Answer provided by John Heard, MAFRD Crop Nutrition Specialist
Our initial experiences of soybeans and high nitrogen soils in Manitoba were negative. High soil N tends to inhibit nodulation while at the same time this nitrogen tends to grow slightly larger vegetative plants. However after flowering that nitrogen is often insufficient to provide full yield and protein potential for the crop. So we adopted a thumb rule that if soils had more than 60-75 lb nitrate-N/acre, producers should consider growing a crop other than soybeans. The other crops should benefit more from the N than the soybeans, which would still take up the nitrogen but may not express full yield and protein. Nitrogen was also seen to trigger iron deficiency chlorosis in soybeans grown in wetter, high lime soils.
But more recently farmers and agronomists have observed that soybeans may perform well on some high N soils. This may be because soybeans have been grown more often and a native reserve of rhizobium exists in many of these cropped soils. With industry partners (AGVISE Labs and ToneAg Consulting) we made observations at 13 field demonstrations in 2013 where high soil N levels were simulated with N application (Heard et al, 2013). Our observations were:
- Nitrogen at 50-100 lb N/ac reduced nodulation at all sites, but most severely at the virgin or first time soybean sites. Nodule numbers were still generally sufficient on those fields with a previous history of soybeans (Figure 1).
- Few sites were harvested for yield, but nitrogen affect was more severe on the first year soybeans (reduced yield or lower protein).
Figure 1: Average rhizobium nodules per root from 13 demonstrations in 2013.
So if one needs to replant soybeans on a field already fertilized with nitrogen consider:
- Whether well nodulated soybeans have been grown in the past.
- Treating fields with a history of soybeans as virgin fields by applying full rate of inoculant
- Even if fields have high N in the spring, soybeans will largely deplete those reserves during the season
One may still wish to avoid planting soybeans if it is a virgin field or if there is risk of iron deficiency chlorosis.
There was no advantage to supplementing properly nodulated soybeans with additional nitrogen at these sites. In US studies, additional nitrogen appears warranted “sometimes” when yields are very high (>65 bu/ac) or when nodulation failures occur due to acid soil, drought or other adverse weather conditions.
Heard, J., J. Lee and R. Tone. 2013. Nitrogen and soybeans: Friends, foes or just wasted fertility? Manitoba Agropnomists Conference 2013.