineralization is of economic importance. It consists of two steps:

1. Ammonification - the release of N as ammonium (NH +) from organic molecules through enzyme activity of bacteria & fungi (Figure 5).
2. Nitrification - the release of N as ammonium (NH +) from organic molecules through enzyme activity of bacteria & fungi (Figure 5).

From Figure 5 it can be deduced that urea, being an organic molecule, also requires mineralization before the N therein is released for uptake. This process of urea breakdown, however, is done by a very specific group of bacteria that are able to release an enzyme called “urease”. Low microbial activity is the reason why the use of urea is not recommended as fertilizer on coarse sandy soils. Furthermore, it is worthwhile noticing that two hydrogen ions (H+) are released to the soil for every NH + -ion that is nitrified. The implication is that the mineralization of all organic matter (especially urea) acidifies the soil. Where the use of urea or organic matter applications is frequent, maintenance lime applications should be considered for soils with a pH(KCl) lower than 6,0.

The amount of N, as well as the rate by which it is released, is determined by the C:N ratio of the soil organic matter. The C:N ratio is an expression of the amount of C compared to N that are in the soil, but does not give an indication of the forms that the C and N are in. The value of knowing this ratio is that one can deduce the potential N- availability to both soil micro-organisms and plants. The lower the C:N ratio, the more of the N in the organic matter will be available for mineralization and, therefore, for plant nutrition. On the other hand, a high C:N ratio causes periodic plant N deficiency because the soil micro-organism population is under-supplied by N and as a result consume all the available N, both in the organic and mineral form. This is called immobilisation of mineral N. The addition of organic matter with a high C:N ratio to the vineyard soil can therefore lead to a periodic N deficiency in the vines. In Table 2 an indication of the C:N ratio of some fresh organic materials are given with an indication of its corresponding rate of N-release.

The Rhizosphere

Related to a grape production system, the rhizosphere is the most important facet of soil microbiology. It is the region of soil immediately adjacent to, and directly affected by, the vine roots.

Active growing roots exude organic compounds (exudates) into the soil. These compounds are a food source for micro-organisms, causing their number to be up to 500 times higher than in the rest of the soil (McLaughlin et al., 2000). The characteristics of the exudates changes according to the age of the plant, season, chemical inputs, water availability and nutritional status. The rhizosphere is therefore a constantly changing environment, in which the relative number of each member group adjusts according to the nature of the exudates and the environmental conditions. For example, when a vine wilts due to water stress, more amino acids are released by the roots to the rhizosphere. Likewise, some plants release organic acids into the rihizosphere in response to iron (Fe) or P deficiency, which then solubilise Fe, P, Zn and other metals, thereby enhancing their bioavailability (McLaughlin et al., 2000).

Chemicals also affect the microbial situation of the rihizosphere. It happens in three ways, namely (1) through its direct influence thereon by being incorporated into the soil; (2) by being absorbed by the plant leaves and translocated to other plant parts where it alters the plant’s metabolism and, as a result, affects the composition of the exudates; and (3) by being absorbed by the plant, translocated to the roots and exuded by the roots into the rhizosphere. For example, application of a Bordeaux mixture to the leaves of bean plants was found to increase the Cu in the rhizosphere, reducing the bacterial numbers therein (Wilhelm, 1966).

Relating to soil pathogenic infections, the rhizosphere is often called “the first line of defence”. Pathogens can only cause a disease by entering the root from the rhizosphere. For it to successfully establish itself as a member of the rhizosphere, it must be able to compete with the other saprophytes for nutrients. But because of the high numbers of microbial organisms and their enhanced activity, which is associated with antagonistic and competing reactions, the rhizopshere generally protects the roots against infection. However, if the balance of this micro-habitat is disturbed by a chemical or detrimental environmental conditions (drought, water-logging, nutritional deficiencies) the effectiveness of the saprophytes to compete with the parasites/pathogens reduces and results in root-infections. In terms of the root environment, biological control of fungal diseases would therefore entail the maintenance of a high activity of beneficial organisms in the root-zone.

Managing Soil Biological Fertility

Although management practices are known to impact the biology of vineyard soils, there is limited knowledge to support the development of detailed management strategies. A particular practice may have the desired result in one situation but have little effect in another because biological communities respond to the interaction of multiple factors including food sources, moisture, historical land use and physical habitat.

A common denominator is that soil potential improves when the number and complexity, or diversity, of the soil biological community increases. There, however, is also an economic limit to the amount of diversity that one might want to obtain. Some practical ideas on increasing and maintaining biological activity and diversity are therefore suggested below.

Firstly, supply organic matter to the soil. It is known that over time, the organic matter content of cultivated soils decreases, especially during the first years after development (Figure 6). Organic matter can be added to the soil by utilising crop residue maximally, i.e. do not remove shoots after pruning, by applying compost or manure (if available and affordable) and by planting cover crops. Surface residue, like shoots and stubble, encourages the decomposers, especially fungi because they have an advantage over bacteria in digesting surface residue. Earthworms and arthropods are also benefited. In addition to above ground residue produced, cover crops increase the sub-soil organic matter content through its dense root system. The rhizosphere of cover crop roots also increases the biological activity of the soil significantly. Compost inoculates the soil with a wide variety of organisms while animal manure provides food and mineral nutrients to both larger organisms and micro-organisms. A combination of manure and plant matter will obviously support a larger mixture of soil organisms (Anonymous, 2005).

