Sulphur: Sources, Distribution and Forms | Soil

After reading this article you will learn about:- 1. Sources of Sulphur 2. Distribution of Sulphur 3. Forms 4. Deficiency 5. Toxicity.

Sources of Sulphur:

Sulphur is mostly present as sulphides, sulphates and organic fractions associated with N and C. The primary source of sulphur is sulphide bearing plutonic rocks which on weathering these sulphides convert into sulphates, some of which dissolves, precipitates and reduces to even elemental sulphur depending upon the conditions.

Besides, atmosphere is recognised as an another source of sulphur in soils where sulphur dioxide is produced and subsequently brought down by rain. Fertilizers, organic manures, and irrigation water etc. are also important sources of sulphur which contribute considerable amount of sulphur to the soil.

The important sulphur bearing minerals in rocks and soils are appended below:

Name – Chemical formulae

Gypsum–CaSO₄.2H₂O

Anhydrite – CaSO₄

Epsomite – MgSO₄.7H₂O

Iron pyrite – FeS₂

Sphalerite – ZnS

Chalcopyrite – CuFeS₂

Galena – PbS

Arsenopyrite – FeS₂.FeAs₂

Besides these above minerals, some other sulphur containing minerals are also found throughout the world and all these minerals contribute sulphur to soils.

Distribution of Sulphur:

Total sulphur in soils varies widely (19 to 4000 mg kg^-1) and is present both in mineral and organic forms. However, the total sulphur content is usually of little value for its short term fertility of a soil. Among various forms of sulphur, organic sulphur fraction is the most dominant and constitutes about 80-90 per cent in Indian soils.

With an increase in the depth of the soil, the amount of organic as well as inorganic sulphur content decreases. Since India is a sub-continent having differential tropical climate and soils supporting different agro ecosystems, the amount of sulphur content varies with soil types and even within soils. Sulphur deficiencies are scattered in more than 90 districts covering about 20-30 million hectares of the arable land in India.

Intensive cropping, introduction of high yielding varieties use of high analysis chemical fertilizers particularly sulphur free and other modern agro- techniques joined hands to deplete the secondary and micro-nutrients. Among the secondary nutrients, the deficiency of sulphur with increasing frequency from different parts of India has been reported.

The total sulphur content in alluvial soils range from 47-851, 26-302, 127-169 and 63-90 mg kg^-1 in Bihar, Punjab, Uttar Pradesh and West Bengal with mean values of 576, 140, 143 and 74 mg kg^-1 respectively. The soils of West Bengal were relatively poor in total sulphur content than that of soils of above three states. The range of total sulphur content in hill soils of West Bengal was 187-258 mg kg^-1.

In West Bengal, the total sulphur content of Terai soils was lower (112-165 mg kg^-1) as compared to hill and coastal (226 mg kg^‑1) soils. The coastal saline soils of West Bengal contained about three times more than that of normal soils (74 mg kg^-1) because of the accumulation of sulphate salts from the sea water. However, the depth wise distribution of different forms of sulphur in West Bengal is appended in Table 21.4.

From various distribution studies of sulphur it has been found that with an increase in depth of the soil the amount of total as well as organic sulphur content decreases.

The distribution of different fractions of sulphur in 110 soil samples belonging to 25 soil series of West Bengal was studied (Table 21.5) and found that the mean values of sulphate-sulphur, organic sulphur, non-sulphate sulphur and total sulphur varied from 4.2 to 27.2; 26.9 to 94.8; 5.0 to 31.3 and 46.2 to 141.1 mg kg^-1 respectively.

Out of 25 soil series comprising of 48 soil samples which include block area of Chanda and Purbasthali, Simlapal.

Arsha, Bolpur and Jhargram in the districts of Burdwan, Bankura, Purulia, Birbhum and Midnapur (before division) respectively recorded mean values which were deficient in sulphate- sulphur based on 10 mg kg^-1 as critical limit (0.15% CaCl₂ each extractable sulphur). Such deficiency may be due to very coarse soil texture as well as very low clay content.

Forms of Sulphur:

Sulphur is present in the soil as its various forms viz. inorganic and organic fractions. It occurs in various oxidation states starting with +6 in H₂SO₄ and its derivatives (oxidised form of Sulphur) to -2 in H₂S and its derivatives (reduced sulphur). Besides, sulphur exists in solid, liquid or gaseous phases. However, various forms of sulphur are described below very shortly.

