Strategies for soil sampling
The commonest scenario that I come across is to provide a sample of soil for analysis of both chemical and microbiological constituents.The objective when gathering growing media for testing should always be to ensure the highest standard of quality and, at the same time, obtain a sample that is representative of the topsoil in a given field or plot. To achieve this, planning and preparation is essential.
Select the right tools
If you plan on taking a number of samples, it will reward you to make an investment in suitable apparatus.
Choosing fields to sample
Choosing where to start on a large estate with multiple fields can be difficult. Instead of attempting to cover every crop in one year, I recommend planning soil tests in accordance with rotations. Certain plants will affect the microbiological communities - brassicas are known to negatively impact soil fungal populations, so crop order needs
to be taken into account. The decision to select fields can be taken a step further by prioritising under-performing or previously untested plots.
Plan your route
For a general assessment of soil condition, a ‘W’ transect is the popular method. This means gathering a portion of material from each point on a ‘W’ shape, avoiding field margins. Spending a few minutes drawing the route on a reference map will provide a better perspective and speed up the actual collection. Your path should consider the inclusion or exclusion of hot spots or zones with known issues. If the area is uniform, 40 ha may be a reasonable limit before taking another sample. If the field is much larger, it is less likely to be consistent and will contain variances such as texture, structure, hydrology, elevation and drainage. Think in terms of quality – will the accuracy of
readings be compromised if not tested separately?
Gather a sample
I recommend starting the process earlier in the day when UV levels and temperatures are lower. It also means that you can make the last post! Once at the first point, check for surface contaminants before inserting the auger. I tend to recover material from below the top 2 cm of the core for additional consistency. If you are using a spade, dig a V-shaped hole to the required depth and remove the slice before partitioning it. A trowel will be easier for extracting fine roots for assessments of mycorrhizal colonisation on plant roots, though if applied carefully, an auger can still be effective. Aim to sample the top 10 cm for microbiology testing (where it is aerobic) and top 15 cm for chemical analyses. If testing both aspects, select a point at around 12 cm. Once bagged, move on to the next transect point. By the time the fifth (final) batch of material is gathered, there should be more than 200 g of soil in the bag. Chemical and microbiology assays will require more than 400 g, so for this reason, I often double up cores at each transect point and aim for more, rather than less material. Once finished, squeeze enough air out of the top of the bag to ensure that it closes correctly, and then place it in a padded envelope for secure and swift delivery to the laboratory.
Note keeping
It is vital to take effective notes to produce a process that is replicable in the future,
and there are many good apps such as Evernote that include a GPS, text and voice notes and photograph functions which can help make this easier. Due to the amount of sampling that I do, I have developed my own series of sample forms which allow me to geo-reference samples (with a phone’s GPS) so that I can revisit sites in the future. I can then export information into spreadsheets and use them in my own soil analytics and visualisation software called i-Soil. Whether you plan to commission external tests from a dedicated laboratory, or undertake your own assessment using a microscope or field-based kits, it pays to put time and effort into
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