Secondly, rotate crops. This practice regularly puts a different food source into the soil that results in a wider variety of organisms and prevents the build-up of a single pest species. Within a vineyard context, this can be achieved through rotation of cover crops. For example, on a sandy soil saia oats can be followed up after two years by grazing vetch, which is followed up by pink seradella (personal communication: J. Fourie).

Thirdly, protect the habitat of soil organisms. Soil organisms need air, moisture, a constant food supply and protection against adverse elements. Soil compaction changes the movement of air and water through soil, and may cause a switch from aerobic to anaerobic organisms. It also reduces the space available for larger organisms to move through the soil. The ecosystem is therefore affected, with the result that the population of fungi, larger predators and arthropods declines. Among the nematodes, the predatory species are the most affected, followed by the fungal feeders and then the bacterial feeders. Root-feeding nematodes, however, are the least sensitive to compaction – perhaps because they do not need to move through the soil in search for food (Anonymous, 2005).

Clean cultivation leads to the migration of most arthropods or to their starvation. The soil climatic conditions are also adverse to any biological activity, while cover crops will stabilize the soil moisture and temperature conditions and maintain the biological habitat. Although tillage enhances bacterial growth in the short term by aerating the soil and mixing the organic matter with the soil, it leads to an accelerated loss of C as CO2 and it triggers explosions of bacterial predators like protozoa. Over the long term the soil organic matter that fuels the organism population reduces and as a result the populations of fungi, earthworms, nematodes and arthropods will decline due to a lack of food, rather than the (also detrimental) mechanical action of tillage.

Pesticides also impacts non-target organisms. It feed some organisms and harms others. Few pesticides have been studied for their effect on a wide range of soil organisms, so that the net effect of moderate chemicals is not well understood. We do know, however, that many of the well-known pesticides reduce soil biological complexity (van Zwieten, 2004). Herbicides often do not affect soil organisms directly, but the weed loss (in clean cultivation) changes the food sources and habitats available to organisms. Fertilisers provide some nutrients to organisms and favour those species that can utilize mineral nutrients. The pH and salt effect of fertilizers like ammonium nitrate and potassium chloride reduces populations of bacteria, fungi and nematodes temporarily (Anonymous, 2005). On the other hand, responsible fertilizer use increases plant growth, and therefore organic inputs into the soil which benefits biological activity. This is especially applicable to cover crops.

In conclusion, it is important to consider the fact that soil biological processes can develop slowly and it can take time before changes are noticed. Some processes can take place at enormous rates, eg. bacterial population growth after soil cultivation, affecting the soil within days, while other processes takes many months or even years to impact the soil.

Practical considerations to keep in mind

The presence of organic matter is a prerequisite for microbial activity. One therefore must ensure its presence. Clean cultivation leads to a loss of soil organic matter, increases soil density and soil erosion, while temperature extremes also develop in the topsoil. All are detrimental conditions to soil biological activity. Rotated cover crop cultivation and the maintenance of pruning material in the vineyard should be an integral part of a viticulturalist’s soil management strategy.

The use of organic matter and urea acidifies the soil, regular monitoring of the soil pH.

The C:N ratio of applied organic matter will determine whether N is released rapidly, slowly or whether mineral N in the soil is immobilized periodically. As a general rule, an estimated 50% of all organic N applied to soil will be released during the course of the first year. Thereafter 25% is released in the second year and 10% in the third. Through annual additions of organic material the N released from the soil organic matter will therefore increase continuously. Mineral N-fertilisation will therefore have to be decreased.

The health of the rhizosphere determines to a large extent the health of the vine, particularly its roots system. Many chemicals affect it and as a result, soil conditions that contributes to the rapid restoration of the rihzosphere ecosystem benefits the health and performance of the vine. A high microbial activity in the soil, maintained by the presence of organic matter, is the crucial aspect.

Literature Cited

Anonymous, 2005. Soil biology and soil management. www.extension.unm.edu

Cowan, D. 2005. Building soil organic matter. Agri-Food Laboratories CCA,

Dickinson, C.H. & Pugh, G.J.F., 1974. Biology of plant litter decomposition. Academic Press, London, 775pp.

Gregorich E.G., Carter M.R., Doran J.W., Pankhurst C.E. and Dwyer L.M., 1997. Biological attributes of soil quality. In: ‘Soil quality for crop production and ecosystem health’. (Eds. E.G. Gregorich, M.R. Carter) pp. 81-113. Elsevier, Amsterdam.

Johnston, A.E., 1973. The effects of ley and arable cropping systems on the amounts of soil organic matter in the Rothamstead and Woburn Ley arable experiments. Rothamstead Experimental Station Annual Report for 1972, 2,131-159.

