Fertilizer Management and Bean Density Drive Productivity–profitability Trade-offs in Maize–common Bean Intercropping on a Tropical Andosol in the Western Highlands of Cameroon

The intensification of maize (Zea mays L.)–common bean (Phaseolus vulgaris L.) intercropping requires balancing nutrient supply, interspecific competition and returns to fertilizer investment. The individual and interactive effects of NPK fertilizer regime, supplemental urea, and bean density on system productivity and economic performance on a tropical Andosol of the Cameroon Western Highlands were evaluated. Two independent field experiments were conducted on separate fields (C1: April–July; C2: July–October) using a split-split-plot design with three replicates. Five NPK regimes (0; 300 and 600 kg ha−1 of 20-10-10 + 6SO3; and 300 and 600 kg ha−1 of 14-23-14 + 5S + 1B2O3) were combined with 0 or 100 kg ha−1 urea and three cropping systems: sole maize at 55,555 plants ha−1 (S1), and maize at the same density intercropped with bean at 22,222 (S2; two plants hill−1) or 33,333 plants ha−1 (S3; three plants hill−1), while maintaining the same spatial arrangement. Mean combined physical grain yield was 7.384 t ha−1 in C1 and 6.746 t ha−1 in C2; these differences were considered descriptive because the experiments were conducted on separate fields. The NPK × urea × cropping system interaction significantly affected combined physical grain yield in both C1 (P = 0.037) and C2 (P = 0.028). NPK14_600 × U100 × S2 produced the highest yields, reaching 9.476 and 8.295 t ha−1, respectively. Increasing bean density by 50% from S2 to S3 reduced bean yield by 8.5% in C1 and 14.0% in C2 and further reduced maize yield. The productivity optimum, however, did not coincide with the economic optimum. The aggregated economic assessment identified NPK14_300 × U0 × S2 as having the highest benefit-to-cost ratio (BCR = 2.59; fertilizer economic return = 158.85%), compared with a BCR of 1.33 for the highest-yielding strategy, NPK14_600 × U100 × S2. Thus, among the configurations tested, S2 was common to the strategies achieving the highest physical productivity and economic return, whereas fertilizer intensity determined the trade-off between these objectives.