Nitrogen (N) plays a key role in agriculture systems by increasing crop yield. An understanding of the rate of N mineralization in the soil for Morinda citrifolia production will help to achieve a cost effective N fertilizer application. Hence, an incubation study was conducted to investigate the effects of organic amendments on soil N release.

The experiment was a 2 x 8 factorial experiment: top soil and river sand amended with 160 kg N ha-1 biochar, biochar + Poultry manure (PM), PM, N:P:K 15:15:15, biochar + N:P:K, PM + N:P:K, biochar + PM + N:P:K and control. Data collected were subjected to Analysis of Variance (ANOVA) using Genstat statistical software package. Significant means were separated using Duncan’s Multiple Range Test at p<0.05. Results indicated that top soil had a significantly higher level of NH4 + N mineral release than river sand throughout the 98 days of incubation period. Soil with N: P: K 15:15:15 (10.02 mg/kg) at 14 days after incubation (DAI) had the highest.

However, at 98 DAI, soil amended with PM (1.45 mg/kg of NH4+N) was significantly higher (p<0.05) compared to all other amendments. Top soil for raising seedlings increased the release of NH4+ - N, while River sand increased the release of NO3--N. It was recommended that poultry manure could be used for soil amendment and Noni in top soil will sustain the release of N+4–N and NO2–N mineral to the plant over a longer period of time.