A series of experiments was carried out in England and Wales in the period 1972-1982, to compare the relative effectiveness of various commercially available water-insoluble phosphate fertilisers with that of superphosphate. Most of the trials were on permanent grassland, but a few used barley and swedes as test crops. Ground Gafsa rock P, although only approximately 50% as effective as superphosphate in the year of application, gave somewhat higher responses in the following (residual) 2 years on unlimed, acid grassland. In these circumstances, grinding finer than 150 pm did not appear to be worth the extra cost. Granulation of finely ground Gafsa rock P slightly reduced its effectiveness for grass in the first year, but markedly reduced its efficiency for cereals. Mixtures of medium grade basic slag and Moroccan rock P were only slightly inferior to high grade basic slag, but mixtures of low grade slag and ground rock P (Moroccan or Gafsa) were somewhat less effective than the finely ground Gafsa rock P, especially in the first year. Granulated ground underacidulated rock P (Gafsa or Moroccan), was as effective as very finely ground Gafsa rock P, however a bulk blend of granulated Gafsa rock P and triple superphosphate was more effective than Gafsa rock P, but less so than superphosphate in the first year. Calcined aluminium calcium P was the least effective of the fertilisers tested, being especially slow in action, and only 25% as effective as superphosphate in the first year.
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