Use of Biochar and Compost Improves the Productivity of Nutrient-poor Tropical Soils

Amendments are usually needed to support crop growth in highly weathered soils. Two biochars (0, and 2%) and composts (0, and 2%), were applied to two tropical soils of Hawaii (Leilehua series, Ultisol and Wahiawa series, Oxisol). Chinese cabbage (Brassica rapa cv. Bonsai) was used as the test plant in two greenhouse trials. The experimental design was a factorial in a completely randomized design with three replicates. Plant fresh and dry weights, soil pH, EC, total N, and other nutrients in soil and plant tissue were measured. The results showed that the interaction between biochar and compost additions significantly affected the pH in both soils. Cabbage growth was almost double in the acid Ultisol when amended with 2% lac tree (Schleichera oleosa) biochar and 2% vermicompost compared with either the lime or the vermicompost treatment alone. Soil pH was increased by one unit on average, and EC (in 1:10 soil-to-water ratio) was increased from 0.35 to 0.47 dS/m and 0.30 to 0.37 dS/m in the Ultisol and Oxisol, respectively. Exchangeable aluminum was decreased from 2.6 cmolc/kg to virtually zero in the high-Al Ultiso. Mehlich-3 extractable Mn and Fe in the high-Mn Oxisol decreased from 806 and 64 to 360 and 37 mg/kg, respectively. The enhanced cabbage growth in these nutrient-poor soils was likely due to the improvement of nutrient retention/availability, increasing soil pH, and decreasing Al/Mn toxicity by the additions of biochar and compost.