ORGANIC-CARBON EFFECTS ON AVAILABLE WATER CAPACITY OF 3 SOIL TEXTURAL GROUPS

A major cause of soil productivity loss through erosion is attributed to changes in soil water-holding properties or a reduction in thickness of the plant rooting zone. The objective of this study was to measure the effect of management-induced changes in soil organic carbon (OC) concentration on soil characteristics affecting the available water capacity. Four sites each of moderately coarse (sandy), medium, and moderately fine or fine (fine) soils were sampled in four increments to 0.457 m within each of two cropland management systems (conventionally cultivated and stubble mulched) and each of two virgin grassland management systems (grazed virgin and relict virgin). Measurements of the water concentration by weight (P(w)) at field capacity (FC) and permanent wilting point (PWP) were made on disturbed soil samples. Bulk density decreased with increasing OC concentration. The magnitude of change was greatest in sandy and least in the medium-textured soils. For all soils combined, a change in sand fraction accounted for about 75% of the change in P(w) at both the FC and the PWP. The change in P(w) was greater at FC than the PWP. A unit change in OC concentration in the sandy soils caused a greater change in P(w) at FC than at the PWP, but in the medium and fine soils the change in P(w) at FC essentially paralleled the change at the PWP. An increase in OC concentration did not change the available water capacity in the sandy group and decreased it in the medium and fine textural groups. Loss of soil productivity induced by erosion in the northern Great Plains is probably more closely associated with a decline in nutrients and biological activity than from a change in available water capacity.