Calibration And Interpretation Services

The results of long-term soil test calibration studies on different soil types are then utilized to establish recommended amounts of plant nutrients to apply to a particular crop at a given soil test level. For instance, if the soil test P level is in the range of 0–10 ppm (which is low), the P recommendation for a 150 bu/acre corn crop may be 100 lb/acre of P2O5; whereas, if the soil test P level is above 40 ppm (very high), the recommendation may be 0 to 20 lb/acre.

In this example to the right, more than 85 percent of the fields testing very low in a particular plant nutrient may give a profitable yield response to the added nutrient. At the very high level, there is only a 15 percent probability of a profitable yield increase to the added nutrient. These values are arbitrary, but they illustrate the idea of expectation of response.

The tools of site-specific precision management now allow growers to manage more homogeneous areas within fields. Some of those areas have much higher yield potentials than the database with which most of today’s soil tests were calibrated. This lack of calibration for high-yielding areas is one of the factors driving interest in using yield monitors and global positioning satellites to conduct strip trials to determine the adequacy of existing soil fertility programs. New precision ag tools have the ability to develop algorithms that allow for management of multiple site-specific zones within individual fields. This means a balanced crop nutrition prescription can be delivered to each square foot of every field.

When interpreting soil test results, several things should be kept in mind:

• The chances of getting a profitable response to fertilization are much greater on a soil that tests low in a given nutrient than on one that tests high.
• This does not rule out the possibility of a profitable response from nutrient application at a high level of fertility or lack of a profitable response on soils of low fertility.
• Soil tests are better at predicting the probability of a profitable response to nutrient application than predicting the actual quantity of nutrient that will be needed in any one year.
• Research in the United States and Europe shows that in any one season, a soil testing low in a nutrient often will not yield as well as a soil testing at an optimum level, no matter how much fertilizer is applied that year.
• Interpretation of soil test results and recommendations often becomes a matter of how to improve the fertility status of soils testing less than optimum. How much will be needed to change the soil from low to medium or high in that element? What will be the most economical level at which to maintain the nutrient status of the soil?
• With top-level management practices, yields increase and the probability of a response at any given soil test likewise increases.
• Wise use of soil testing incorporates a long-term approach to fertility management, in which site-specific soil test target levels are established for each field and nutrient management plans developed to reach and maintain the target levels.