NITKOGENOUS MANUEES 259 



16 or 20 per cent, of sulphate of ammonia. One cwt. 

 of the latter should therefore have the same manurial 

 value as 5 or 6 cwts. of soot. 



Some farmers consider that it has a valuable mechani- 

 cal effect on clay soils, but no mention is made of 

 its having any effect good or bad on light soils. Soot 

 is very largely used in gardens, but seems to be appre- 

 ciated as a preventive against the ravages of slugs and 

 insects, even more than for its manurial effects. 



NITRATE OF SODA. 



Occurrence. Immense natural deposits of nitrate of 

 soda or Chili saltpetre, as it is sometimes called, have 

 been discovered in the arid rainless district on the borders 

 of Chili and Peru on the west coast of South America. 

 The deposits are found always near the coast at a height 

 of three or four thousand feet above sea level. It' 

 seems plain that they must have been formed by the 

 nitrification of organic matter which would be promoted 

 by the presence of sodium carbonate and other alkaline 

 salts and basic substances with which the soil is impreg- 

 nated. The process of nitrification has already been 

 described. In this country the nitrates do not accumu- 

 late in the soil but are washed out by the excess of rain 

 water which passes into the drains. In the desert region 

 in which these deposits are found, the climate is ex- 

 tremely dry ; the rainfall is never sufficient to produce 

 any flow of drainage, and the nitrates are not washed 

 out of the soil but only carried down a little below the 

 surface. 



It is more difficult to account for the enormous quantity 

 of organic matter from which it is supposed the nitrate 

 has been produced. Two possible sources have been 

 suggested, viz., guanos, deposits of which are found 



s2 



