A natural fertile soil is one that
I. Has the correct proportions of sand and clay particles,
ii. Has adequate humus and mineral salts,
iii. Has adequate humus and mineral salts,
iv. Has a good crumb structure,
v. Is well drained and aerated, and
vi. Is neutral or slightly alkaline.
Acidic soils are usually not fertile as acidity makes the mineral salts very soluble. Rain washes these salts into the deeper soil layers, out of reach of plant roots.
In a natural ecosystem, soil fertility is maintained by the activities of the organisms living in it. The materials that the organisms remove from the soil are returned to it when they die and their remains are broken down by decomposers.
Farming reduces soil fertility because crops are harvested and used by us. Their remains are not returned to the soil. Farmers , therefore , have to make sure that the fertility of the soil is maintained by adding fertilizers.
In a small farm, the farmer can maintain soil fertility by adding organic manure such as farmyard manure, compost and green manure. Organic manure increases the humid content of the soil and so provides the necessary nutrients and maintains crumb structure.
Farmyard manure is a mixture of livestock droppings and decaying plant remains such as straw used for animal bedding. Compost consists of the decayed remains of plants and animals. Green manure are plants , especially grasses and legumes, that are grown to be ploughed under and buried in the soil so that they decay to form humus and release nutrients.
In large scale farms , addition of organic manure is not sufficient to maintain soil fertility. Inorganic fertilizers containing the essential mineral salts are also added to the soil.
Farmers also practice crop rotation to maintain soil fertility.
