Contents preface (VII) introduction 1—37

3.3. CHEMICAL PROPERTIES OF SOIL
For satisfactory crop yield, soils must have sufficient plant nutrients, such as nitrogen, carbon, hydrogen, iron, oxygen, potassium, phosphorus, sulphur, magnesium, and so on. Nitrogen is the most important of all the nutrients. Nitrogenous matter is supplied to the soil from barnyard manure or from the growing of legume crops as green manures, or from commercial fertilisers. Plants absorb nitrogen in the form of soluble nitrates.
Soils having excess (greater than 0.15 to 0.20 per cent) soluble salts are called saline soils and those having excess of exchangeable sodium (more than 15 per cent or pH greater than 8.5) are called alkaline (or sodic) soils. Excessive amounts of useful plant nutrients such as sodium nitrate and potassium nitrate may become toxic to plants. Saline soils delay or prevent crop germination and also reduce the amount and rate of plant growth because of the high osmotic pressures which develop between the soil-water solution and the plants. These pressures adversely affect the ability of the plant to absorb water.
Alkaline (or sodic) soils tend to have inferior soil structure due to swelling of the soil particles. This changes the permeability of the soil. Bacterial environment is also an important feature of the soil-water-plant relationship. The formation of nitrates from nitrogenous compounds is accelerated due to favourable bacterial activity. Bacterial action also converts organic matter and other chemical compounds into forms usable by the plants. Bacterial activity is directly affected by the soil moisture, soil structure, and soil aeration. Compared to humid climate soils, arid soils provide better bacterial environment up to much greater depths because of their open structure. Besides, due to low rainfall in arid regions, leaching (i.e., draining away of useful salts) is relatively less and the arid soils are rich in mineral plant food nutrients, such as calcium and potassium.
Soils become saline or alkaline largely on account of the chemical composition of rocks weathering of which resulted in the formation of soils. Sufficient application of water to the soil surface through rains or irrigation helps in carrying away the salts from the root-zone region of the soil to the rivers and oceans. When proper drainage is not provided, the irrigation water containing excessive quantities of salt may, however, render the soil unsuitable for cultivation. Saline and alkaline soils can be reclaimed by: (i) adequate lowering of water table, (ii) leaching out excess salts, and (iii) proper management of soil so that the amount of salt carried away by the irrigation water is more than the amount brought in by irrigation water.