McLaughlin, M.J., Hamon, R.E., McLaren, R.G., Speir, T.W., & Rogers, S.L., 2000. Review: A bioavailability-based rationale for controlling metal and metalloid contamination of agricultural land in Australia and New Zealand. Aust. J. Soil Res., 38, 1037-1086.

Van Zwieten, L., 2004. Impacts of pesticides on soil biota. In: “Soil Biology in Agriculture.” Proc. of a Workshop on Current Research into Soil Biology in Agriculture, (Ed. Lines-Kelly, R.) NSW Dept. of Primary Industries, Tamworth, Australia.

Wilhelm, S., 1966 Chemical treatments and inoculum potential of soil. Annual Review of Phytopathology, 9 (4), 53-78.

In the light of decreasing availability of water for irrigation, producers are compelled to investigate the possibility of using water sources previously not considered, i.e., boreholes or wastewater. The quality of these sources often raises concern, making producers unsure whether they can rely on them for long-term crop production or new developments.

In this document, the most common factors that determine whether water is suitable for irrigation, as well as possible mitigation options, are discussed.

Water salinity

The first quality parameter that must be considered is total dissolved solids (TDS), which is a measure of the total quantity of various inorganic salts dissolved in water.

The TDS is assessed by measuring the electrical conductivity (EC) of the water. The principle of electrical conductivity measurement is based on the fact that salts in water increase the rate at which electricity is conducted in the water – the higher the salt content, the higher the conductance.

The TDS is typically calculated mathematically by laboratories from the EC by using a factor that will differ according to the unit in which the EC is expressed. The applicable factors are provided in the table below, with the TDS being expressed as milligrams per liter (mg/L), which is equal to parts per million (ppm).

Interpretation of the total salt content of water is done, using a scale of EC values – the higher the EC, the less suitable the water becomes for crop production. Different EC ranges are used to classify the water’s quality – the different classes that are internationally used as an indication of the water’s suitability for crop production are provided in the table below. The EC, however, does not indicate the type of salts that are present in the water.

Sodium content of the water

Even though the water’s total salinity places it in salinity Class C1, C2 or C3, it can still be unsuitable for irrigation due to disproportionately high sodium (Na) in the water. In addition to being a salt that can be damaging to plant tissues in extremely high concentrations, Na is harmful to soils since it displaces Ca and Mg on clay particles which results in clay dispersion and breakdown of soil structure. A decreased water infiltration and/or drainage, with an exponential increase in the rate of salinification of the soil, is the end result.

To evaluate the risk that Na poses to the soil, the ratio of Na in relation to calcium (Ca) and magnesium (Mg) in the water is used, being expressed as a factor called the sodium adsorption ratio (SAR).

The SAR is a value that is mathematically calculated by the laboratory from the water Na, Ca, and Mg analysis results. An interpretation of the SAR values is provided in the table below.

Irrigation water analysis reports will therefore often contain a so-called “irrigation class”, which is expressed as “C…S…”. For example, highly saline water (EC > 2.25) with a low SAR (<5) will be classified as C4S1 water. From the tables above, these classifications can therefore be interpreted.

Residual sodium carbonate index

When the pH of irrigation water exceeds 8.0, a useful alternative measure of the risk that sodium poses is the residual sodium carbonate (RSC) index. In this index, the bicarbonate (HCO3–) and carbonate (CO32-) concentrations in the water are brought into consideration. The logic being that precipitation of soil Ca and Mg with HCO32- and CO32- in the irrigation water subsequently leads to an increase in the relative proportions of sodium to the other cations in the soil solution. This situation, in turn, will increase the sodium hazard of the soil-water to a level greater than indicated by the SAR value of the irrigation water. A high RSC of irrigation water, therefore, results in an increase in the soil’s exchangeable sodium percentage (ESP), with the consequent dispersion of clay, as referred to above.

On its own, high HCO3– and/or CO32- levels in irrigation water also increases the water pH, cause clogging of irrigation systems due to calcite or lime deposition, and a whitish deposition form on leaves and fruit from the droplets of irrigation water.

In arid and semi-arid regions underground water often has a high pH and RSC. The higher the pH of the water, the higher the HCO32- and CO32- concentration will be, with a progressive shift towards CO32- as the pH increases, leading to clogging of irrigation systems. The guidelines below can be used to assess the risk of drip irrigation systems becoming blocked due to calcite/lime deposition.

A calculation of the RSC index, to assess the risk of soil sodification should be done if the HCO32- and CO32- concentration of the water exceeds 120 mg/L and 15 mg/L respectively, and the SAR > 3.

The RSC index can be calculated from normal irrigation water analysis results, using the following formula: RSC index = ((HCO3/61)+(CO3/30))–((Ca/20)+(Mg/12)) where the concentrations are in mg/L, and the resulting RSC index value is expressed in milliequivalents per liter (meq/L).

