Many Louisiana soil samples show significant nutrient deficiencies, enough to affect potential yield of the state’s major crops.

But producers can’t arbitrarily bump fertilizer applications and expect to solve the problem, says an LSU AgCenter agronomist.

Dan Fromme, state specialist for corn and cotton at the AgCenter in Alexandria, addressing the annual Louisiana Agricultural Technology and Management Conference recently in Marksville, said several factors influence soil fertility and crop response.

“Many factors control soil productivity,” Fromme says. “Failure to employ sound production practices reduces the potential benefit of fertilizer and limits productivity.”

Among the issues Fromme says producers must consider are:

• Soil Compaction — “I always pack a soil penetrometer.”

• Too much water (excessive rainfall or furrow irrigation)

• Temperature

• Date of planting

“We always see good years and bad years and it’s not always about fertilizer, insects, and diseases,” he adds.

The nutrient issue is real, however. Fromme says soil tests indicate 87 percent of phosphorus and 76 percent of potassium samples tested below critical levels for major crops in Louisiana.

“Less than 30 percent of Louisiana soybean acres received potassium or phosphorus in 2015,” he says.

Related:Data a key tool for farm profitability

Soil testing also indicates sulfur and zinc levels are often deficient.

Fromme says the basis of adequate soil health remains routine sampling. Adequate testing includes exact measurements in parts per million. “Classify the degree of nutrient sufficiency — very low, low, medium, high and very high,” he adds.

Sorghum response to phosphorus is significant, improving more than 500 pounds per acre with 30 to 60 pounds of phosphorus, applied deep compared to broadcast on the surface. Fromme says banding phosphorus 6 inches deep may reduce rate by 40 to 50 percent.

Response to Fertilization

With very low soil nutrient levels, producers can expect less than 50 percent of normal yield. With fertilization, a 90 percent likelihood of plant response is expected.

A low nutrient level likely results in 50 to 75 percent of normal yield. Potential for yield response is 60 to 90 percent with fertilization.

Medium fertility level suggests a yield potential of 75 to 90 percent of normal and yield response probability at 30 to 60 percent.

At a high level, producers may expect 100 percent of normal yield, but additional fertilization may be necessary to maintain that high rating. Probability of a yield response is 0 to 30 percent.

At very high ratings, yield potential is 100 percent, no fertilization is needed, and no yield response is likely.

Takes Time to build

Rebuilding nutrient levels may take time, Fromme says.

“When the soil test is below the critical level (CL), it may be desirable to apply phosphorus or potassium at rates that increase soil test above the CL,” he says.

“Generally, applications of 10 to 30 pounds P₂O₅ per acre are required to increase soil test phosphorus level 1 ppm, depending on soil properties influencing P fixation capacity.”

He adds that 5 to 15 pounds of  K₂O per acre are needed to increase soil potassium level 1 ppm. “From an economic standpoint, this may take several years. Apply the amount that you can afford each year.”

Other factors also affect soil fertility. Soil pH makes a big difference.

pH Factor

Improving soil pH may take several years, too, Fromme says. In one study, four applications over a 12-year period were needed to increase pH to adequate levels at 4 to 6 inches.

He says lime should be incorporated to improve distribution and soil-lime contact. “Fix pH issues before committing to no-till systems.”

Reduced tillage offers other challenges. Fromme says broadcast applications often only increase soil test level phosphorus at the surface 1-inch depth and increase the proportion bound in less soluble compounds.

For potassium, Fromme says fall applications “should be avoided in coarse-textured soils, especially those with Cation Exchange Capacity (CEC) less than 6 milliequivalent (meq) per 100 grams. In no-till, surface applications are effective with little to no incorporation.”

Other issues affecting soil fertility include soil depth, water holding capacity, and soil type.

He says adding more fertilizer may not be the answer to nutrient deficiencies, and a soil sample is necessary to determine nutrient levels.

“Our goal is to ensure that the most limiting factor is one out of our control.”

That’s usually the weather.