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.