Poor crop nutrition contributes to the low yields of root crops in the South Pacific region where symptoms of nutritional disorders are frequently observed. This study was therefore undertaken to assess the effect of inorganic fertilizers on sweetpotato [lpomoea batatas (L)] and taro [Colocasia esculenta (L.) Schott] yields. It indicates the importance of the correct diagnosis of the nutritional problems at each site for improved nutrition management and describes procedures used to identify and correct nutritional limitations to growth. Nutrient deficiencies were detected using nutrient omission pot trials and the amounts of fertilizer nutrients required to correct deficiencies were determined in a series of pot and field trials. Placement of P fertilizers and number and timing of N applications were found to increase fertilizer efficiencies. Critical leaf concentrations of N and P were determined. Leaf concentrations sampled during early vegetative growth were found to be good indicators of the effect of N or P deficiency on yield. Economic analysis of fertilizer applications demonstrated a high level of profitability.