Large quantities of organic matter are traditionally used in sweet potato mulch mounds on volcanic ash soils in the highlands of Papua New Guinea. Two trials investigated compost x fertilizer interactions using a 24 factorial combination of compost (C), urea (N), triple superphosphate (P) and potassium chloride (K). At Taluma (altitude 2570 m) there was a highly significant (P < 0.01) positive C x K interaction in addition to positive effects of C, N and Pon tuber yield. At Kandep (2350 m) there was a significant (P = 0.05) negative C × N × K interaction, in addition to positive effects of C and P and a negative effect of N on yield. The occurrence of consistent compost × fertilizer interactions, whether positive or negative, would have implications for the development of subsistence agriculture in PNG.