Agronomic use of calcined christmas island iron/aluminium phosphates i. field trials

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Abstract
Christmas Island “C” ore when calcined at 550°C contains up to 24% aluminium and iron and 15% phosphorus which is not water-soluble but is up to 95% soluble in neutral ammonium citrate. Samples vary in total phosphorus content and in the percentage of phosphorus soluble in neutral ammonium citrate, depending on the proportions of crandallite, millisite, and apatite in the “C” ore sample. In field trials at Otara finely ground calcined “C” ore—“Calciphos” —gave yields equal to or slightly less than those obtained from superphosphate on both limed and unlimed plots at equal rate of applied P. Yields with a mixture of Calciphos powder and mature superphosphate (1∶1) were equal to, or better than, those from superphosphate alone. Pelletising the finely ground Calciphos reduced pasture responses to 41% of those obtained from granulated superphosphate. Materials containing the highest percentage of phosphorus soluble in neutral ammonium citrate do not necessarily give highest yields, indicating that other factors besides the neutral ammonium citrate solubility may be involved.