Soil fertility changes in a red-brown earth under irrigated pastures. III Water relations of the mat and surface soil

Abstract

The water relations of the surface mat of organic matter and the underlying 2.5 cm of soil from three annual and three perennial pastures of the same age were examined before and after mixing by cultivation. In either circumstance, the water relations of the 0–2.5 cm soil horizon did not differ significantly between the various pastures.

The mats retained a considerable amount of water over a wide range of matric potentials. The organic carbon contents and amounts of water retained in the mats at –1520 J/kg matric potential were linearly related; at –9.8 J/kg and at –0.098 J/kg matric potentials they were linearly related, but the regression coefficients for mats from annual pastures were significantly greater than for mats from perennial pastures.

Incorporation of the mat into the soil slightly increased the total porosity of the 0–2.5 cm soil horizon and changed the pore size distribution towards larger pores.

Available water capacity (AWC) was defined as the depth of water held between the limits –0.098 and –1520 J/kg matric potentials. The AWC of the 0–7.6 cm soil horizon plus overlying pasture mat was less than that of the equivalent depth of cultivated soil with incorporated mat for all pastures. With the more conventional limits of AWC between –9.8 and –1520 J/kg matric potential, more water was available in the 0-7.6 cm soil plus mat than in the equivalent depth of cultivated soil, except in the case of an annual grass pasture.