Soils - Part 9: Fundamentals of Soil Testing

Correlation and calibration are the terms scientists use to classify the research needed to develop a soil test. The distinction between correlation and calibration is explained below.

Correlation is a relationship between the amount of nutrient extracted from soil by a laboratory test and nutrient uptake by plants in the greenhouse or field and/or crop yield. If such a relationship cannot be established, the chemical procedure has little or no usefulness. Sometimes the relationship can be established only for one nutrient and one crop and on a particular group of soils. If this limitation is known and recognized, then the soil test should only be used for those limited conditions.

A useful correlation has been established between the Bray and Kurtz No. 1 phosphorus test and percent of maximum yield of soybeans, corn, and wheat grown under Nebraska climate conditions and non-calcareous soils. The correlations shown in Figure 9.2 are useful in determining when soil phosphorus, as assessed by soil test, is adequate for maximum yields.

Figure 9.2. Correlation between extractable phosphorus and percent of maximum yield for soybeans, corn, and wheat.  The intersections of the dotted and solid lines indicate the critical point where further fertilizer additions will not economically increase yields.

Calibration is a means of establishing a relationship between a given soil test value and the yield response from adding nutrient to soil as fertilizer. The data in Table 9.2 give the pounds of alfalfa hay produced from several rates of applied phosphorus when the assessed soil phosphorus level was eight ppm. Such field experiments are repeated where soil phosphorus levels will range from two to 30 ppm. Optimum P2O5 application would be from 60 to 80 pounds per acre at this site. However, many trials are needed to establish general recommendations for a region. When similar experiments are conducted over many sites, analysis of crop response allows researchers to predict needed fertilizer at various soil phosphorus levels. The result of the calibration effort is to determine the amount of fertilizer phosphorus needed at various soil levels to produce maximum yields.

Table 9.2.  An example of calibration showing the relationship between a given soil test and the yield response from adding a nutrient to a soil1 (Alfalfa)

The critical point is shown in Figure 9.2. The critical point is where soil test values delineate responsive soils — those where fertilizer additions increase yields — and nonresponsive soils. There are many ways to determine critical values, but their calculation is beyond the scope of this lesson.

When the calibration procedure is completed, one can place soil phosphorus levels into categories of very low, low, medium, or high. These categories simplify recommendations and probably reflect the reality of the variable nature of soils. Table 9.3 gives an example interpretation for small grain.

Table 9.3.  Categories of soil phosphorus levels showing chances of small grain yield increases from fertilzers applications.

Figure 9.3.  Theoretical crop response by soil test level to applied nutrients.