Effect of pH and concentration on the complexation of calcium ions with humic acid extracted from composted oil palm bunches

The reaction (complexation) between calcium ions at certain concentrations (1.000, 0.500, 0.250 and 0.125 M) and pH (2, 4, 6, and 8) and humic acid obtained from composted oil palm ( Elaeis guineensis Jacq) bunches were investigated in this study. The results obtained showed that at maximum total calcium concentrated (1.000 M) few or no Ca2+ was complexed to the humic acid at pH 2 and 8 respectively, while some degree of complexation was exhibited at pH 4 and 6. Similarly, at 50% total calcium level, more Ca2+ reacted with the humic acid at pH 4 and 6 respectively. Interestingly, all the points on the plotted graph of Log(10) [Ca2+] versus Log10 [Ca-T] converged at the lowest total Ca2+ concentration. Furthermore, the study revealed that depending on the total Ca2+ concentration, effective interaction occur at and between pH 4 and 6 with the humic acid. This feature can be employed in regulating the release or retention of Ca2+ by the application of the humic acid in calcareous or limestone rich soils.