The following indicates the suitability of water for irrigation on account of the RSC index (meq/L):

Management and mitigation possibilities

From the above tables, it is implicit that while it is possible to use a wide variety of water quality types for irrigation, management procedures become more demanding as the water quality decreases – the suitable range of soils and crops also become more restrictive. The possible management options to prevent the build-up of salt in the soil, and ensure long-term crop performance, are:

Soil drainage: To avoid the build-up of salts, the soil must be permeable – permeable soil can be leached to wash out the salts. In this regard, sand is much more forgiving than heavy soil.

Leaching: Salt build-up in soil is avoided through leaching. Through regular monitoring of salt levels in the soil (measuring the soil’s EC or resistance), the need for leaching (or its effectiveness when practiced) can be determined. Generally, the higher the salt level of the water, the more arid the climate, the greater the leaching requirement will be.

Irrigation frequency and duration: Irrigation scheduling has a huge impact on salt accumulation or reducing levels in the soil. Short, regular cycles will more likely result in salt build-up in the topsoil because of evaporation. On the other hand, long cycles leading to deep penetration of water, mitigate the build-up of salts in the plant root zone.

Soil and water amelioration: Irrigation water can be modified with gypsum to improve its SAR. The quantity of gypsum needed for adding to irrigation water depends upon the quality of water (the SAR and RSC) and the quantity of water required for irrigation during the growing season of the crop. The addition of gypsum increases water salinity, so this is only an option when the water’s salinity is not too high, and when the soil is well-drained. Soils can also be modified using gypsum to ensure proper drainage (it helps to maintain and improve the soil’s structure), but this should only be used when the soil’s exchangeable sodium percentage exceeds 10% – otherwise, the soil is actually further salinified.

In this information sheet, only three water quality parameters were discussed, i.e., total salinity, SAR, and RSC. These are regarded as the most common restricting factors. Water quality problems can also be associated with the presence of other constituents, like excessive levels of iron (Fe) or manganese (Mn), that can result in drip irrigation being blocked.

Crust formation that typically occurs on the surface of soil that is irrigated with water that has a high RSC index or an excessive SAR – the result is excessive run-off, i.e. poor water infiltration

Both sulphur (S) and boron (B) are essential plant nutrients. Sulphur is a macro-nutrient of which roughly about 30 kg/ha is utilized by orchards, while B is a micro-nutrient and only about 120 g/ha is annually taken up by trees. For inexplicable reasons, many producers have neglected to analyse their soil and leaf samples for both of these two nutrients – in this information sheet the role of these two nutrients in tree crops, as well as the value of knowing the S and B concentrations of soils, are discussed.

Sulphur in plants:

Sulphur (S): In the tree, S is important for the production of amino acids, proteins, and chlorophyll; so deficiency symptoms express as a light green to yellow colour, sometimes very light yellow to almost off-white. Deficient plants also have retarded growth.

Sulphur in soil:

In soil, S is found in a large variety of minerals. Near the coast, S-containing spray – or, in industrial areas, emissions – ends up in the soil. It can vary between 1 and 100 kg/ha/year, which is enough to meet the nutritional needs of trees. A considerable amount of S is also found in the organic fraction of the soil, with N:S ratios varying between 10:1.2 and 10:1.5. This S is released through mineralisation. Citrus trees, for instance, require slightly more sulphur than phosphorus to produce optimally. From this, it should be clear that S nutrition ought to receive attention and be routinely incorporated into the fertilisation programme. In many areas that are far from industries, sulphur (S) deficiencies are becoming more common. Deficiency also often appears on very sandy, low pH soils where it leaches out more easily. Any S deficiency in soil can easily be addressed with a once-off application of 1 ton/ha gypsum. But, bear in mind that recovery from deficiencies in the plant takes place gradually over more than one season. It is, therefore, better to be proactive and to ensure sufficient S levels in the soil from orchard establishment. It should therefore be a standard practice to test for the soil’s S concentration.

Boron in plants:

Boron (B): In the tree, B is involved in sugar/carbohydrate transport and cell division, and since it is immobile in the tree, symptoms develop in young growing sites, such as apical meristems, terminal younger leaves, flower development, fruit set, and young fruit development.

Boron in soil:

In the soil, B is generally associated with micas, clay, and especially sesquioxides, while the organic fraction can also have a meaningful B content. It occurs in the soil solution mainly in an undissociated, electroneutral boric acid (B(OH)3) form and is mobile in soil – i.e., acidic soil can become depleted of B owing to leaching from rainfall or excessive irrigation. In calcareous (or over-limed) and saline soils, the plant availability of B is also reduced. Boron deficiencies in soil are widespread but very common on acidic, sandy soils. The margin between sufficient and deficient supply of B in the soil is quite narrow. Soil applications, therefore, are risky. Even though the correct quantity is applied, poor distribution can lead to areas of toxicity. Some crops are very sensitive to excessively available soil boron. When taken up in excess, leaf symptoms can be confused with excessive heat stress, salinity, K deficiency, or even biuret toxicity.

