128 NATURE TEACHING. 



brown grains or powder containing from 10 to 

 14 per cent, of nitrogen. It also contains small 

 amounts of potash and phosphate. Dried blood 

 being insoluble, cannot be used at once by the 

 plant but requires to be altered first. It is there- 

 fore more lasting in its action. 



PHOSPHATIC MANURES. 



1. Phosphate of lime occurs in nature (as 

 an insoluble substance), in bones and in cer- 

 tain mineral deposits. These are sometimes 

 finely ground and used as manure without any 

 further treatment, but, as certain changes are 

 necessary before this insoluble phosphate can 

 be used by plants their action is slow. More 

 frequently the phosphatic mineral, or the bones, 

 are treated with strong sulphuric acid, which 

 renders the phosphate of lime soluble. Thus 

 prepared the manure is known as super-phos- 

 phate. Super-phosphate contains from 25 to 45 

 per cent, of phosphate of lime in a soluble con- 

 dition. 



2. Basic phosphate, Thomas' phosphate, 

 or basic slag is a form of phosphate of lime ob- 

 tained as a by-product in the manufacture of 

 steel. It is a heavy, brownish or purplish- 

 grey powder which should be as fine as flour. 

 Unlike super-phosphate, which is acid, basic 



