Agriculture Reference
In-Depth Information
2.2.4.1
Water retention in different soils
On either a gravimetric‚ or mass per unit volume basis‚ the equilibrium water content
corresponding to any given water potential is strongly influenced by both soil texture
(textural porosity) and structure (structural porosity). At a given potential‚ the gravimetric
water content depends on the size-frequency distribution of the soil pores and‚ in
particular‚ the total interconnected pore space‚
i.e
.‚ that which can be filled with water.
Since soils differ widely in pore space distributions‚ the total store of water in soils
also varies accordingly.
At high potentials‚ much soil water is held in the larger pores and soil structure
greatly influences the amount of water retained. The effect of structure is clear in strongly-
structured soils‚ but only at high potentials since structure controls the volume of
the large inter-aggregate pores that retain gravitational water. In a strongly-structured
Hawaiian soil‚ Sharma and Uehara (1968) found that almost all the water stored between
the aggregates had drained from the soil after drying to -0.029 MPa and that at lower
potentials‚ water was largely derived from within the aggregates.
At lower potentials the lesser importance of structure means that the shape of
the characteristic draining curve is largely dependent on texture and the specific surface
area of the soil. Soils with high clay contents have large internal surface areas and
a greater capacity to retain water at all potentials (Figure I.25). Coarse-textured soils
retain less water overall and have less plant-available water than the finer textured
loams and clays.
