The role and deficiency of sulfur in the mineral nutrition of agricultural crops

Sulfur is a key element for plant nutrition, which, in terms of its importance, ranks alongside nitrogen, phosphorus, and potassium. For a long time, agronomists did not pay due attention to sulfur, as it was abundantly supplied to fields as a concomitant component of simple fertilizers. Practically until the 1970s, the overall sulfur balance in domestic agriculture remained positive. However, the shift to concentrated fertilizers and intensive technologies led to a gradual depletion of the soil reserves of this element.

Reasons for deficiency and diagnosis of sulfur in the soil

Today, a lack of sulfur is observed in many regions. Environmental standards and changes in the cultivation technology of crops have become limiting factors. The main reasons for sulfur deficiency in the soil are the following factors:

• An increase in crop yields, which has led to greater removal of the element from the soil.
• A reduction in the use of non-concentrated fertilizers containing sulfur as an impurity.
• A decrease in sulfur emissions into the atmosphere due to reduced use of coal and the installation of cleaning filters at industrial enterprises and thermal power plants.
• A significant reduction in the use of sulfur-containing products for plant protection against pests and diseases.

To monitor the situation in the fields, it is necessary to conduct regular soil analysis. The supply of available sulfur compounds is determined using a 1.0 N KCl extraction. The results obtained allow for a prompt assessment of soil status across three levels.

• Low availability — less than 6 mg/kg
• Medium availability — 6–12 mg/kg
• High availability — more than 12 mg/kg

Soils that are light in terms of texture are at risk, as well as gray forest, podzolic, and sod-podzolic soil types. They account for about 25% of all agricultural land in the country. A low content of available sulfur is also found in some chernozems, so one cannot blindly rely on natural soil fertility. Natural sulfate sulfur reserves in the plow layer vary significantly depending on the zone.

Crop requirements and atmospheric reserves

The need for sulfur directly depends on the biological characteristics of a specific crop. Plants absorb it from the soil exclusively in mineral form as sulfates. The average annual removal of sulfur with harvests ranges from 10 to 20 kg/ha. The highest level of removal is demonstrated by members of the Brassicaceae and Liliaceae families; legumes and solanaceous crops have moderate needs, while cereals require the least amount of sulfur.

An important feature of sulfur nutrition is the ability of plants to absorb sulfur through leaves from the air in the form of sulfur dioxide (SO3). Due to this source, crops can cover up to a third of their need for the element. In total, about 142 million tons of sulfur enter the atmosphere annually: 44 million tons rise with water dust from the sea, 30 million tons are released during the decomposition of organic matter in swamps and estuaries, and the rest has a technogenic origin.

Since sulfur dioxide (SO3) is twice as heavy as air, it does not travel well over long distances. Because of this, the sulfur concentration in the air near large industrial centers is 2–3 times higher than in rural areas. An SO3 level of 0.2 mg/m³ of air is considered optimal for the development of most crops.

For precise planning of a fertilization system, an agronomist needs to prepare balance calculations. They account for sulfur removal with the planned harvest and its losses from leaching out of the root zone, comparing them with the intake of the element from the soil, precipitation, and fertilizers. This approach allows for accurately forecasting the application rates of sulfur-containing fertilizers for each crop.

 Sulfur Content, % dry Crop Economic part Harvest, cwt/ha matter Sulfur removal, kg/ha Poaceae Grain 25.0–40.1 0.09–0.24 Wheat Straw – 0.09–0.23 } 7.6–13.0 Grain 25.3–35.8 0.07–0.26 Barley Straw – 0.15–0.48 } 7.7–14.9 Grain 30.0–35.8 0.13–0.34 Oat Straw – 0.17–0.26 } 5.1–6.5 Grain 60.0-65.0 0.15–0.17 Maize Stems and leaves – 0.19 } 9.0–18.0 Grain 60–70 0.08–0.17 Rice* Straw – 0.17–0.27 } 8.0–16.0 Legumes Pea Grain 12.8–23.0 0.22–0.23 5.7–34.2 Alfalfa Hay 50.0–100.0 0.10–0.56 8.0–30.0 Clover Hay 40.0–100.0 0.14–0.29 15.0–30.0 Amaranthaceae Beet: Root crops 651.0 0.14 – Table Leaves – 0.49 } 25.0 Root crops 350.0–540.0 0.06–0.42 – Sugar Leaves – 0.14–0.97 } 15.0–30 Solanaceae Tubers 170.0–230.0 0.10–0.30 Potato Tops – 0.15–0.50 } 9.0–37.0 Apiaceae Root crops 540.0–590.0 0.15–0.24 Carrot Leaves – 0.40 } 20.0 Alliaceae Bulbs 150.0–350.0 0.26 Onion Leaves – 0.65 } 18.0–30.0 Bulbs 215.0 0.80 Garlic Leaves – 0.34 } 107.0 Brassicaceae Cabbage Heads 250.0–1053.0 0.70–1.53 30.0–134.0 Root crops 457.0–500.0 0.48 30.0–40.0 Turnip Tops – 0.73 – Root crops 200.0–400.0 0.56 Swede Tops – 0.75 } 28.0–64.0 Root crops 581.0– 764.0 0.36–0.58 Rutabaga Tops – 0.59–0.92 } 38.0–79.0 Seeds – 0.70–1.70 Mustard Above-ground mass 10.0–16.2 0.28–0.32 } 14.1–36.2 * supplemented by A.Kh. Sheudzhen

Sulfur enters the soil from the atmosphere as part of precipitation, dust, and through adsorption directly from the air. In the European part of Russia, 5–10 kg/ha of sulfur falls with atmospheric precipitation, and in some areas up to 15–17 kg/ha; in Eastern Siberia and the Far East, 2–3 kg/ha, and near large industrial centers up to 25–45 kg/ha.

