Taking Soil Test Samples

One of the most important steps in soil testing is collecting the soil sample. The soil sample is the first part of the soil testing process and the foundation for information derived from laboratory analyses, soil test interpretations, and recommendations. Also, soil sampling is the largest source of errors in the soil testing process.

Soil samples can be collected through much of the year, although fall (September to December) or spring (February to April) are the best times. Fall sampling will often result in a faster return of results and recommendations. Fall sampling will also allow the grower time to have the fertilizer applied well before planting the next crop. However, fall sampling results in lower pH and soil test K levels when conditions are dry. In either case, a field should always be sampled the same time of the year in order to make historical comparisons.  Most fields should be sampled every three to four years.

A soil probe, auger, garden trowel, or a spade and knife are all the tools you need to take the individual cores that will make up the “field” sample. You will also need a clean, dry, plastic bucket to collect and mix the sample cores. Be sure not to use galvanized or rubber buckets because they will contaminate the sample with zinc. Soil sample boxes or bags and information forms for submitting samples are available at all county Extension offices.

Collect at least 10 soil cores for small areas and up to 30 cores for larger fields. Take the soil cores randomly throughout the sampling area and place them in the bucket. Do not sample:
• back furrows or dead furrows,
• old fencerows,
• areas used for manure or hay storage and livestock feeding,
• areas where lime has been piled in the past.

One commonly overlooked component of soil sampling is the depth of soil to be tested. Most plant nutrients accumulate at the soil surface. This nutrient stratification is a result of past broadcast fertilizer applications and decomposition of plant residue on the soil surface. Because there is a higher concentration of nutrients on the soil surface, soil test values usually go down as the sample depth is increased. To obtain accurate and consistent (between different years) results, samples must be taken to the following depths for these areas:
• Tilled Areas—Take soil cores to the depth of the tillage operation (usually 6 to 8 inches).
• Non- or Reduced-Tilled Areas—Take soil cores to a depth of 3 to 4 inches for pastures, no-till planting (where fertilizer or lime remains on the soil surface), and minimum-till planting (where fertilizer is incorporated only in the surface 1 to 2 inches).
• Lawns and Turfgrasses—Collect soil cores to a depth of 3 to 4 inches.

After all cores for an individual sample are collected and placed in the bucket, crush the soil material and mix the sample thoroughly. Allow the sample to air dry in an open space free from contamination. Do not dry the sample in an oven or at an abnormally high temperature. When dry, fill the sample container with the soil.

Sampling and preparing the soil for submission is only half of the process. The other equally important part is filling out a sample information sheet so that the desired crop, tillage, and other information can be considered when making the fertilizer recommendation. The sample information sheet contains all the important information required to provide accurate lime and fertilizer recommendations. Sample information sheets for the University of Missouri Soil Testing Laboratory can be found on the web here.  Tests and costs associated with each test is found here.

For additional information, contact your local county extension office.