Soil Testing After A Dry Growing Season

University of Illinois assistant professor Fabián Fernández said that many farmers are asking questions about adding nutrients to the soil in preparation for next year’s crop. The fact that grain yields and nutrient removal levels are lower than normal due to the drought and that some fields have been baled or harvested for silage complicates the interpretation of soil test results for phosphorus (P), potassium (K), and pH.

In Illinois, P and K removal rates, which are calculated by multiplying seed yield by seed nutrient concentration, are used to estimate appropriate fertilization rates. Historically, farmers based their calculations on the average yield for the field. Now many producers use yield monitors to estimate removal rates for different parts of a field and then use variable-rate applications.

Some farmers collect seed samples for analysis to calculate nutrient concentrations and removal rates for different parts of the field. Others use the standard removal rates published in the Illinois Agronomy Handbook. Fernández noted that the removal rates in the Handbook are at the upper end of the range of removal rates of current hybrids and varieties.

Whether a 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, obtaining a good yield estimate is very important.

“While drought, soil fertility status, and yield level could affect the nutrient concentration of grain, our research has shown that yield is the most important factor impacting P and K removal rates,” Fernández explained. “Even though seed P and K concentrations varied widely, we have observed a positive linear relationship between yield level and P and K removal for both corn and soybean.”

Soil testing for P, K, and pH is used to determine the need for fertilization or limestone applications. Farmers are asking if the drought will affect how they should interpret this fall’s soil test results.

“This is a two-fold issue,” Fernández explained. “One is procedural: obtaining an appropriate sampling depth. The other relates to natural processes: leaching of nutrients out of plant materials and equilibration of nutrients in the soil.”

At the procedural level, it is difficult to control the sampling depth when the soil is dry. Moreover, if the soil surface is dry and crumbly, the top portion of the core may be lost during sampling. Conservation tillage systems such as no-till, and even chisel-plow, do not mix the soil, so nutrients tend to become stratified with higher concentrations in the soil surface. Tests using a sample less than 7 inches deep will overestimate fertility while those using a sample deeper than 7 inches or one that lost a portion of the top will underestimate fertility.

One of the main issues related to natural processes is that P and K cycling from the crop to the soil is an important mechanism of soil nutrient replenishment.

Corn and soybean return approximately one-fourth to one-third of the total P taken up, and about two-thirds of the total K taken up, to the soil. K cycling has the greatest impact on fall soil sampling because this nutrient remains in an inorganic form in plant tissues and readily leaches out of the plant with precipitation once the crop reaches maturity.

“We should not be as concerned about P because P is in organic form in plant tissues and those materials need to be decomposed before P is released back to the soil,” Fernández advised.

Fernández warns that this year it is not safe to assume that lower removal rates and lower yields will result in greater P and K levels in the soil because drought limits the equilibration of nutrients in the soil. During the season, plants have extracted P and K from the more easily available nutrient pools. Because of the lack of moisture, the soil has, to a large extent, not been able to replenish those pools from less available nutrient pools.

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