56 



TROPICAL AGRICULTURE 



CHAP. 



the foliage becomes greener, abundance of fresh leaves are 

 thrown out, and the succeeding crop is large. Sulphate of 

 ammonia was formerly one of the " waste products " of the 

 gas manufactories. Nitrate of soda, or " Chili saltpetre " is 

 a white salt-like substance found in enormous beds, some- 

 times eight feet thick, in the rainless parts of Peru and Chili. 



Phosphates 

 in the soil 

 liable to 

 exhaustion. 



Worn-out 

 pastures. 



" Boning-' 

 land. 



Fermenta- 

 tion of bones. 



Phosphatic Manures. — Phosphorus, in the form of 

 Phosphoric acid, is an important plant food, and it is classed 

 as one of the secondary organic elements. It cannot be 

 obtained from the air ; and, as it exists in the soil usually 

 in very small quantities, it is one of those substances that are 

 liable to be exhausted by continuous cropping, and thus it 

 must be supplied in the form of a manure. It is present in 

 all general manures, and some of the special manures contain 

 it in large quantities and hence they are called phosphatic 

 manures. Phosphorus is a very important constituent of the 

 bones of animals and the milk of cows, and when cattle are 

 grazed on pasture land for any length of time they exhaust 

 the phosphoric acid in the soil by taking it up in the grass, 

 and by " fixing " it in their own bodies. This is the principal 

 reason that pastures become " poor " or " worn out " or " used 

 up," and it has been found by experience that the application 

 of phosphatic manures to a worn-out pasture will soon 

 restore it to a luxuriant condition. The principal phosphatic 

 manures are bones^ superphosphates^ and 7-cdiiced phosphates. 



Bones contain about half their own weight of phosphate of 

 lime, all of which has been taken by animals from the soil in 

 their food. For a long time past bones have been used as 

 manure. They were broken into small pieces with hammers 

 and spread over the land, and this was called " boning " the 

 soil. But bones do not readily crumble away, and so they 

 were ground to powder, and in this way they acted quickly. 

 It was found afterwards that they would ferment if moistened 

 with water and kept in a heap exposed to the air ; and, as 

 the fermentation causes the bone to decompose, or break 



