Effect of Sulfate Application on Alfalfa Productivity in Northern Quebec.

Alfalfa (Medicago sativa L.) has high requirement for sulfur. The critical value of sulfur in alfalfa, from the early bud stage to the early flowering stage, is 0.25%. Under this value, the plants are considered deficient and sulfur fertilization is recommended. Symptoms of S deficiency in alfalfa have been observed since 2014 at Normandin Research Farm, Quebec, Canada. In 2016, a trial has been established to determine the effect of sulfur fertilization, added as potassium sulfate, on the productivity of alfalfa. Four potassium rates (40, 80, 120 and 160 kg ha^-1 of K₂O with and without sulfate) and one control were evaluated. The addition of potassium sulfate in spring 2016 did not affect DM yields of alfalfa at the first cut. Moreover, the sulfur content of alfalfa at the beginning of flowering was not significantly different from one treatment to another and was around 0.10%. In the second cut, the plots receiving sulfur fertilization obtained the highest yields. Seasonal yields were significantly different from control treatment when 28 kg ha^-1 of sulfur were applied. Thus, the 80 kg ha^-1 K₂O, in the form of potassium sulfate, yielded 3.4 Mg DM ha^-1 compared to 2.3 Mg DM ha^-1 for the control treatment. This dose also met the sulfur requirements of alfalfa, as the sulfur content of the plant was greater than 0.25%. These results demonstrate that the addition of sulfate could have a beneficial effect on alfalfa productivity when the sulfur content in the plant is less than 0.25%. The trial is repeated in 2017 and results will be presented.