SOURCES OF PLANT-FOOD 



count of its source, and it has been a common 

 practice to label the bags " bone-phosphate," 

 or " dissolved bone," or such other designation 

 as would imply an organic source, but the acid 

 phosphate is made out of rock-phosphate, regard- 

 less of the name given. The prejudice against 

 the rock as a source of plant-food is giving way. 

 It is our chief and cheapest source of supply. 

 The combination of sulphuric acid with rock-phos- 

 phate in the production of acid phosphate pro- 

 duces sulphate of lime, known as gypsum or land- 

 plaster. The amount of gypsum in a ton of acid 

 phosphate varies, but may be roughly estimated 

 by the buyer as two thirds of the total weight of 

 the acid phosphate. 



The tendency of gypsum is, in the long run, to 

 make a soil acid, and its use necessarily hastens 

 rather than retards the day when a lime deficiency 

 will occur. The influence in this direction is not 

 great enough to be a very material factor in de- 

 ciding upon a carrier of phosphoric acid. If 

 a soil has little lime in it, a state of acidity soon 

 will come anyway, and the increase in amount of 

 required lime will be small. The cheapness of 

 acid phosphate, as compared with animal bone, is 

 the decisive factor. 



[181] 



