The distribution and extraction of soil nitrogen as a function of soil particle size

Abstract

Two surface soils, a red-brown earth and a lateritic podzolic, have been fractionated with respect to soil particle size and the distribution of organic matter and nitrogen through these fractions determined. Each fraction was extracted sequentially with 0.1 m sodium pyrophosphate and twice with 0.5 n sodium hydroxide (at 20°C and 60°C). The distribution of nitrogen through the residues and extracts was then determined. Most of the organic matter was present either as recognisable organic particles in various stages of humification or as components intimately associated with clay minerals. The nitrogen content of the organic matter was found to increase as the particle size decreased, the organic matter associated with clay having the highest nitrogen contents. The humic acid extracts did not reflect this variability and gave relatively constant values for nitrogen content. The unextracted organic matter had a low nitrogen content showing that the nitrogenous components were preferentially extracted. Recovery of nitrogen was incomplete due to partial volatilization during alkaline extractions.