Influence of Various Composted Organic Amendments and Their Rates of Application on Nitrogen Mineralization and Soil Productivity Using Chinese Cabbage (Brassica rapa. L. var. Chinensis) as an Indicator Crop

There is a diversity of locally available nitrogen (N)-rich organic materials in Samoa. However, none of them was evaluated for their N supplying capacity after composting in Samoan Inceptisols for vegetable cultivation. Thus, N-releasing capacity of five composted organic amendments (OAs) namely macuna, gliricidia, erythrina, lawn grass and giant taro, and their two application rates (10 and 20 t ha(-1)) were assessed through a laboratory incubation and a crop response study using Chinese cabbage as a test crop. Among the OAs, composted mucuna was characterized by a higher total N (2.91%), organic C (63.6%) and NO3-N content (341 mg N kg(-1)). A significant difference in N mineralization was observed among the OAs as well as application rates. The highest N mineralization was recorded in composted mucuna followed by gliricidia, erythrina, lawn grass, and giant taro. A crop response study also showed a similar trend. Mucuna treatment had the highest biomass yield and N uptake followed by gliricidia, erythrina, lawn grass, and giant taro. Leguminous composted OAs @ 20 t ha(-1) performed substantially better in all the plant growth and yield parameters, and N uptake compared to 10 t ha(-1) that was not the case for non-leguminous OAs. Thus, non-leguminous OAs should be applied at 10 t ha(-1). All the composted leguminous OAs showed promising results while mucuna was the best in both the application rates. Therefore, mucuna can be promoted to supply N for crop cultivation in Samoa, other Pacific Islands and tropical countries where N fertilizer is costly and not easily available.