Transformation of Sulphur: 10 Factors | Soils

This article throws light upon the ten important factors affecting the transformation of sulphur. The factors are: 1. Type and Amount of Clay 2. Oxides and Hydrous Oxides of Fe and Al 3. pH 4. Organic Matter 5. Texture 6. Salt Content 7. Alternate Wetting and Drying 8. Cations and Anions 9. Presence or Absence of Vegetation 10. Interactions of Sulphur with Other Nutrients.

Factor # 1. Type and Amount of Clay:

Soils containing higher amount of clay, the availability of sulphur decreases and such decrease may be due to higher retention of soluble sulphate sulphur by clay minerals usually through the adsorption mechanism. However, the retention of sulphur is more in soils containing higher amount of 1: 1 type of clay minerals.

Factor # 2. Oxides and Hydrous Oxides of Fe and Al:

It is the most important component of soil which can retain a substantial amount of sulphate sulphur on their surfaces rendering sulphur unavailable to the plant. Example, in an acid laterite soils of West Bengal where this soil component is dominant, the availability of sulphur decreases due to its higher adsorption.

Factor # 3. pH:

Soil pH is considered as an another important factor affecting the availability of sulphur in soils. In strong acid soils, the availability decreases due to higher retention through adsorption. However, the retention of sulphur becomes negligible with an increase (acid soil) or decrease (alkali soil) in pH towards neutrality.

But in general, the oxidation of applied sulphur takes place rapidly under acidic soil conditions, because an optimum pH range for the activity of Thiobacillus sp. in carrying out the oxidation process is 4-7.2. However, the amount of released sulphur is directly proportional to soil pH up to a value of 7.5.

Factor # 4. Organic Matter:

It is also an important factor affecting the sulphur availability in soils. Soils containing an adequate amount of organic matter have higher ability to supply sulphur to the plant because of greater release of sulphate sulphur resulting from the mineralisation process. Again, in some soils organic matter adsorb sulphur and becomes less available to the plants.

In general, total sulphur in most of Indian soils is function of soil organic matter content which is significantly and positively correlated. It is evident that higher amount of total sulphur in surface than in the sub-surface soils of India are found due to recycling of organic waste materials.

However, such increased accumulation of sulphur in soils is dependent on nature and type of organic residues undergoing decomposition. The release of sulphur from the organic waste materials is chiefly a microbial process which also dependent on C or N/S ratio of the material.

Factor # 5. Texture:

It has been found that the sulphur content in soils is a function of clay content. Clayey and clay loam soils are believed to contain a higher amount of sulphur than that of sand and sandy loam soils. Such variation is mainly due to textural differences and also partly from the association of organic matter with clay colloids.

Factor # 6. Salt Content:

Accumulation of sulphur is usually brought about by the accumulation of sulphate and such accumulation is favoured in soils of high salt content like coastal saline and alkaline soils of West Bengal. The significant positive correlation was found between total sulphur and sulphate sulphur content of the soil.

Factor # 7. Alternate Wetting and Drying:

An enhancement of sulphur availability in soils due to alternate wetting and drying is important under actual field conditions. Variation in soil moisture ranging from field capacity to saturation affects the mineralisation of sulphur mainly ester sulphates in the soil organic matter. Drying after soil saturation or submergence, reduces the sulphur content in soils.

Factor # 8. Cations and Anions:

Various cations like H, K, Ca, Mg and Na and anions like OH^–, PO₄^3-, NO₃^–, and CI^– etc. affect the availability of sulphate sulphur in soils through adsorption mechanism. Example, acid soils containing higher amount of H⁺ retain a greater amount of sulphate sulphur whereas, soils fertilised with sufficient amount of phosphorus reduce the sulphate adsorption with subsequent release of sulphate in the soil solution.

Factor # 9. Presence or Absence of Vegetation:

It is evident that a greater amount of sulphur is mineralized in cropped than that of un-cropped soils. Such increase in sulphate sulphur in soils may be ascribed to the “Rhizosphere effect” resulting from the excretion of amino acids and sugars by plant roots and the resultant stimulation of microbial activity.

On the other hand in an un-cropped or fallowed soils, the release of sulphur may not take place because of rapid immobilisation resulting from the conversion of sulphate sulphur into organic form.

Factor # 10. Interactions of Sulphur with Other Nutrients:

Besides various soil factors, interaction of other nutrients with sulphur affects the availability of sulphur to plants. The nutrient which frequently interact with S are N, P, K, Ca, Mg, Zn, B, Mo and non-essential element like Se.

The interaction between N and S has been found to be synergistic. Application of N increases the concentration and uptake of S and vice-versa. A similar positive interaction has been found between S and P.

Such positive interaction may be attributed to the promotion of root development by phosphorus resulting higher uptake of native as well as applied sulphur. It is evident that per cent utilisation of applied S increased with P application whereas the application of S itself has very little effect.

The application of Zn (10 kg ha^-1) increased S utilization by crops enhancing the concentration of S in soils in most cases confirming the application of Zinc @ 10 kg ha^-1 increased the availability of sulphur in soils and vice-versa.

However, other combinations of nutrient interactions like S × MO, B × S, S × Se etc. may affect the availability of both native and applied sulphur in soils. Some other factors like time and cultivation, amendments like gypsum, lime etc. also affect the transformation of sulphur in soils and hence uptake by crops.