| Summary: | Studies were conducted to determine the appropriate level of NPK 15:15:15 fertilizer application that will produce the most profitable yield in a humid forest ultisol. This study involved the use of Dioscorea rotundata Poir fertilized with NPK (15:15:15) at rates of 0 (0 kg N + 0 kg P + 0 kg K), 100 (15 kg + 6.79 kg P + 12.45 kg K), 200 (30 kg N + 13.58 kg P + 24.90 kg K), 300 (45 kg N + 20.37 kg P + 37.35 kg K) and 400 kg ha-1 (60 kg N + 27.16 kg P + 49.80kg K) using a randomized complete block design. Results from the trial showed that leaf area index values ranged from 1.24 to 2.88 at 16 weeks after planting (WAP) and 2.20 to 3.10 at 24 WAP respectively for the unfertilized and 400 kg NPK ha-1. The corresponding values for dry matter accumulation were 1.29 to 3.70 t ha-1 and 2.97 to 5.67 t ha-1 at 16 and 24 WAP respectively. These parameters were responsible for higher tuber yield and relative yields for plants fertilized. The greatest yield (19.16 tonnes hectare) was produced from plots treated with 300kg ha‑1 fertilizer. The production cost increased from N 390440.00 - N 417920.00, revenue from N 1036000 - N 1155000.00, gross margin from N 680000.00 - N 773200.00, net income from N 645560.00 - N 732080.00 and benefit-cost ratio from 1.65 - 3.73 as fertilizer application rate increased from zero to 400 t/ha. The optimum fertilizer application rate was 200kg ha-1 based on the fact that it had the greatest gross margin (N 1547600.00), net income (N 1507720.00) and benefit-cost ratio (3.73).
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