MANURES. 243 



The chemistry of the phosphates of lime has been very clearly 

 set forth by Professor S. W. Johnson in his report on manures, 

 made to the Agricultural Society of Connecticut, and it may be 

 worth while to reproduce here, in a very brief form, the principal 

 features of this portion of the report. 



A single atom of phosphate of lime contains one atom of 

 phosphoric acid and three atoms of lime. Any process which 

 will remove from the compound two atoms of the lime, leaving the 

 whole amount of phosphoric acid, will convert it into superphos- 

 phate of lime, which is very much more soluble than is the origi- 

 nal or basic phosphate; and it is the custom in the manufacture of 

 superphosphate of lime to apply such an amount of sulphuric 

 acid as will remove these two atoms of lime, the result being a 

 compound containing superphosphate of lime and sulphate of lime 

 or gypsum ; and when no other matters are added to increase the 

 rapidity of the action of the manure, this is the composition of 

 the pure superphosphate of lime of commerce. It contains very 

 much more lime and sulphuric acid than phosphoric acid, but the 

 latter is in such a state of solubility as will allow it to be carried 

 by rains very readily into the soil, and if applied while plants are 

 actually growing, it may be taken up by them without delay. 



Ordinarily, however, when superphosphate of lime is applied 

 to the soil, it immediately hunts out particles containing potash 

 or lime or magnesia or soda, with which its unsatisfied phosphoric 

 acid may again combine ; and it is not likely that the true super- 

 phosphate ever remains for any considerable length of time as an 

 element of the soil ; and the question may readily arise, why is 

 it worth while to resort to such an expensive and troublesome 

 process to reduce the phosphate of lime to the superphosphate, 

 when we are almost certain that within a short time after it is 

 applied to the soil it will have returned again to the condition of 

 the comparatively insoluble phosphate ? The reason why this is 

 worth while is to be sought only in the degree of fineness to 

 which the article is reduced by the chemical changes through 

 which it has passed. 



Professor O. N. Rood, of the Troy University, at the request 



