Quantitative assessment of the spatial variability of morphological and physical properties of some alluvial soils at Lincoln College, Canterbury : A thesis submitted in partial fulfilment of the requirements for the degree of Master of Applied Science in the University of Canterbury [Lincoln College]

Abstract

The complex soil distribution across part of a Lincoln College Farm relates to the previous history and pattern of alluvial deposition. Depth to mottles (DM), depth to gravels (DG), and thickness of loamy sand and/or coarser-textured layers (TS) are used to classify the soils and delineate the area into Eyre, Templeton and Wakanui soil-series simple mapping units. Geostatistical analyses of the grid data reveal that values of each morphological parameter are spatially dependent, though to different extents. Most variation of DM, for instance occurs between 30 m and 430 m, whereas a large amount of the variation in TS is present within less than 30 m. The morphological parameters also vary anisotropically, with the direction of maximum variation for DM and DG being NE-SW across a major abandoned channel hollow. Similar patterns are reflected in the soil maps of the study area and of adjacent larger regions, where mapping units are elongated in a NW-SE direction. Geostatistical methods are more efficient than conventional in determining optimal sampling strategies for future soil survey and variability studies: less samples are needed to achieve the same level of precision. The morphologically based soil classification system is generally effective in separating soils into (series) taxonomic units in terms of soil physical properties. Examined hydraulic properties [e.g. "field -saturated" hydraulic conductivity (Kfs) differ between typical profiles of each taxonomic unit : these observations are statistically substantiated by data from taxonomically-pure "window areas" of the three soil series. The differences are mainly attributable to spatial changes in soil texture and pore-size distributions. Different amounts of variation in physical properties, however, are still present within each taxonomic unit. The variation in physical properties amongst the combined window areas is reduced, though, to differing extents by the classification and delineation into separate taxonomic units. More than half of the variance in moisture content at both topsoil and subsoil depths amongst Templeton and Wakanui taxonomic units for instance is accounted for by the classification. and is thus clue to differences between the two soils. Little contribution however is made by the classification in reducing the heterogeneity of Kfs, in topsoils. The classification is particularly effective in separating Wakanui from Templeton taxonomic units in terms of subsoil Kfs an important property controlling water movement storage and related soil-forming processes.