A review of data from more than 150 studies from the U.S. and other regions showed that high yields were linked to production systems in which corn plants took up key nutrients at specific ratios -- nitrogen and phosphorus at a ratio of 5-to-1 and nitrogen and potassium at a ratio of 1-to-1. These nutrient uptake ratios were associated with high yields regardless of the region where the corn was grown.
"The agricultural community has put a lot of emphasis on nitrogen as a means of increasing yields, but this study highlights the greater importance of nutrient balance," said Tony Vyn, Purdue professor of agronomy. "We will not be able to continually boost global corn yields and achieve food security without providing adequate and balanced nutrients."
While corn producers in the U.S. have long relied on nitrogen fertilizers to improve yields, they should not overlook other nutrients such as potassium and phosphorus, Vyn said.
"Growers need to be as concerned about the amount of potassium available to their plants as they are about nitrogen," he said. "Corn's demand for nitrogen and potassium is similar. We need to focus on the nitrogen-potassium balance because that's where we have the greatest deficiency in terms of application, especially in the eastern Corn Belt."
The main obstacles to closing corn yield gaps -- that is, reaching the potential yield projected for a particular soil and climate -- around the world are the inaccessibility and cost of fertilizers and the inherent nutrient deficiencies of soils in many regions in which corn is grown, said Ignacio Ciampitti, assistant professor of agronomy at Kansas State.
"On the global scale, the potential yield response to balanced nutrient applications is big," he said. "But growers outside the U.S. should also focus on developing an integrated management program that considers factors such as optimum planting dates, plant densities and pest management."
The study revealed a sharp difference in the "indigenous" supply of nitrogen in soils in the U.S. and in other regions. In cases where no additional nitrogen fertilizers were applied, U.S. corn took up an average of 120 pounds of nitrogen per acre -- compared with about 52 pounds of nitrogen per acre in regions outside the U.S. The high level of indigenous nitrogen in U.S. soils may be due to factors such as an inherently greater amount of organic matter in the soil, a history of fertilization in the Corn Belt, and the use of superior corn hybrids, Vyn said.
But despite the higher nitrogen content of U.S. soils, corn plants in the U.S. were not more efficient at absorbing nitrogen fertilizers from the soil than those in other regions. Nitrogen recovery efficiency, the measure of how much applied nitrogen the above-ground portion of a plant absorbs from the soil, was the same -- 48 percent -- for the U.S. and other parts of the world.
Vyn said nitrogen recovery efficiency in the U.S. is not higher partly because increasing fertilizer application rates can create a "declining return:" The more fertilizer applied, the more difficult it becomes to extract the same percentage of the nutrients in the corn.
Optimizing the timing of applications and developing alternate nitrogen sources may improve the recovery efficiency of U.S. corn production systems, he said.
Data collected from 1976 to 2012 also revealed that the efficiency with which individual corn plants absorbed and used nitrogen, potassium and phosphorus stayed relatively consistent despite plants being grown at much higher densities.
"On a per-plant basis, corn plants are not taking up more nutrients than they were in the past," Ciampitti said. "They may be taking up less because they are grown closer together, but they are more efficient at producing more grain with the same amount of nutrient uptake."
Still, growers need to keep an eye on the amount of nutrients removed at harvest to ensure soil nutrient levels don't drop to the point where future yields could suffer, Vyn said.
"Growers should not rely too heavily on modern genetics to give them the yields they expect without spending a considerable amount of effort on maintaining nutrient availability throughout the growing season."
Ignacio A. Ciampitti, Tony J. Vyn. Understanding Global and Historical Nutrient Use Efficiencies for Closing Maize Yield Gaps. Agronomy Journal, 2014; 0 (0): 0 DOI: 10.2134/agronj14.0025