(A) Total Sulphur:

The total sulphur content in soils vary widely depending upon the nature and properties of primary minerals from which the soil is derived, organic compounds, sulphate ions adsorbed and present in the soil solution. It is mostly found in arid and semi-arid soils or areas with deficient in precipitation where salt accumulates. Sulphur is usually present as sulphates, sulphides and organic combinations.

Sulphides of the plutonic rocks on weathering get converted into sulphates and also are adsorbed by soil colloids or precipitated as insoluble salts of Ca, Mg, Na etc. or depleted by plants and soil micro-organisms and again some portion of it form organic combinations with C, N and P or remaining parts are reduced to sulphides and elemental sulphur under anaerobic conditions.

(B) Inorganic Forms:

The inorganic forms are readily soluble-sulphate, adsorbed sulphate; insoluble sulphate co-precipitated with CaCO₃ and reduced inorganic sulphur compounds. Since plants derive sulphur primarily from soil as dissolved sulphate, easily soluble sulphate plus adsorbed sulphate represent the readily available fraction of sulphur in soils that is utilised by plants.

(i) Easily Soluble Sulphate:

Sulphur is usually taken up by plants as the SO₄^2- ion. Concentrations of 3 to 5 mg kg^-1 SO₄—S in the soil has been found to be adequate for most plant growth. In sulphur deficient soils or soils having low sulphur supplying capacity, the easily soluble sulphate content lies between 5 and 10 mg kg^-1. However, the coarse textured soils like sandy and sandy loam soils are frequently deficient in sulphur and contain sulphur less than 5 mg kg^-1.

(ii) Adsorbed Sulphate Sulphur:

It is an important fraction of sulphur in soils containing considerable higher amounts of hydrous oxides of Al and Fe. As for an example, Ultisol, Oxisol and Alfisol contain appreciable amounts of adsorbed sulphate sulphur since the ability to adsorb this form of sulphur is high due to presence of higher amount of oxides and hydrous oxides of Fe and Al in such soils.

Adsorbed sulphate sulphur has been found to contribute significantly to the sulphur requirement of crops grown in highly weathered soils since it is readily available. But in certain soils, where sulphate is strongly adsorbed on soil colloids, this fraction of sulphur may not be so easily available to plants and it may be released into the soil solution over a longer period of time.

It has been also found that the concentrations of this fraction of sulphur are found higher at depths ranging from 15 to 75 cm below the surface. Certain deep rooted plants can utilise this fraction of sulphur from the sub-surface soil layer. Adsorbed sulphate sulphur can account for up to one-third of the total sulphur in subsoils while the same fraction represents less than 10 per cent of the total sulphur in the surface soils.

Sulphate sulphur adsorption by soils is considered as a beneficial mechanism because it protects sulpher from various harmful reactions like forming most insoluble sulphur compounds through chemical reactions and leaching in high rainfall areas.

(iii) Sulphate Co-Precipitated with Calcium Carbonate and other Precipitated Forms:

Sulphate sulphur attached to calcium carbonate is considered as an important fraction of the total sulphur particularly in calcareous soils. This fraction of sulphur is thought, to be relatively unavailable to plants, particularly when calcium carbonate is present as its coarse particles.

However, the solubility and availability of this fraction of sulphur are found to be affected by various factors viz. size of calcium carbonate particles, moisture content in soils, common ion effects, ionic strength etc.

However, the common ion effect most likely contributes more towards the formation of this fraction of sulphur. During preparation of soil sample in the laboratory through grinding higher amount of this fraction is subjected to chemical extraction by a particular soil test method than is the actual in situ in the field conditions.

In addition, selenite, a crystalline form of gypsum, has been found in poorly drained sub-soils. Extremely insoluble barium and strontium sulphates are also found in some soils.

(iv) Reduced Inorganic Sulphur:

It is evident that sulphate sulphur form is the stable fraction in well-drained upland soils and very low amounts of sulphide forms are known to occur. Under water logged conditions, sulphide fraction is known to be the dominant resulting from the reduction of sulphate sulphur.

