| Plant and animal wastes degrade in soils to form relatively stable humified compounds, which form ion complexes that affect the bioavailability of elements in the soil solution. Hydroponic studies with wheat and rice were conducted to characterize the effect of humic acid (HA) on phosphorus (P), iron (Fe), fluorine (F), and iodine (I) bioavailability. Ferrihydrite [Fe(OH)3] precipitation was greater on root surfaces without HA or synthetic chelates. Oxides such as ferrihydrite strongly adsorb P and provide exchange sites for metals. HA reduced this precipitate and increased P and Fe uptake.;Humic acid had no effect on F toxicity in rice, where solution levels above 0.5 mM F inhibited growth. Data supported the hypothesis that in moderately acidic solutions (pH < 6), F uptake is primarily as HF rather than F -. Doubling solution Ca caused a 10-fold increase in root surface CaF2 precipitates, but the additional Ca did not decrease F toxicity. Calcium levels above 1 mM caused HA to flocculate over time, but the addition of F reduced flocculation by competing with HA for Ca. The majority of shoot F was apparently associated with the middle lamella, suggesting that F may bind with phosphates and pectate-Ca.;Organic matter promotes aqueous iodine (I2(aq)) reduction to I-, a less toxic species. HA reduced I2(aq) toxicity by 50%. In solutions without HA, 6.5 muM I2(aq) was more toxic than 30 muM I-. Humic acid had no effect on I- Uptake or toxicity, where I- and IO3- were toxic to rice at 10 and 100 muM, respectively.;These data were used to model element cycling through plants in a regenerative human life support system for NASA's Advanced Life Support program, where HA, P, Fe, F, and I from plant residues and human wastes are recycled to the crop production system. |
