Sampling Procedures for Field-Based Samples
A soil sample is a collection of individual cores from a known area. A soil core is a continuous length of soil taken with a soil probe to a specific depth. Soil samples are taken as a surface sample and sometimes a deep sample. All the soil from the length of the soil sample is saved. Individual soil cores are combined and mixed. Usually there is more soil in the collecting bucket than room in the sample bag or box. It is important that individual cores are mixed thoroughly before the sample box or bag is filled.
One statement summarizes the guiding principle for proper sampling: Take lots of individual cores for compositing into a single sample, and take lots of samples. Practical considerations must balance what is most applicable. A desirable number of surface samples is at least 15 cores. The University of Nebraska soil test sampling instructions state that an acceptable sample should be taken from every 20 acres and each sample should consist of 10 to 15 cores (see Resources for Soil Testing, Management, and Calculating Fertilizer Rec for more detailed procedures).
Figure 9.1 suggests that six soil samples be collected from the 80 acres which has been divided into three fields. Notice that the various areas from which soil samples are collected correspond to the types of soil in the field. Fewer samples will need to be collected from fields that have fewer differences in soils. The soil survey for a particular county shows the variability of soils in a given field and can be a valuable guide when collecting soil samples. In the past, spreading fertilizer differentially based on sampling was extremely difficult. It was thought that it is better to know the differences in the field, even if managing them differently was a problem.
Figure 9.1. Typical variability by soil mapping unit.
When collecting soil samples for field-based recommendations, do not take soil cores from odd areas, such as:
— old fertilizer bands in row crops
— field borders
— dead furrows, draws
— terraces, old fence rows, or roads
— manure spots
— unusual areas such as alkali spots
However, if you want to sample these areas separately to compare to the rest of the field, the results might help interpret problems or suggest how to treat these areas differently.