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 changes is not well understood. Organic matter additions to soil have been reported to both increase and decrease soil pH. An investigation of 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). Soil pH was measured with time and increases of up to 4 pH units were observed followed by attainment of a steady state at or above the original soil pH, depending on the type of organic matter added. The final pH 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 material. 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 strongly dependent on the kinetics of the organic matter decomposition, which are, in turn, biologically mediated. Decomposition of Ca-containing organic molecules was shown to have an effect on pH analogous to that of mineral lime (CaCO3).