Some South Dakota soybean fields that were planted early and have had sufficient moisture have reached early pod setting stage (R3) and are looking good. What is the potential for increasing soybean yield by adding some late-season N at the pod filling stage? In fact, this is not late for the soybean plant. At early pod set, the soybean plant has reached only 25% of its maximum dry weight and only accumulated about 30% of its total N. The plant will now be rapidly adding pods and filling seed that will require much more nitrogen. When growing conditions are ideal and little stress, there is some evidence that the biological nitrogen fixation that is taking place in the soybean root nodule cannot keep up with the plant's N needs.
Applying N at the R3 stage has produced significant, profitable yield increases for irrigated soybean in studies in Nebraska and Kansas. The Kansas study reported six of eight sites that had significant increases (5-10 bu/a) to in-season N applications of 20-40 lb./a. The six responsive sites were generally low in organic matter and available soil nitrogen and had yields above 60-65 bu/a.
A number of studies from MN, WI, SD, Ill, and IA showed no significant advantage to late N applications for soybean. However, none of these studies had yields above 60 bu/a. Soybean N need can reach almost 300 lb./a for a 65 bu/a yield with about 50-60% of the N coming from nitrogen fixation. The remaining N (120-150 lb./a) must come from either soil inorganic N or mineralization of soil organic matter or breakdown of previous years residue. Supplemental fertilizer N would be needed if the soil cannot meet this N demand. It is assumed that the lack of response from the late-season N studies listed above is because the soil did meet the plant's N need at these relatively lower yields (<60 bu/a). In addition, surface soils in mid-summer tend to be dry and active uptake of added N is reduced until rainfall is received therefore limiting any yield responses.
For the producer who wants to try some late-N applications for soybean, the following suggestions would be made:
- Select fields with excellent yield potential and where soybean has had little stress and growth is exceptional at R3.
- Select fields that have adequate soil moisture or are irrigated.
- Apply from full bloom (R2) thru beginning pod (R3) stages.
- Apply 20 – 40 lbs. of N either as liquid or dry urea.
- Preferably, the N should be applied under the soil surface to avoid leaf burn and N volatilization losses.
- If applying liquid without injection, use drop nozzles and dribble on soil surface to limit leaf burn.
- If treating the whole field, be sure to leave harvest strips where no N is applied so the practice can be economically evaluated.
The chances of an economical yield response will be increased under irrigation and for soils low in organic matter.