Method
Plant Available Water
Marked spatial differences in agricultural productivity occur within a region because of different soil properties. The vertical sequence of soil properties regulates water entry into, storage and retention in, and redistribution within and out of the soil profile.
The following considerations are important in the context of this Guide:
- Respective thicknesses of various soil horizons.
- Surface properties (e.g. crusting, sealing, cracking, tillage) which affect its infiltrability.
- The sequence of soil horizons in regard, for example, to the distributions of clay, sand and silt percentages within the soil profile. These drainage characteristics of the soil, or impeding layers within the soil profile, either natural or man-made, may cause drainage/waterlogging problems.
- The water holding capability of the soil, which finds expression, inter alia, through the
- soil profile’s depth, its
- texture (sand, silt, clay make-up) and its
- soil water content at specified/critical soil water conditions, viz. at its
- permanent wilting point, i.e. the soil water content constituting the lower limit of soil water available to the plant, at its
- drained upper limit, previously termed ‘field capacity’, being the soil water content held by capillary forces that are great enough to resist gravity after natural percolation from the soil has ceased, and when the soil water content is at
- total porosity, i.e. at saturation when all pore spaces are filled with water
- The plant available water of the soil, as an integrator of many of the above attributes.