Much of the earth's rain and snow fall on the surface of the soil. The nature of the soil is very important in determining the fate of this precipitation. Depending on the type of soil, some of the precipitation will flow over the surface of the soil, much of it will enter the soil where it will be held for a period of time, and some of it passes straight on through the soil to the groundwater, rivers, lakes and eventually, to the oceans. The period that the water is in the soil is a crucial one for agriculture and food production, for all plant life and for many of the organisms that live in the soil.
All soils have what is known as a soil water regime. This regime is dependent on the water holding capacity of the soil, the demands on that moisture by plants and animals and the availability of the soil water to them. Water entering the soil is held with different degrees of strength depending on the texture of the soil, i.e. whether the soil is clayey, loamy or sandy, and the structure of the soil, i.e. the network of pore space.
After heavy rainfall, the soil is usually very wet and the water is held in the soil at what is known as field capacity. The field capacity of a soil is the maximum amount of water that a particular soil can hold for plants. After the rainfall has ceased, the soil will begin to dry out as much of the surplus water has drained through the soil. If there is no further rainfall for some time, the soil will continue to dry out and may reach what is known as the permanent wilting point (PWP) when plants will begin to wilt because of lack of moisture. This situation is a feature of many soils in low rainfall and drought situations. The amount of water and the tenacity with which it is held in the soil is a very important factor in how a soil behaves and what it can be used for.