When is N available to corn from soybeans or a cover crop of rye?
Apr 17, 2014
Question: When is the nitrogen in soybean nodules available to another plant? If it isn't available immediately; when in the following year is it? When would the nitrogen stored in a cover crop of rye that is killed at the 6" to 8" range in the spring be available to that year’s corn crop?
Answer: There really isn’t that much nitrogen (N) available in the soybean nodule. The nodule is where N is made for the soybean plant but then it’s sent throughout the plant. When we talk about N coming from the soybean plants, it’s actually from the whole plant decomposing. You harvest most of the N and take it away in the soybean. In other words, the soybean you haul out of the field has a lot of N in it. The nodule itself is no diff from the root, the stem, the leaves, and the pod as far as that whole plant breaking down. Farmers mistakenly think that N is in the nodules only. Farmers also often think if they leave the nodules in the ground they can still take an N credit, but that’s not really the case. The reality is the N is in the plant residue. Think of it like a cover crop, you have to decompose the entire cover crop to get that N back. In the recycling of a bean crop, that recycling is temperature sensitive depending on when harvest takes place. As we try to track that here in central Illinois, it looks like it’s somewhere in that five- to seven-week range, once soil temperatures are above 60 degrees F. There’s actually more N left in a cornstalk field than there is in a bean field, but the cornstalk field is so much slower in breaking down the N. As for the cover crop of rye, the bigger the rye gets the longer it takes for it to be available for a corn crop. With a cover crop of rye at the 6- to 8-inch range, again it depends on temperature and some other factors, but typically you’re in that 40- to 50-day range before the N will be available to the corn crop.
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