Effects of Residual Maize Root Biomass and Diammonium Phosphate (DAP) Fertilizer on Maize Yield, Soil Properties, and Economic Returns across Successive Cropping Seasons

Maize production in smallholder farming systems is increasingly constrained by declining soil fertility, reduced crop productivity, and the high cost of inorganic fertilizers. Recycling crop residues and utilizing residual fertilizer nutrients may provide a sustainable approach to improving nutrient-use efficiency and maintaining crop productivity. This study evaluated the effects of maize root biomass and diammonium phosphate (DAP), applied individually and in combination, on maize growth, yield, selected soil properties, and economic performance over two successive cropping seasons. The experiment was conducted using a randomized complete block design, and data were analysed using R statistical software (version 4.5). Agronomic growth and yield parameters were assessed alongside soil pH, nitrogen (N), phosphorus (P), potassium (K), electrical conductivity (EC), and organic matter content. In the first season, root biomass recorded the highest biomass production (13,192 kg/ha) and fresh cob yield (4,822 kg/ha), whereas root biomass + DAP produced the highest grain yield (2,515 kg/ha), closely followed by DAP (2,511 kg/ha). In the second season, DAP recorded the highest biomass production (4,818 kg/ha) and fresh cob yield (2,544 kg/ha), while root biomass + DAP again recorded the highest grain yield (1,967 kg/ha). Seasonal analyses showed that treatment effects on grain yield were not statistically significant in either season (p=0.961 and p=0.686), although the number of leaves in the first season approached significance (p=0.052). Across the combined analysis, grain yield was significantly affected by treatment (F=4.24, p=0.006), but not by season (p=0.127) or the season × treatment interaction (p=0.192). Dunnett’s comparisons showed that root biomass + DAP significantly increased grain yield relative to the control by approximately 211 kg/ha (p=0.003; 95% CI: 61.988–360.230 kg/ha), whereas DAP (p=0.192) and root biomass alone (p=0.822) did not produce significant increases. Biomass was significantly higher than the control under DAP, root biomass, and root biomass + DAP (all p<0.001). Soil analysis indicated acidic conditions across all treatments, with pH ranging from 4.84 under DAP to 5.37 under root biomass. Root biomass recorded the highest organic matter content (4.17%), while DAP had the lowest (2.53%). Total nitrogen was highest in the control (0.091%), DAP recorded the highest phosphorus concentration (29.0 mg/kg), and root biomass + DAP had the highest potassium concentration (172 mg/kg). Economically, root biomass alone recorded the highest gross margin at UGX 439,600/ha, whereas DAP produced the highest numerical grain yield but a negative gross margin, although the reported marginal rate of return favoured DAP (2.48). Overall, combining root biomass with DAP showed promise for increasing maize grain yield, while root biomass alone offered the most favourable gross margin under the conditions studied. Nevertheless, the observed seasonal responses and soil differences require further validation over several seasons. Additional trials across multiple locations and seasons are recommended to establish the consistency, agronomic benefits, and economic viability of integrating root biomass with inorganic fertilizer for sustainable maize production.