PHYTOAVAILABILITY OF PHOSPHATE ADSORBED ON FERRIHYDRITE, HEMATITE, AND GOETHITE

Iron (hydr)oxides are important phosphate adsorbents in soils. However, the dependency of phytoavailability of adsorbed phosphate on mineral type and properties has not been established. To examine the influence of mineralogy, one ferrihydrite, two hematites, and two goethites were prepared. They differed widely in morphology (from euhedral crystals to granular aggregates), specific surface area (15-266 m2 g-1) and microporosity (0-87% of total surface area). Their relative affinity phosphate, which is inversely related to the concentration exponent in the Freundlich equation, b, decreased in the order goethite (b = 0.11, 0.09) > hematite (b = 0.13, 0.17) > ferrihydrite (b = 0.21). Phytoavailability of the adsorbed phosphate was studied by growing sunflower (Helianthus annuus L.) in pots containing suspensions of the phosphated Fe (hydr)oxides in equilibrium with a concentration of 1 mg P L-1. The fraction of the adsorbed phosphate that was phytoavailable, B1, (1) was lower for goethite (B1 = 0.43, 0.38) than for hematite (B1 = 0.73, 0.49) and ferrihydrite (B1 = 0.79), (2) was not negatively affected by microporosity, and (3) decreased, in general terms, with increasing relative affinity for phosphate. Similar trends were observed for the production of dry matter. These results, together with previous reports showing that goethites usually have higher relative affinity for phosphate than hematites, suggest that phosphate could be more available to plants in hematitic than in similar goethitic soils.