Of the six micronutrients, B and molybdenum are the easiest to correct by foliar application just before or during flowering or to maintain the level with a foliar application during October. However, since one can easily induce B toxicity with foliar sprays, it is necessary to establish what the soil and leaf B concentrations are – this allows the producer to make an informed decision whether they need to apply foliar B to their orchard(s)

‘n Grond se vermoë om water vas te hou is ‘n funksie van die grond se tekstuur. Tekstuur word bepaal deur die hoeveelheid en verhouding van sand, slik en klei deeltjies tot mekaar. Hierdie verhouding is ‘n funksie van die moedermateriaal waaruit die grond ontstaan het, en bly dus konstant vir ‘n bepaalde grond. ‘n Eenmalige laboratoriumtoets om ‘n grond se waterhouvermoë te bepaal word dus aanbeveel.

Die onderstaande tabel wys die invloed van tekstuur op grond eienskappe van belang by waterbestuur:

Die berekeninge hieronder het ten doel om jou te help om riglyne te ontwikkel waarteen jy die besproeiingsvolumes wat jy tans toedien kan evalueer. Daar is min produsente wat besproeiingsmonitors in elke blok het, en in sommige gevalle word ‘n algemene riglyn vir ‘n hele plaas gebruik. Effektiewe verbruik van water gaan toenemend belangrik word, en dit is elkeen se verantwoordelikheid om dit goed te bestuur.

• Die waterhouvermoë van ‘n grond word uitgedruk in mm water/m grond diepte.
• Die eenheid van reën/besproeiing is mm, en 1 mm reën of besproeiing is gelyk aan ‘n toediening van 10m3 water/ha.
• Die tabel hieronder toon dan dat ‘n growwe sand, 50mm water oor ‘n diepte van 1m kan vashou.
• Dit is gelyk aan 500m3/ha, of 500m3/10 000m3
• Op hierdie punt is al die grondporieë met water gevul, en enige addisionle water sal van die oppervlak afloop, of verby ‘n diepte van 1m dreineer.
• Ons besproei baie selde wanneer die grond kurkdroog is. Gewoonlik word die water aangevul wanneer die gewasse 50% van die water onttrek het. In hierdie voorbeeld beteken dit dan dat slegs 250m3 water nodig sal wees om weer die grond te versadig.
• Indien die gewas se wortels net 60cm diep is, word die watervolume wat in die grond vasgehou word met 40% verminder tot 150m3 – ons bestuur immers die water in die wortelsone.
• By wingerd en vrugte speel rywydte ‘n rol, want dit is slegs die plantrye wat besproei word.

Op ‘n growwe sand, met ‘n 3m ryspasiëring (33rye/100m), ‘n worteldiepte van 80cm, en ‘n benatte plantry van 1m wydte, word die waterhouvermoë van ‘n ha area soos volg bereken:

Ander faktore wat besproeiingsvolumes kan beïnvloed sluit die volgende in:

Organiese materiaal verhoog ‘n grond se waterhouvermoë. Klip verminder die grondvolume en verlaag dus waterhouvermoë. Bewerking en verdigting vernietig grondstruktuur, en het dus ‘n negatiewe invloed op die grond se waterhouvermoë. Die effektiwiteit van die besproeiingstelsel. Periodieke loging van oortollige soute. Bespreek hierdie inligting met jou besproeiingsadviseur ten einde water en krag verbruik te optimaliseer.

Al die blokke is op dieselfde besproeiingskedule. ‘n Vermorsting van water, kos en krag.

In hierdie skrywe deel ons graag wat in ‘n onlangse verslag van die Bemestingstofvereniging van Suid-Afrika (FERTASA) gepubliseer is met julle. Produsente is soms besorg oor die kwaliteit van hul kunsmis, te wete of dit werklik die verklaarde hoeveelheid N, P, K, Ca, Mg en spoorelemente op die etiket bevat. In ‘n opname wat verlede jaar gedoen is, is die werklike ontleding van 101 kunsmisprodukte van 16 verkillende verskaffers se produk se etiket vergelyk. Die 98 korrel-, en 3 vloeibare produkte is op plase gemonster.

Volgens wet, mag die totale plantvoedsel (N+P+K) in ‘n produk, nie met meer as 14g/kg (1.4%) varieer van die geregistreerde waarde van die produk nie. Dit is ‘n hoë standaard gegewe die skaal van operasies in kunsmisaanlegte. Tog is dit belangrik dat boere, en plante , kry wat aanbeveel is.

Die bevindinge kan soos volg opgesom word:

• Die oorgrote meerderheid verskaffers en hul produkte se stikstof (N)-konsentrasie stem ooreen met, of is hoër as, dit wat verklaar word. Die onderstaande figuur wys dat gevalle waar die N-konsentrasie laer is as die verklaarde waarde, dit slegs met 5% of minder te laag is.
• In die geval van fosfaat (P) is die prentjie egter effens slegter, veral in die Somerreënvalstreke (Noorde) – 40% van die produkte se konsentrasie is laer as die verklaarde waarde. Die blou blokkies is produkte van FERTASA lede, terwyl die groen driehoekies van nie-lede is.

• Wat kaliumbemestingstowwe betref, is die meerderheid produkte se kalium (K) inhoud hoër as die verklaarde konsentrasie – 25% s’n is egter effens minder as wat die etiket aandui.

