University of Illinois Assistant Professor Fabián Fernández says 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 droughtand 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(pdf). Fernández notes 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 says. “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 explains. “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 in. deep will overestimate fertility while those using a sample deeper than 7 in. or one that lost a portion of the top will underestimate fertility.