The nature of humic acid‐apatite interaction products and their availability to plant growth

Abstract

Abstract An investigation was conducted to determine the nature of decomposition products resulting from the interaction between humic acid and apatite and assess their availability to plant growth. Interaction analyses were performed by shaking 200 mg apatite with 0 to 800 mg/L HA or FA solutions at pH 5 or 7 for 0 to 12 hr. Phosphorus concentrations were determined in the supernatants by spectrophotometry. The nature of P‐humic acid complexes was determined by 31P NMR analysis. Availability of these dissolution products was studied by growing corn plants in aerated hydroponics to which 200 mg apatite and 0 to 800 mg/L HA were added at pH 5 or 7. The results indicated that the rate of dissolution of apatite was parabolic in regression with time, and increased by increasing the amounts of HA or FA applied from 100 to 800 mg/L The dissolution reaction was influenced by pH, because larger amounts of PO4 3‐ions were detected at pH 5 than at pH 7. 31P NMR spectroscopy indicated the presence of P‐humic acid complexes, previously believed to be humophosphate esters. The PO4 3‐ ion was complexed by HA at pH 7 or above, but PO4 3‐ appeared to be released again as adsorbed and free ions at pH <5.0. Plant performance corresponded with increased PO4 3‐concentrations at pH 5.0. No significant improvement over the control was observed in the growth of corn plants by apatite + HA treatments at pH 7. However, plant growth was increased significantly over the control by apatite + HA treatments at pH 5.0. Better growth performance of corn plants were noticed by apatite + HA than by KH2PO4 treatments at pH 5.0.