Prior to the precipitation that started Wednesday, May 25th, it had been unusually dry this spring (Figure 1 below) and there are nitrogen fertilization implications.
Farmers and agronomists may be wondering about the fate of previously applied surface fertilizer. The 2 main concerns would have been stranding at the soil surface where it is isolated from the crop root zone, and volatilization loss.
Rainfall is the only solution for the positional stranding at the soil surface. For spring seeded crops this rainfall will likely have moved N into the root zone before any malnutrition of the seedling. However surface applied N to fall rye and winter wheat may have been stranded during the main yield building period, since both crops are at the boot stage now (Figure 2 below). So in the Carman area, topdressed N applied after April 17 was probably stranded. Nitrogen moving into the root zone now will contribute more to protein than yield.
The other concern is of volatilization loss of surface applied N. But much of the surface applied N to spring seeded crops was onto dry soil – and we have had several field demonstrations to measure volatilization loss using dosimeter tubes (see photo below). In the cases we have followed, the soil surface must have been dry enough to prevent measurable hydrolysis (where the urea molecule is cleaved into carbon dioxide and 2 ammonia molecules). In fact urea pellets were observed intact for several weeks on the soil surface. So unless a light shower had been received, I anticipate losses were minimal. Where some soil moisture was present, the losses would have proceeded until the soil surface dried up.
The best way to sense any N shortage in the crop is comparing to a N Rich Strip. Many farmers no longer overlap fertilizer so this is difficult to detect. If there is little to no colour difference between a N rich strip and the general field, then losses are probably minimal.
Submitted by: John Heard, Crop Nutrition Specialist, Manitoba Agriculture