Effects of fallow replacement green manuring with annual legumes on soil water reserves
Legume green manures are used in crop rotations to add organic matter, enhance N availability and to provide ground cover for soil conservation. In drought-prone regions like the Palliser Triangle, annual rather than perennial legumes could be used for fallow replacement green manuring in short rotations to avoid excessive depletion of soil water reserves. Results from a 6-year study of green manuring with 4 annual legumes in combination with snow trapping on a Brown loam at Swift Current were used to assess water use during legume growth, water use efficiency (WOE) of green manure production and amounts of soil water remaining for subsequent spring wheat. Each spring Black lentil, Tangier flatpea, Chickling vetch and Feedpea were seeded into wheat stubble with tall stubble trap strips along with continuous wheat and wheat-fallow plots. Legumes were incorporated in some and chemically desiccated in other plots as soon as they reached full bloom. Water use by the 4 legumes was generally related to DM production and did not differ from the amount used by wheat during the same vegetative growth period. Most of the water used was extracted from the 0- to 60-cm soil depth. On average, inoculated legumes used only 12% more water but produced much more DM than uninoculated legumes. Thus rhizobial seed inoculation and the enhanced N2-fixation elicited a doubling of the WUE by these green manures. Chickling vetch and feedpea were able to use water at a 20% greater efficiency than well fertilized spring wheat. Contrary to expectations, levels of soil water recharge between legume bloom and the following spring were always as high for incorporated as for desiccated green manures. At seeding time there was generally 20% less water under continuous wheat than in the fallow soil. In green manured soils, water reserves in spring did not differ between legume species and were usually 15% lower than in fallow plots with wheat trap strips. Wheat yields after green manures were affected primarily by weather and legume management but not by legume species. After incorporated green manures grain production was, on average, 12% and 17% greater than after conventional fallow or desiccated green manure, respectively. The presence of wheat strips in the fallow phase increased subsequent grain yields by an average 20%. We concluded that green manuring with annual legumes, when combined with snow trapping, offers a more bio-resource efficient and soil conserving alternative to conventional summer fallowing for wheat production within the Palliser Triangle.
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