Costs of crop fertilizers are continuing to remain at all-time highs. Fortunately, 2021-2022 LSU AgCenter research indicates there could soon be lower recommended rates of phosphorous and potassium fertilizer in medium- to-high-tested soils for both Louisiana soybeans and corn, which would save producers money.

Researchers have been looking at current soybean recommendations that were originally made more than 20 years ago, according to Rasel Parvej, an AgCenter soil fertility specialist. Higher-yielding genetic plants with improvements have been developed since that time.

Parvej has continued to develop a critical soil test level that indicates crop yield response to fertilization. This is the exact amount of fertilizer — not too little, nor too much — needed to reach maximum yield for current soybean varieties.

The trials were conducted at 32 locations in 2020-2021 and are ongoing at another 20 different sites for 2022. Tests were performed on all different soil types that range from sandy loam to clay.

“I’ve found with the two years of data so far, we need to revise our current phosphorus and potassium recommendations,” said Parvej.

He said when the soil phosphorus level was below 20 parts per million (ppm), or 40 pounds per acre of phosphorus, and fertilizer was applied, there was a yield response. However, the probability of yield response to fertilization was much higher when the soil test was 15 ppm (30 lbs./acre) or less of phosphorus.

When the phosphorus level was more than 20 ppm, there was no increase in yield with added fertilizer.

“The current soil-test-based phosphorus fertilizer recommendations are fairly accurate, but we may need to fine-tune our critical level and fertilizer rate above critical level,” said Parvej.

The existing potassium recommendations for Louisiana soybean vary with soil types and geographical position. For example, if a soil is tested at 150 ppm (300 lbs./acre) of potassium, the current recommendations call for potassium fertilizer application for clay soil, but not for sandy or silty soil.

Parvej’s research has shown that the current potassium recommendations are too complicated, and there is no added benefit of potassium to yield if a soil tests over 110 ppm (220 lbs./acre). Another important finding showed there were similar yield responses on all the soils that were covered, meaning producers could use one recommendation for every field.

Parvej has also compared the current recommended depths (0 to 6 inches) for soil testing against other depths (0 to 8 and 0 to 12 inches), because deeper soil might have more nutrients, such as potassium, available to the plant. His research, to this point, has found the recommended 0 to 6 inches of soil still works well for determining the plant’s nutrient needs.

“But what we will change is our critical level,” he said. “That will change for both phosphorus and potassium.”

Matt Foster, AgCenter corn, cotton and grain sorghum specialist, has conducted a research project about fertilizer applications for commercial corn for the last two years. The 2021 test sites were on sandy, silty soil in Tensas Parish and on heavy, Sharkey clay soil in West Carroll Parish.

There was no yield response for potassium or phosphorous fertilizer applications in the clay soil, but there were yield improvements on the silty soil.

“Similar to Parvej’s phosphorus findings for soybean, based on 32 site trials, anything below 15 ppm of phosphorus, we’re seeing improved yield for corn. But anything above it, we see no improvement in yield,” said Foster.

Also similar to Parvej’s soybean findings, Foster’s corn results showed potassium applications above 110 ppm (220 lbs./acre) had no statistical impact on yield compared to non-treated corn — on both soil types.

Based on the data from these few corn trials, Parvej will propose research on at least 30 to 40 additional sites to replicate these findings and to revise current phosphorus and potassium recommendations.

“This tells you updating our current soil-test-based recommendations is very important, so you can predict whether you’d see a response from fertilizer,” said Parvej. Randy LaBauve