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UBC Theses and Dissertations

An evaluation of soil and water management practices on a lowland soil with poor natural drainage Heinonen, John Stanley


Low-lying areas of the Lower Fraser Valley tend to have poor natural soil drainage. Excess moisture, because of a high water table and ponded surface water, restricts the range of crops that can be grown and influences farm management decisions regarding the nature and timing of field operations. When economic considerations lead farmers to commence field operations when the soil is too wet, the result can be structural degradation of the soil, leading to the formation of surface seals and crusts and a compact ploughpan. The aim of this study was to evaluate the effectiveness of various management practices in controlling the water table, to gain an understanding of the mechanisms governing excess water accumulation, persistence, and removal from lowland soils, and to make management recommendations from these. Monitoring was conducted on two adjacent farms on Westham Island. They differed in their management of excess water: one relied on surface ditching, subsoiling, mole drains, and cover crops, while the other used a system of perforated subsurface drainlines emptying into a ditch that was pumped to keep the water level low. Soil-water pressure potentials were measured automatically and manually with piezometers located in selected depressions and surrounding slightly higher ground from February to April 1981. Depth of ponded surface water and water level in the ditches draining each farm were monitored. Rainfall was recorded with four rain gauges. Bulk densities and saturated hydraulic conductivities of the surface crust, ploughpan, and subsoil were measured. Soil penetration resistances and aggregate stabilities (wet sieving) were measured as indices of compaction and structural stability. Earthworm numbers and biomasses were measured in both farm. Land drainage through a system of subsurface drainlines was found to lower the water table more rapidly after rainfall, and reducing the depth and duration of ponding compared to undrained land. Drainage resulted in more trafficable and opportunity days for field operations. Drainage dramatically increased earthworm populations which in turn improved the drainage via the pathways for water movement provided by burrows. Although there was a more severe ploughpan in the undrained farm there was no evidence that it impeded water movement. There were no differences in aggregate stabilities between farms. The presence of ponded water in depressions was found to be due mainly to a low permeability surface seal which was more severe in the undrained farm. The presence of the seal resulted in surface runoff to low areas and slowed water infiltration there. Subsoiling and moling may also aggravate excess water problems by directing water to low areas. Surface ditches were found to be ineffective in removing ponded water. Using a cover crop, which reduced the severity of crusting and ponding, was a more effective practice than subsoiling when the soil was undrained.

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