THE NATURE OF HUMIC ACID-APATITE INTERACTION PRODUCTS AND THEIR AVAILABILITY TO PLANT-GROWTH

An investigation was conducted to determine the nature of decompositio n products resulting from the interaction between humic acid and apati te 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-h umic acid complexes was determined by P-31 NMR analysis. Availability of these dissolution products was studied by growing corn plants in ae rated hydroponics to which 200 mg apatite and 0 to 800 mg/L HA were ad ded at pH 5 or 7. The results indicated that the rate of dissolution o f apatite was parabolic in regression with time, and increased by incr easing the amounts of HA or FA applied from 100 to 800 mg/L. The disso lution reaction was influenced by pH, because larger amounts of PO43- ions were detected at pH 5 than at pH 7. P-31 NMR spectroscopy indicat ed the presence of P-humic acid complexes, previously believed to be h umophosphate esters. The PO43- ion was complexed by HA at pH 7 or abov e, but PO43- appeared to be released again as adsorbed and free ions a t pH less-than-or-equal-to 5.0. Plant performance corresponded with in creased PO43- concentrations at pH 5.0. No significant improvement ove r the control was observed in the growth of com 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.