i8o CHEMICAL FERTILIZERS 



only produced at the first stages of mixing ; once mixed and 

 screened, there is little risk of a second caking (see p. 12). 



The general experience in the past of occasional difficulties 

 with the manufacture of ammoniated superphosphate, owing 

 to its tendency to cake, is connected very largely with the 

 small relative amounts of other forms of nitrogenous matter 

 that are used in these mixtures. During recent years, 

 however, the scarcity of organic nitrogenous fertilizers, and 

 the Government controlled low price of ammonium sulphate, 

 have made ammonium sulphate the most easily obtainable 

 form of nitrogen, and other forms of nitrogen have dropped 

 out of use. The presence of comparatively large quantities 

 of ammonium sulphate in admixture with superphosphate 

 in the absence of other materials has produced an extra 

 tendency to " set." The setting frequently takes place so 

 slowly, that a compound fertilizer may appear, when placed 

 in bags at the factory, to be in a fit condition for sowing on 

 the land, but, after storage or transit it is found to have set 

 into hard lumps, which may necessitate cutting away the 

 bags. When the manufacturer has much space at his dis- 

 posal and can manufacture in advance, the difficult}- of 

 setting disappears, because he stores the fertilizer until it has 

 finished setting, and then regrinds it before dispatch, but 

 recent pressure of trade has often rendered this precaution 

 impracticable. Experiments that have been conducted to 

 determine the conditions under which superphosphate and 

 ammonium sulphate undergo the process of setting, show that 

 mixtures containing 5-6 % of ammonia give the greatest 

 trouble. Much depends, however, upon the amount of 

 moisture in the ammonium sulphate that is used for mixing, 

 samples containing 4 % of water setting much more rapidly 

 than those containing only 1-2 %. A difficulty occurs 

 where setting only takes place to a slight extent; the mix- 

 tures are left damp. When sulphate of ammonia containing 

 3-4 % of water is used, the mixtures set rapidly, appear 

 lighter in colour, feel dry and gritty, and can be easily re- 

 ground. Further trials to discover whether the addition 

 of ground Gafsa phosphate produces much influence on the 



