Soils and Plant Nutrition

Analyzing the total N, the C/N ratio, and inorganic N (ammonium, nitrate) provides an insight into the nitrogen supply to soil microflora and plants. The total N content ranges from <0.02% (subsoils) to > 2.5% (peats). A-horizons of mineral soils contain 0.06-0.5% N. Nitrogen, phosphorus, and/or potassium deficiency may limit the microbial decomposition of pollutants in soil. Optimum conditions are achieved at a C:N:P ratio of 100:10:2. Nitrogen is an important nutrient for plants and soil microorganisms. Ammonium and nitrate in the soil are the N sources that are immediately available to plants. These are produced by the mineralization of organic compounds or are added to the soil as fertilizer. Besides ammonium and nitrate, nitrite may also be present, although usually at very minor levels except in neutral and alkaline soils that have recently been treated with ammonium salts or ammonium-forming fertilizers.

Soil phosphorus is, like nitrogen, potassium, calcium, and magnesium, an important nutrient for soil organisms and plants. It exists as inorganic and organic fractions (the proportions of each fraction can vary between 5 and 95%). The soil organic P fraction is derived from plant residues, soil flora, and soil fauna tissues and residues that resist rapid hydrolysis. Inorganic fractions consist of Ca, Al, and Fe phosphates. The most prominent phosphate mineral in soils is apatite. The total concentration in soil is generally in the range from 200 to 800 mg kg-1. A considerable amount of P is also bound in the amorphous mineral fraction. Soil microbes are involved in the mineralization of P from organic debris. Extracellular phos-phatases are produced by microorganisms and roots and contribute to the mineralization of organic P. Phosphorus deficiency can limit the growth of plants and the microbial decomposition of pollutants in soil.

Without using very large quantities of fertilizers, it would not be possible to maintain agricultural production at the levels that are currently required. Because of this, Europe, America, and Japan have been using fertilizers for a long time. In Japan, roughly half of the plant food comes from bulky organic manures and half from fertilizers. Most of their straw is used to prepare manures and composts, and the Japanese have one of the highest consumptions of fertilizers per unit area of arable land. Bulky organic manures are also a major source of plant food in Europe and America. All practicable measures should be adopted to increase their supply in India too, but fertilizers are required to supplement them. Farmyard manure and composts have their virtues, but we cannot afford to make a fetish of them.

Certain chemical elements known as micronutrients or trace elements are crucial to the growth and health of plants in very small quantities, but are toxic to them at higher levels. When these elements are not taken up by the plants symptoms of diseases appear. Deficiencies of manganese, zinc, and copper are widespread in citrus trees. Such diseases are cured by applying the deficient element to the soil and spraying the trees. Some of these trace elements are now being incorporated in fertilizer mixtures.

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