Composition of Soil

The chief components of soil are inorganic matter (90 - 95%) and organic matter (5 - 10%). Besides, soil contains water and air. The composition of soil varies considerably depending on the type of soil.

8.5.1 Inorganic Components of Soil

Most of the inorganic components of soil exist as silicate minerals. The common elements in soil are oxygen (46.6%), silicon (27.7%), aluminium (8.1%), iron (5.0%), calcium (3.6%), sodium (2.8%), potassium (2.6%) and magnesium (2.1%). The silicates which are commonly found in soil are quartz (SiO₂), orthoclase (KAlSi₃O₈), albite (NaAlSi₃O₈) and epidote (4CaO (AlF)₂O₃∙6SiO₂∙H₂O), Besides iron oxides, FeO(OH), magnetite (Fe₃O₄) are also present in many soils. Certain soils contain titanium and manganese oxides. Another common component of soil is calcium carbonate.

Soil also contains mixtures of minerals as carbonates, sulphates, chlorides and nitrates. Some soils contain clay minerals, which are hydrated aluminium and iron silicates. These clay minerals bind cations such as Ca²⁺, Mg²⁺, Na⁺, K⁺, NH₄⁺, so that these cations are not leached by water and are available in soil as plant nutrients.

8.5.2 Organic Components of Soil

The organic components are responsible for the fertility of the soil. The major organic compounds in soil are organic matter, polysaccharides, amino sugars, nucleotides, amino acids, proteins, organo - sulphur and phosphorus compounds. The humic materials constitute the most important compounds which form complexes with a number of cations, so that these are retained in the soil. The major organic compounds in soil are given below:

(i) Humus : The humic substances, in general, are high molecular weight macromolecules.

(ii) Lipids : These are extractable by organic compounds and constitute fats, resins and waxes. The physical properties of soil depend on these components.

(iii) Saccharides.∙ These are the major food materials for soil microorganisms. Theirexamples being cellulose, starches, hemicelluloses, etc.

(iv) Nitrogen containing Organics : These provide nitrogen fertility in soil. Their examples include amino acids, amino sugars, etc.

(v) Phosphorus containing Compounds: These are source of phosphorus in soil. Their examples are phosphate esters, phospholipids, etc. elements are required at trace levels. If present at higher levels, they have toxic effects.

From soil, most of the nutrients find their way into plants and when the plants / crops are consumed by humans; these nutrients are used in various life processes. When plants, animals and human beings die, the nutrients return to the soil. In this way, most of the elements are recycled.

8.7 Sources of Essential Elements in Soil

The essential elements on which the fertility of soil depends are nitrogen, phosphorus and potassium.

Nitrogen is made available to the soil in the form of organic material resulting from the decay of dead plants and animals and human wastes. The organic material is hydrolysed to NH₄⁺, which in turn is oxidized to NO₃^- by various natural processes in soil. The plants use NO₃^-from soil. When nitrogen is added to soil in the form of fertilizers (like ammonium sulphate, urea, etc.) nitrifying bacteria convert NH₄⁺ into NO₃^-, which is used by plants. Some leguminous plants have the inherent ability to fix atmospheric nitrogen through nitrogen fixing bacteria in their root nodules. Legumes are capable of adding considerable quantities of nitrogen to the soil.

Phosphorus is essential for the growth of plants. The main sources of phosphorus are rocks from which it finds its way into soil by weathering or mining. The decay of dead organisms also releases phosphorus in the soil. In soils, the plants utilize phosphate as H₂PO₄^- and HPO₄^2-.

Potassium is essential for plants. It activates some enzymes and plays a vital role in the water balance in plants. It is, of course, available in the earth’s crust but is not easily available to plants. The fertilizers mostly used contain nitrogen, phosphorus and potassium (NPK) as the major components. These provide the essential elements in the soil.