42 AGRICULTURE. [CH. 



stand what is meant by it. Soluble phosphate is only 

 another name for mono-calcic phosphate, which has 

 been already described as being rapidly soluble in 

 water. If a portion of super-phosphate of lime be 

 added to a proper quantity of water, and especially if 

 tepid water be employed, in a few hours the whole of 

 the mono-calcic phosphate is dissolved out of it. 

 Thus the soluble phosphate is separated from the rest 

 of the manure, and its quantity is estimated by 

 chemical analysis. 



80. Purchases are frequently based upon the strength 

 of the super-phosphate, which is often sold as con- 

 taining, say 25 per cent, of soluble phosphate. It 

 would of course be a perfectly reasonable contract 

 that a super-phosphate of lime should contain say 

 25 per cent, of soluble or mono-calcic phosphate, but 

 in the great majority of cases something different from 

 this is really intended. The strength intended is more 

 correctly stated when expressed as equal to 25 per 

 cent, of tri-calcic phosphate rendered soluble. 

 You may take it that 20 cwt. of tri-calcic phosphate 

 rendered soluble will only yield about 15 cwt. of 

 mono-calcic phosphate, so that 25 per cent, in the 

 trade sense, viz., 25 per cent, of tri-calcic phosphate 

 rendered soluble, really means only 19 per cent, of 

 mono-calcic phosphate in the sample. 



8 1. It may be convenient to notice at this stage the 

 practice of selling manures upon the " unit system," 

 which is becoming more and more general, viz., a 

 sale of super-phosphate at an agreed price per unit. 

 When a super-phosphate is thus sold which contains 

 25 per cent, of tri-calcic phosphate made soluble, this 

 represents 25 units at the agreed price, it may be 

 35., 45., or more per unit, per ton. If it contains 

 30 per cent, this would represent 30 units, and thus 

 the number of units always corresponds with the 

 amount per cent. A super-phosphate containing 25 

 units and sold at 45. per unit would produce $ per 



