Effect of Preceding Crops On N Requirements of Spring Wheat in Northwestern Ontario.

General recommendation for spring wheat production in Ontario is to apply 70 kg N ha^-1. However, N requirements of spring wheat could be affected by several factors; primarily by the preceding crops that could add, leave/or remove N from the soil. A field experiment, in split plot design with four replications, was conducted at the Thunder Bay Agricultural Research Station, Thunder Bay, during 2008-’11. Five crops, alfalfa, soybean, silage corn, barley, and wheat were assigned to the main plots split for 4 rates of N application; 0, 40, 80 and 120 kg ha^-1. Averaged over three years, the results revealed that spring wheat following alfalfa, silage corn and soybean without application of N, produced as much grain yield as that following barley and wheat supplied with 80/or 120 kg N ha^-1. At zero N, grain yields of proceeding wheat after alfalfa, silage corn, soybean, barley and wheat were 3,980, 3,834, 3,723, 3,139 and 3,139 kg ha^-1, respectively. The corresponding grain yields at 80 kg N ha^-1 were 4,239, 4,207, 4,124, 3,905 and 3,751 kg ha^-1, respectively. Increasing the rate of N from 80 to 120 kg ha^-1 didn’t improve the wheat grain yield further after any of the crops. Averaged over years and preceding crops, wheat grain yields at 0, 40, 80 and 120 kg ha^-1 were 3,551, 3,857, 4,045 and 3,965 kg ha^-1, respectively. Averaged over years and N rates, wheat grain yields after alfalfa, silage corn, soybean, barley and wheat were 4,138, 3,999, 4,039, 3,667 and 3,428 kg ha^-1, respectively. The corresponding nitrate N test values in spring before seeding wheat were 19.7, 17, 16.3, 11.7 and 13.7 ppm, respectively.