382 SOILS 



available for plants. This material is considered 

 one of the best phosphates for general use, espe- 

 cially on moist soils rich in humus and poor in lime. 

 It is produced in this country in considerable 

 quantities. 



Superphosphates. Any phosphate, either bone 

 or mineral, which has been treated with acid to 

 render its phosphoric acid more soluble is called 

 a superphosphate. One popular superphosphate 

 dissolved boneblack has been mentioned. Dis- 

 solved bone, made by treating raw ground bone 

 with sulphuric acid, is another. It contains 13 to 

 15 per cent, of available phosphoric acid and 2 to 

 3 per cent, of nitrogen. The most common super- 

 phosphate is that made by treating ground rock 

 phosphate with sulphuric acid. The great fault 

 of the raw rock and raw bone phosphates is their 

 slowness; plants derive little benefit from them 

 the same season that they are applied. To over- 

 come this, the manufacturer mfces some strong 

 acid with them, usually sulphuric acid. This 

 makes most of the phosphoric acid in them 

 soluble. 



Contrary to the belief of some, a well-made 

 superphosphate contains no free acid that will 

 make the ground "sour." The acid used in 

 making it is all combined with lime, making 

 gypsum, which is itself a valuable dressing 

 for some soils. However much difference 

 there may be in the agricultural value of 

 raw bone phosphates and raw mineral phos- 

 phates, a superphosphate made from one is as 

 good as a superphosphate made from the other, 

 per pound of phosphoric acid; the kind of raw 

 material does not count after it has been treated 

 with acid. 



