CATION AND NITROGEN CONTENTS OF ORGANIC-MATTER DETERMINE ITS SOIL LIMING POTENTIAL

The effect of organic matter addition to soil in relation to soil pH c hanges is not well understood. Organic matter additions to soil have b een reported to both increase and decrease soil pH. An investigation o f the effects and mechanisms involved was undertaken. Plant materials of different types were incubated with an acid Cecil topsoil (clayey, kaolinitic, thermic Typic Kanhapludult) (pH(KCl) 4.01) in a glasshouse experiment at rates equivalent to 50 and 200 Mg ha(-1) (dry mass). So il pH was measured with time and increases of up to 4 pH units were ob served followed by attainment of a steady state at or above the origin al soil pH, depending on the type of organic matter added. The final p H was well correlated with the amount of basic cations present in the organic matter (r(2)=0.92). The initial short-term fluctuations in pH were due to mineralization and nitrification of the N added in the mat erial. A separate factorial study using simple organic sources of Ca ( calcium gluconate) and N (beta-alanine) supported this contention. The short-term organic Liming effect stemming from N transformations is s trongly dependent on the kinetics of the organic matter decomposition, which are, in turn, biologically mediated. Decomposition of Ca-contai ning organic molecules was shown to have an effect on pH analogous to that of mineral lime (CaCO3).