Water Retention

The soil matrix retains water by two mechanisms. First, water can be adsorbed on particle surfaces (especially clay particles due to their reactive large surface area), and second, water can be held in soil pores by capillarity.

Water entering the soil

When water enters dry soil, the most rapid flow will initially take place in large pores. Fine porous systems, such as aggregates with a high clay content, will wet up more slowly.

Water draining from soil

Pores larger than 30 µm in diameter cannot retain water against the downward pull by the force of gravity. A saturated soil will lose water by drainage depending on the volume of soil space represented by such large pores.

Water holding characteristics of soil

The soil matrix contains pores of a large range of sizes and shapes, as a result of soil-specific particle size distribution (soil texture) and aggregate size distribution (soil structure).

Water is held more tightly in smaller pores than in larger pores. Clay soils retain more water and for longer periods than sandy soils. High-swelling clays (montmorillonite) can adsorb very large amounts of water, as can organic matter, which by weight can retain more water than soil.

Plant Available Water

The amount of water plants absorb from soils is determined by a number of plant, climatic, and soil variables. An important soil characteristic that affects plant water uptake is soil moisture tension or matric potential. The range of plant-available water is defined as that between field capacity (a matric potential of -10 kPa) and permanent wilting point (a matric potential of -1500 kPa).

A soil is at field capacity when, after saturation, all water has been drained from macropores by gravity. The wilting point is reached when, in drying soil, water is held so tightly that the rate of its supply to plants will be so slow that the plants will stay wilted.

Non-limiting Water Range

The non-limiting water range (NLWR) is the range of water content for which plant growth is not seriously reduced by water availability, aeration (AL), or soil strength (SSL). Soils with a large NLWR (usually limited by permanent wilting point (PWP) and field capacity (FC)) are relatively easy to manage; excess water can drain away, and there is good water storage. Soils with a small NLWR are more difficult to manage; for good plant growth, the water content needs to be held in a narrow range (e.g., by frequent irrigation). Amelioration may be possible to widen the NLWR.