• Sover dit kalsium (Ca) en mengsels van N:P:K aangaan, was die resultate soos volg: Uit 20 Ca-produkte, was slegs een se Ca-konsentrasie onder die verklaarde waarde.

In die geval van N:P:K mengsels, is 87 produkte ontleed en 23 van hulle se totale voedingselementkonsentrasie was tussen 1 en 5% laer as wat dit moet wees.

Samevattend kan gesê word dat al die N- en Ca-produkte se konsentrasies goed is. Dit geld ook vir K- en S-produkte in die Wes-Kaap. Verder lyk dit asof die Wes-Kaap se produkte ‘n bietjie minder variasie toon, veral wat P betref.

Dit blyk dat, produsente oor die algemeen (>80%) meer plantvoedsel kry as wat die produkte se etikette aandui en waarvoor hulle dus betaal. Enkele uitsonderings kom wel voor, maar hierdie aantal probleemgevalle het in die afgelope 3 jaar van 20% tot 14% afgeneem.

As guidelines upon which soil analyses are interpreted and fertilisation is applied, the basic cation saturation ratio (BCSR) concept (now better known as the Albrecht system) has become popular amongst various plant nutritionists. It is based on a presupposition that plants will only grow optimally if there is a balanced ratio of cations (Ca2+, Mg2+ and K+) for each soil according to its cation exchange capacity (CEC). Fertilisation is therefore done according to the soil’s needs, not the plants’.

This development is unfortunate. Apart from the fact that it has no proper scientific foundation with regard to plant nutrition, a line is being drawn through all the proper scientific work done on plant nutrition through the years. In a review article, Kopittke & Menzies (2007) trace the BCSR concept back to the late 1800’s and found that since its origin no research data has been able to prove the existence of any “ideal” basic cation saturation ratio. Instead, they found that promotion of the BCSR concept resulted (and will result) in inefficient use of resources and fertilisers.

Instead, research by various local and overseas scientists has shown that the Sufficiency Level of Available Nutrients (SLAN) concept, where a minimum concentration of available nutrients in the soil is required for optimal plant nutrition also applies to vines. Producers are again reminded that although the “ideal” soil varies dramatically from region to region and soil type to soil type, it generally looks as depicted in the table below. It is based on a minimum level of nutrients that is required in the soil to supply the vine of its nutritional requirements. This is an excerpt from an article by Van Schoor, Conradie & Raath (2001):

References

Kopittke, P.M. & Menzies, N.W., 2007. A review of the use of the basic cation ratio and the “ideal” soil. Soil Sci. Soc. Am. J., 71, 259-265.

Van Schoor, L., Conradie, W.J. & Raath, P.J., 2000. Guidelines for the interpretation of soil analysis reports for vineyards. Wynboer, 135, 101-103,

In the world of soil science, all particles of 2mm and larger, are regarded as course fragments or stones. In a soil analysis report the amount of stones in a soil is expressed on a volume basis, e.g., 30% of the soil sample received at the lab consisted of fragments ≥ 2mm.

Stones are inert. They do not hold water or nutrients, but they take up space, and as such, reduce the soil’s capacity to store water and nutrients.

Water (and air) is stored in the pores between the soil particles. Stones reduce the number of pores available for storing water and thus reduce the soil’s water holding capacity. In a stony soil, irrigation must, therefore, be more frequent and in smaller volumes.

Nutrients are adsorbed to the surface of soil particles, especially clay, silt, and organic particles. More stones equal less soil particles, which equals a smaller nutrient holding capacity in the soil. Fertilisers must be applied in smaller installments and more frequently if the soil is stony.

Having a good knowledge about the stone content of your soils is very important – it impacts on your irrigation scheduling and nutrient management. For this reason, the measurement is included is Labserve’s analyses.

The lab can only report on the stones received with the sample. It is therefore important to estimate the amount of stone within the soil profile, as it is often not practical to include it in the lab sample. The estimated and/or the measured amount of stone must be taken into consideration when calculating irrigation volumes, lime requirements, and fertilizer applications. This will ensure that resources such as water and fertilizers are managed optimally, and that conditions in the root zone remain favourable for root growth.

Soil clay minerals and organic matter tend to be negatively charged, thus attracting the positively charged ions (aka cations) on their surfaces. As a result, the cations remain within the soil root zone and are not easily leached. The adsorbed cations may easily exchange with other cations in the soil solution, hence the term “cation exchange.” The adsorbed cations replenish the ions in the soil solution when concentrations decrease due to uptake by plant roots. CEC should not be confused with T-value. Soil analyses results often display a T-value, which is the sum of ALL exchangeable cations, including free (in solution) cations and those on the exchange sites. The T-value is therefore often larger than the CEC.

*If you wish to have your soil CEC measured you will have to ask the lab specifically at an extra cost.

