Ontario Grain Farmer October 2021

NITROGEN (N) DEFICIENCY in soybeans caused many fields to turn yellow in late June and July. A dry spring delayed nodulation, then heavy rainfall saturated soils just as plants required large amount of N during the rapid growth phase. There is often a lag phase when nodulation and N fixation is insufficient to supply enough N for the plants demands early in the season. This year, symptoms were evident later in the season and more pronounced than usual. So, a number of factors come together in 2021 to cause this temporary N deficiency. A large demand for N, limited available soil N due to excess moisture, and insufficient biological N fixation. The good news is that most fields recovered as soils dried out and biological nitrogen fixation caught up to demand. Unfortunately, in some areas where fields continued to receive heavy rainfall or drainage was poor, plants could not fully recover. Some farmers questioned if N fertilizer could be added to these fields to help them through this yellow phase. Although applying N fertilizer does help green up the plants, there is no economic benefit. Numerous trials have been conducted in Ontario and the average yield response to N fertilizer is less than two bushels per acre. The addition of extra N may also cause excess vegetative growth leading to white mould and lodging later in the season. While a small amount of N fertilizer does make the crop look greener, it does little to improve yields; and a large amount of N fertilizer leads to significant problems later in the season. The best advice is to wait it out and let soybeans recover naturally. Biological N fixation is absolutely necessary to grow a successful soybean crop. l Soybean N deficiency CROP SIDE WITH... Crop side ONTARIO GRAIN FARMER 35 OCTOBER 2021 TILE RUN SOYBEANS SUFFERING FROM N DEFICIENCY. PICTURE TAKEN JULY 7, 2021. SOYBEANS THAT HAVE FULLY RECOVERED FROM MID-SEASON N DEFICIENCY. PICTURE TAKEN FROM SAME LOCATION AUGUST 11, 2021. PHOTOS COURTESY OF HORST BOHNER. Horst Bohner, Soybean Specialist, Ontario Ministry of Agriculture, Food and Rural Affairs