A critical step in soil testing is sample collection. It is important to obtain a representative sample; a poor sample may result in erroneous soil test results and poor recommendations. The first step is to determine the area that will be represented by the sample. Soil physical appearance, texture, color, slope, drainage, and past management should be similar throughout the area. It may be helpful to draw a map of the farm and identify areas where you will sample separately. Using a clean bucket and a spade, auger, or sampling tube collect at least 12-15 subsamples to a depth of 6-8 inches from random spots within the defined area and place them in the bucket. This may be accomplished by walking a zig-zag pattern across the sampling area and collecting a subsample periodically. Avoid sampling field edges, old fence rows, areas where manure or lime were stockpiled, and other non-representative areas. Next, break up any lumps or clods of soil, remove stones and plant debris, and thoroughly mix subsamples in the bucket. This step is very important, because only a few tablespoons of your sample will actually be used for testing. Once the sample is thoroughly mixed, scoop out approximately one cup of soil and spread on a clean piece of non-adsorbent paper to air-dry (brown paper bags work great). Once the sample is dry, place it in a labeled container provided by the lab (or a zip lock bag), and complete a sample submission form. For each sample, indicate the crop to be grown, recent field history and any concerns.
Soil pH and extractable levels for certain nutrients (e.g., P and K) vary throughout the year. While the seasonal fluctuation is not typically large enough to significantly influence interpretation and recommendations, it can make it difficult to compare soil test values from the same field over time. For this reason, it is a good idea to be consistent about timing of sample collection from one year to the next. Although soil samples can be taken any time, many prefer to take samples in late summer or fall because this allows time to apply any needed lime, plan a fertility program and order materials well in advance of spring planting. Avoid sampling when the soil is very wet or within six to eight weeks after a lime or fertilizer application. Routine soil analysis should be conducted once every two to three years.