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Soil Mulch Effects on Seedbed Temperature and Water During Fallow in Eastern Washington¹

ABSTRACT

Depth of dry soil mulch affected summer soil temperatures and seed-bed water at the end of fallow in the dryland wheat region of eastern Washington. Increasing the depth of the tillage mulch from 6 to 11 cm reduced summertime seed zone drying sufficiently to benefit wheat emergence. Reduction of drying was greatest when the seed zone had good capillary continuity with the deeper soil layers. Drying depth and intensity was greater with a cloddy soil mulch than with a fine mulch.

The seed zone water-conserving effect of a fine soil mulch was related to the lowered temperatures and temperature gradients across the seed zone associated with the increased mulch depth. The deeper soil mulch conserved seedbed water through increased resistance to water flow from moist layers to the atmosphere, and through increased thermal insulation of the moist soil below the dry mulch. Surface-applied straw (4,000 kg/ha) decreased seed-zone temperatures under the shallow mulch only.

The key factor in conserving seedbed water through extended periods of hot, dry weather is that the loss rates from the seed zone be balanced by upward unsaturated flow from deeper layers. As the soil dries, the relative importance of loss rates through evaporative loss across the dry layer and thermally induced flow downward increases because of decreasing return flow from the deeper layers along matric potential gradients. Under these dryland conditions, it appears that seedbed water could be best conserved through using a soil mulch of maximum resistance to vapor and liquid water flow, and maximum thermal insulation, overlying a seed zone having good capillary continuity with the deeper soil layers. The stratification produced by moderately deep rod-weeding appears to provide such a mulch.

¹ Contribution from the Northwest Branch. Soil and Water Conservation Research Division. ARS, USDA, in cooperation with the Washington College of Agr. Res. Cen., Pullman, Washington 99163. Scientific Paper 3896.

² Soil Scientist, SWCRD, Senior Experimental Aid. Dep. of Agronomy and Soils, Washington State University, and Soil Scientist Research Assistant, SWCRD, respectively.

Received for publication August 14, 1972. Accepted for publication December 22, 1972.