COMMERCIAL FERTILIZERS 139 



132. Appearance of Phosphate Rock. — In appear- 

 ance phosphate rock varies greatly. The soft, white 

 phosphates of Florida resemble marl, and are nearly 

 as soft. The Tennessee rock, as a rule, is hard and 

 resembles in appearance ordinary Limestone. The ordi- 

 nary South Carolina rock is between these two ex- 

 tremes, being of a dirty gray color and moderately 

 hard. 



133. Composition of Phosphate Rock. — All phos- 

 phate rock, wherever it occurs and whatever its appear- 

 ance, contains some compounds of phosphorus, and its 

 value as a fertilizer depends on the amount and condi- 

 tion of these compound- Ifosi of the rock used for 

 fertilizers contains phosphorus combined with calcium 

 and oxygen, which compound is known as calcium 

 phosphate or bone phosphate. Some samples of phos- 

 phata rock contain as much as 98 per cent calcium 

 phosphate, and some rock contains almost none at all. 

 Good rock should contain at least 50 per cent oi 

 cram phosphate. Pari oi the phosphorus may be com- 

 bined with iron and aluminum, and is then known as 

 iron and aluminum phosphate. The best grades of 

 contain but little Iron and aluminum, but in some 

 Varieties of n><k all the phosphorus is combined with 

 these two elements. Such rock, while of some value 

 for fertilizers, is not nearly so valuable as the calcium 

 phosphate rock, 



des compounds of phosphorus, phosphate rock 

 contains varying quantities of sand, organic matter, 

 iron and sulphur, and minute quantities of several other 

 substances. 



