Nitrogen and Sulfur Fertility Effect on Pacific Northwest Dryland Canola (Brassica napus) Production.

A four site-year experiment (2 growing seasons 2007-2009 and 2 locations-Davenport, WA and Pullman, WA) was conducted in the dryland (annual precipitation:  358 – 534 mm) region of eastern Washington State to evaluate nitrogen (N) and sulfur (S) fertility requirements for winter and spring canola (Brassica napus).  For each site-year N and S rate responses as well as N application timing efficacy was evaluated.  In each site-year, winter canola was seeded and in all but one site-year, winter canola suffered failure due to winter kill and spring canola was reseeded.  Five N ((NH₂)₂CO) rates and two S (NH₄SO₄) rates were applied.  Analysis of variance of all site-years for grain and oil yield revealed an NxS interaction and no significant differences owing to N timing.  Analysis of variance of individual site-years for grain and oil yield revealed an NxS interaction for the first year at Pullman, no significant effect for the second year at Pullman, a significant main effect for N for the first year at Davenport, a significant main effect for N and S for the second year at Davenport, and significant differences owing to spring N application for both grain and oil yield at Davenport for both years.  Grain yield response data plateaued and oil yield response data peaked and declined with excess N, so for individual site-years grain and oil yield data were fitted using the Mitscherlich and quadratic models, respectively, using least squares fit.  Soil N supply that would lead to 90% of maximum grain or oil yield was in the range of 0 – 90 kg ha^-1 applied N suggesting that dryland canola may be either very efficient at scavenging residual soil N or limited in yield by other growth factors such as water.