Soil pH and CEC

Soils dominated by clays with variable surface charge are typically strongly weathered. Weathering is the process whereby soil is formed as rocks and minerals break down. The fertility of these soils decreases with decreasing pH which can be induced by acidifying nitrogen fertiliser such as ammonium sulphate [NH4(SO4)2]. As nitrate-nitrogen (NO3–) is taken up by plant roots one hydroxide (OH-) molecule is produced. This binds with one hydrogen (H+) molecule to form water (H2O). Nitrate is an anion (negatively charged ion) and can easily be leached from the upper soil layers because of over-irrigation and soils will slowly acidify over time. Clearing vegetation or harvesting of high yielding crops like wheat also removes basic cations, which will increase soil acidity over time. Soil pH change can also be caused by natural processes such as decomposition of organic matter and leaching of cations by rainfall. The lower the CEC of a soil, the faster the soil pH will decrease with time. Liming soils at low rates will maintain exchangeable plant nutrient cations. High CEC soils are generally well buffered, and pH changes much less from crop production. Higher lime rates are needed to reach an optimum pH on high CEC soils due to their greater abundance of acidic cations at a given pH.

Nutrient availability and CEC

Soils with a low CEC (1–5 meq/100 g, sand) are more likely to develop deficiencies in K, Mg and other cations, while high CEC soils (>30 meq/100 g, clay) are less susceptible to leaching of these cations. For plant nutrition, the critical factor is whether the net amount of Ca or K in the soil is adequate for plant growth. The addition of organic matter will increase the CEC of a soil but requires many years to take effect.

With large quantities of fertilisers applied in a single application to sandy soils with low CEC, loss of nutrients is more likely to occur via leaching. In contrast, these nutrients are much less susceptible to losses in clay soils. However, as low CEC soils are poorly buffered, a single application of high amounts of fertiliser will cause the EC (salt content) of the soil to spike and may cause root damage to citrus trees. This is especially prevalent in older citrus orchards with a shallow root system. During late winter in the Western Cape when soils are cool, root activity is low, and the uptake of nutrients are reduced. Root damage due to successive fertilizer applications during periods of slow uptake may cause minimal to severe leaf drop. Therefore, it is important to split fertiliser applications and wash in the fertiliser properly to the correct depth.

In the case of old citrus orchards with shallow root systems, it might be necessary to start slowly with fertilizer applications and increase applications gradually as temperatures and uptake of nutrients increase once trees become more active.

The aim of plant analysis (leaves, fruit etc.) is to relate the mineral content of the plant (or part thereof) with its physical appearance, growth rate and yield or quality of the harvested product, and also with the nutrient content of the soil. Precise sampling procedures with respect to the selected plant part and the growth phase (time of sampling) are required. The interpretation of results depends on the assumption that a significant biological relationship exists between the elemental content of the soil, the plant and its growth rate or production.

The purpose of plant analysis therefore is:

• An aid to evaluate the supplying capacity of the soil for nutrients
• To evaluate the response of the plant to fertiliser applications
• To evaluate the relationship between the nutritional status of the plant as an aid to estimate fertiliser requirement, e.g. to diagnose for deficiencies of nutritional elements.

Sampling protocol

Plants consist of complex structures of which the elemental composition is not homogenous. It is known that leaves, stems, shoots and fruit, as well as the position of the organ on the plant, differ in elemental composition. Mixing of different organs in a compound sample is therefore not recommended. The choice of the plant part to be sampled needs to be properly selected.

Refer to Table 1 to ensure that the correct plant part and number of leaves are sampled, during the most appropriate time.

Trees/plants at the edge of the block and at the end of rows should not be sampled - they may be coated with soil/dust particles and/or differ in their nutrient status and irrigation regime from the rest of the block. Do not sample diseased, insect damaged, or dead leaves. Leaves should not be sampled during the hottest part of the day since it can influence leaf composition. Collect samples in the morning, place the leaves in a clean plastic or paper bag and keep it cool until delivered to the laboratory. Do not freeze it under any circumstances.

Trees/plants in a problem spot, as well as adjacent healthy ones should be sampled separately. To identify seasonal trends in perennial crops, it is desirable that routine samples are always collected from the same trees - trees should be clearly marked for this purpose.

Die gebruik van blaaranalises en grondanalises is die akkuraatste tegniek om element behoeftes van ‘n boord of wingerd te bepaal, saam met alle ander inligting van ‘n betrokke blok. Dit bied die versekering van optimale produksie en kwaliteit van ‘n gewas deurdat elke seisoen se bemesting- en blaarvoedingsprogramme op ‘n wetenskaplike basis gegrond is. Die doel van blaar- en grondontledings is dus om bemestingsprogramme so aan te pas dat voedingsprobleme en die duur gevolge daarvan voorkom kan word.

Die waarde van blaar- en grondontledings vir voedingsprogramme van vrugtegewasse

Die gebruik van blaaranalises en grondanalises is die akkuraatste tegniek om element behoeftes van ‘n boord of wingerd te bepaal, saam met alle ander inligting van ‘n betrokke blok. Dit bied die versekering van optimale produksie en kwaliteit van ‘n gewas deurdat elke seisoen se bemesting- en blaarvoedingsprogramme op ‘n wetenskaplike basis gegrond is. Die doel van blaar- en grondontledings is dus om bemestingsprogramme so aan te pas dat voedingsprobleme en die duur gevolge daarvan voorkom kan word.Middel tot einde Januarie is die tyd wat blaarmonsters geneem word vir alle kernvrugte, steenvrugte, wingerde, bloubessies en ander alternatiewe gewasse soos vye. Middel tot einde Maart is die regte tyd om blaarmonsters te neem van sitrus.