The majority of atmospheric sulfur enters with snow during the winter period and is leached in significant quantities during spring by meltwater and percolating water.

Sulfur reserves in soils can also be replenished through its input with organic fertilizers. However, the input of sulfur via organic fertilizers in most cases does not lead to a significant improvement in the soil's supply of this element, as various types of these fertilizers generally contain less than 0.2% of it by dry matter weight. With mineral fertilizers, 8–12 kg/ha of sulfur is applied. An analysis of the assortment of fertilizers in the Russian Federation shows that the amount of sulfur in them is not only not increasing but is actually decreasing.

In irrigated agriculture, a certain amount of sulfur enters the soil with irrigation water, but in most cases, it does not exceed 5–10 kg/ha.

In balance calculations, it is necessary to account for the fact that a significant amount of sulfur can be leached out with drainage water and migrate through the soil profile to groundwater, as the SO42– anion is weakly absorbed by the soil, especially in soil with a light particle-size distribution. Annually, 15–25 kg/ha of sulfur is leached from the soil.

Application rates for sulfur fertilizers depend on the biological characteristics of the crops, the targeted yield, the soil fertility and particle-size distribution, and the sulfur content in the atmosphere and irrigation water. In most cases, they are as follows:

When using sulfur fertilizers, one should consider the sulfur content in plants and the N:S ratio in protein, which can be used to assess the supply of this element. The critical sulfur content in wheat seed is 0.17%, in rice – 0.23%, in potato tubers – 0.11%, in clover leaves – 0.11–0.32%, in alfalfa – 0.2%, and in cotton during the budding stage – 0.5%. The critical N:S ratio in wheat seed is 14.8, in rice – 12.6, in barley – 13.1–16.4, and in clover – 15–18.5.

Visual signs of sulfur deficiency in plants closely resemble the symptoms of nitrogen starvation. In agricultural practice, this often leads to diagnostic errors, excessive application of nitrogen fertilizers, and lower yields. The difference is that with a nitrogen deficiency, the lower, older leaves turn yellow and die off, whereas with sulfur deficiency, the plant growth points are affected, younger upper leaves turn pale green, and leaf veins take on a lighter shade. Visual symptoms of sulfur deficiency in plants can be observed as early as 2–3 weeks after seedling emergence. A sulfur deficit in the mineral nutrition of plants leads to morphological changes: smaller leaf size, poor development of the root system, and shortening and lignification of stems.

Sulfur fertilizers are applied for winter ploughing or during Pre-sowing tillage">pre-sowing tillage. In case of sulfur deficiency, the following are often carried out:

• row application of sulfur fertilizers;
• foliar top dressing of actively growing plants with a 0.5–2% aqueous solution of sulfates.

When the plants' requirements for nitrogen, phosphorus, and potassium are met, most crops respond well to improved sulfur nutrition. Yield increases from the application of sulfur fertilizers are:

• winter wheat grain – 2–4 centners/ha;
• winter rye – 1.5–3.0 centners/ha;
• barley – 2–3 centners/ha;
• oats – 1.5 centners/ha;
• clover hay – up to 15 centners/ha;
• potato tubers – up to 30 centners/ha;
• rutabaga roots – 30–50 centners/ha;
• turnip – up to 30 centners/ha;
• forage cabbage green mass – up to 40 centners/ha.

The use of sulfur fertilizers improves product quality.

In our country, specialized sulfur fertilizers are not produced. However, many mineral fertilizers, organic fertilizers, chemical soil amendments, and industrial waste products contain sulfur.

Table 73 – Sulfur content in fertilizers, amendments, and industrial waste

Sources of sulfur and guidelines for the use of sulfur fertilizers

The effectiveness of sulfur fertilizers directly depends on how well plants are supplied with nitrogen. The higher the level of nitrogen nutrition, the more strongly crops respond to sulfur application. For example, the productivity of cultivated pastures increases proportionally to the share of nitrogen in the complete fertilizer composition when gypsum or elemental sulfur is applied together with nitrogen.

To compensate for a sulfur deficit, mineral and organic fertilizers are used, as well as chemical soil amendments and industrial waste. The content of the active ingredient in these materials varies significantly.

Elemental sulfur (S) in its pure form is still rarely used. Plants cannot assimilate it directly — first, soil microorganisms must convert it into a sulfate form. The speed of this conversion depends on the fineness of the fertilizer's grind, as well as the soil and climatic conditions in a specific field.

Gypsum (calcium sulfate) provides plants with sulfur in an available sulfate form and simultaneously serves as a source of calcium. Currently, it is mainly used as a soil amendment to neutralize saline soils. Phosphogypsum is obtained as a byproduct during the production of double superphosphate; it is advisable to use it as a local sulfur fertilizer. The average chemical composition of phosphogypsum includes 42.3–51.6% SO3, 31.5–37.2% CaO, 0.5–3.5% total P2O5, 0.2–3.0% water-soluble P2O5, 0.2–0.6% Al2O3, 0.1–0.3% Fe2O3, and 0.1–0.4% F.

The timing and methods of gypsum and phosphogypsum application depend on the biological characteristics of the crops:

• For clover and winter cereals, fertilizers are applied superficially in early spring.
• For spring crops, they are incorporated into the soil during pre-sowing tillage.
• For potatoes, corn, root crops, and cruciferous crops, the application rate is increased and applied during pre-sowing tillage or autumn plowing.

• Application rate for clover and winter cereals — 300 kg/ha
• Application rate for spring crops — 300 kg/ha
• Application rate for potatoes, root crops, corn, and cruciferous crops — 500–600 kg/ha