With more than 20 different commercially available sulphur-containing fertilizers, it may be a challenge for farmers to choose the best option. Farmers should speak with a local agronomist or extension agent to determine how to best identify and correct deficiencies in their fields.
Elemental S based fertilizers are the most concentrated S carrier. Modern technologies increased their use in direct applications or as additives to N-P-K fertilizers.
The use of elemental S to reduce soil pH and to reclaim sodic soils is well known. However, its function as a fertilizer source of S has greatly increased its use in agriculture with the limited production and availability of other sulphur-containing fertilizer materials.
Elemental S is a yellow, inert, water-insoluble crystalline solid. Commercially it is stored in the open, where it remains unaltered by moisture and temperature changes. When S is finely ground and mixed with soil, however, it is oxidized to SO4-2 by soil microorganisms. The effectiveness of S in supplying S to plants compared with SO4-2 depends on several factors, including particle size, rate, method, and of application; S oxidizing characteristics of the soil; and environmental conditions. S oxidation rate increases as particle size is reduced. The finer the S particle size, the greater the surface area and the faster the SO4-2 formation. Thus, increases in the S surface area results in increased SO4-2 availability to crops.
When S is finely ground and mixed with soil possessing a high oxidizing capacity, it is usually just as effective as other sources. Time of application is especially important with S products. Finely divided S should be worked into the soil as far ahead of planting as possible. Placement of S can often affect its oxidation rate, with broadcasting, followed by incorporation, being superior to banding. Uniform distribution of S particles throughout the soil will 1) provide greater exposure of S particles to oxidizing microorganisms, 2) minimize any potential concerns caused by excessive acidity, and 3) provide more favorable moisture relationships.
If S is placed on the soil surface and compared with SO4-2 placed similarly, the SO4-2 may initially give better responses. Because of its solubility, it can move into the root zone with percolating waters. The S must first be oxidized to SO4-2 . This is not a rapid process, particularly when left on the soil surface. In the case of granular elemental S products, a period of exposure on the soil surface to wet-dry and freeze-thaw cycles is required to disrupt the granules and disperse the S. This dispersion process prior to soil incorporation is essential for satisfactory conversion of S to plant available SO4-2 . With the exception of perennial crops, topdressing of S sources is not normally recommended, and in all cases, elemental S should be applied well in advance of crop need.
A variety of S-bentonite fertilizers have been produced to improve the effectiveness of granular elemental S products by incorporating, typically 10% by weight of swelling clay such as bentonite. Particles of S-bentonite are sized for blending with solid N,P, and K fertilizers. When it is applied to soil, this bentonite component imbibes soil moisture, causing fertilizer granules to disintegrate into finely divided S, which is more rapidly converted to SO4-2 . This material has gained wide acceptance as a source of plant nutrient S for high analysis, bulk blend formulations because it provides elemental S in an acceptable physical form that can be converted easily into the SO4-2 form in soil.
Because of variable oxidation rate of these S sources during the first growing season after application, it should be incorporated into soil prior to planting. When it is applied just before seeding and on severely S deficient soils, some SO4-2 should also be provided. Repeated use of elemental S containing fertilizers tends to gradually enlarge the population of S-oxidizing microorganisms, resulting in a corresponding increase in the rate of SO4-2 formation.
In view of the growing concern about S deficiency in world agriculture and its implications for fertilizer technology and management, research has been initiated by fertilizer companies to study the effectiveness of modified phosphates and nitrogenous fertilizers with incorporated elemental S. These studies show that nitrogen and phosphate fertilizers such as TSP, MAP or DAP may be modified by incorporating elemental S to become effective and economical sources of fertilizer S.
Recently, a new sulphate and elemental S-enriched MAP fertilizer was developed in North America. This granular fertilizer containing 50% elemental S and 50% sulphate-S provides readily available S for early plant uptake and residual S for later in the growing season. They are excellent sources of N, P and S, and satisfactory for bulk blending with other granular fertilizers or direct application, particularly for topdressing legumes when both P and S are required.
Sulphur enriched SSP are popular in some countries, such as New Zealand. Ordinary superphosphate is enriched with S to make mixtures containing 18 to 35% S. The added S is superior in its residual effect to the CaSO4 already in the SSP. This S-enriched SSP has received attention in the area with high leaching losses of plant nutrients because of its potential for reducing SO4-2 leaching loss and also providing available SO4-2 to meet crop needs during the whole growing season.
One way of achieving controlled release of soluble forms of plant nutrients is to coat them with relatively insoluble and affordable material such as elemental S. Sulphur coated urea (SCU) is a controlled release N fertilizer consisting of an S shell around each urea particle. It contains 77% to 82% of urea (36% to 38% N) and 14% to 20% S coating.
Sulphur coated urea has the greatest potential for use in situations where multiple applications of soluble N sources are needed during a whole growing season, particularly on sandy soil under high rainfall or irrigation. It is advantageous for use on sugarcane, pineapple, grass forages, turf, fruits such as cranberries and strawberries, and rice under intermittent or delayed flooding.