Minerals get depleted in the soil due to several reasons :
(i) Overcropping which withdraws minerals from the top layers of the soil.
(ii) Use of high yielding varieties which require higher quantity of mineral nutrients.
(iii) Non-rotation of crops causing deficiency of minerals at a particular level in the soil.
(iv) Leaching or washing down of minerals along with gravitational water.
(v) Precipitation of minerals due to change in soil pH.
Restoration of Mineral Fertility : It is done through two methods, natural and artificial.
(i) Natural replenishment : In soil under natural vegetation, mineral fertility is restored naturally through :
(a) Recovery of minerals from decomposition of fallen leaves, twigs, dead roots, dead animals and animal excreta.
(b) Slow process of weathering of bed rocks.
(c) Burrowing habit of some animals like earthworms which bring sub-soil over the surface of top soil.
(d) Biological nitrogen fixation.
(ii) Artificial replenishment: It is carried out by human efforts :
(a) Addition of farmyard manure. Manure contains humus. Humus form colloidal particles in soil for increased hydration, crumb structure, aeration, loosening the soil, slow release of minerals and organic substances that stimulate plant growth.
(b) Green manuring.
(c) Crop rotation or sowing of alternate crops successively in the same field. It helps in maintaining nitrogen fertility of the soil.
(d) Addition of fertilizers.