Welcome rains have turned to downpours in some parts of Ontario. In extreme cases, fields have received in excess of 12 inches of rain over a two week period. Excess moisture has resulted in a tremendous number of fields turning yellow (Picture #1). This yellowing is not the result of root rots but largely a nitrogen deficiency. Although root rots are a significant problem in certain fields, the majority of yellow plants are a result of wet feet. The fact that tile runs are so evident in water-logged fields is clear evidence that that this is a nitrogen deficiency (Picture #2).
Another factor that has added to this N deficiency is that soybeans are in their rapid growth phase, attempting to quickly put on tri-foliates while flowering is in full swing. This rapid growth requires nitrogen. The early season dry conditions actually inhibited nodulation, so a number of factors have come together to cause these symptoms. A large demand for N, limited available soil N due to excess moisture, and insufficient biological N fixation have all added to this yellowing.
The obvious question that needs to be addressed is, should supplemental nitrogen be applied now to help the crop recover? In most cases the answer is “No.” Fields will recover once soils dry out and natural N fixation catches up to demand. There is often a lag phase when nodulation is behind N demand in other years. This year the problem is just more obvious. The main problem with applying nitrogen fertilizer is that it provides no economic benefit. On average the yield response to N fertilizer is about 2 bu/ac, unless there is a complete nodulation failure or severe disease. In those cases responses can be higher. The other problem is that a small amount of nitrogen is not enough to really help the plant. It might look better at first, but yield will not improve enough to justify the cost. Biological N fixation is absolutely necessary to grow a profitable crop. The good news is that soybeans will recover as naturally available N in the field becomes available, soils dry, and more importantly N-fixation swings into full gear.
Risks of Supplemental N Application
The main concern with applying N fertilizer is that excess vegetative growth will lead to white mould and lodging later in the season. With wet conditions many fields are already at risk of white mould, so the addition of N fertilizer is problematic. It is entirely possible that supplemental N fertilizer will do more harm than good. So while a small amount of N fertilizer now will make the crop look better, it will do little to improve yields; and a large amount of N fertilizer now will lead to significant problems later in the season. The best advice is to wait it out – soybeans will recover. Your money would be better spent on a foliar fungicide if the field has a history of white mould.