Sulphide Form:

Under flooded as well as an intense anaerobic soil conditions, there may be higher accumulation of sulphide form of sulphur. However, the magnitude of such accumulation may be greater in soils containing higher amount of organic matter. It is evident that sulphate form of sulphur serves as an electron acceptor for sulphate reducing bacteria and such reduction is dependent on oxidation-reduction potential as well as pH of the soil.

However, there may be little or no accumulation of sulphide at Eh value > -150 mv and pH range between 6.5 to 8.5. In coastal region soils, sulphide accumulation is not usually found. In certain rice soils where organic matter content is high and low in active iron content, sulphide forms and exist as free which pose problem to the rice roots.

Elemental Sulphur:

It is an intermediate product of oxidation process of sulphide. It is largely a chemical process. However, this form of sulphur may form in soils of incomplete oxidation of sulphur due to alternate flooding.

(C) Organic form of Sulphur:

Majority of sulphur in most soils are present as organic fractions. It is evident that about 80-90 per cent of the total sulphur is present as its organic pool. Organic form of sulphur is usually low in sub-soil horizon than that of surface soil.

However, the total sulphur as well as organic sulphur content in soils varies with organic carbon and total nitrogen content and usually the association of carbon, nitrogen and sulphur is found in the ratio of 110:10: 1.5 respectively. It has been also found that the total nitrogen and organic sulphur are believed to be more positively correlated as compared to organic carbon and organic sulphur.

Nitrogen and sulphur ratio (N: S) in most soils lies between 6 to 8: 1. It is also evident that this organic form of sulphur plays an important role in supplying available sulphur to the plant. Various types of organic sulphur are still unaccounted.

However, three broad groups of organic sulphur compounds have been recognised as follows:

(i) Hydro-iodic acid

(ii) Reducible sulphur,

(iii) Carbon-bonded and

(iv) Eesidual or unidentified sulphur.

Hl-reducible (non-carbon bonded) sulphur. Sulphur in this chemical pool is extracted by hydro-iodic acid and this fraction of sulphur is not directly attached to carbon.

It is believed that this form of sulphur is largely present as sulphate esters and ethers with C—O—S linkages, e.g. arylsulphates, alkylsulphates, phenolic sulphates, sulphated poly­saccharides, choline sulphate, sulphated lipids etc. Out of total organic sulphur, about half of the amount is present as this fraction.

Carbon Bonded Sulphur:

In this fraction, sulphur is directly attached to carbon and this fraction of sulphur can be determined by reduction to sulphide with Raney nickel. However, this procedure has some limitations. It fails to reduce all carbon-bonded sulphur compounds. In-spite of such limitation, this method is still useful for determination.

This fraction of sulphur is consisting of sulphur-containing amino-acids, cystine and methionine, amounting to about 20 per cent of the total organic sulphur fractions. In addition, some other oxidised forms of sulphur like sulphoxide, sulphenic, sulphinic and sulphonic acids and some other heterocyclic sulphur compounds are present in this form of sulphur.

Residual or Unidentified:

When organic form of sulphur is not extracted or rather not reduced either by HI acid or Raney nickel, and then it is called residual or unidentified sulphur. This fraction however, accounts for about 30 to 40 per cent of the total organic sulphur in most soils. Since it is the most stable form of sulphur, it has very little practical implication in plant nutrition.

Deficiency of Sulphur:

It is evident that sulphur is an essential constituent of proteins and hence its deficiency in soils and plants in particular leads to an inhibition of protein synthesis. In view of such deficiency in sulphur in plants, non-S containing amino acids namely asparagine, glutamine and arginine accumulate in plant tissues.

Besides, sulphur deficiency in plants contains low sugar resulting from the poor photosynthetic activity. In field crops it is very much difficult to distinguish deficiencies of S and N. In sulphur deficient plants the amount of SO₄—S contents are very low in contrast to accumulation of amide N and NO₃—N. In N deficient plants, the amount of N contents are very low while that of S contents are normal.

However, plants suffering from sulphur deficiency resemble those with nitrogen deficiency as mentioned earlier that the leaves are light green or yellow in colour. They are frequently small and spindly with slender stalks, and have stunted growth and delayed maturity particularly with cereal grains.

In case of leguminous crops, nodulation is frequently reduced and in many cases, high red tints develop at the leaf margins.

