Soil fertility under shifting and semi-continuous cultivation in the Southern Highlands of Papua New Guinea

People in the Southern Highlands of Papua New Guinea cultivate some land se mi-continuously within a regime of shifting cultivation; the staple crop is sweet potato (Ipomoea batatas). The fertility of Tropepts, variably affect ed by falls of volcanic ash, was investigated to give further understanding of how subsistence farmers avoid the soil constraints that commonly prompt abandonment. While organic matter, N and K all decrease significantly with time under cultivation, they do not reach critical levels. Phosphorus also decreases significantly ol;er time, although contents are small throughout . Other nutrients show no significant variation with period of use or aband onment. These findings comply with the diversity of crops cultivated early in the life of 'gardens': followed by sweet potato, cultivated as a virtual monocrop in long established 'gardens'. If continues yielding adequately r egardless of decrease in nutrient availabilities, notably because nutrient ratios remain favourable for tuberisation and because of sweet potato's tol erance of small phosphorus concentrations. It appears that burning of veget ation significantly increases available minerals and helps maintain a suppl y which is adequate for longterm sweet potato monoculture. Similar situatio ns are postulated for other areas of less-weathered soils within the tropic s.