In a corn-soybean rotation, the corn will need less nitrogen (N) following the nitrogen-fixing legumes than in a continuous corn operation. Thus, a nitrogen “credit” is applied to manage N properly. Most people account for soybean credits when making fertilizer decisions, but often assume the extra nitrogen is coming from the soybeans themselves. The real reason is a bit more complicated and involves several factors including soil nitrogen availability, residue amount, and microorganism preferences.
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Source: Minnesota Crop News