A mistake gave extension specialists a rare opportunity to test the yield effect of post-emergent nitrogen applications on non-inoculated soybeans.
In 2011 a virgin soybean field was inadvertently seeded without inoculation. Most of the field was then fertilized following emergence in June with 100 pounds of nitrogen per acre as urea (46-0-0).
Some small plots in an adjacent unfertilized area were treated with different applications for comparison. One was an untreated check plot, and the other plots received Agrotain-treated urea at 50 and 100 pounds of nitrogen per acre, either at flowering in mid July or at pod fill in early August.
In June, a severe iron deficiency chlorosis (IDC) was observed in the areas that received 100 pounds of nitrogen per acre at emergence. High soil nitrate is known to be a contributing factor of IDC along with factors such as salinity, carbonate levels and saturated conditions.
Soybean roots in the field did not develop nodules, yet plants did not appear pale in colour or nitrogen deficient. Nitrogen levels in plant tissue at early flowering were sufficient and soil organic matter levels were high. The combination of soil nitrate and mineralization from soil organic matter provided sufficient nitrogen for a modest check yield of around 31 bushels per acre.
Soybean yields did not respond to the early-applied nitrogen, but the yield increased 24 and 32 per cent with late-applied nitrogen at 50 and 100 lb./ac., respectively.
The higher nitrogen rates increased protein, decreased oil content and increased seed size. However, even at 100 pounds of nitrogen per acre, seed protein was 40 per cent, which is less than protein usually achieved with good nodulation.
Soybeans responded favourably to late nitrogen applications even though visual deficiency symptoms were not severe. Response would be expected to be greater in fields with lower nitrogen and organic matter levels. If farmers need to make nitrogen applications to non-nodulated soybeans this should be done at the pod-filling stage for maximum benefits. †
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