No. 23 Article 7/October 8, 2010

Take Advantage of Good Soil Conditions to Apply Phosphorus and Potassium--If You Need Them

In the last article that I wrote about phosphorus and potassium applications for this fall, I did not talk much about fertilizer rates. I decided to cover that topic this issue. I finished the last article with a brief mention of the importance of soil testing. I will start this one by reemphasizing this practice. The most important thing to know before you get started with an application of phosphorus or potassium is to how much nutrient is needed--if any at all. The best way to know is by conducting a soil test, which measures the availability of the nutrient for the crop.

The soil tests for phosphorus and potassium have been correlated and calibrated to determine what fertility level is needed to minimize a yield reduction due to insufficiency of these nutrients. In a yield response curve, the critical soil test level is the point at which near maximum yields are obtained. If soil test levels are below that critical value, the crop is likely not to produce maximum yield because it is limited by the lack of that nutrient. On the flip side, if the test shows the nutrient is above the critical level, the crop is less likely to respond if additional fertilizer is applied.

Sometimes a soil test will indicate excessively high levels, in which case reducing or eliminating applications for a while may be desirable. When soil test levels are very high, the chance of yield response to additional fertilizer is low. As long as the crop has what it needs--which you can know from the soil test--it won't care if the fertilizer was applied recently or several years ago, or how much you paid for it! Thus, the recommendation is to try to build levels to at least the critical value if the test is low, and then to maintain levels or try to increase them slightly above critical. Normally, producers who farm their own land prefer to build test levels above the critical point (the "feed the soil" approach), while those who rent land typically try to maintain test levels (the "feed the crop" approach). Whichever approach you use, what is important is to ensure that crop yield is not limited by nutrients.

Table 2 outlines the critical levels for phosphorus and potassium in corn and soybean in the different regions of Illinois. Phosphorus-supplying power regions are broadly defined by parent material and degree of weathering (Figure 2), while potassium-supplying power regions are broadly defined by function of soil cation exchange capacity (CEC; Figure 3). Table 3 shows average phosphorus and potassium removal rates in seed for corn and soybean. Additional information on phosphorus and potassium recommendations can be found in the Illinois Agronomy Handbook.

Table 2. Recommended soil test levels for phosphorus and potassium in Illinois.


Test level (lb/acre)



Maintenance range

No application needed





  Low supply power




  Medium supply power




  High supply power








  High CECb




  Low CEC (including sands)




aTo ensure adequate fertility even in years with high yield potential, it is recommended that phosphorus test levels be increased slightly above the critical level before a maintenance program is used.
bCEC: cation exchange capacity.

Figure 2. Phosphorus-supplying power regions of soils in Illinois.

Figure 3. Cation exchange capacity (CEC) of soils in Illinois. Shaded areas are sands with low CEC.

Table 3. Average nutrient removal in seed of corn and soybean.












If your field needs an application and your budget does not allow for a full rate, it is better to apply some rather than none. Another alternative is to apply the nutrients as a starter fertilizer in the spring. Of course, these practices should be viewed as temporary measures and should not replace a sound fertility program. Applying no phosphorus or potassium 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. Applying phosphorus and potassium 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 2), you can likely forego the application for one year without risking yield loss. If the test levels are very high above the point where an application is not recommended (Table 2), you can likely withhold an application for more than one year and still have adequate fertility to maximize yields.

Traditionally, most farmers apply phosphorus and potassium at the same time to save time and money and to reduce the trips across the field. While this makes sense when both nutrients are needed, be sure to determine if such is the case. If soil testing shows a need to apply only one of the nutrients or less of one and more of the other, don't worry about applying more of what you don't need. In a recent survey of soil fertility throughout Illinois, we determined that overall many fields have sufficient phosphorus fertility built in but are low in potassium. Of course on a field-to-field basis, we saw some fields needing phosphorous and not potassium, and other fields needing both nutrients. These results underscore the importance of having recent soil test data to determine what kind of application is needed, if any at all. If you determine that an application is needed, this fall looks like an appropriate time to do it. Unlike the last two years or so, many fields are already harvested, soil conditions are adequate to drive on the field, and fertilizer prices are lower.--Fabián G. Fernández

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