Biochar and Humic Acid Effects on Soil Properties and Soybean Growth in Coastal Sandy Soil

Abstract

Coastal sandy soil remains constrained by its low capacity to retain water and essential nutrients, particularly nitrogen, which is highly susceptible to leaching. This study aimed to evaluate the effects of rice husk biochar and humic acid application on soil chemical properties and their relationship with soybean growth. The experiment was conducted at the Research Farm of the Faculty of Agriculture, Universitas Pembangunan Nasional "Veteran" Yogyakarta, using a Completely Randomized Design (CRD) with a two-factor factorial arrangement and three replications, resulting in 48 experimental units. The first factor was rice husk biochar (B) at four dosage levels, and the second factor was humic acid (H) at four dosage levels. The results showed that rice husk biochar and humic acid application improved soil organic C, cation exchange capacity (CEC), and total nitrogen, which was followed by better soybean growth. The highest values were observed under the B3H3 treatment (20 ton ha^-1 biochar and 3,15 L ha^-1 humic acid). This combination increased soil organic C from 0.51% to 1.11%, CEC from 1.1 cmol(+) kg⁻¹ to 1.72 cmol(+) kg⁻¹, and total nitrogen from 0.21% to 0.44%. The improved soil chemical fertility was accompanied by better soybean performance, with a plant height of 88.72 cm, 27 leaves, and 23.67 pods per plant. Overall, applying rice husk biochar and humic acid improved soil chemical properties, particularly organic C, CEC, and total nitrogen, thereby supporting soybean growth.