On water availability in saline soils

Abstract

We present a formulation for the effect of the osmotic pressure of the soil solution on the availability of soil water for plant uptake in the extreme case that the reflection coefficient of root-cell walls is always unity. We also present a new equation to fit the water retention curve, which allows for an inflection point and is solidly anchored at both the wet end (saturated water content) and the dry end (water content at 150 m head, the permanent wilting point). By differentiating the fitting-equation one finds the differential water capacity, which is subsequently multiplied by a weighting function to account for the impediment caused by soluble salts. The weighted differential water capacity is then integrated over the entire range of the matric head from zero to infinity. This produces the integral water capacity and constitutes the total amount of water the soil can hold and release to a hypothetical plant that behaves like a perfect osmometer. We illustrate the approach using data found in the literature for a wide range of soil textures. In this paper the lower boundary of water availability in the presence of soluble salts is defined and calculated as would be registered by a perfect osmometer (reflection coefficient of unity). The upper boundary of water availability is found by setting the weighting coefficient at unity at all times, which implies a reflection coefficient of zero, and in turn that the salts in the soil solution have no influence on the availability of water (as would be registered by a tensiometer). The upper and the lower boundaries constitute the envelope within which the actual availability of water to real plants occurs, and implies a variable reflection coefficient plus the occurrence of active plant osmo-regulation. This establishes a framework within which water availability to real plants experiencing real osmo-matric conditions can be evaluated.