From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Summer 2003, No. 5


A high soil test doesn’t mean nutrient management is not needed. There are hidden hunger areas in many of our most productive fields. Fields that have been carefully managed according to recommended soil test goals often have areas where nutrients have been depleted to levels that suppress yield potential. If the field-average soil test is maintained at the recommended goal, and nutrient applications are based on field-average soil tests, 50 percent or more of the field may be below the threshold of optimum productivity. Often those deficit areas have the highest yield potential, so that in most years, nutrient removal is greater than field-average fertilizer applications.

The new technologies associated with site-specific management systems provide a more detailed measurement of variability in many factors affecting yield. This variation is really the basis of management opportunities to improve yields and profits. Yield monitor maps provide one of the most detailed data sets available for a field. Collecting data on a 1-second interval, a 6-row combine provides about 250 data points per acre, or 10,000 points for 40 acres. Compare that to soil test data sets, which are commonly on a 2.5-acre grid, which provide 16 points in 40 acres. So the yield data give a much clearer picture of the variability within the field. Knowing and mapping that variability is the first step in being able to manage it, so a yield monitor should be one of the first investments to be made in site-specific technology.

The soil is the source of many factors impacting yield, and variation in soil properties is thus a primary source of yield variation. Water and nutrient relations, rooting restrictions, and acidity are among the key variable factors. A detailed soil survey map...preferably Order 1...provides information on this variability. Available from the Natural Resources and Conservation Service (NRCS) office in each county, the soil survey is a good general guide. After studying the map, farmers and their advisers can often explain much of the yield variability observed. In some cases, soil survey is a good basis for plotting nutrient management zones.

Electrical conductivity is a simple measurement that is useful in mapping variability in the soil. It is another example of a measurement that provides a large number of data points. The actual numeric measurement is usually not as valuable as the relative change measured across the field. Many different chemical and physical properties of the soil affect conductivity, so the procedure is not often able to identify specific sources of change, but simply mapping the conductivity is helpful in delineating management zones, refining soil survey maps, and guiding sampling for more specific analysis of spatially variable yield components.

Remote sensing is another spatially-intensive data source to help identify and understand yield variability. Either aerial or satellite imagery can be used to document variability in bare soil and in reflectance of the crop at different growth stages. Variation in reflectance, and variation in the rate-of-change in reflectance of the crop, provide clues to yield potential changes. Remote sensing can help track the timing of crop changes, too, which is critical to many management changes.

The tools available to today’s farmers, their advisers, and their input suppliers provide a means of fine-tuning management. Which technologies fit a given farm depends on the spatial variability of the factors and how well they can be manipulated. Determining which ones fit a given field is, itself, a site-specific management challenge. Rewards go to those willing to expend the extra effort to learn the tools and how to employ them. At this point, the technology is moving ahead of the agronomic information and management commitment needed to fully realize its potential.


For more information, contact Dr. Harold F. Reetz, Jr., Midwest Director, PPI, 111 E. Washington Street, Monticello, IL 61856-1640. Phone: (217) 762-2074. E-mail:

A-B Summer 03-5.pdf

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