The Soil Testing Process

Soil testing involves more than just a chemical analysis. For the results to be meaningful to a grower, four steps must receive careful attention.

Step 1: Taking A Good Sample

Recommendations based on a soil test can be no better than the soil sample from which they are made. Growersare urged to take great care to be sure that the sample submitted represents as accurately as possible the area from which it is taken. Generally a sample should be a composite of subsamples taken from 10 to 20 spots in the area. Samples from plowed fields should be taken to plow depth, while those from sod or areas not to be plowed should be taken to a depth of 2 inches.

Soil Sampling

Step 2: Analyzing the Sample

This is the chemical extraction and testing procedure used by the laboratory. Although laboratories may use different extraction and analysis techniques, the procedures used must be correlated to plant growth and nutrient uptake in Alabama. In addition, quality control by the staff is essential for reliable and accurate results. The Auburn University Soil Testing Laboratory uses procedures recommended and published by the Southern Extension and Research Activity on Soil Testing and Plant Analysis. (SERA-6).

Step 3: Interpreting the Analysis

The analytical results must be related to plant growth or yield. Extensive soil test calibration research on the crops and soils of Alabama has been conducted and will continue. For each nutrient, crop, and soil, a good calibration must show that plant growth, yield, or nutrient uptake increases as the level of an extractable nutrient increases up to a point where further increases in soil test levels fail to show significant or economical increases of plant growth or yield.

Step 4: Using the Results

When growers receive a soil test report and appropriate recommendations, they must make certain practical decisions which may result in a modification of the given recommendation. Some of these decisions may involve the following:

  1. Using readily available fertilizers or ordering custom blended fertilizer.

  2. Applying the same fertilizer grade to all fields or group of fields or ordering separate fertilizers for each field (or portion of a field) sampled.

  3. Using premium fertilizers which contain secondary and micronutrients or applying only those micronutrients specifically recommended for the crop.

  4. Splitting fertilizer and/or lime applications.

  5. Using starter fertilizers and foliar fertilizers to supplement recommendations.

  6. Modifying nitrogen recommendations based upon comments on report.

  7. Applying fertilizers with other materials such as herbicides.

  8. Modifying recommendations based upon current economic conditions.

These and many other considerations affect how the soil test results are used, and is a decision the grower or crop advisor must make.

Auburn University's Soil Testing Program is a joint program of the Cooperative Extension Service and the Agricultural Experiment Station. The Cooperative Extension Service has primary responsibility for education on soil testing and distribution of supplies. The Agricultural Experiment Station conducts soil test calibration research and operates the Soil Testing Laboratory.

Last Updated: Feb. 24, 2011

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