An annual soil test to determine nutrient needs for cotton is more important than ever and farmers probably should dig a little deeper to determine if they have residual nitrogen that could reduce fertility costs significantly.

Dennis Coker, Texas AgriLife Department of Soil And Crop Science, says cotton producers have to be careful not to leave out essential nutrients but can manage crop fertility better when they know what’s already in the soil.

He recommends annual soil tests “because of increased fertilizer costs and the need to know what’s in the soil bank. The soil bank is a season-long resource,” he said.

Coker, speaking at the recent Texas Plant Protection Association annual conference in College Station, said several factors affect availability of some nutrients in the soil. Rainfall, variety selection, day and nighttime temperatures, soil texture, pH and organic matter content all may affect nutrient uptake, leaching potential and volatilization.

Price volatility is a good reason to soil test, he said. Since 2003 the price of anhydrous ammonia has increased from 15 cents a pound to 35 cents a pound; urea has gone from 27 cents a pound to 54 cents a pound; and UAN 32 has increased from 28 cents a pound in 2003 to 62 cents a pound in 2011, and that’s down from 2008.

He said the 2011 drought may have left nitrogen in the soil. “We had a lot of cotton planted in exceptional drought areas,” he said. A lot of corn also failed, possibly leaving residual nitrogen. “In any given season, a certain amount of nutrients are removed by crops,” Coker said. But some nutrients, such as phosphorus, potassium, calcium, magnesium, sulfur and micronutrients might carry over.

Residual nitrogen

Nitrogen is subject to leaching, runoff and volatilization, but enough may remain in the soil, especially following a poor crop year, to produce cotton the following season.

“More than 10 years of research has demonstrated that residual nitrogen measured to a 24-inch depth, can be credited at 100 percent,” he said. “Residual soil nitrogen in the major production regions of Texas is often substantial with cotton showing a yield response to supplemental nitrogen fertilization at only 23 percent of study sites.”

He said sulfur and boron also are easily leached deeper into the soil, down to 24 to 48 inches.

“Look deeper into the profile to determine the amount of nutrients available,” he said. “We recommend deep profile sampling.”

He said identifying residual nitrogen and adjusting supplemental application rates accordingly has benefits other than cutting back on fertilizer. “Farmers may be able to use less plant growth regulator, insecticide, and harvest aid. They also may be able to harvest earlier.”

Coker said tests evaluating advantages of slow-release nitrogen have shown no significant yield advantages.

He said phosphorus is a very stable, immobile element that is not prone to leaching or volatilization. He recommends band injecting phosphorus at about a six-inch depth.

He said additional work is needed “to determine if nitrogen losses can be reduced under high loss potential conditions.”