Biochar as an Amendment to Acid Soils

Abstract

The capacity of biochar to improve acid soil productivity and enhance nutrient retention was the main focus of this study. The specific objectives were to characterize six wood-derived biochars, to assess biochars’ liming effects on Hawaiian and Indonesian acid soils, and to study nutrient retention of biochars. Six and another two biochars were collected, characterized, and then were used to evaluate their liming effect on a Hawaiian and two Indonesian acid soils with Desmodium intortum and soybean (Glycine max) as test plants, respectively. Two biochars in combination with two composts (both at 2%) as nutrient sources were used to investigate their nutrient retention with pak choi (Brassica rapa) as the test plant. The results showed that six wood-derived biochars were different in their properties, including ash content, pH, cation exchange capacity (CEC), CaCO3 equivalent, basic cations and surface functional groups. Based on their CaCO3 equivalent, leucaena (Leucaena leucocephala) and lac tree (Schleichera oleosa), Hilo mixed wood and she oak (Casuarina junghuhniana), and mahogany (Sweitenia mahagoni) and mountain gum (Eucalyptus urophylla), were grouped into the highest, moderate, and lowest liming potential biochars, respectively. Additions of six biochars at 2% and 4% with or without 2 cmolc/kg of lime to a Hawaiian acid soil increased soil pH and CEC, reduced exchangeable Al, enriched plant nutrients and enhanced Desmodium growth with lac tree and leucaena being most effective, followed by she oak and Hilo mixed wood biochars. Similar results were obtained from lac tree wood and rice husk biochars (4 and 8%) applied to two Indonesian acid soils. Addition of lac tree wood and Hilo mixed wood biochars in combination with vermicompost or thermocompost to a Ultisol and a Oxisol of Hawaii showed a positive interaction effect on EC, P and K, cabbage fresh and dry matters. Biochars increased soil pH, plant tissue Ca, retention of K, Ca and Mg, and reduced exchangeable Al in both soils. Overall, the liming capacity and nutrient retention potential of selected biochars have been positive.