Resistance of Humus to Decomposition

Abstract

AN organic soil (previously under wattle followed by grass) containing 8.51 per cent organic carbon (Walkley and Black¹, corrected) has been submitted 204 times to the following cycle : (a) moistened to field capacity ; (b) allowed to decompose for 5 days at 25° C. in the macro-respirometer²; (c) oven-dried at 100° C. for 48 hr. After 105 such treatments it was calculated³ that no further decomposition would occur, on moistening, after about the 250th treatment and that 73 per cent of the organic carbon originally present would have been mineralized. After 204 such treatments, however, the soil had become completely structureless and on moistening formed cohesive lumps, a condition adverse to aerobic decomposition. The experiment was therefore concluded and the soil analysed. A comparison of the results for the soil before and after the 204 treatments is given in Table 1.