Yields varied with soil type and responded significantly to all manuring methods. Response to composting was less on soils of higher inherent soil fertility, and fertilizer response increased as the soil levels of the supplied nutrient decreased. Responses to phosphate and potash fertilizer application were predominantly asymptotic; maximum yield increase on any soil was by a factor of 3 to ca. 20 t fresh tuber yield/ha. In organic manuring experiments, yield responses to compost application were predominantly linear; maximum response was by a factor of 4 to a yield of ca. 17 t/ha. Improving sweet potato productivity by increasing rates of composting may have limited potential if it leads to an unacceptable increase in the burden of work for women. Despite large yield increases from phosphate and potash application, only potash application was economic.

The efficiency of nutrient use for P and K applied in organic manuring was greater than in inorganic manuring, but this advantage decreased as the influence of resorted or limestone parent material in soil formation increased. Composting may be particularly suitable for use on volcanic ash soils because it supplies nutrients directly to the plant, thereby overcoming P fixation and poor base nutrition on these soils. Phosphate nutrition was the dominant factor affecting fertility on these soils and regression analysis showed that this nutrition was significantly affected by indigenous mycorrhizal infection; increased infection was associated with increased yield and reduced response to phosphate fertilizer application.