Glasshouse pot trials with ryegrass, subterranean clover, and radish as indicator plants showed that calcined Christmas Island iron/aluminium phosphates (Calciphos) when finely ground and mixed into the soil gave yields equal to those from the use of granulated superphosphate. Localising the powder in spots significantly reduced the yields only in early harvests. Plant response was greatest to material calcined between 500°c and 700°c, and yields were lower from material calcined below 500'c than from that calcined above 700°c. In pot trials pelletising Calciphos severely reduced yields. Bentonite, smectite, and sulphur were added in varying proportions to Calciphos in an attempt to make pellets which readily disintegrated on moist soil, so making the phosphate more available to plants. The improvement obtained was not sufficient to warrant the extra cost of pellet formulation and decrease in phosphate content.
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