Varying plant population within a corn field means you also must vary your nitrogen rate. Deciding how much nitrogen to apply is challenging—multiple applications, including sidedressing, are required.
Almost overnight, it seems, variable-rate application technology (VRT) for seed and fertilizer has arrived. The new equipment brings the potential for higher yield and less pollution of natural resources. The equipment is ready, but are you?
"Variable-rate application of seed and fertilizer is a systems approach to farming," says Farm Journal Field Agronomist Ken Ferrie. "To make the equipment pay for itself, you have to think the system through and put all aspects of it into play."
What is variable-rate? "An environmentally sound variable-rate program usually involves moving seed and nitrogen from areas of excess to areas where you’re not applying enough," Ferrie says. "You’re not changing the total amount applied as much as you are moving resources around within the field itself."
Effectively juggling those resources requires excellent management. "If you’re overapplying nitrogen in part of a field by 90 lb. per acre and you move that fertilizer to places where you’re underapplying, the return can be big in terms of reduced nitrogen loss and increased yield," Ferrie says. "But if heavy rain causes you to lose 100 lb. of nitrogen, the areas you moved the nitrogen from are going to suffer. The areas you moved it to are going to win; they may still suffer, but not as much."
In other words, the purpose of variable-rate application is to manage inputs more precisely.
Nitrogen management. "Deciding how much nitrogen to apply is challenging because nitrogen is controlled microbially in the soil," Ferrie says. "In 2010, many growers applied more than 200 lb. of nitrogen per acre, but they still had yellow corn. It’s not a matter of applying a certain amount—it’s knowing when and where to apply it on a particular field or part of a field."
Microbial activity determines how much nitrogen your soil provides, Ferrie explains. That can range from 30% to 80% of a corn crop’s requirement.
"As they decompose the old crop, microbes immobilize nitrogen in the soil and make it temporarily unavailable to plants," Ferrie says. "Later in the season that nitrogen is mineralized, making it available once again.
"This process is tied to the amount and source of carbon in the old-crop residue [soybean residue breaks down more easily than corn] and the nitrogen reserve in your soil. Understanding this nitrogen cycle helps you know how much nitrogen you need to apply and when to apply it, based on soil type and population."
Each plant needs a certain amount of nitrogen. "There’s going to have to be a certain amount of nitrogen taken up by each plant," Ferrie explains.
"Increasing plant population is like putting more cattle in a pen—you have to put out more feed. So a higher population requires more nitrogen, which must come either from fertilizer or from the soil.
"In a silage study, 29,000 plants per acre produced 21 tons of silage, and 33,000 plants per acre produced 27 tons of silage," Ferrie says. "Silage analysis showed the 33,000 plants removed
60 more pounds of nitrogen from that field—the uptake into the plant itself.
"The rate of nitrogen per bushel leaving the field may be the same, but if you have 36,000 plants and 34,000 plants per acre, both producing 190 bu. of corn, the amount of nitrogen per plant has changed. And nitrogen drives the whole process of growth."
- February 2011