By Jennifer Stewart, Purdue University
Soil nitrogen left behind by the 2012 drought-stricken corn crop could mean that the state's wheat growers might be able to apply less fertilizer this spring, a group of Purdue Extension agronomists say.
Corn plants stressed by extreme heat and too little water yielded less grain and left more nitrogen in soils than in normal years. Wheat has potential to scavenge the nutrient more so than a subsequent corn crop.
In their new publication, "Carryover Nitrogen - Potential Impact on Wheat Fertilization," Shaun Casteel, Jim Camberato and Chuck Mansfield discuss the topic and why it's good news for wheat growers who might not have to buy as much fertilizer.
"Spring fertilization rates necessary to optimize yield may be lower than what is needed following normal corn crops," Casteel said. "Wheat planted in the fall has an advantage in that it will accumulate some nitrogen prior to dormancy. Wheat's primary advantage is the established root system that can take up nitrogen in the early spring before corn is even planted."
In a more normal year, Indiana wheat growers don't have to consider leftover nitrogen because winter and spring precipitation removes it from the crop root zone, Camberato said.
"Most of the nitrogen remaining in the soil at the end of the season is a highly leachable form of nitrate," he said. "Nitrate is repelled by soil particles, so it moves downward with water. Indiana typically receives 18 to 24 inches of rainfall between October and April, which is sufficient to remove most of the nitrate from the root zone of the state's soils."
This year, parts of Indiana have received less rainfall than that - especially in the northeastern and northwestern parts of the state where rain has ranged from 10 to 20 inches. Nitrogen carryover potential is much higher in these regions than in southern Indiana.
Soil sampling and lab analysis are the most accurate way to estimate the amount of nitrogen left behind. But because deep soil sampling is labor-intensive, the more common approach to adjusting nitrogen fertilizer rates has been to use young wheat plants as an indicator of soil nitrogen supply instead of sampling soils directly, Casteel said.
Virginia Tech and the University of Kentucky provide guidelines to growers for sampling wheat at growth stage Feekes 5 and adjusting the fertilization rate based on the tissue nitrogen concentration.
"Sampling wheat at the proper growth stage is important because the tissue concentration changes rapidly with growth during this time period," Casteel said. "The only guidelines for adjusting nitrogen fertilization are at Feekes 5, which is an ideal time to fertilize from crop production and nitrogen efficiency standpoints. Unfortunately, this timing is risky in wet soils and is later than when many fertilize wheat."
Casteel, Camberato and Mansfield's full report, including tables and photos, is available for free download: Carryover Nitrogen - Potential Impact on Wheat Fertilization.