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Issue No. 22, Article 7/September 4, 2009

Phosphorus and Potassium Applications for the 2010 Crop

The 2009 growing season is another challenging one. Late planting along with conditions both cooler and wetter than normal prevailing for most of the summer will push harvest later than most years this fall. However, it is not too late to start thinking about the 2010 growing season and what would need to happen this fall to be ready for next year. As we all know, in the last two to three years phosphorus (P) and potassium (K) prices have fluctuated a lot. I would venture to say that people have paid more attention than ever before to managing these nutrients during this period. As you approach fall, ask yourself whether you need to apply P and K, and, if so, how much? Would it be better to do a biennial application by applying enough nutrients for two crops, or are yearly applications preferable? Is it better to apply P and K in fall or spring? I will try to answer those questions from an agronomic standpoint, with the final decisions being yours as you evaluate the agronomic details, current prices, and your farm operation strategies.

Whether you need to apply P and K depends on your soil test levels. A soil test provides a measurement of the availability of a nutrient for the crop. The tests for P and K have been correlated and calibrated to determine the levels of nutrients needed in the soil to produce certain levels of yield for various crops. In a yield response curve, the critical soil test level is the point at which near-maximum yields are obtained. If test levels are below that critical value, the crop will likely not produce maximum yield because the nutrient is deficient and reduces yield potential. On the other hand, if test levels are above the critical value, additional fertilizer is less likely to produce a yield response, because the soil has enough nutrient available to provide what the crop needs. When test levels are above the critical value but not excessively high, applying what the crop removes is always a good idea to maintain that level of fertility.

So in deciding how much you need to apply, if any at all, there is no substitute for soil testing. If your test indicates that the crop will have all that it needs, adding "fresh" fertilizer is not going to make a difference to the crop (though, again, it would be wise to apply fertilizer to maintain fertility at adequate levels). In contrast, if the test levels are excessively high, reducing or eliminating applications for a while may be desirable. Finally, if test levels are below the critical point, it is important to apply enough fertilizer to at least reach that point. Traditionally, most farmers apply P and K simultaneously to save time and money and to reduce the number of trips across the field. But remember that if after testing the soils you find only one of the nutrients is needed, don't worry about applying more of what you don't need. Additional information on P and K recommendations can be found in the new edition of the Illinois Agronomy Handbook (available for purchase at www.pubsplus.illinois.edu, or call 800-345-6087 during business hours).

Is an annual application better than a biennial one? Our research indicates that as long as you apply the needed fertilizer to ensure soil test levels show adequate nutrients to supply what the crops will need, there is no difference in benefit in terms of yield response. However, if you prefer a biennial application, it is better to do it before the corn crop and to have soybeans in the second year as a residual feeder. An annual application is most effective when soils fail to build up or when the potential for nutrient fixation in the soil is high. Fortunately, with most soils in Illinois this is not a cause for concern.

Is a fall application of P and K better than a spring application? Again, this timing does not make a difference in the availability of these nutrients in typical Illinois soils. A fall application is often preferred because more time and equipment are available than during the busy spring planting season, soil compaction problems tend to be less because soils are typically drier in the fall, and P and K applications combined with tillage operations are more feasible in the fall. One potential drawback for fall applications is the fact that the nitrogen accompanying P in MAP and DAP is more susceptible to loss. However, the amount of N present in these applications is not very high, and the benefits of a fall application typically outweigh any small N losses that might occur.--Fabián G. Fernández

Fabián Fernández

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