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.
Sulphate-containing fertilizers provide most of the fertilizer S applied to soils. These materials have the advantages of supplying S primarily as a component of multi-nutrient fertilizers in a form, SO4-2, that is immediately available for plant uptake. The most readily available and popular sources are ammonium sulphate (AS), single superphosphate (SSP), potassium sulphate, and potassium and magnesium sulphate.
Ammonium sulphate (21-0-0-24S) is one of the oldest of N and S-containing fertilizers, and is still popular around the world. Ammonium sulphate is mostly produced as a co-product of other industries. An estimated 70% of global output originates from the production of caprolactam, an intermediate for the manufacture of synthetic fibers. A small amount is recovered from coke oven gas, with most of the remainder produced synthetically from sulphuric acid and ammonia. The product is used directly or for blending with other fertilizers. Improvements in the ammonium sulphate formulation processes allow for increasing shares of larger-sized granular material, which is easy to handle and suitable for bulk blending. This has greatly increased application options and spreading performance. Ammonium sulphate is also popular in Europe in the manufacture of compound fertilizers, now deliberately being added to increase their S content.
The main advantages of ammonium sulphate are low hygroscopicity and chemical stability. It is a good source of both N and S. Its use may be undesirable in acidic soils already in need of liming and when it is applied at nitrogen rates. Also, when applied as direct application as an N source, much more S is applied incidentally than is required by most crops.
Ammonium sulphate now can be obtained as a granular material, which is easy to handle and suitable for bulk blending with other fertilizer materials. Segregation problems can occur in bulk blends when its product size is not well matched with those of N, P, and K materials. This difficulty can be minimized by carefully controlling uniformity and sizing of ammonium sulphate used in blends.
It can also be used in clear liquids to make solutions of fertilizer containing N and S. Sulphur concentrations in solutions based on ammonium sulphate can vary from 1%-9%. In NPS blends formulated with ammonium sulphate, the usual S concentrations range from 1% to 3%.
Single Superphosphate was once the most important P source in the world and still is a major P fertilizer in China, India, Australia and New Zealand due to its P and S contents. It is composed of 50% by weight each of monocalcium phosphate and gypsum or its lower hydrate. SSP contains 6% to 9.5% P (12% to 22% P2O5 ) and 10 to 14% S and is an excellent source of S. The occurrence of S deficiencies has been delayed in many areas of the world because of the involuntary addition of S when a large amount of SSP was used to supply P in the past. Its Ca content, ranging from 18% to 21%, can be important in soils low in this nutrient.
In the 1960s, there was a change in the structure of the phosphate industry as more concentrated phosphate materials, such as diammonium phosphate (DAP), replaced SSP. It also became more economical to process rock at the mine source rather than at the agricultural center, as had been the case of SSP. In contrast with ammonium sulphate, SSP usually is consumed in the country where it is produced. Furthermore, its production is declining, as the majority of new phosphate fertilizer plants under consideration worldwide include compound fertilizers and ammoniated phosphates rather than SSP. However, because of the low energy requirements for production and increasing demand for S, SSP use may increase in the future in some regions on a limited scale.
The most common grade of ammonium phosphatesulphate has the analysis 16-20- 0-14S. It is composed of about 40% monoammonium phosphate and 60% ammonium sulphate. Other products of this type include 13-39-0-20S, 19-9-0-20S, and 23-20-0-7S. The latter contains some urea. They are made by several processes, including reacting a mixture of phosphoric acid and sulphuric acid with ammonia, and introducing ammonium sulphate solutions and H2SO4 into a H3PO4 plant reaction circuit.
Direct application of 16-20-0-14S to forage crops, particularly legumes, is practiced in many countries. It is also popular for in-row applications on small grains and rapeseed/canola. This product is frequently used for formulating bulk blends.
Potassium magnesium sulphate is a double salt and contains 18% K (22% K2O), 11% Mg and 22% S. It has the advantage of supplying both Mg and S and is frequently included in mixed fertilizers for the purpose on soils deficient in these two elements. They are particularly useful when low levels of chloride are desired, as is often the case for crops such as tobacco, potatoes, peach, some legumes and turf grass. It is suitable for direct application, bulk blending and inclusion in suspensions.
Potassium sulphate is the major potash fertilizer containing S. It is a material containing 42% to 44% K (50 to 53% K2O) and 17% S. It is produced by different processes, depending on the original raw material. Most K2SO4 is recovered directly from potash salts or brines. About 25% of world K2SO4 capacity is based on the reaction between potassium chloride and H2SO4. Although K2SO4 can be costly in some markets, compared to KCl, it remains a widely used specialty fertilizer for valuable cash crops such as potatoes and tobacco, which are sensitive to chloride. It also has the advantage of supplying S.
Magnesium sulphate containing 13% S and 9.8% Mg has limited use as a source of Mg in clear liquid fertilizers and foliar sprays. Significant amounts of S will also be provided when it is used to supply Mg.
Micronutrient sulphate salts are also incidental carrier of S. For example, in the group consisting of Cu, Fe, Mn, and Zn, concentration of S varies between 13% and 21%.
Gypsum (CaSO4.2H2O) is a natural mineral which is found mostly in the state of Rajasthan. Agriculture grade gypsum contains 13% S, in the plant available sulphate form. It is particularly preferred for groundnut cultivation where its Ca is also useful during pod formation.