Sources of sulfur supply to the soil
When developing the system of fertilization of crops and in crop rotation, sulfur as an element of plant nutrition has not been given due attention before, since the range of mineral fertilizers produced by the domestic chemical industry contains a sufficient amount of sulfur as a co-element or in the form of impurities. Significant amounts of sulfur also enter the soil as a result of technogenic pollution, mainly in industrial areas, and due to volcanic activity.
From the atmosphere sulfur enters the soil with precipitation: in large industrial areas – more than 100 kg/ha, in rural areas – 2-3 kg/ha. In the European part of Russia with atmospheric precipitations falls down to 5-10 kg/ha of sulfur, in separate regions – to 15-17 kg/ha, in Eastern Siberia and Far East – 2-3 kg/ha, near large industrial centers – up to 25-45 kg/ha. In Pre-Urals with precipitation came 16 kg/ha of sulfur, in Donbass – 54 kg/ha, in Moscow suburbs – 17-136 kg/ha per year. With precipitation of more than 10 kg/ha of sulfur per year, plants are usually provided with this element, so, according to calculations, the overall balance of sulfur in agriculture is positive.
Plants can absorb gaseous sulfur compounds from the atmosphere through their leaves. The foliar uptake accounts for up to 30% of total uptake.
In the future, sulfur may become an element limiting yields and product quality. It depends on the introduction of progressive cultivation technologies, the use of high doses of mineral fertilizers, aimed at the realization of the potential productivity of plants and accompanied by an increase in the removal of nutrients from the soil, including sulfur. A large amount of sulfur can be washed out with precipitation beyond the root layer, since SO42- anion is poorly absorbed by the soil, especially by light soils with a granulometric composition.
With sulfur-containing fertilizers some amount of sulfur enters the soil. For example, ammonium sulfate contains 24% S, potassium sulfate – 17.6% S, potassium magnesia – 18.3% S, schoenite – 15.9% S, magnesium sulfate – 28-30% S. However, they do not play a significant role in providing sulfur for sod-podzolic soils, as they are used in limited quantities. Sulfur is included in the nitrophoska sulfate and superphosate.
Phosphogypsum contains 22% S, is a waste product of chemical plants producing double superphosphate, similar in composition to gypsum, but contains impurities of phosphorus and some other elements. It can serve as a sulfur fertilizer of local importance. The disadvantages of phosphogypsum – high humidity up to 30-35%, an impurity of fluorine and strontium. Therefore, when using it, it is necessary to constantly monitor the accumulation of these elements in the soil, plants and products, not allowing them to exceed the maximum allowable concentration (MAC).
Gypsum contains 18.6% S, is a fast-acting, well available to plants neutral sulfuric acid salt of calcium. It is used mainly for reclamation of saline soils.
Elemental sulfur as a fertilizer is used little. It becomes available to plants only after conversion into sulfate form by microorganisms. The rate of this process is influenced by the fineness of grinding, temperature and humidity of the soil, microflora activity, soil type, and the content of other elements. Elemental sulfur is less susceptible to leaching from the arable layer and has a longer persistence than gypsum and sulfate forms.
Sulfur is contained in manure up to 1 kg SO42- per 1 ton. However, the proportion of areas fertilized with manure is small.
Application of sulfur fertilizers
The effect of sulfur and sulfur-containing fertilizers on crop yields and product quality depends on the sulfur content in the soil, fertility, biological characteristics of the crop, and weather conditions.
Features of sulfur fertilizer application:
- In soil, up to 85-90% of sulfur is in organic form as part of humus and other organic compounds, 10-15% – in the form of SO42-, which is assimilated by plant roots. Sulfur of soil organic compounds as a result of mineralization due to microbiological activity is converted into mineral sulfur. This process is called sulfification, which has a seasonal character with a minimum in spring, maximum in summer, and attenuation in autumn. The release of nitrogen and sulfur occurs in the same ratio as they are in humus and organic residues. At present, there are no criteria for assessing the availability of sulfur in plants. For example, legumes and crucifers are not deficient in sulfate when the sulfate content is more than 11-14 mg/kg, and cereals – more than 7 mg/kg.
- When applying sulfur-containing fertilizers, consider the critical levels of sulfur in plants and the ratio N:S, by which you can estimate the lack of sulfur. The critical sulfur content in wheat grain is 0.17%, in potato tubers 0.11%, in alfalfa – 0.2%, in cotton during the phase of budding – 0.5%. The critical N:S ratio in wheat grain is 14.8, in barley 13.1-16.4, in clover 15-18.5.
The efficiency of sulfur fertilizers is influenced by weather conditions, especially in early spring. Yield gains from application of sulfur were higher in years with low spring temperatures and abundant rainfall, i.e. when sulfification processes slowed down and mineral sulfur reserves were washed into the lower soil layers and became unavailable for plants.
- Therefore, in early spring on sod-podzolic soils mineral sulfur, as well as mineral nitrogen, is in short supply. Regardless of the total sulfur content in the soil, spring crops respond well to sulfur fertilizer applied before sowing. Overwintering plants, especially clover and alfalfa, also respond well to sulfur fertilizer applications in the spring.
- Neutral forms of sulfur-containing fertilizers – gypsum, phosphogypsum and simple superphosphate – are most effective on sod-podzolic soils. By the action of gypsum and phosphogypsum are of equal value. Sulfate forms of nitrogen and potassium fertilizers, as well as elemental sulfur are inferior in effectiveness as they have an acidifying effect on the soil solution. Phosphogypsum increases yields of intensive crops such as corn, fodder rutabaga, and fodder sprouts that take out large amounts of nutrients, including sulfur, as well as leguminous grasses and lupine. Yield gains from applying phosphogypsum and other sulfur fertilizers increase in years with high yields and in years with cold springs.
- The timing and methods of sulphur fertilizer application depend on the biological characteristics of crops: under winter cereals – preplanting, spring cereals – under pre-sowing cultivation, clover – in early spring on the regrowing plants, row crops equally respond to the pre-and post-sowing application.
- Most crops respond well to sulfuric fertilizers with a sufficiently fertilized background and the systematic application of nitrogen, phosphorus and potassium fertilizers in the rotation. Yield gains from the use of sulfuric fertilizers are: winter wheat grain – 0.2-0.4 t/ha, winter rye – 0.15-0.3 t/ha, barley – 0.2-0.3 t/ha, oats – 0.15 t/ha, clover hay – up to 1.5 t/ha, Potato tubers – up to 3.0 t/ha, rutabaga roots – 3.0-5.0 t/ha, turnip hay – up to 3.0 t/ha, green mass of fodder cabbage – up to 4.0 t/ha. The quality of plant products increases – the content of protein, dry matter, starch in potato tubers, the share of marketable products.
- Sulfur-containing fertilizers contribute to the absorption of other nutrients.
For the majority of cultures the optimum dose of sulfur is 50-60 kg/ha on sandy soils, for cruciferous crops on loamy soils – 100-120 kg/ha of sulfur. It is applied in fall for autumn autumn plowing, in early spring for pre-sowing tillage, and in spring during grass growth. If there is a shortage of sulfur, sulfur fertilizer is added to rows during planting and foliar feeding with 0.5-2% sulfate solution.
Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific and Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.