The procedures outlined herein are employed in the University of Wisconsin Soil and Forage Analysis Laboratory, Marshfield, and the Soil and Plant Analysis Laboratory, Madison. Several private soil testing laboratories also follow these procedures, including all Wisconsin DATCP Certified Soil Testing Laboratories.

A laboratory test is only as good as the research upon which it is based. These procedures have been modified from the research efforts of a great many individuals, at the University of Wisconsin and elsewhere, and adapted for routine analysis. This is a continuing process. As new information is uncovered through research, soil, plant, and forage tests must be updated to include the latest findings. Consequently, the procedures found herein are revised from time-to-time as needed to keep them current.

Soil test correlation: The first step in developing a soil test is to find a suitable extracting solution. This is the objective of a correlation study. A large number of the more important agricultural soils are collected. These soils are then cropped in the greenhouse, where most of the variables can be controlled. After a specified period, the assay crop is harvested; the amount of the element to be tested that is taken up by the crop is measured.

From knowledge of the chemistry of the element in the soil, several different possible extracting solutions are used to extract the element from the soil. An ideal extractant would remove the same amount of the element as is taken up by the plant. This is rarely achieved in practice, but a close correlation between plant uptake and the amount of the element extracted chemically is sought. In some cases, a regression equation that considers other soil properties may improve the prediction of plant availability of the element in question. Once a suitable extractant has been found, the effects of shaking time, solution-to-soil ratio, reagent concentration, etc. must be studied before the test can be run on a routine basis.

Soil test calibration: After a soil test procedure has been developed through greenhouse and laboratory experimentation, it is necessary to calibrate the test on a large number of sites under field conditions. The objective of soil calibration is to determine the amount of nutrient that must be added to the soil at different soil test levels of that nutrient to obtain maximum yield.

Because variables such as climate, insects, disease, drainage, etc. cannot be controlled as closely in the field as in the greenhouse, it is necessary to repeat field calibration studies three to five years before definite conclusions can be drawn.

Correct usage of soil, plant, and forage results depends on 1) a sample representative of the area or batch from which it was taken, 2) an accurate laboratory analysis, and 3) the correct interpretation of lab results. The laboratory analysis should be the most accurate step unless gross analytical errors go undetected or poor laboratory technique is allowed. Built –in checks can minimize these possibilities. The interpretation of the lab results depends on knowledge of the relationship between the test value and plant, soil, and animal response.

The greatest source of error is usually the sample itself. Since physical samples may be extremely heterogeneous, it is important that the sample tested be truly representative. Procedures for taking representative samples may be found in the following articles:

• Manure Sampling Instructions
• Sampling Soils for Testing (A2100)
• Sampling Garden Soils and Turf Areas for Testing (A2166)
• Sampling for Plant Analysis
• Focus on Forage: Sampling and Evaluating Total Mixed Rations
• Sampling Soils for Preplant (2 feet) and Pre-sidedress (1 foot) Soil Nitrate Tests

The interpretation of soil test results and the procedure for making lime and fertilizer recommendations are covered in detail in 'Nutrient Application Rate Guidelines for Field, Vegetable, and Fruit Crops in Wisconsin' (A2809).