Co-compost biochar as a soil ameliorant: improvement of soil chemical characteristics and maize yield in Ultisol

Adding co-compost biochar could increase the effectiveness of ameliorant compared to biochar and manure. The research aim was to measure the effectiveness of co-compost biochar to improve soil chemical properties and maize yields in Ultisol. The field study at Taman Bogo Research Station was conducted for two planting seasons (December 2017–July 2018) with treatments: 1) control, 2) cacao shell (CS) biochar (BC), 3) corncob (CC) BC, 4) co-compost CS BC, 5) co-compost CC BC and 6) manure, with four replicates and a 10 t ha−1 dose of ameliorant. The results showed that co-compost biochar CS and CC for two planting seasons (PS) increased pH, total N, K, Ca, and decreased Al. However, the application of BC CS resulted in a more significant improvement in the soil’s chemical characteristics. The dry grain from the biochar and co-compost BC application increased significantly (7.41 t ha−1 in PS1 and 6.73 t ha−1 in PS2) compared to the control (4.46 t ha−1 PS1 and 3.82 t ha−1 PS2), but BC CS and co-compost CS gave more stable results. The co-compost biochar formulation was more effective in improving acidic soil and maize productivity when biochar was produced from the low-quality feedstock.