Everyone wants to apply the right amount of fertilizer at the right time in the right place. But be careful using early season crop sensors teamed with variable rate nitrogen applications. They can fool you more often than not, says Iowa farmer Clay Mitchell.
"If I had to choose one tech that fails most frequently it's variable rate nitrogen," says the Geneseo, Iowa farmer and Harvard graduate in biomedical engineering. "With variable rate N, there's a myth you jump in to early on, that the color of the plant is a good proxy for future nitrogen need. Once you agree to this premise, you're off and running, but there's a lot of stresses that relate to plant color."
Discolored corn might be a nitrogen problem. Or it might be something else. In low lying areas where it's damp in the spring, with the highest mineralized nitrogen, plants can be a little stunted and yellow. But that's the area that has the most nitrogen in the soil, so as soon as it warms, plants snap out of it, says Mitchell.
"So not only would you be wrong in your assessment based on plant color, you would be flipped from what you need," he says.
Another problem: Even if you detect N stress and it caused a plant discoloration, it's too late to avoid stress on the plant. Sometimes N is pooled deeper in the soil profile, and early season plants may be stressed until roots hit that N and boom - they have all the N they need.
Another stress that looks like an N problem is sulfur deficiency, which causes streaking. "There's a lot of that in the Midwest, and putting on more N doesn't help," says Mitchell.
Adding to the riddle these days is the huge variation in hybrid development; some are more yellow than others at any given point in the season.
"Some tech people have adopted variable rate N and will not give up no matter what," concludes Mitchell. "Even so, this is one precision technology that has the highest rate of attrition