Blaaranalises word meestal gebruik vir:

• Kwantifisering van totale element konsentrasies in blare (sluit in N, P, K, Ca, Mg, S, Mn, Zn, Cu, Fe en B).
• Indikasie van vrugtegewasse se voedingstatus.
• Bevestig tekorte, oormate of wanbalanse.
• Toon effektiwiteit van bemestingsprogram.
• Kan verskillende kunsmisstowwe se doeltreffendheid aandui.

Grondanalises word meestal gebruik vir:

• Die bepaling van organiese materiaal inhoud, pH en ander ekstraheerbare elemente (sluit in P, K, Ca, Mg, Na, Cu, B, Zn, Mn en ander).
• Data opbou van ‘n aantal jare se analises om grondveranderinge te observeer.
• Bepaling van die beste manier om ‘n tekort van ‘n element wat in blaaranalises aangedui word reg te stel.
• Die bepaling of ‘n tekort van ‘n element in die blaarontledings chemies verwant is en of nie-chemiese faktore ‘n rol speel in voedingstof opname van ‘n plant. Die opname van voedingstowwe deur ‘n plant kan belemmer word deur oorsake soos droogte kondisies, vloedtoestande, wortel-beskadiging en koue of hittegolf toestande.

Blaar- en grondmonstersneming volgens grond, groeikrag en produksie:

Blaar- en grondmonsters word gewoonlik per blok geneem, waarvan die grootte van die blokke enigiets tussen 0.5 – 10 ha kan wees. Die korrekte manier om die posisies en hoeveelheid blaarmonsters in ‘n blok te bepaal is om verskille in groeikrag en produksie te monitor. Wanneer daar verskille in grondtipe ook teenwoordig is moet monsterneming aangepas word. Boomouderdom, boomhoogte, onderstamme, grondkaarte, lugfoto’s en satellietfoto’s kan verder gebruik word om besluite te neem vir eenhede wat apart gemonster moet word.

Foto 1: In die foto kan gesien word dat daar twee duidelike dele is in die blok waar groei en grondtipe verskil. Die blou gedeelte toon normale groeikrag en die geel gedeelte swak groeikrag. Dit sal dus voordelig wees om ‘n aparte grondmonster en blaarmonster van die twee dele van die blok te neem vir analisering.

In the agricultural sector well-informed, objective discussions are rare. Labserve acknowledges the significance of fostering such dialogues and is committed to openly sharing our knowledge, as well as that of other industry experts, pertaining to the challenges that confront us all. Our primary objective is to introduce actionable initiatives for consideration and engage in productive discussions that involve the collective input of stakeholders. Thank you to everyone who attended and participated to make this event a huge success.

The topics discussed were:

• The reality of high output regenerative agriculture. By Corné Kruger – Jindilli Macadamias
• Laboratory tour and processes. By Larne Auerswald – Labserve
• Sampling; simply difficult! By Michael Bufe – Labserve
• Followed by an open discussion and Q&A session

Please feel free to download the presentations on the right, on desktop, else below on mobile:

On 15 February 2024 we hosted our 2nd Technical Discussion, this time in Nelspruit, at LVCC (Lowveld Country Club). In the agricultural sector well-informed, objective discussions are rare. Labserve acknowledges the significance of fostering such dialogues and is committed to openly sharing our knowledge, as well as that of other industry experts, pertaining to the challenges that confront us all. Our primary objective is to introduce actionable initiatives for consideration and engage in productive discussions that involve the collective input of stakeholders. Thank you to everyone who participated to make this event a huge success.

The topics discussed were:

• Sampling; simply difficult! By Michael Bufe – Labserve
• Management of soil acidity. By Dr. Pieter Raath – CRI/Labserve
• Regenerative agriculture? By Bennie Diedericks – Resalt/Soilution
• Pesticide resistance: a rose story. By Tom Murray – Woolworths
• How to generate Carbon credits and how to use them. By Dr. Philip Goyns – Promethium Carbon
• When BBBEE makes commercial sense. Johan Blignault – SABEE

Please feel free to download the presentations on the right, on desktop, else below on mobile:

On 21 September 2023 we had a fantastic turn out at our Labserve Mycro Stellenbosch branch for a technical discussion. In the agricultural sector well-informed, objective discussions are rare. Labserve acknowledges the significance of fostering such dialogues and is committed to openly sharing our knowledge, as well as that of other industry experts, pertaining to the challenges that confront us all. Our primary objective is to introduce actionable initiatives for consideration and engage in productive discussions that involve the collective input of stakeholders. Thank you to everyone who participated to make this event a huge success (despite the cold weather).

Please feel free to download the presentations on the right, on desktop, else below on mobile: