Nitrogen Release Patterns of Some Locally Made Composts and Their Effects on the Growth of Chinese Cabbage (Brassica Rapa, Chinensis Group) When Used as Soil Amendments

ABSTRACT.

Due to strong demands from organic farmers, landscapers, and home-gardeners, production and utilization of composts have rapidly increased. So, there is the need for their characterization (compost quality evaluations, particularly their nutrient releasing characteristics). In this study, three vermi-composts, six thermo-composts, and a green manure were incubated at 10 g/kg with a Mollisol and an Oxisol at room temperature. Unamended soils served as controls. Nitrate (NO₃-N) was extracted and measured at 3, 7, 14, 21, 28, 42, and 56 days during the incubation. The results showed that the potentially mineralizable N (referred to as N₀) varied widely among the composts, but the mineralization rate (k) remained nearly constant, ranging from 0.02 to 0.03/day, suggesting that approximately 2 to 3% of N₀ was transformed into NO₃-N per day. Cumulative NO₃-N release was linearly related to the square root of the total N in the composts. Relative absorbance at 260 nm of a 0.01M NaHCO₃ extract failed to predict N₀, but could clearly separate vermi-composts (low absorbance readings) from thermo-composts (high absorbance). A subsequent greenhouse experiment using pak choi (Brassica rapa, Chinensis group, cv. Bonzai) as the test plant grown on the same two soils amended with 10 g/kg composts or green manure demonstrated the beneficial effects of the organic inputs when compared with the control (soil alone) or a complete mineral fertilizer treatment (providing 80 mg N/kg as 16-16-16 commercial fertilizer with 0.5% micronutrients). Organic amendments provided more benefits to cabbage growth in the infertile yet better drained Oxisol than in the fertile but poorly drained Mollisol.

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Funding

The authors would like to thank the Western SARE (Sustainable Agriculture Research & Education) for funding this project.