No. 21 Article 5/September 7, 2012

Soil Testing After a Dry Growing Season

With the growing season ending, farmers are turning their attention to next year's crop. The drought in most of Illinois and the region will result in lower-than-normal grain yields and nutrient removal in that grain. In addition, because of crop failure and concerns over insufficient available feed for livestock production, some fields have been baled or harvested for silage. An earlier article discussed nutrient removal for those materials (issue 17, July 27). All these variables have the potential to affect test results and complicate the interpretation of results for phosphorus (P), potassium (K), and pH.

Removal rates. In Illinois we use P and K removal rates to estimate fertilization rates needed to maintain fertility levels. The removal rate is the product of seed yield and nutrient concentration of the seed. Historically, farmers used the average yield for the field, but now with yield monitors it is easier to estimate removal rates for different parts of a field, and many are using variable rate applications to match P and K removals for the various yield levels. Some farmers may be inclined to collect seed samples for analysis in various parts of their field in order to calculate nutrient concentrations and removal rates, while others may prefer to go with an easier and less expensive approach and use the standard removal rates published in the Illinois Agronomy Handbook (Adobe PDF). Recent studies have shown that the long-standing removal rates in the handbook represent the upper end of the range for current hybrids and varieties (Table 1).

Table 1. Pounds of P2O5 and K2O removed per bushel of corn and soybean yield according to the Illinois Agronomy Handbook and to values (including descriptive statistics) measured from new hybrids and varieties in Illinois.


Handbook values

New values*





Mean (std dev)


lb P2O5/bu







0.27 (±0.051)







0.69 (±0.056)


lb K2O/bu







0.19 (±0.026)







1.17 (±0.074)

*Based on approximately 1,500 corn seed samples representing 289 hybrids and 16 relative maturities and approximately 3,450 soybean seed samples representing 658 varieties and 36 maturity groups. Samples were collected between 2007 and 2011, with the great majority collected in 2009. The Illinois Agronomy Handbook is published on-line.

While drought, soil fertility status, and yield level could affect the nutrient concentration of grain, our research has clearly shown that the most important factor impacting P and K removal rates is yield level. Even though seed P and K concentrations varied widely, we have observed an increasing linear relationship between yield level and P and K removal for both corn and soybean. Thus, whether a good seed sample is taken to represent the inherent or drought-induced variability of the field or a removal rate value from the table is used, the most important thing is to ensure an adequate estimate of yield.

Soil testing. Soil testing for P, K, and pH is the fundamental method of determining the need for fertilization or limestone applications. The drought this year has created concerns about the interpretation of soil test results this fall. This is a two-fold issue. One is procedural: obtaining an appropriate sampling depth. The other is the action of natural processes: leaching of nutrients out of plant materials and equilibration of nutrients in the soil.

First let's talk about the procedural issue. It is difficult to control sampling depth when soil is dry. Also, if the soil surface is very dry and crumbly it is difficult to keep the top portion of the core during sampling. Since conservation tillage systems, like no-till, as well as chisel-plow do not mix the soil, nutrients tend to become stratified, with higher concentrations in the soil surface. A sample less than 7 inches deep will result in overestimating actual fertility, while a sample deeper than 7 inches or one that lost a portion of the top will result in underestimating.

Now let's talk about the issue of natural processes. While with lower yields one would expect less removal of P and K, it is not safe to assume a translation into greater P and K test levels, for two reasons. First, it is important to remember that nutrient cycling from the crop to the soil is an important mechanism of nutrient replenishment to the soil. Corn and soybean return to the soil roughly 1/4 to 1/3 of the total P taken up and about 2/3 of the total K taken up. For fall soil sampling, K cycling is most important, because this nutrient remains in inorganic form in plant tissues and readily leaches out of the plant with precipitation once crops reach physiological maturity. We need not be as concerned about P, because it is in organic form in plant tissues, and those materials need to be decomposed before P is released back to the soil.

The second reason it is not safe to assume that lower removal rates with lower yields result in greater P and K test levels is that drought limits the equilibration of nutrients in the soil. During the season, plants have extracted P and K out of the more easily available pools, but because of the lack of moisture, the soil has to a large extent been unable to replenish those pools from less available nutrient pools--what could be called "nutrient reserve pools." While some of the moisture we are beginning to receive now will certainly help the replenishment process from the nutrient reserves in the soil, those processes take time. Normally, the longer one can wait in the fall to collect samples, the more reliable K test values will be.

All the variables I have discussed make it difficult to predict with precision how much soil test results may need to be adjusted. My predictions are that soil test levels for K will end up being lower than actual and for P will be about normal to slightly lower than actual.

Soil pH. Finally, I have received several questions about the effect of drought on soil pH. Studies looking at pH variability over years and seasons (encompassing a variety of wet and dry conditions) have shown no consistent trend, so we cannot assume that dry conditions will in every case affect soil pH. That said, slightly lower pH is possible with dry conditions in soils that are normally neutral to slightly acidic. Differences of as much as 0.3 pH units are possible, as lack of rain can induce salt accumulation in the soil solution and increase its hydrogen ion concentration, resulting in lower (more acidic) pH. However, buffer pH values, used to determine the amount of lime applications, are largely unaffected by the small amount of salt that might accumulate in the soil during a dry season. So even if there is an overprediction of acidity that calls for a lime application, the amount of lime to be applied should not be affected by the dry conditions.--Fabián G. Fernández

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