Nitrogen for Soybean in Kentucky: Principles and Practice

By John Grove

Nitrogen is contained in chlorophyll, and chlorophyll is needed for photosynthesis, where carbon dioxide (CO2) and sunlight are turned into plant carbon/dry matter. Nitrogen is also contained in protein, so amino acids, the building blocks of protein, depend upon plant N.

After water, N is most often limiting to crop production. Notably, soybean can form a symbiotic relationship with bacteria, Bradyrhizobium japonicum, which biologically fix dinitrogen (N2) gas making up 78 percent of our atmosphere. The fixed N is quickly used by the plant. Well nodulated soybean (Figure 1, left) will also take up soil ammonium (NH4+) and nitrate (NO3-) released by mineralization of soil organic N. With these two N sources, soy-bean’s N supply is sufficient. Soybean does have a high N demand, about 3 pounds of N for each bushel of grain, and more is required for roots, stems, and leaves. A 70 bushel per acre soybean yield will require about 330 lb N/acre.

Nitrogen is mobile, so deficiency symptoms (Figure 2) generally occur on older leaves. The N deficient crop will exhibit reduced growth and show pale green leaves that eventually turn yellow/brown and senesce prematurely. Little to no nodulation (Figure 1) is the common cause of soybean N deficiency. Soybean under stress (drought, flooding) can exhibit the deficiency symptoms of several nutrients, including N.

Dealing with soybean N deficiency requires looking at soybean root systems to find out if these are well nodulated and healthy. After being split open with a knife, a healthy nodule that is actively fixing N should be pink to bright red. Nodules on plants under drought or flood stress might not be pink or red. If nodules seem healthy, you then determine if other nutrients or growth factors cause symptoms. When nodules are lacking, root growth is not limited and N deficiency is observed prior to flowering, N fertilizer may be needed. About 200 to 250 lb N/acre will be needed for a normal yield because of soybean’s high N requirement. All N sources are equal, but all should be applied directly to the soil as high foliar N rates will result in crop burn. Volatilization loss is possible from surface broadcast urea or surface drib-bled UAN.

When the crop is well nodulated, and with soil N from organic matter, there is no fertilizer N need. Still, soybean yield has been rising, and there are times when soil N supply is relatively low. Questions about nodulated soybean’s need for fertilizer N continue and have been the subject of ongoing UK re-search, in both double-crop and full-season soybean production. Judy and Murdock (1998) examined double-crop soybean response to 30 to 40 lb N/acre applied as dribble banded UAN solution between full bloom and very early pod formation at six locations. Their hypothesis was that soil N supply to double-crop soybean was low as residues of the previous wheat crop decompose. Yield of the check (not N fertilized) treatments ranged from 38 to 45 bu/acre, averaging 41 bu/acre. The yield response to applied N ranged from -1.3 to +2.8 bu/acre and averaged +1.5 bu/acre. They found that the N application was unprofitable at 4 of 6 locations (using then current prices).

More recent work, from the 2019 and 2020 production seasons, has been done with full-season soy-bean. I thank Drs. Edwin Ritchey, Chad Lee, and Carrie Knott for their contributions to this newest data set. Financial support from the Kentucky Soybean Board is gratefully acknowledged. The interpretation of the results is entirely my own. These latest trials did not have soybean N nutrition as the primary, or only, study objec-tive, but particular treatment combinations (total of 9) permitted an evaluation of the benefit of added N to soy-bean yield. Added N rates ranged from about 10 to 40 lb N/acre. The N sources included ammonium sulfate, ammonium thiosulfate, UAN and urea. One objective was to determine if greater soybean yield potential increased the possibility of a positive response to added N fertilizer.

Grain yield for the check (no N fertilizer) treatment ranged from 49 to 83 bu/acre, averaging 69 bu/acre. The yield response to applied N ranged from -3.7 to +6.3 bu/acre and averaged +1.0 bu/acre. The N application was unprofitable in 7 of 9 locations (using then current prices). Further, there was no relationship between soybean yield potential and the crop’s response to N fertilizer. Both negative and positive yield responses were observed at both ends of the range in yield potential.

In summary, there continues to be no need to apply fertilizer N to nodulated soybean. Average yield responses to the practice remain only slightly positive and are generally uneconomical – a mixture of both more negative and positive outcomes for individual field sites. There are better places for a grower to invest production expense monies.

Judy, C., and Murdock, L.W. 1998. Late season supplemental nitrogen on double-cropped soybeans. Soil Sci. News Views 19 (2). https://uknowledge.uky.edu/pss_views/10