Humus Decomposition in East African Soils

Abstract

THE macro-respirometer described by Swaby and Passey¹ has been considerably modified while still retaining its chief advantages. The modified apparatus (Fig. 1) is relatively cheap, easy to make and sensitively reflects the magnitude of organic matter decomposition over any desired period. The respirometer vessel is an 8-oz. sample bottle in which the soil (usually 50 gm.) is placed. A small container with about 10 ml. of 2 M sodium hydroxide absorbs the evolved carbon dioxide. The unit is immersed in a thermostatically controlled bath 24 in. × 12 in. × 15 in. deep which holds fourteen such units. The hydrogen evolved at the cathode is collected in a 100-ml. burette. The electrodes consist of platinum strips, and the anode was easily prepared by inserting the platinum through a small slit in a 2-in. length of polythene tubing (Fig. 2) about 5 mm. in diameter. This was then joined to the end of the glass tubing (of slightly smaller diameter) leading from the respirometer, using Chatterton's compound. The platinum outside the polythene tubing was bound to the side with fine wire, leaving some for the terminal, and also sealed with Chatterton's compound. The electrodes were joined to a 6- or 12-V. accumulator.