One of the main functions of soil is to store moisture and supply it to plants between rainfalls or irrigations. Evaporation from the soil surface, transpiration by plants and deep percolation combine to reduce soil moisture status between water applications. If the water content becomes too low, plants become stressed. The plant available moisture storage capacity of a soil provides a buffer which determines a plant’s capacity to withstand dry spells.

Forms of Soil Water Storage

Water is held in soil in various ways and not all of it is available to plants.

 Chemical water is an integral part of the molecular structure of soil minerals. It can be held tightly by electrostatic forces to the surfaces of clay crystals and other minerals and is unavailable to plants.

The rest of the water in the soil is held in pores, the spaces between the soil particles. The amount of moisture that a soil can store and the amount it can supply to plants are dependent on the number and size of its pore spaces 

Gravitational water is held in large soil pores and rapidly drains out under the action of gravity within a day or so after rain. Plants can only make use of gravitational water for a few days after rain.

Capillary water is held in pores that are small enough to hold water against gravity, but not so tightly that roots cannot absorb it. This water occurs as a film around soil particles and in the pores between them and is the main source of plant moisture. As this water is withdrawn, the larger pores drain first. The finer the pores, the more resistant they are to removal of water. As water is withdrawn, the film becomes thinner and harder to detach from the soil particles. This capillary water can move in all directions in response to suction and can move upwards through soil for up to two metres, the particles and pores of the soil acting like a wick.

When soil is saturated, all the pores are full of water, but after a day, all gravitational water drains out, leaving the soil at field capacity. Plants then draw water out of the capillary pores, readily at first and then with greater difficulty, until no more can be withdrawn and the only water left is in the micro-pores. The soil is then at wilting point and without water additions, plants die.

The amount of water available to plants is therefore determined by the capillary porosity and is calculated by the difference in moisture content between field capacity and wilting point. This is the total available water storage of the soil. The portion of the total available moisture store, which can be extracted by plants without becoming stressed, is termed readily available water. Irrigators must have knowledge of the readily available moisture capacity so that water can be applied before plants have to expend excessive energy to extract moisture.

The amount of soil water available to plants is governed by the depth of soil that roots can explore (the root zone) and the nature of the soil material. Because the total and available moisture storage capacities are linked to porosity, the particle sizes (texture) and the arrangement of particles (structure) are the critical factors. Organic matter and carbonate levels and stone content also affect moisture storage.   Poor structure, low organic matter, low carbonate content and presence of stones all reduce the moisture storage capacity of a given texture class.

Clays store large amounts of water, but because they have high wilting points, they need significant rain to be able to supply water to plants. On the other hand, sands have limited water storage capacity, but because most of it is available, plants can make use of light showers regardless of how dry they are before the shower. Plants growing in sand generally have a more dense root system to enable them to access water quickly before the sand dries out.

Measuring Soil Water Holding Capacity

Firstly, establish the depth of the root zone, either by observing the depth to which roots from the previous crop have extended, or by noting the depth to a restrictive layer. The roots of most annual field crops occur in the top 120cm of soil, if there are no restrictive layers. Some perennial species may extend roots to 600cm or more if soil conditions are ideal and moisture is present.  Secondly, use Figure 3 to calculate the water holding capacity of each soil layer in the root zone. For example, 25cm of clay loam with an available water of 1.8mm water per cm of soil, can store 45mm of available water.  The water holding capacity of a soil is calculated by summing the capacity of each layer in the root zone.