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Issue No. 1, Article 9/March 26, 2010

Phosphorus and Potassium: To Apply or Not to Apply?

The challenges of last year's growing season described in the article about nitrogen are not restricted to that nutrient. Farmers face similar concerns about phosphorus (P) and potassium (K) fertilization. Soil testing was not performed in most fields last fall, let alone P and K applications. After a few years of high P and K fertilizer prices, everyone was ready to get back on track applying these nutrients once prices came down last year. Unfortunately, between muddy conditions that turned ice-hard once temperatures started to drop in late November to early December and the late harvest, most fields could not be sampled to determine how much P or K was needed. So the relevant questions are these: Since I have not applied or have substantially reduced the rate of P and K application in the last few years and I have been unable to sample the soil, is there a way to know approximately what my soil-test levels are? Should I soil-sample now in the spring, or should I wait until fall? If I don't soil-sample now, how much P and K should I apply?

How much have soil-test levels dropped? Mostly because fertilizer prices were high, many farmers have not applied all the P and K that were removed given the high yields we experienced the last two years or so. How much soil-test levels have dropped would depend on where they started before the drawdown phase, especially for P.

Let me stress that what I am presenting here is approximate values to assist in making general decisions when reliable soil test information is unavailable. They are not a substitute for testing soils periodically to track changes under your specific soils and management. In general, the higher the initial test value, the more rapid the decline in test levels if no fertilizer is applied. Usually when test levels are very high (about 140 lb P per acre), soil P levels will drop 9 lb P per acre in the first year if no P is added. If test levels are high (about 70 lb P per acre), the drop will be approximately 5 lb P per acre in the first year. If test levels are at the critical level (about 30 to 45 lb P per acre), the drop will be approximately 2 to 3 lb P per acre in the first year.

Potassium is typically too variable to determine how quickly it will decline depending on starting levels. A limited number of studies have shown that on average, one can expect the soil test to drop about 13 lb K per acre for each year that K is not applied. Conversely, in Illinois P test levels will increase on average 1 lb per 9 lb of P2O5 applied, and K levels will increase on average 1 lb per 4 lb of K2O applied. Again, these are approximate values to help producers make some decisions for this year in lieu of testing information, but they do not eliminate the need to determine soil-test levels by properly collecting and analyzing soil samples in the future.

When to soil-test? While it is possible to collect soil samples any time of the year for P and K analysis, be aware of potential problems. Potassium levels fluctuate depending on soil moisture conditions and freezing and thawing. Levels in the late fall and winter tend to be higher than for the typical fall sampling time. Samples collected in summer, on the other hand, tend to be lower than in fall. These changes make it difficult to compare test levels to previous years and accurately determine fertilizer needs. Unfortunately, since the changes in K level due to time of sampling are related to several complicated processes in the soil, there is no simple rule to adjust the test result up or down depending on when samples were collected. As far as handling a wet sample at the time of collection, there are no differences. It is the interpretation of results from the lab that need to be considered carefully. In my experience samples collected during winter to very early spring will typically test anywhere from 25 to 100 lb per acre higher than those collected during the typical time in fall after crops are harvested (October to early November). An inflated test level, if taken as the actual value, will result in a lower application than actually needed. For phosphorus, the time of sampling is not much of an issue.

For growers who typically collect soil samples in the fall but could not do so last year, we recommend using recent sampling data along with information on how much P and K has been applied by fertilization and how much has been removed with the grain harvested to determine how much should be applied this spring. Typically, corn removes 0.43 lb of P2O5 per bushel and 0.28 lb of K2O per bushel, whereas soybean removes 0.85 lb of P2O5 per bushel and 1.3 lb of K2O per bushel. If growers do not have historical field fertility records, such as in the case of farming a new field, or they have not been applying at least maintenance rates in the last two or three years, soil sampling and analysis should be conducted in the spring to determine in general terms fertilization needs.

How much should be applied? Adequate fertility for P and K depend on the soil. Fertilizer recommendations for Illinois were developed on a 7-inch sample depth. We have looked at different depths of sampling to adjust for different tillage but have found no justification for adjusting sampling depths. For P, we recommend maintaining fertility in the following ranges: 30 to 60 lb per acre in soils with high P-supplying power (mostly the western part of the state); 40 to 65 lb per acre in soils with medium P-supplying power (central Illinois, with arms extending to northern and southern Illinois), and 45 to 70 lb per acre for soils with low P-supplying power (roughly the northeast and southeast corners of the state). For K, generally speaking we recommend 260 to 360 lb per acre for the southern third of the state and sandy soils and 300 to 400 lb K per acre for the northern two-thirds of the state.

The need to fertilize a soil depends on test levels. A recent soil fertility survey in nearly 600 fields in more than 50 Illinois counties showed that for P, approximately 59% of soils were above the level where additional P is not recommended; 19% were below the critical level, or the point at which yields are near maximum; and 22% were at maintenance levels, at which applications should replenish what is being removed by the crop. For K, 31% of the fields were above the point at which additional K is not needed to maximize yield; 45% were below the critical level; and 24% were at maintenance levels. From this information we suspect that in general more attention needs to be paid to K than to P, but fertilization decisions should always be made case by case.

When P and K are needed, we have found that it does not make a difference if the application is done in the fall or the spring. The only concern is if a high rate of K is applied right before planting under the row, because it can cause some salt injury, especially for soybean. Another question for a few farmers this year (since few acres of wheat were planted) is whether soybeans planted after wheat need to be fertilized. The P requirements of wheat are much larger than those of soybean, so if the wheat was fertilized adequately there should not be concerns for the soybean crop. If a producer is concerned that K levels may be too low, it would be beneficial to collect samples, analyze them, and interpret the results (paying attention to the factors mentioned earlier) to get a general idea of where the fertility is and whether there is a need to apply K.--Fabián G. Fernández

Fabián Fernández

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