Sulphur is thus essential in plant nutrition and its deficiency can seriously affect the yield and quality. In sulphur deficient plants greater decrease in chlorophyll content in leaves, inhibition of protein synthesis and carbohydrate metabolism, composition of proteins, supply of mineral nutrients, nitrogenase activity in root nodules etc. take place which ultimately affect the growth, yield and quality of crops and produces.

As for an example, a decrease in the cysteine content of cereal grains like wheat reduces baking quality of flour, since disulphide bridging during dough preparation is responsible for the polymerinsation of the glutelin fraction. Besides tllis, in Brassicaceae the content of glucosinolates and their volatile metabolites is believed to be related with the supply of SO₄—S.

With an increase in the glucosinolate content in plants beyond the level at which SO₄—S supply affects the growth. However, with regards to quality, such increase in the content of glucosinolate is plants can be considered as favourable (e.g., because it enhances the taste of vegetables, making them spicer) or unfavourable (e.g., because it decreases acceptability as animal feed).

Plants when grown at sites with low sulphur supply these new cultivars (derived from improved breeding technique) are, however, more sensitive to S deficiency than the traditional crop varieties with high glucosinolate content. This higher sensitivity to sulphur deficiency may be explained partly by the role of glucosinolates as transient storage compounds of sulphur.

However, the deficiency symptoms of some of the important crops are given below:

Rape/Mustard:

Leaves turn yellow initially and become cup shaped at the later stage. Underside of leaves turn reddening. Stems of the crop also become reddish tint.

Groundnut:

Shortening of height of plants is found. Plants turn pale in colour and more erect than the plants not deficient in sulphur. Trifoliate leaves of groundnut turn into a ‘V’ shaped appearance. Older/matured leaves may remain green colour while that of colour in young leaves turn pale.

Tomatoes:

Plants are smaller associated with stunted growth and lighter in colour. Different parts of plants turn into yellow colour. However, petioles and stem exhibit a prominent red colour due to acute S-deficiency.

Rice:

Leaf sheath at the initial growth stage and with the progress of growth, the leaf blades become yellowish. With severe deficiency, the whole plant turns into chlorotic at the tillering stage. Stunted growths with reduced number of tillers are observed. At the maturity of crop, effective numbers of tillers are very few with shorter length containing very little number of grains per panicle.

Tea:

“Tea yellows” is usually found due to S-deficiency. The colour of leaves turn yellow with smaller size and shorter internodes, and ultimately, the whole plant appears shrunken. Leaves curl up and their margins and tips turn brown when sulphur deficiency is acute. Axial buds produce dwarf yellow leaves.

Toxicity of Sulphur:

High SO₂ concentrations in the atmosphere may be toxic to plants. It is found that the critical SO₂ level for plants is 120 µg m^-3.

Sulphur in the form of SO₂ may be removed from the atmosphere mainly in precipitation or by direct interaction of the gas molecules with soil and vegetation, in a process known as dry deposition.

The amount of sulphur brought to the earth in precipitation varies widely from one place to other and is influenced by

(i) The volume of precipitation (rainfall);

(ii) Adjacent to industrial areas,

(iii) Proximity to sea and

(iv) The prevailing winds.

In industrial areas, however, concentrations of SO₂ are several times higher than that of normal levels. The cause of SO₂ toxicity may be from various sources. Sulphur dioxide (SO₂) absorbed by the leaves dissolves in the moist surfaces of mesophyll cells in the stomatal cavities. The resulting sulphurous acid (H₂SO₃) dissociates giving rise to H⁺, HSO₃^– and SO₃^2-.

Sulphate (SO₄^2-) ions are produced by a free radical chain reaction and then assimilated in plants. It is evident that a major reason for the toxic effect of sulphur dioxide (SO₂) is that at high levels of exposure, SO₂ gas and the S anions (HSO₃^– and SO₃^2-) can accumulate and uncouple photophosphorylation.

The disruption of chloroplast membranes can also result from SO₂ toxicity. Sulphur dioxide (SO₂) toxicity in plants is characterised by necrotic symptoms in leaves. Due to toxicity of SO_x and NO_x in the atmosphere, very frequently acid rains are produced which also damage the crop to a great extent.