Sulfur Deficiency Symptoms
Sulfur deficiency symptoms in most crops first appear on younger, upper leaves. Signs unique to canola are delayed and prolonged flowering, pale, whitish flowers; erect leaves on the upper parts of the plant; young leaves cup inward and develop a pink, reddish or even purple color on the underside; seed pods will be small and poorly filled.
SOIL TESTING. This will provide a general indication of a soil's sulfur status, but it's best if combined with an early-season plant analysis (tissue test). Because sulfur is a mobile soil nutrient, Canadian specialists recommend taking soil samples from increments of 0-6 inches, 6-12 inches, and 12-24 inches. North Dakota specialist Dave Franzen suggests a 0-6 inch and 6-24 inch sampling. On hilly acreage, it's a good idea to take separate samples from mid-slopes and low-lying areas because sulfur deficiency is more common on knolls.
"The degree of sulfur deficiency can vary throughout the field," says Jack Dobb, agronomist with the Ministry of Agriculture and Fisheries, British Columbia. "Even if the average reading indicates that a soil contains enough sulfur, the crop may still benefit from sulfur fertilization."
PLANT ANALYSIS. An early-season plant analysis is an excellent complement to soil testing. Samples must be taken early, however, to leave ample time for corrective action. Canadian agronomists recommend sampling the crop at the rosette stage. Consult your local diagnostic lab for specific sampling procedures.
As a general rule, the canola plant should contain one part sulfur for every seven parts nitrogen for optimum performance. This is a smaller ratio than for most other field crops because the crop's sulfur demands are greater. Applying additional nitrogen without sulfur can throw off the nutrient balance and create, or intensify, a sulfur deficiency. In severe situations, Canadian research has shown that adding nitrogen without sulfur may even reduce crop yield.
"By itself, sulfur deficiency can really hurt yields," says Marvin Nybord, soil specialist at the University of Alberta. "Where farmers get into trouble, though, is when they continue to apply nitrogen to sulfur-deficient fields. That makes the sulfur deficiency even worse."
TEST STRIP. Placing a marked strip (a drill width, for example) the length of a field makes a good visual evaluation. Look for darker leaves and a larger seed head. With sulfur improving crop vigor and the number of seeds set, the farmer can realize improved yield and a better return.
FERTILIZER TIMING AND PLACEMENT. Sulfur fertilization application can be easily incorporated into other applications. To get canola off to a fast start, apply a small amount at planting. The balance could then be sidedressed, for instance. Broadcasting before planting or, for winter canola, topdressing in February or March, are other effective ways to apply sulfur.
BE SELECTIVE. Price, availability, and other nutrient needs will help determine the best source of sulfur. However, dealers should make sure that their customers use the right type of sulfur at the right time.
In general, elemental sulfur is converted over time to the sulphate form, which plants require. The time involved in this conversion may be several months or more, depending on the specific sulfur source and the particle size. Since most crops require sulfur in a deficiency situation rapidly, not slowly, elemental sulfur is not recommended for initial deficiency correction by itself, without most of the sulfur requirement being met by a sulfate-S form.
Sulphate sulfur is readily available to the crop and is most suitable for rapid correction of a sulfur deficiency. Dealers should consult their fertilizer supplies for application guidelines of sulfur fertilizers in their area.
The NDSU Extension Service has available a color circular detailing sulfur deficiency symptoms of all stages of canola. The circular is available from the NDSU Extension Service through circular SF-1122.