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

Spring 1998, No. 3


Farmers who can use site-specific management to determine the variability in nutrient needs within individual fields have a unique opportunity to improve their fertilizer use efficiency. To effectively use the new variable-rate technologies, the recommendations should be based upon detailed soil tests, yield maps, and other data sets. If possible, these data should be geographically-referenced using the global positioning system (GPS) to identify the exact location of each data point. Maps showing the variability of each nutrient within the field can be prepared from these data sets, and then used to prepare maps of variability in fertilizer needs across the fields. These are used to develop the database to guide the fertilizer application, adjusting the rates on-the-go, either applying nutrients individually or in combinations of materials.

Efficiency is improved because the amount of fertilizer applied is more closely matched to the estimated nutrient requirement for optimum yield for specific areas within the field. Field-average management tends to over-apply nutrients in low-yield areas of the field and under-apply in high yield areas. With site-specific management, rate adjustments are made on the basis of smaller, more uniform areas. Research has shown that one-acre grids are recommended for most intensive grain production areas with a long history of significant fertilization or manuring. Variable-rate application maps are provided to the fertilizer applicator operator to be used in guiding the on-board controller to adjust rates to match the recommendation as the applicator moves across the field. For best results, a database and map of the amounts actually applied should be generated to verify the rates used and to help build a record of the variable-rate applications made in the field. These actual application maps should closely match the recommendation maps.

All of the maps produced should be cataloged in a geographic information system (GIS) data management package for use in future recommendations and other types of analysis. This is part of the complete site-specific system needed to fully utilize the technology and gain the most advantage over conventional field-average nutrient management. For best efficiency, variable-rate applications of lime and nitrogen should be made as well as potash and phosphate. Initially, most people have focused on phosphorus and potassium, but the best return on the technology investment is often obtained from lime and nitrogen. Recent analysis of a field study by the University of Illinois showed a gross gain of $16.31 and $12.90 per acre, respectively, for two fields studied. Subtracting $8.00 per acre added cost for soil testing, mapping and variable-rate application, the net gain for variable-rate application on these fields was $8.31 and $4.90 per acre, respectively. Approximately 60 percent to 75 percent of this gain came from the variable-rate nitrogen application.

Another study projected that if phosphorus and potassium application rates are based on more precise guidelines than those used for field-average management, the variable-rate application will pay-off in approximately 60 percent of the fields. If variable-rate nitrogen, lime and/or herbicides are also used, the pay-off probably can be dramatically increased. Variable-rate application, based on site-specific data, is not just a good agronomic practice, it also holds substantial profit potential.


For more information, contact Dr. Harold F. Reetz, Jr., Midwest Director, PPI, 1497 N 1050 East Road, Monticello, IL 61856-9504. Phone (217) 762-2074. E-mail:
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