The soil consists of mineral and organic matter. The mineral matter consists of pebbles, sand and clay. The organic matter consists of dead and decomposed remains of plants and animals. It is called humus. The clay and humus and soil solution are important source of minerals.

I. Clay and humus: Clay is an inorganic material. It consists of aluminium and silicon oxides. Humus consists of carbonaceous residue. It is derived from lignin of cell walls. Both clay and humus particles are Similar in size. They are hydrophilic colloids. The surface of these particles is negatively charged. So they can absorb cations like calcium, potassium. magnesium and others.

2. Soil solution: Soil solution contains both cations and anions:



  • ┬áCations: Some cation is dissolved in the soil solution. Mostly root of plants absorbs these ions. A dynamic equilibrium is maintained between the cations present in the soil solution and the ions adsorbed on the colloidal particles. The plant root takes up ions. Therefore, the soil solution becomes deficient in cations. So the colloids (clay and humus) release cations and replenish the soil solution. Therefore, the soil colloids constitute a reserve source of cations.
  • Anions: Anions like nitrate, phosphate and sulphate are not adsorbed on the colloids. Therefore, they are present only in the soil solution. Therefore, anions are easily leached out with irrigation or rain water. Tints it is necessary to frequently replenish the anions by fertilizers.

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