Clay mineralogy and potassium status of selected soils in the glacial Lake Agassiz region of central Manitoba

The clay mineralogy of the soil is important in that it can affect potassium (K) availability, particularly if vermiculite is present. Large areas of Glacial Lake Agassiz in Manitoba contain fine-grained lacustrine sediments that support boreal forest vegetation. The purpose of this investigation was to identify the clay mineralogy of several commonly occurring soils and document its influence on soil fertility, particularly K availability. The soils were classified as either Eluviated Eutric Brunisols or Orthic Eutric Brunisols. X-ray diffraction analysis verified that vermiculite, hydrous mica and kaolinite were the principal clay minerals in these soils. Vermiculite was demonstrated to have hydroxy Al-interlayers and the amount of vermiculite decreased in the deeper soil horizons. Smectite was present in the Btj horizons of the Eluviated Eutric Brunisols, while trace amounts of chlorite were present in every inorganic horizon. Quantity/intensity curves for K and an experimental estimate of the K-fixation potential indicated that the Ae horizons of several Eluviated Eutric Brunisols supported higher K activities in the soil solution and that K-fixation was greater in subsurface horizons. Potassium fixation and the quantity/intensity curves for selected horizons of the Orthic Eutric Brunisols were nearly identical and each showed a reduced intensity to maintain the aqueous activity of K.