Inorganic Elements and Compounds of Soil:
The chief inorganic constituents of soil are the compounds of following elements—Al, Si, Ca, Mg, Fe, K and Na. Soil also contains smaller amounts of compounds of following inorganic elements—B, Mn, Cu, Zn, Mo, Co, I, F, etc. Most of these inorganic salts exist in soil in the form of weak solution. Soil solution may contain complex mixtures of minerals as carbonates, sulphates, nitrates, chlorides and also organic salts of calcium, magnesium, sodium, potassium, etc.
The chemical nature of nutrient solution depends on the nature of the parent matter through which water has percolated and climatic conditions of the region and it affects the types of vegetation of that region. For example, temperate soils with high rainfall has H ions in abundance, due to which leaching of basic nutrient ions like Ca, Mg and K occurs and fertility of soil is greatly reduced.
Soils with suboptimal concentration of nutrients are called oligotrophic and those with more or less optimal concentrations of nutrient solutes as eutrophic. The oligotrophic soils cause many physiological disorders in plants which have very poor root system and fruiting. To compensate the efficiency of nutrients, fertilizers containing N, P and K salts are added to such soils.
Organic Matter of Soil:
The chief organic component of soil is humus which chemically contains —amino acids, proteins, purines, pyrimidines, aromatic compounds, hexose sugars, sugar alcohols, methyl sugars, fats, oils, waxes, resins, tannins, lignin and some pigments. Further, humus is black coloured, odourless, homogeneous complex substance.
As soil is composed of crystalloids and colloids, therefore, it exhibits all the physico-chemical properties which are related with these two soil particles. Colloids for example exhibit absorption, electrical properties, coagulation, Tyndal phenomenon, Brownian movement, dialysis, etc.
Many chemical properties of soils centre on soil reaction. As regards their nature, some soils are neutral, some are acidic and some basic. The acidity, alkalinity and neutrality of soils are described in terms of hydrogen-ion concentrations or pH values.
A pH value of 7. 0 indicates neutrality indicates alkaline condition and a value below (0—6. 9) indicates acid conditions. Normally, pH value of soils lies between 2-2 and 96. Surface soils are more acidic. Warm, dry climate soils are strongly basic. In India, acidic soils (pH below 5.5 to 5.6) occur in the high rainfall areas of Western Ghats, Kerala, Eastern Orissa, West Bengal, and Tripura.
Manipur and Assam the saline, alkali or basic soils (called ‘Usarcontain pH upto 8.5) of India, occur in U. P., West Bengal, Punjab, Bihar, Orissa, Maharashtra, Madras, M. P., A. P., Gujrat, Delhi and Rajasthan.
pH value of soil controls the concentration and availability of other minerals also. For example, certain plants require considerable amounts of calcium (calciphytes) and thus grow on basic soils. Plants requiring low calcium amounts are called oxylophytes. At low pH, generally copper, iron, zinc, manganese and aluminium become toxic. Highly acidic and highly saline or alkaline soils are often injurious for plant growth and micro-organisms, etc. Neutral or slightly acidic soil, however, is best for the growth of majority of plants.