LSU graduate student Diego Mayorga applies granular sulfur fertilizer to corn as part of a fertilizer study headed by LSU AgCenter soil scientist Brenda Tubaña at the Central Research Station in Baton Rouge. Photo provided by Brenda Tubaña

When agricultural producers mention fertilizer, the big three normally come to mind — nitrogen, phosphorus and potassium. But there are other nutrients and considerations that are important to improve crop yields.

LSU AgCenter researchers across the state are looking at the various nutrients needed to produce a heathy crop. The studies are being conducted in Winnsboro, Bossier City, St. Joseph and Alexandria.

At the Macon Ridge Research Station in Winnsboro, AgCenter soil fertility specialist Rasel Parvej leads the project that is looking at the importance of timely soil sampling to determine optimum fertilizer rate for maximum yield and profit.

“Nowadays, crop consultants or farm-service reps prefer to collect soil samples mostly within a few weeks after summer crop harvest due to the chances of rain in the early to late winter months,” he said.

Soil sampling immediately after crop harvest may not account for all of the nutrients that are going to be available during the next summer crop because crop residue of high biomass producing crops such as corn, rice, etc. contains significant amounts of phosphorus and potassium that are eventually added to the soils once the residue breaks down with rainfall and soil management such as tillage.

The corn biomass after harvest is loaded with nutrients, but taking a soil sample before the biomass breaks down will give a false reading as the nutrients have not been released in the soil yet. This can take at least two to three months, depending on rainfall amount and soil tillage.

“After harvest, we suggest producers to wait about two months before soil sampling,” Parvej said. This way the soil test values will be higher, especially for potassium, considering the nutrients released from crop residue. This will reduce fertilizer rates for the next summer crop production.

Parvej said his study is being conducted in Winnsboro and St. Joseph. He starts taking soil samples every 15 days immediately after harvest until the next summer crop planting. The results of this project will help model the variation of soil test nutrient values across the winter months and find the optimum time to collect soil samples for maximum soil test nutrient values that will result in reducing fertilizer rates.

From the same project, researchers are also evaluating the nutrient release rate from the crop residue after harvest by collecting crop residue in every 20 days starting immediately after harvest until the next summer crop planting. At the end of this project, they will be able to pinpoint how much nutrient each crop residue contains at harvest and how fast it is released back to the soil.

On the LSU main campus, AgCenter soil scientist Brenda Tubaña is looking at sulfur application rates and the potential use of biostimulants.

“Plants require 14 essential mineral nutrients that are absorbed by the roots,” she said. “Sulfur is one of these nutrients with a unique role in facilitating the assimilation of inorganic nitrogen into organic forms.”

Studies have shown that the Clean Air Act changes made in the 1980s caused a decrease in sulfur dioxide in the atmosphere, which is also causing sulfur deficiencies in the soil.

“So, we have cleaner air to breathe, but we no longer have this source of free sulfur for our plants,” she said.

In addition to decreases in the concentration of sulfate in rainfall, sulfur deficiency can be further exacerbated by the practice of burning residue, such as corn, which also leads to sulfur losses to the atmosphere.

This insufficient supply of sulfur can reduce nitrogen use efficiency of a crop, she said.

There has been an increasing interest in the use of biostimulants, which include diverse formulations of substances and microorganisms.

“We identified a silicate solubilizing bacteria that is capable of dissolving phosphates and produces plant growth promoting compounds,” she said. “The bacteria are known as Win-54, because it was discovered at Winnsboro.”

At the Red River Research Station near Bossier City, AgCenter soil scientist Syam Dodla is looking at the micronutrients needed to improve corn grain production.

Dodla said the micronutrients are only required in small quantities but are important as major nutrients to increase the grain yields. At present, these nutrients are not given enough importance as part of soil fertility improvement.

“There are six micronutrients that we wanted to evaluate and test their potential to improve the grain yields,” Dodla said. “These micronutrients are copper, manganese, iron, boron, zinc and molybdenum.”

The pH level plays a key role in the availability of nutrients, Parvej said.

“If the soil pH is slightly acidic to neutral (pH 6.5 to 7.0), there may be no need to apply micronutrients except molybdenum (Mo) because there will already be enough micronutrients available to the plants, and plants need a small quantity of micronutrients,” Parvej said.

Dodla said the soil loses a certain amount of nutrients anytime the crop biomass is removed from the field. Over time, these nutrients will have to be replaced.

“What we already know is that the alkaline soils tend to have more micronutrient deficiencies,” he said. “But there is not enough research to develop recommendations for the micronutrients.”

Correcting the soil pH to between 6 and 7 is important to eliminate many nutrient deficiencies or toxicities. Lime or calcium carbonate can be used to increase the pH of acidic soils and elemental sulfur can be used to decrease the pH in alkaline soil, he said.