At first glance, soil sampling would seem to be a relatively easy task. However, when you consider the variability that likely exists within a field because of inherent soil formation factors and past production practices, the collection of a representative soil sample becomes more of a challenge.
Before heading to the field to take the sample, be sure to have your objective clearly in mind. For example, if all you want to learn is the average fertility level of a field to make a uniform maintenance application of P or K, then the sampling approach would be different than sampling for pH when establishing a new alfalfa seeding or sampling to develop a variable rate P application map.
In some cases, sampling procedures are predetermined and simply must be followed. For example, soil tests may be required for compliance with a nutrient management plan or environmental regulations associated with confined animal feeding operations. Sampling procedures for regulatory compliance are set by the regulatory agency and their sampling instructions must be followed exactly. Likewise, when collecting grid samples to use with a spatial statistics package for drawing nutrient maps, sampling procedures specific to that program should be followed.
Regardless of the sampling objectives or requirements, there are some sampling practices that should be followed:
Figure 1 The level of accuracy of the results of a soil test will depend, in part, on how many subsamples were taken to create the composite sample. In general, a composite sample should consist of at least 10-15 subsamples. For better accuracy, 20-30 cores, or subsamples, should be taken and combined into a representative sample. This chart shows that if 21 cores per sample are taken, the results will be within 15% either way of the actual mean value.
Soil test results for organic matter, pH, and non-mobile nutrients (P, K, and Zn) change relatively slowly over time, making it possible to monitor changes if soil samples are collected from the same field following the same sampling procedures. There can be some seasonal variability and previous crop effects, however. Therefore, soil samples should be collected at the same time of year and after the same crop. For example, in a corn-soybean rotation, sampling after soybean harvest in the fall would be an excellent sampling system
Soil sampling has much to offer if done properly, but it all starts with the proper soil sample collection procedure, fall after harvest is an excellent time for soil sampling.
Dorivar Ruiz Diaz, Nutrient Management Specialist