Using a new equation to fit the water retention curve, we account for the effect of the soil solution osmotic pressure on the availability of soil water to a plant where the reflection coefficient of root-cell walls does not vary from unity. The differential water capacity is multiplied by a weighting function to account for soluble salts and then integrated from zero matric head to infinity. This produces the integral water capacity, IWC, and constitutes the total amount of water the soil can hold and release to a (hypothetical) plant that behaves like a perfect osmometer. The upper boundary of water availability was found by setting the weighting coefficient always equal to unity, which implies a reflection coefficient of zero and implies that the soluble salts have no influence on the availability of water (as would be registered by a tensiometer). The lower boundary of water availability in the presence of soluble salts is defined and calculated as if the reflection coefficient were unity. The upper and the lower boundaries constitute the envelope within which the actual availability of water to real plants (with variable reflection coefficients) in variously saline soils occurs. This establishes a framework within which water availability to real plants experiencing real osmo-matric conditions can be evaluated. The approach is illustrated using data for a wide range of soil textures.