Phosphorus Redistribution in Saskatchewan Soils
Letkeman, Lorne Peter
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Phosphorus (P) is one of the least soluble and least mobile nutrients found in the soil environment. Despite its low solubility and mobility there has been considerable redistribution of P in soil profiles over the time of soil formation. The soil-forming processes thought to be largely responsible for soil P redistribution are leaching and biocycling, the translocation of P to surface soil horizons by roots. The intensity of both leaching and biocycling is primarily controlled by soil moisture conditions. It was the purpose of this study to examine the redistribution of P in soil profiles along an environmental gradient and over toposequences, as these factors can strongly influence soil moisture conditions. Soil profiles were sampled from uncultivated land near Swift Current (Brown soil zone), Aberdeen (Dark Brown soil zone), and Spiritwood (Gray Luvisol), making up an environmental gradient from the semi-arid southwest to the subhumid central region of Saskatchewan. At the Swift Current and Aberdeen locations soil profiles were taken from the top and bottom of catenas on native pasture, while at Spiritwood soil profiles were taken from a native forest and an adjoining alfalfa field which had been cultivated for about 50 years of continuous wheat crops. All the soils were formed on uniform deposits of glacial till parent material. Using soils of glacial till parent material was essential as only these soils allow for the distribution patterns of P with soil depth to be attributed to soil-forming processes and not systematic variations in the parent material composition. Redistribution of soil P was found in all the soil profiles sampled. Soils were found to generally accumulate P in the surface horizons, experience P depletion from the rooting zone (upper B horizons) and, in the well-drained soils, P accumulations were found in the lower profile (lower B, and upper C horizons). The profiles taken from the grassland depression landscape positions were found to show greater P1 redistribution than the upland positions and they also contained considerable P accumulations in the subsoil (50-150 em depth). It was determined, by mass balance, that as much as 130 g m-2 ofP had been moved out of the depletion zone while only 60 g m-2 could be accounted for in the surface horizon accumulation. It was concluded that over one-half of the P lost from the depletion zone had translocated down into the subsoil below as the result of leaching. Phosphorus fractionation analysis indicated that the subsoil accumulation was made up of primarily secondary Pi forms. This suggested that deep leaching of P may be occurring as the result of soluble Pi translocation, and not by organic P or clay translocation as others have suggested. Greater P redistribution was found in the Swift Current depression soil profile, which was part of a large water catchment basin, than the Aberdeen depression soil profile, which was part of a much smaller catchment basin indicating that local topography has a greater control over soil moisture conditions than the influence of a narrow environmental gradient. Both Gray Luvisol soils at Spiritwood showed a loss of P from the solum. Pedogenic indexing indicated that the native soil had lost about 7% of its estimated original P content. The cultivated soil was found to have lost between 25 and 35% of its original P according to a mass balance approach of quantitatively calculating P redistribution. Of the 150 to 200 g m-2 of P determined to have been lost from the solum only 100 g m-2 was estimated to have been lost due to crop removal. It was concluded that between 8 and 16% of the original P in the solum was lost from the cultivated soil as the result of processes other than those related to agricultural practices.