Triple superphosphate (21.6% P), potassium metaphosphate (25.0% P) and magnesium ammonium phosphate (19.0% P) were compared as phosphate fertilizers alone and in combination, and as powders and granules, in two experiments on acid soils. Basal nutrients were adjusted to allow for the amounts of N, K, and Mg in the test fertilizers. Immediate effects of the fertilizers were measured by early potatoes and barley, and residual effects by radishes and ryegrass.
All powdered fertilizers, alone or in combination, were equivalent to powdered superphosphate for early potatoes. The value of granular fertilizers for potatoes increased with the proportion of superphosphate in the mixtures. Superphosphate equivalents of the granular fertilizers were: magnesium ammonium phosphate, 14%; potassium metaphosphate + magnesium ammonium phosphate, 36%; potassium metaphosphate, 53%; superphosphate + potassium metaphosphate + magnesium ammonium phosphate, 54%; superphosphate + magnesium ammonium phosphate, 60%; superphosphate + potassium metaphosphate, 81%.
Residues from powdered fertilizers, applied to potatoes, were equivalent, for radishes, to residues from powdered superphosphate. Residues from granular fertilizers were more effective for radishes than residues from powders.
All powdered fertilizers were equivalent to powdered superphosphate for barley cut green. Granular magnesium ammonium phosphate, either alone or mixed with potassium metaphosphate, was slightly less effective than powder.
Residues from all powdered fertilizers applied to barley were equivalent for ryegrass. Residues from granular fertilizers, especially potassium metaphosphate and magnesium ammonium phosphate, produced more grass than residues from powders.
Apparent recoveries of P by the four crops ranged from 24% to 32% and were more from granules than from powders. Residues from all fertilizers increased 0.5M-NaHCO3-soluble P in soils, and mean yields of ryegrass, the final test crop, increased by 3.2 to 5.1 cwt dry matter/acre/ppm NaHCO3-soluble P in the soils.