Figure 4.1 soil textures and the available water
.
Texture Clay
Silty
Clay
Clay
Loam
Loam Sandy
Loam
Loamy
Sand
Sand
MAD
0.3 0.4 0.4 0.5 0.5 0.5 0.6
Table 4.1 Maximum allowable depletions for different soil textures
For example, let us suppose your soil is a sandy loam. From table 4.1, MAD = 0.5, From Figure
4.1 (or a soil surrey) PWP = 13% and the field capacity is 25%. Using equation [4.1] yields;
AD = (25-13) x 0.5 = 6% . The lower limit would then be calculated by equation [4.3].
LL = 25 - 6 = 19%.
This means for a sandy loam, if the soil moisture drops below 19%
(or Hydra Probe reading of 0.19), the crop may become stressed and it is time to irrigate. Never
let the soil moisture drop below the permanent wilting point.
4.4 Soil Water Salinity (g/L NaCl)
The Soil Water Salinity (g/L NaCl)the salinity of a 1:1 soil/water slurry. It is the “total dissolved
solids” (TDS) in the form of dissolved salts. The soil salinity is primarily used with the portable
versions of Hydra Probe. The soil water salinity calculation in equation [4.4] is not applicable for
in situ measurements in a permanent/semi permanent installation in soil. The soil water salinity
is approximated:
Soil Water Salinity (g/L)
≈
EC (S/m) x 6.4 [4.4]
29
