Do soybeans need N fertilizer?
While these results don’t prove that adding N fertilizer doesn’t increase soybean yields, it clearly shows that we can’t count on such an increase, and it certainly calls into question the wisdom of making such applications, at least with our current state of understanding. It is possible that soils often contain more N than we realize, especially under good mineralization conditions, which are also good growing conditions. It is also possible that we don’t really understand the photosynthetic capacity of soybeans under field conditions, and that our guess that yield is limited by photosynthetic rates when the plant is also fixing its own N is just incorrect.
The usual signal of N deficiency in crops – light-green leaves – is rarely seen in soybean plants during the period of podsetting and seedfilling, unless the crop is under prolonged drought stress. Late in seedfilling, leaves start to mobilize their N as chlorophyll and photosynthetic proteins break down, and much of this N moves to pods and into seeds as photosynthesis winds down. If there were a way to get more N into the leaves early in this process, it might be possible to maintain photosynthesis a bit longer and move more material into seeds. It is clear that getting this to happen consistently will be difficult, especially under an unpredictable water supply.
Until and unless we find a way to learn to make N application to soybeans work consistently, or in most cases to work at all, this practice increases both economic and environmental risk. Under dry late-season conditions such as we experienced in 2013, much of the N we apply will stay in the soil, and become part of the mobile pool of soil N going into the fall.
One way to get a better look at this over a wide range of fields and soils is to put strips trials in farm fields. These can be done using aerial or ground application, but ground application is easier to track, if more difficult to do. If you have interest in running such a trial, I’ll be glad to suggest a design.
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