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Issue No. 4, Article 8/April 17, 2009

Phosphorus and Potassium Applications This Spring

With the high price of fertilizers and the late harvest last fall, many fields did not receive the typical phosphorus (P) or potassium (K) applications. The question being asked is, Should I apply these nutrients this spring? The answer depends on several factors. The guiding principle in fertilizer decisions is that you should definitely apply nutrients when the chance for yield increase is large and the expected yield will pay for at least the cost of the fertilizer.

Soil test information is important in guiding these decisions. The critical level for a nutrient is the point at which its application will result in a significant increase in yield, or the point at which withholding its application will result in a large decrease in yield. For corn and soybean the critical levels for P and K are listed in Table 1; they vary between regions of the state. For P, the supplying power of the soil in different regions is considered (Figure 1); for K the cation exchange capacity (CEC) of the soil is taken into account (Figure 2). If your field needs a nutrient and your budget does not allow for a full application, apply at least a portion rather than none. Another alternative that can help when a full rate cannot be applied is to apply nutrients as a starter fertilizer. Of course, these practices should be viewed as temporary alternatives and should not replace a sound fertility program.

Figure 1. Subsoil phosphorus supplying power of soils in Illinois.

Figure 2. Cation exchange capacity (CEC) of soils in Illinois. Shaded areas are sands with low CEC.

Not applying any P or K results in a gradual decline in soil-test levels because the crop removes these nutrients in the grain that is taken out of the field. Table 2 shows average removal values for corn and soybean. Applying P and K to maintain test levels at an optimum is considered a good practice for sustained profitability over time. However, if your field is slightly above the critical level in the optimum (maintenance) range or above (Table 1), you can likely forgo the application for one year without risking yield loss. If test levels are very high above the point where an application is not recommended (Table 1), you can likely withhold an application for more than one year and still have adequate fertility to maximize yields. Again, having recent soil test data is important in determining whether an application is needed or not. In a recent survey in Illinois we determined that many fields have sufficient fertility built into them to warrant withholding P and K fertilization. Also we observed that in some fields it is necessary to apply one nutrient but not the other. Applying only the nutrient that is needed rather than a blend of both is another strategy to lower costs when fertilizer prices are high.

Finally, a word on application timing. Studies have shown that applying P and K in either fall or spring is an effective method. Thus, there is no problem if you didn't apply these nutrients in the fall and plan to do it in the spring. Studies have also shown that as long as you apply adequate amounts for both corn and soybean, there is no difference if the application is done ahead of corn for both crops or is done annually just to feed each crop individually. Since the cost of application relative to the cost of fertilizer is smaller now, doing annual applications is certainly an option if you cannot afford the upfront cost to feed both crops.--Fabián Fernández

Fabián Fernández

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