2 The change of wilting coefficient with soil depth
Plants absorb water from the soil,which cause soil water content root reduce. Soil dry become severe and soil water stress in the soil layer near root. At the same time, the water moves slowly from the from the soil layer nearest root to the soil layer near root in the soil matrix driven by gravity and water potential gradients. Wilting coefficient for Huangmian loess soil is the water content at -1.5 MPa in a given soil layer (Yang and Shao, 2000). In the terrace land, 23-years-old red plum apricot tree root develops to a considerable soil depth and suck water in the dry year on National high quality red plum apricot demonstration area. Once a soil layer in which soil water content equals or less than wilting coefficient, the soil layer become dried soil layer,The dried soil layer happened in the soil layer deeper than maximum infiltration depth is permanent dried soil layer in which the soil water cannot be recovered. The permanent dried soil layer reduces the soil moisture mobility and blocks up the intercourse between soil water in the soil layer upper and below the permanent dried soil layer. So soil water management should pay attention to soil water in the soil layers from surface soil to maximum cumulative infiltration depth.
Plant root water absorption is a process in which plant root and soil particle contends for soil moisture. Along with plant growth and root water absorption, soil water content reduces and soil water stress increase in the soil around root. When the soil water content in a soil layer reduces to wilting coefficient, the soil water potential in a soil layer surrounding the root reach balance with the water potential in plant root cell, and plant cannot absorb the water from the soil layer anymore. This balance point is wilting coefficient. The relationship between volumetric soil water content, w, and soil water suction, S, is determined as: w = aS -b, See Figure 4. It can be seen that the volumetric soil water content dropping with the increasing soil water suction from 0.01 Ba, 1Ba = 1×105 Pa , to 15.0 Ba, such as in the 10 cm soil layer, volumetric soil water content dropping from 38.37% to 7.98% with the increasing soil water suction from 0.01×105Pa to 15.0×105 Pa,.
(Figuer 4 Here)
Where, θ is soil water constant and S is soil water suction. According to the relationship between θ and S, the wilting point at the suction of -15 MPa can be estimated. The determination coefficient, R2, changes from 0.981 in 140-180 cm soil layer to 0.991 in the 0-10 cm and 10 -30 cm soil layer. The change of wilting coefficient with soil depth is shown in the Figure 5. It can be seen from Figure 5 that field capacity at the suction of -0.33 MPa drops from 28.11% in 5 cm to 17.87 % in 160 cm soil layer and then rises gradually to 21.82% in 400 cm with increasing soil depth. The wilting coefficient at the suction of -15×105 Pa drops from 7.98 % in 0-10 cm to 6.68 % in the 120 cm.