In an ideal world there should be fertile soils with abundant but not too much water. There are few areas around the world that are blessed with this combination and these are the prime agricultural areas of the world. In most areas of the world there is not this ideal combination. For example, in the arid desert areas of the world there is generally insufficient rain entering the soil. Here, if plants are to succeed, irrigation is practiced with water brought in from outside the region and added to the soil at crucial periods of the year. Unfortunately, these waters can contain many salts and their use over hundreds of years has given rise to salinisation which has damaged land almost beyond repair.

Dye tracers used to see where rain goes.

In some parts of the world also, soil erosion has become a major problem. The condition of the soil will regulate to a great extent what happens to the incoming water. If the soil structure is poor and the surface compacted there will be a much greater tendency to run-off and flow over the surface. If there is rapid run off, overland flow can lead to extensive soil erosion and development of rills and gullies. In some areas such run-off is difficult to avoid because of the tendency for heavy storms to fall on poorly structured soils. Much of the soil erosion of the world, which is a matter of increasing concern, is due to heavy rainfall on dry poorly structured soils. The result is often severe soil erosion as seen in the illustration.The other outcome of rapid soil run-off is flash flooding. With most of the incoming rainfall flowing over the soil surface into the rivers and lakes, the inputs often lead to extensive and damaging flooding.

The soil-water interaction is a critical one, particularly for agriculture, natural and semi-natural vegetation, organisms that live in and above the soil, and increasingly for the infrastructure that is needed to support the growing population. We need to understand well the interaction between soil and water so that we can control it and ensure that it is maintained and available for future generations.

Sursa: www.soil-net.com

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.