Research Highlights

Double-checking Kentucky Soil Fertility Recommendations for Soybean Production

By Carol Brown

There are a lot of moving parts when it comes to farming. To get optimum productivity, producers must find the right combination of seed, soil, moisture and nutrients. Researchers in Kentucky are focusing on the nutrient element in the farming equation.

Kentucky Extension Soils Specialist Edwin Ritchey is leading a three-year project that is reevaluating the state’s soil fertility recommendations for soybean production. The study, in its final year, is funded by the Kentucky Soybean Promotion Board.

“The University of Kentucky Cooperative Extension Service developed recommendations about 40 years ago for soil fertility including potassium (K) and phosphorus (P),” says Richey. “We want to make sure these recommendations are still accurate or if changes need to be made.”

Ritchey and his team are conducting tests at the UK Princeton Research and Education Center on fields that have tested low in P and K values. The first two years, the study was conducted only on fragipan soil and this year they added testing on a limestone-derived soil. These soil types are representative of the most common types of soil for row crop production in western Kentucky, Ritchey says.

Farmers can find the existing fertility recommendations in the UK College of Agriculture, Food and Environment soils publication 2020-2021 Lime and Nutrient Recommendations. This is the document in which Ritchey’s team is focusing. The publication includes recommendations for all types of crops grown in the state including soybeans.

The team is evaluating data from soil samples, plant tissue nutrient content and grain nutrient content as well as yield to detect nutrient removal rates and comparing data with previous years. An economic analysis is also being completed to determine costs associated with fertilizing. But, Ritchey says, that is difficult as input costs and other variables change every year.

Adding P and K to soybean fields may or may not be necessary to increase yield, and the research team is working to ensure farmers have the right information to make this management decision.

“There are two ways to lose money with fertilization,” Ritchey says. “Fertilizing too much could mean farmers are spending unnecessary economic resources on something they don’t need. Also, over-applying phosphorus could be a potential environmental concern. Under-fertilizing could cost farmers yield.”

So far, the team is finding the current recommendations are accurate, but data is still being analyzed. And changing the recommendations takes some doing. Ritchey says that based on the findings they currently have, there’s not enough evidence to support a change in the recommendations. He is working on the answers to why this may be.

“Soybean plants are yielding more, so they remove more nutrients from the soil,” he says. “And as breeders improve soybean varieties, I think plants have become more efficient at utilizing the nutrients that are present.”

There are recommendations available made by private companies that are higher than the University’s, Ritchey says, and if farmers use these, they probably won’t lose yield. But they may be spending a lot more on fertilizer that they don’t need.

Farmers should conduct routine soil testing to remove the doubt of how much P and K to apply to their soybean crop. The tests can be done either in the fall or spring, but the tests should be done consistently at the same time of year when testing multiple years. Farmers can contact their local Extension expert for further information on conducting and submitting soil tests.

Published: Feb 28, 2022