Screening Hawaii's Locally Produced Composts for Their Nitrogen Release and Plant Growth Response under Different Soils

Variability in compost stability and maturity limits utilization of these potentially beneficial plant growth promoting local amendments by Hawaii growers. Lab incubation and greenhouse experiments were conducted to screen locally produced composts for their nitrogen release and plant growth response in different soils. Eleven different composts were selected to be studied at different application rates (0, 5, and 10 t/ha for the lab incubation, and 0, 10, and 30 t/ha for the greenhouse trial) in two soils (Mollisol and Oxisol). Both experiments had Complete Randomized Design (CRD) with three replicates. Lab incubation was carried out for two months. Soil and composts were mixed thoroughly and moisture was maintained at field capacity through out the experiment period. Sub-samples were taken at 0, 1, 2, 4, 6, and 8 weeks. Samples were analyzed for pH, Electrical Conductivity (EC) and Nitrate (NO₃-N) content. The greenhouse trial was conducted using Chinese Cabbage (Brassica rapa var. chinensis). Seedlings were transplanted after two weeks of seeding, and plants were harvested a month after transplanting. Plant height, fresh and dry weights were recorded. Mineralization varied (50% to 70% of total N). Differences in C:N ratio explain some but not all. NO₃-N and EC showed a positive significant correlation (R = 0.78). Cabbage dry weights were significantly different under the application of different composts and rates of application.