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Fluid Journal : Early Spring 2012
somewhat slower. Elemental S materials provide a more gradual release of sulfate into the soil because the S must first be oxidized to the sulfate form. This reduces the immediate risk of leaching losses, but S availability to the crop is delayed, and crop growth will not be affected until oxidation occurs. Cool, dry soils and the relatively short growing season that occurs in northern climates may restrict the oxidation process, so that availability of elemental forms can be delayed for one to several years. However, a residual benefit may occur for subsequent crops. Timing and placement. The timing of S fertilizer applications, placement of the S sources, and fertilizer rates for specific crops are all management decisions that require careful consideration. The growth and development of cereal grains, oilseed crops, and various legumes are quite different, so the demand for S varies considerably with growth stage. In general, a sufficient S supply is needed during the early growth stages of cereal grains to ensure proper tiller development. In contrast, the greatest demand for S by canola occurs during flowering and seed set. Sulfate sources should be applied at or near the time of planting to provide sufficient S for early growth. In soils with low organic matter content, sandy texture, or rapid water movement through the profile, fall applications of sulfate materials should be avoided to reduce the risk of leaching losses. Soil or foliar applications of sulfate sources can also be used to correct S deficiencies during the growing season. To be effective, in-season soil (top-dress) applications depend on rainfall or irrigation to move the S into the root zone. Fall application of elemental S allows fertilizer granules to break down with freezing-thawing and wetting-drying cycles, thus aiding oxidation of elemental S during the growing season. However, even with fall application, conversion of elemental S to sulfate may be too slow in the northern Great Plains to optimize yield of a spring crop (Figure 2). Effective fertilizer placement options for S also depend on the fertilizer source and the soil characteristics. Sulfate will move readily through the soil in the soil water. Therefore, sulfate sources that are broadcast with or without incorporation can provide readily available S to the crops, although stranding of the fertilizer at the soil surface can occur under dry conditions. Row or band applications at the time of planting are also effective, if only small amounts of fertilizer S are required. Care must be taken to avoid damage from excessive sulfate in contact with the growing seedling. Since elemental S must be converted to sulfate prior to crop uptake, placement of elemental S should encourage oxidation. Elemental S sources should generally not be applied in bands because banding reduces the contact between S and the oxidizing microorganisms in the soil. Elemental sources may be left on the soil surface to allow weathering, but should subsequently be tilled into the soil to blend the fertilizer with soil in the rooting zone to encourage oxidation. Under reduced tillage, the combination of lower soil temperatures during the spring and early summer period and the removal of the mixing action of cultivation may slow the conversion of elemental S to sulfate, reducing the rate at which it becomes available for crop growth. On the other hand, a more active microbial population near the soil surface under reduced tillage and the maintenance of higher soil temperatures through the fall period may enhance oxidation of elemental S. Dr. Grant is Research Soil Scientist, Agriculture and Agri- Food Canada, Brandon, Canada, and Dr. Kovar is Research Soil Scientist, USDA-ARS, Ames, IA. "Sulfur is a critical nutrient for canola, pulses, and forage legumes, such as alfalfa." FLUID JOURNAL ARTICLE ARCHIVES 18 years of agricultural research and studies available, dating back to 1993, the inaugural date of the Fluid Journal. Spanning our first year, 3,850 visits and 81,034 page views in 28 countries since we've joined the Realview web. We publish 4 times a year, each edition packed with valuable agronomic research that will help you maximize yields. Visit our archives at: www.ﬂuidfertilizer.com
Late Spring 2012