The challenge of collecting a representative soil sample
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:
- A soil sample should be a composite of many cores to minimize the effects of soil variability. A minimum of 10 to 15 cores should be taken from a relatively small area (two to four acres). Taking 20-30 cores will provide more accurate results. A greater number of cores should be taken on larger fields than smaller fields, but not necessarily in direct proportion to the greater acreage. A single core is not an acceptable sample.
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.
- A consistent sampling depth for all cores should be used because pH, organic matter, and nutrient levels often change with depth. Sampling depth should be matched to sampling objectives. For example, K-State recommendations call for a sampling depth of two feet for the mobile nutrients – nitrogen, sulfur, and chloride. A six-inch depth is suggested for routine tests for pH, organic matter, phosphorus (P), potassium (K), and zinc (Zn).
- When sampling a specific area, a zigzag pattern across the field is better than following planting/tillage pattern to minimize any past non-uniform fertilizer application/tillage effects. With GPS system available, georeferencing of core locations is possible. This allows future samples to be taken from the same locations in the field.
- When sampling grid points for making variable rate nutrient application maps, collecting cores in a 5-10 foot radius around the center point of the grid is preferred for many spatial statistical software packages.
- Unusual spots obvious by plant growth or visual soil color/texture differences should be avoided. If information on these unusual areas is wanted, then a separate composite sample should be taken from these spots.
- If banded fertilizer has been used on the previous crop (such as strip till), then is suggested that the number of cores taken should be increased to minimize the effect of an individual core on the composite sample results, and obtain a better estimate of the average fertility for the field.
- For permanent sod or long-term no-till fields where nitrogen fertilizer has been broadcast on the surface, a three- or four-inch sampling depth would be advisable to monitor surface soil pH.
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