To break and pulverize the soil and make it soft and mellow, so the roots of plants may enter it in search of food, and get a firm hold for the support of the plant which is above ground.
To make the soil open and porous, so that it can more readily absorb rain as it falls on the surface.
To check loss of water by evaporation.
To admit air to the roots of plants. Also to allow air to act chemically on the mineral and organic matter of the soil and make them available to the crop.
To raise the temperature of soils in the spring, or of damp soils at any time.
To mix manures and organic matter with the soil. The more thoroughly manure is distributed through the soil the more easily plants will get it and the greater will be its effect on the soil.
To destroy the insect enemies of the plant by turning them up to the frost and the birds.
To kill weeds. Weeds injure crops:
They waste valuable moisture by pumping it up from the soil and sending it out into the air through their leaves. In this way they do their greatest injury to crops.
They crowd and shade the crop.