Adsorption and availability of phosphorus in response to humic acid rates in soils limed with CaCO₃ or MgCO₃

Adsorção e disponibilidade de fósforo em resposta a doses de ácido húmico em solos corrigidos por CaCO₃ ou MgCO₃

ABSTRACT

Humic acid (HA) may reduce adsorption and increase soil P availability, however, the magnitude of this effect is different when Ca²⁺ prevails over Mg²⁺ in limed soils. The objective of this study was to evaluate the effects of HA rates and carbonate sources on the adsorption, phosphate maximum buffering capacity (PMBC), and P availability in two contrasting soils. Oxisol and Entisol samples were firstly incubated with the following HA rates: 0, 20, 50, 100, 200 and 400 mg kg^-1, combined with CaCO₃ or MgCO₃, to evaluate P adsorption. In sequence, soil samples were newly incubated with P (400 mg kg^-1) to evaluate P availability. The least P adsorption was found when 296 mg kg^-1 of HA was added to Oxisol. Applying HA rates decreased maximum adsorption capacity, increased P binding energy to soil colloids and did not alter PMBC of Entisol. Available P contents in Oxisol increased with HA rates, but it did not change in Entisol. Choosing the right HA rate can decrease PMBC up to 40% and increase the Oxisol P availability by 17%. Application of MgCO₃ instead of CaCO₃ decreased P adsorption in both soils. Thus, a positive correlation between Ca²⁺ content and PMBC was verified. Optimum rate of HA and the preponderance of Mg²⁺ instead of Ca²⁺ in soil volume fertilized with P are effective practices to reduce adsorption and increase P availability, especially in clayey Oxisol.

Index terms:
Organic ligands; liming; humic acid-metal-P complexes; Langmuir isotherm; phosphate maximum buffering capacity.