Mar 10, 2017

Agronomy & physiology Agro-physio-australia
Abstract                                                                         Back to Table of contents

Eighty percent of irrigated cotton in Australia is grown under the management system of retained beds.  In this system wheel traffic is restricted to furrows and beds are only lightly cultivated.  Furrow compaction becomes a problem when hills need replacing either due to poor alignment of beds or when wheels compact the "shoulders" of beds.  As the soils have high clay contents and contain swell/shrink clay minerals, compaction can be repaired by wet/dry cycles.

A laboratory and field trial was undertaken to assess soil structure repair with repeated wet/dry cycles.  In the laboratory, intact soil cores (0.3 m dia. x 0.5 m high) from the furrows of a Vertisol were subject to wet/dry cycles.  Wetting was by rainfall and flooding, and drying was by evaporation.  Structure repair was assessed by measuring the changes in four soil physical properties:  total water infiltration (at each wetting event), torsional shear strength, water infiltration by disc permeameter and the digitising of soil cracks from photographs of the soil surface.

Seven flood wet/dry cycles gave an almost 14% increase in total water infiltration relative to the amount of water added to the soil at the first wetting occasion.  Total infiltration dropped significantly, though, after nine flood wet/dry cycles and this was associated with a 50% reduction in surface cracks, as fine cracks developed with increased wet/dry cycles. Rain wetting increased percent crack (per unit surface soil area) up to nine wet/dry cycles, but was not associated with significant or large increases in total water infiltration. Infiltration from the disc permeameter was almost double for flood wetting than rain wetting, reaching a maximum for flood wetting after five wet/dry cycles, that was maintained to after nine wet/dries.  Rain wetting gave slightly lower shear strength of the soil surface than flood wetting, with both wetting types giving the maximum decrease in shear strength after five wet/dry cycles with only a small further reduction to nine.

The study demonstrated soil structure repair solely by wetting and subsequent water evaporation.  Flood wetting is more effective in repairing the soil to depth than rain wetting as demonstrated by greater water infiltration into the flood-wetted soil with repeated wetting/drying.  Rain-wetting gave a coarser surface structure with no related increase in infiltration; the large surface cracks rapidly closing with wetting, so forming a surface seal.

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