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• Hygroscopically bound water is defined as that water that is bound by forces > 5 MPa;

• The permanent wilting point is defined as the water status when water is bound by 1.5 MPa. This is the water potential at which a sunflower plant is no longer able to replace the water lost through transpiration from the water in the soil and thus wilts. Depending on the adaptation of plants to dry habitats the permanent wilting point may be at values between 0.7 and 3 MPa;

• Field capacity is defined as water bound at 0.05 MPa. At this point water can no longer be retained by soil particles against gravity and therefore drains out of the soil.

The amount of water in the soil that is available for plants corresponds to the water content between field capacity and wilting point. Loam and clay soils differ not only in the amount of available water (for loam with 0.2 g g_1 almost ten times higher than in sandy soils), but also in the amount of water which is not available for plants in dry soils; this has consequences for the availability of water to plants with decreasing precipitation (Fig. 2.2.4 B). In areas with high precipitation, sandy soils are drier habitats than clay soils, because sandy soils retain less water. With less precipitation the limit at which water can no longer be taken up by the plant, because of the capillary and matrix forces are reached sooner in clay soils than in sandy soils. In arid regions sandy soils are therefore "moister" for plants than clay soils. This is modified in nature by the frequency of precipitation.

The transport of water in the soil to the root occurs along the potential gradient, whereby the rate of flux in the soil is determined by the hy-

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