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UBC Theses and Dissertations

Till-derived podzols of Vancouver Island Lewis, Terence

Abstract

Moderately well drained, non-lithic podzol soils developed in glacial tills of five lithologies, (andesitic, basaltic, intrusive, limestone and schistose), were characterized throughout the dry and wet subzones of the Coastal Western Hemlock biogeoclimatic zone of Vancouver Island. The soils are benchmark soils that occupy large areas, which could be used to assess the effects of local site differences and various land management practises. Analysis of the range of Vancouver Island podzols is followed by the synthesis of a Vancouver Island model of podzols, which is typified by large accumulations of sesquioxides throughout a thick B horizon, and yet the frequent absence of any Ae horizon. Accumulations of organic matter, both as mor surface horizons and within the B horizon; as well as acidity and low base saturation are also characteristic, as in the long-standing classical Ae/B model of podzols. However, the classical model requires the presence of an Ae horizon and generally has a much thinner, less sesquioxidic B horizon than the Vancouver Island model of podzols. It is suggested that the present concept of podzol genesis, which focuses on the translocation of sesquioxides and organic matter from Ae to B, be broadened to encompass the two rather divergent models of podzols. The proposed concept envisages podzol formation as resulting from three groups of processes - the in-situ weathering of parent materials and solum, which involves losses of the bases and silica and residual enrichment with iron and aluminum; horizonation processes, which redistribute organic matter and sesquioxides within the solum; and anti-horizonation processes, including physical turbation and the biological cycling of elements, which restrict the development of the master B and sub-horizonation within the master B. In the classical podzol, horizonation processes dominate while weathering acts to supply some of the constituents that are translocated. In the Vancouver Island model, in-situ weathering dominates and the translocation of sesquioxides is restricted by relatively rapid insolubilization of organo-metallic complexes by the abundance of weathered iron and aluminum. A duplicate composite sampling method within 0.04 hectare plots was designed in order to generate precise estimates of both the mean and variability of nutrient stocks in the soils. Assessment of both physical and chemical properties in the plots allowed the expression of nutrient status in kilograms per hectare, an expression believed more useful for management purposes. Two-way analysis of variance with interaction involving the geologic and climatic soil-forming factors and one-way analysis of variance to assess the geologic factor within climatic zones were undertaken. The integrity of the subdivision of the Vancouver Island podzols on the basis of till lithology and biogeoclimatic subzone was confirmed since the analyses of variance and Duncan's multiple range tests indicate that no natural grouping of the soils is possible. However, grouping of the soils, on the basis of a few properties, for special purposes, such as fertilization, is feasible. The double composite sampling scheme allowed quantitative estimation of soil variability. The optimum number of samples required to attain specified allowable errors is presented. To attain an allowable error of ten percent requires between three and 62 samples, depending on the soil property of concern. When 16 samples are taken per plot, as in this study, the error is within ten percent for pH in CaCl₂total carbon, total nitrogen, total sulfur and available phosphorus; and within 20 percent for the exchangeable cations and available copper, zinc, iron and manganese. Comparison of values determined from one modal pit with values from composite sampling in replicated plots indicates that modal pit means commonly fall outside of the interval defined by the plots mean ±1 standard deviation. This casts considerable doubt on the adequacy of modal pit sampling for the purposes of generating the estimates of nutrient stocks required for management purposes.

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