CHEMICAL ASPECT OF SOIL

CHEMICAL ASPECT OF SOIL Tiny particles with unusual chemical properties called soil colloids are included in the clay fraction of the soil; these are in the form of thin flakes, which remain suspended indefinitely in water. Colloids may be organic, made up of a very finely divided humus or mineral, in which case they are known as day minerals. Together, the two types make up a clay-humus complex. Most soils have more clay minerals than organic colloids. Clay minerals are of great importance because they are in a state of continuous chemical change, which is fundamental to soil formation.

Unusual chemical properties of colloids result from their vast surface area for a given weight. Colloids have a property of being electrically charged and can therefore attract and hold ions. Ions of calcium, magnesium and potassium are known in soil science as bases. These bases may be given by the colloids to plants, which require them for growth, by a process known. as base exchange. Some bases are more readily given up than others. Particularly the metallic ions, like potassium and sodium, tend to be replaced by hydrogen.

The hydrogen in the soil solution makes for an acid condition. The concentration of hydrogen ions in soil solution is known as the pH of the soil, and is the measure of soil acidity or alkalinity. Soil acidity is thus the property related to the proportion of exchangeable hydrogen in soil with relation to other elements. Over a period of time, this base exchange makes the soil more acid, unless bases ate replenished. The decomposition of plants and animals helps in recycling of bases to the soil, under natural conditions. Artificial supply of the bases in the form
of fertilisers is required where the vegetation is removed by man by cutting or cropping.