Farm Progress is part of the Informa Markets Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.

Serving: East
Corn+Soybean Digest

How Much Phosphorus and Potassium Will Your 2010 Crop Need?

After the 2008 cropping season your intentions to apply phosphorus (P) and potassium (K) may have been waylaid because of high prices. So you planned to wait until after crops were harvested in 2009. But then the weather did not cooperate and you still do not have P and K applied, much less a soil test that indicates how much might be needed. After all, the floods of 2009 may have washed some away. So now, you are asking if you should take soil samples, or wait until the fall, or just apply a typical amount of P and K without the samples. Good questions!

If your nutrient levels are lower than they normally run, you are probably not the only farmer in the Corn Belt in that situation. With the high prices of the past years your nutrient applications may have diminished for economic reasons. University of Illinois Fertility Specialist Fabian Fernandez has published a newsletter that may help your decision-making.

Your initial concern may be how much have nutrient levels dropped since P and K were last applied, and some of that will depend upon the higher yields that you produced over the past two years. Without a soil test, it is nearly impossible to determine where your nutrient levels may be, since there may have been abundant nutrients available a couple years ago, or you might have been at a critical shortage at that time. Fernandez says the higher the initial test level, the more rapid the decline if no fertilizer is applied. If you were at the critical level for P, then the decline is slowed. For potash soil tests show about a 13-lb./acre decline annually when none is applied. The bottom line is the need to know a starting point with a soil test.

When do you conduct a soil test?

Fernandez says that can occur at anytime for P and K, but beware of potential problems.

1) Potash levels fluctuate with soil moisture and freeze-thaw cycles, with levels in late fall and early winter tending to be higher than in mid-fall. He says late-winter and early spring soil samples will test higher than when taken in the fall after harvest. If your test numbers are inflated because of the temperature conditions of the soil, it may result in a lower application than needed.

2) Phosphorus levels will not change with time of year or soil temperatures.

Fernandez suggests that when your soil tests are returned, double check them against your last soil test and the yields that have been taken from those fields. One bushel of corn will remove 0.43 lb. of phosphate and 0.28 lb. of potash, and 1 bu. of beans will remove 0.85 lb. of phosphate and 1.3 lbs. of potash. Compute your yield, calculate the amount of nutrient removal and if the soil test indicates levels of potash well above what you might expect, then consider the time of the year may have influenced the results.

How much P and K should be applied?

Fernandez says that depends on the soil, and while his newsletter addresses recommendations for Illinois, farmers in other parts of the Corn Belt can consult with their Extension personnel for recommendations appropriate for their soil type. When Fernandez analyzed the soil test results from 600 fields over half of Illinois, he found that 59% of the soils had adequate P levels, 22% were at a maintenance level and 19% were at a critical level in need of a phosphate application. For potash, 31% of the fields were above a level that would recommend an application, 24% were at a maintenance level and 45% were at a critically low level. Subsequently, he suggests more attention be paid to K than P.

When should you apply?

Neither fall nor spring will make a difference on when the application is made. But he says a K application under a row of soybeans will result in a salt injury, and for farmers who keep P levels high for wheat, there should be plenty available for soybeans if wheat was not planted.


Many farmers may have concerns about P and K levels, if an application was not made for economic reasons after the 2008 season and not made after the 2009 season because of the late harvest and wet fields. Soil tests will indicate how much P and K, if any, are needed. A general survey of Illinois fields indicated that most fields had satisfactory amounts of phosphate, but nearly half of the fields could be short of potash.

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.