Illinois research zeroing in on optimum soil nitrogen rates

A new study to evaluate the Illinois Soil N Test (ISNT) calls into question traditional soil fertility recommendations and promises a radical new soil-based approach that will benefit crop yields, the environment, and the bottom line for farmers.

In a forthcoming issue of the Soil Science Society of America Journal, scientists at the University of Illinois at Urbana-Champaign will outline how current nitrogen recommendations are faulty, the soybean credit is invalid, and balanced fertility makes for optimum nitrogen uptake. As well, the article highlights the importance of plant populations and crop residue management for proper usage of nitrogen fertilizers.

“Our work involved 102 on-farm nitrogen-response studies conducted throughout Illinois in six growing seasons from 1990 to 2003. A site-by site evaluation of the proven-yield method showed that current fertilizer recommendations are not only wrong, they are scientifically indefensible,” said Richard Mulvaney, a professor of soil fertility.

“We’re on the edge of a revolution in nitrogen fertilizer recommendations,” said Saeed Khan, a research specialist in agriculture and co-developer of the ISNT that estimates the soil’s nitrogen-supplying capacity. “We’re going away from yield-based management to a system that quantifies the main source, which is the soil,”

“The traditional ’proven-yield’ approach says higher yielding areas need more fertilizer nitrogen, whereas crop nitrogen response is typically lowest in these areas. We have found that what matters most is how much nitrogen comes from the soil. Rich soils need less nitrogen from fertilizer, while poorer soils need more,” Mulvaney said.

Balanced Fertility

Mulvaney and his colleagues looked at several sites where a high ISNT value was incorrect in predicting negligible response to applied nitrogen and concluded that balanced fertility is key to efficient crop use of fertilizer nitrogen.

“Two of these sites had a soil pH down around 5. It has been known for a hundred years that acidity inhibits the mineralization of nitrogen,” Mulvaney said. “So although these soils tested high by the ISNT, the nitrogen wasn’t available to the crop because of reduced mineralization.”

Low levels of potassium and/or phosphorous were noted for other high-testing sites that were unresponsive to nitrogen.

“Low potash levels in the soil can limit the utilization of nitrogen,” Khan said. “For example, potassium is involved in numerous enzymatic reactions in the plant, so even though nitrogen may be taken up when potassium is deficient it won’t be utilized efficiently in making amino acids, proteins, and many other essential plant components.”

Plant Populations

A decade ago, the normal plant population for corn was 18,000 to 24,000 plants per acre. Today, it’s more like 30,000 to 35,000 plants per acre.

“We noticed that several of the sites where the ISNT failed in recent years had much higher plant populations. And this makes perfect sense, as a larger soil reserve would be required to feed more plants per acre. So we’ve realized that it would be necessary to adjust the critical test level according to plant density,” Mulvaney said.

“This fact has far-reaching consequences. It means that we now have a basis for both variable-rate planting and nitrogen fertilization,” said Khan. “So, if a farmer has a high ISNT value in part of a field, not an unusual occurrence, he can boost his planting rate somewhat to take advantage of a greater soil nitrogen reserve. Or vice versa, if an area has a low ISNT value, the farmer could plant lower populations or add more fertilizer.”

Crop Residues

Increased plant populations lead to more crop residues after harvest, below as well as on the soil surface. The extra residues have an effect on soil nitrogen cycling and availability, and can increase the critical level for the ISNT.

“Crop residues are about 40 percent organic carbon,” Mulvaney said. “There is an inherent link between microbial cycling of carbon and nitrogen that has long been overlooked in managing nitrogen fertilizers. With higher planting rates, more nitrogen will initially be tied up or immobilized, but some of this will subsequently be released or mineralized as the microbes die and decay.”

The Soybean Credit

Higher planting rates have also impacted the proven-yield practice of reducing nitrogen recommendations by 40 pounds per acre when corn follows soybean.

“We have seen ISNT levels that are higher in corn-after-corn than corn-after-soybeans, which is consistent with the greater nitrogen requirements we observed for a corn-soybean rotation,” Khan said. “So in many cases, the proven-yield method is overfertilizing continuous corn and underfertilizing corn after soybeans.”

The soybean credit originated several decades ago, when planting and nitrogen rates were considerably lower than at present. Soybeans are legumes. They nodulate and therefore fix atmospheric nitrogen, but are more apt to use soil resources when available. Mulvaney maintains that with today’s production systems, soybeans vary widely in their net effect on soil nitrogen availability, which was found to be negative when the ISNT value was high.

Need for Improved Soil Sampling

Although corn roots grow to a depth of seven feet, nutrient needs are often assessed by sampling only the upper seven inches of soil. “And fertilizer is seldom applied below this depth,” Mulvaney said. “So I worry that we are depleting the subsoil, where the crop really needs the fertility, while we concentrate fertilizer where the crop doesn’t benefit from it as much — especially in a dry year. I suspect we have to recalibrate soil tests to the most effective sampling depth — whatever that is.”