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Potassium estimation fixation and release in Lower Fraser Valley soils Fletcher, Harry Francis

Abstract

A study of the potassium status of some Lower Fraser Valley soils was undertaken in order to supply data which would aid in the interpretation of fertility experiments and chemical tests for available soil potassium. Since the rapid chemical tests for available potassium now used in the Lower Fraser Valley leave much to be desired, several promising methods were studied and compared to a standard ammonium acetate extraction gravimetric analysis method for determining exchangeable potassium. The Morgan and Spurway rapid methods for available potassium estimation were not found precise or accurate enough for the purposes of the study. A technique described by Peech and English, which involves extraction of available potassium with normal sodium acetate of pH4.8 and subsequent precipitation of potassium as the di-potassium sodium cobaltinitrite in formaldehyde-isopropyl alcohol solution was tested. A modification of this method was adapted to estimation by the photometer and results for exchangeable potassium, although lower, correlated well with those of the standard determination. A technique which is more rapid and better adapted to studies in which potassium is the only cation to be tested is that described by Bray, who recommends extraction with sodium nitrate and precipitation of potassium with sodium cobaltinitrite in ethyl alcohol solution and subsequent photometric determination. This method extracted slightly more potassium than that of Peech but also showed a high correlation with the standard method used. Both and Bray and Peech procedures were found adaptable to line chart estimation for routine soil testing work. The Bray method was compared to the standard ammonium acetate-gravimetric procedure in potassium fixation and release studies in limed and unlimed soils. Two upland soils and three recent alluvial soils, on which fertility experiments with oats had been conducted, were studied. A greenhouse experiment using lettuce was designed to indicate any existing relation between chemical studies and plant response. In the laboratory fixation was induced by alternately wetting and drying the soils at 110⁰ C. All soils were found to have high fixing potentials when measured by both extraction methods, although the fixation was higher when determined by the standard method. The effect of lime on fixation in both the laboratory and greenhouse soils was determined. The soils generally showed an apparent release of potassium when measured by the ammonium acetate-gravimetric method and a fixation by the sodium nitrate technique. These differences were considered to be due to the relative replacing powers of Na+ and NH+ when Ca++ or H+ dominate the exchange complex. Thus the extraction of potassium by Na+ may be inhibited when calcium is the complementary ion on the colloid. The potassium status of the upland soils was less affected by lime than the lowland soils when fixation or release due to lime was measured by either method. The soils studied failed to release any potassium from the fixed form when leached of their exchangeable potassium and stored for three months in a moist condition, or when wetted and dried at 110 C. No relation was found to exist between exchangeable potassium or the relative potassium fixing and supplying powers of the soils, and oat response to potassium in the field or lettuce growth in the greenhouse.

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