COMPOUND MANURES 181 



result, show that after standing a week with slight pressure, 

 the mixtures are much drier than when no rock phosphate 

 is used. The influence of water is very noticeable, the 

 damper the sulphate of ammonia the quicker does the setting 

 take place, whether rock phosphates have been added or not. 

 Whilst moisture tends to increase the rapidity of setting, the 

 increase is much more marked when the free acid of the 

 superphosphate is reduced to a low figure. If it is desirable 

 to hasten the completion of setting in any of these mixtures, 

 it is necessary to use sulphate of ammonia which contains 

 3-4 % of water, instead of using a very dry sulphate. When 

 rock phosphate, superphosphate and sulphate of ammonia 

 are mixed, the whiteness, dryness and hardness increases 

 with the amount of rock phosphate employed. Compound 

 fertilizers set rapidly if made with about 5 % of ground rock 

 phosphate and about 20 % of sulphate of ammonia contain- 

 ing 3-4 % of water; after eighteen days rest and regrind- 

 ing, further setting need not be feared. As explained 

 on p. 187, there is no agricultural necessity for com- 

 pounding fertilizers containing nearly all their phosphoric 

 acid in soluble form ; the field results are often as good 

 with lower proportions of soluble phosphates, provided that 

 the insoluble phosphates are in a very fine state of division. 

 If for any reason whatsoever it is desirable to retard the 

 setting of mixtures of sulphate of ammonia and superphos- 

 phate, a larger amount of free acid may be present, and there- 

 fore both the superphosphate and the sulphate of ammonia 

 should be more acid than is commonly considered suitable. 

 It will, however, be very difficult to make such a mixture 

 in the form of a dry, fine powder. A fertilizer which has 

 not been allowed to set is moist and sometimes sticky, 

 whereas one which has set and has been ground again is very 

 dry, and altogether in a far better condition for use on the 

 field. So long as the present abnormal state of trade 

 prevails, good condition will have little monetary value ; 

 when business becomes less congested and more normal, 

 there is little doubt that the farmer will, as before, attach 

 much importance to condition in these mixtures. The 



