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Fluid Journal : Early Spring 2012
11 2 Crop requirements S content. The S content of plants differs greatly among crop species, among cultivars within a species, and with developmental stage. In general, the oilseed crops (such as canola and sunflower) and legumes (such as alfalfa and soybean) have a much higher requirement for S per unit of production than the small grains and corn. Wheat, barley, and flax have low concentrations of S varying from 0.15 percent to 0.20 percent at flag leaf (shot blade) and flowering, and have high N:S ratios of 15 to 16 at optimum yield. Canola at flowering and alfalfa at full bud generally have similar S concentrations (0.25% to 0.30% S) and N:S ratios of approximately 12 for canola and 14 for alfalfa at optimum yield. The low N:S ratios of the latter crops reflect their greater S requirement and uptake. S removal. Sulfur requirement is a function of the S taken up in the entire plant, while removal is a function of the S concentration in the tissue or grain that is removed from the field (Table 1). Canola, corn, and oats (among the crops grown for seed) have the highest S requirement and remove the largest quantity of S (15 to 21 lbs S/A) at harvest. The requirements and crop removal of wheat, barley, sunflower, peas, and fababeans are intermediate (9 to 14 lbs S/A). Flax and buckwheat have low requirements for S and remove only a small amount at harvest (7 to 8 lbs S/A). Forage crops remove large quantities of S (9 to 21 lbs S/A), since most of the above-ground biomass is harvested, and there is little return of S to the soil from crop residue. On the other hand, much of the S taken up by grain crops is returned to the soil in the crop residue and can be recycled to subsequent crops. Managing S fertility Availability. When the soil is not able to provide enough plant-available S to meet crop demand, S fertilization can be used to optimize crop yield. A wide range of inorganic and organic sources can be used to supply S. Sulfate or thiosulfate materials provide an immediate source of plant-available S for the crop, but the sulfate is highly susceptible to leaching. Following soil application, thiosulfate rapidly reacts to form tetrathionate, which is not plant-available until it converts to sulfate in a process completed within one to two weeks in warm soils. Under cool soil conditions, the reaction may be Figure 1b: Sulfur deficiency in canola. Figure 2: Canola yield as affected by S source. Table 1. Sulfur removals of various crops at the given yield levels. Crop Plant component Yield S content ton acre-1 lb acre-1 Alfalfa biomass 5.8 30 Canola/rapeseed grain 0.98 12 Cool-season grass biomass 4 16 Corn grain 5.1 13 stover 3.1 9 Cotton lint 0.76 40 Flax seed 0.83 7 Grain sorghum grain 4.2 22 residue -† 16 Lentil seed 0.9 5 Orange fruit 27 28 Peanut tuber 2 21 Potato tuber 25 22 Rice grain 3.5 12 Soybean grain 1.8 12 residue -† 13 Sugar Beet tuber 30 45 Sunflower seed 1.7 6 residue -† 10 Tomato fruit 30 41 Wheat grain 2.4 7 straw -† 15 † Sulfur removals in stover, straw, and crop residues are estimates based on typical values of a harvest index (i.e., the ratio of harvested grain to total plant biomass). In most cases, the crop residues are not harvested and the S would not be removed from the field.
Late Spring 2012