Jc. Lobartini et al., THE NATURE OF HUMIC ACID-APATITE INTERACTION PRODUCTS AND THEIR AVAILABILITY TO PLANT-GROWTH, Communications in soil science and plant analysis, 25(13-14), 1994, pp. 2355-2369
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.