Soil pH and Solubility of Aluminum, Iron, and Phosphorus in Ultisols: the Roles of Humic Acid

Soil reaction (pH), aluminum (Al), iron (Fe) and phosphorus (P) are the soil properties that are related to each other. Their role on the growth and development of plant is very significant. Liming and organic mater are the soil amendments that are frequently used to increase soil pH and P solubility and to suppress the solubility of Al and Fe in the soil. Humic acid is one of the organic fractions which is presumed has a role and is closely related to the changes in the soil chemical properties as mentioned above. Information about the role of humic acid on the soil pH, the solubility of Al, Fe, and P, especially in upland acidic Ultisols is still limited. This study aimed to provide empirical data on the role of various humic acids to soil pH and the solubility of Al, Fe, and P, specifically in upland acidic Ultisols. The study was a laboratory experiment with a single factor which was set by a completely randomized block design and was conducted in two sets of experiments. The first experiment was intended to study the roles of various humic acids derived from several sources (commercial humic acid, humic acids extracted from composted chicken manure, humic acids extracted from composted cow manure, and humic acids extracted from composted goat manure) on the soil pH and solubility of Al, Fe, and P. The second experiment was aimed to study therelationship patterns between application of humic acid (sold commercially) on soil pH and solubility of Al, Fe, and P. The results showed that humic acid was able to increase the soil pH and solubility of phosphorus, while it suppressed the solubility of iron and aluminum with linear patterns of relationships. Humic acid that was sold commercially, at the same amount of C-organics, had greater in increasing the soil pH, solubility of phosphorus, and suppressing the solubility of iron and aluminum in the soil, roles than those derived from organic matter of compost extracts and from organic matter of compost.