06 Feb Soil 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.
- 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.
ARTICLE WRITTEN BY MICHAEL BUFÉ • Labserve Analytical Services Mycro
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