90 AGRICULTURE. 



phosphate, which is not soluble in pure water, 

 but is soluble in the acid soil waters and the 

 acids exuded by the rootlets, and is, therefore, 

 available to plants. So that the mono-calcium 

 and di-calcium phosphates contained in a com- 

 mercial fertilizer together are called the " avail- 

 able phosphoric acid" (see Table). Mono-cal- 

 cium phosphate is immediately available to 

 plants, and will give quick returns ; but that 

 which remains in the soil changes to the di-cal- 

 cium, or reverted form, which is precipitated as 

 a fine powder, and is easily dissolved through 

 the acid reaction of the roots. 



The supply for manufacturing these fertilizers 

 comes largely from South Carolina, which has, 

 perhaps, the richest deposits of rock phosphates 

 in the world. Other valuable deposits are found 

 in Florida, consisting not only of phosphates of 

 lime, but also of phosphates of iron and alumina. 

 Still others are found in Tennessee, Pennsyl- 

 vania, and Virginia. 



Bone-black is obtained by heating animal 

 bones in the absence of air, when the gases and 

 oily matters are driven off, and charred bone or 

 bone charcoal is left. This is used for refining 

 suo-ar ; when it is of no further use for this pur- 

 pose it is sold as a fertilizer. In this form, 

 however, it is slowly soluble, and of little prac- 

 tical value. When bone-black is treated with 

 sulphuric acid a much greater per cent, of sol- 



