INCREASES IN THE CARBON AND NITROGEN CONTENTS OF TROPICAL SOILS UNDER NATURAL FALLOWS

Summary The rate of increase of humus carbon in soils rested under natural fallows has been calculated, assuming that the rate of decomposition of the humus carbon is proportional to the amount present, and that the rate of addition, by humification of litter and roots, remains constant. The rate of decomposition has been derived from a knowledge of the maximum humus level the soil attains under the fallow vegetation, and the rate of addition has been estimated from data on litter production. The rate of increase of nitrogen has also been calculated assuming that the C/N ratio in the soil remains constant. Under the native practice of shifting cultivation in forest the alternation of cropping and fallowing leads to relatively small fluctuations in soil humus at about 75 per cent. of the maximum level. The rates of increase of soil nitrogen at this level are between 20 and 50 lb. per acre per annum in forest and 4 and 10 lb. per acre per annum in savannah. Rates at other levels for which the assumptions made are valid may readily be calculated from the data presented. It is shown that provided vegetation re‐establishes itself rapidly a crop: fallow ratio of about 1:3 should maintain the humus level in forest soils at the 75 per cent. of equilibrium level, a result which accords well with experience.