Water is held in soil in the pores between the soil particles, therefore the maximum amount of water that a specific soil can potentially hold is equal to its porosity (total volume of pores).
There are three types of soil water: Gravitational water, capillary water and hygroscopic water. Each type is affected by different forces that act on the water in soil
Gravitational water is the water that moves through the soil by the force of gravity and drains. Gravitational water moves in the larger pores of the soil and drains quickly.
Hygroscopic water is a thin layer of water, in a vapor form, held tightly to soil particles by surface forces. Hygroscopic water is not available for plants.
Capillary water is the water that is held inside soil pores against gravity. The capillary forces that hold the water inside the pores is a result of the ratio between adhesion and cohesion forces. Adhesion is the tendency of water molecules to stick to other surfaces, and cohesion is the tendency of water molecules to stick one to the other. Capillary forces are stronger when the adhesion is greater than cohesion. Adhesion is stronger in smaller pores.
Fine-textured soils (clay and clay-loam soils) have a larger porosity compared to coarse-textured soils (sand). Therefore, they can hold more water then sandy soils. However, a large portion of the water held in fine-textured soils is not available to plants. This is because the pores in fine-textured soils are small and hold water more tightly.
To absorb soil water, plants must overcome the forces that hold the water in soil pores.
Different soil moisture conditions were defined. These moisture conditions give an indication of the availability of water to plants.
Soil moisture content – a percent volume of water in the soil at a given moment.
In the lab, a known volume of soil is dried and % soil moisture content is calculated in the following way: % moisture content = (weight of wet soil – wet of dry soil)/(weight of dry soil) X 100
Saturation – all soil pores are filled with water. This is not an ideal condition for plants, as plat roots need air.
Field capacity – this is the moisture content of the soil after drainage has stopped. The large pores, that cannot hold the water against gravity are filled with air. By definition, it is the water content retained in the soil at -0.33 bar.
This is considered to be the ideal moisture condition for plants, as water in this condition is easily available.
However, in certain soils, maintaining the soil in field capacity may result in lack of oxygen to the root system or in development of stem and root diseases.
Permanent Wilting point – this is the moisture content of the soil at which plants cannot absorb the water, as it is held tightly in soil pores. By definition, this is the water content of the soil at tension of -15 bar.
The difference field capacity and wilting point is the Available Water. When converted to water amount, it is known as Total Available Water.
Field capacity: 20%
Wilting point: 13%
Root system depth: 20cm
Calculate the Total Available Water (TAW):
% Available water = 20-13=7%
TAW= 0.07 x 1000 mm/m x 0.2m = 14mm (140 m3/ha).
Readily Available Water (RAW) – As water is depleted from the soil, the remaining water becomes more difficult to extract and, at a certain point, the hydraulic conductivity drops down and water flow towards the roots decreases significantly. The Readily Available Water is the water that can be easily extracted by the plant. It is the moisture content of the soil between field capacity and a refill point, which is obtained by multiplying the Total Available Water by a fraction called “depletion fraction” (p). The depletion fraction is crop specific. For many crops, the depletion fraction is set to 0.5.