60 THE FOOD OF PLANTS CHAP. 



of another set of organisms which carry on the process 

 of nitrification), from the decay of vegetable stuff in 

 the soil. By their means insoluble compounds of 

 nitrogen are broken up and the nitrogen converted 

 into nitrous and nitric acids which combine with lime 

 and other bases in the soil. 



If there is not sufficient store of decaying plants 

 in the soil, the crops must be supplied with nitrogen 

 by putting on the soil farmyard manure, which we 

 have seen contains nitrogen, or any other vegetable or 

 animal refuse. 



Two compounds of nitrogen are now also very 

 largely used for this purpose, and are prepared in 

 enormous quantities for the farmer sulphate of 

 ammonia and nitrate of soda. Take a little dry 

 sulphate of ammonia, mix with lime, and notice the 

 smell of ammonia, showing that it contains nitrogen. 

 In order to prove the presence of nitrogen in nitrate 

 of soda we must treat it a little differently. 



Dissolve a little sulphate of copper in water, and 

 put in it a strip of zinc. It is at once coated with 

 spongy copper. 



Dissolve a little sodium nitrate in water, and put 

 in some pieces of zinc covered with spongy copper, 

 and warm gently for some time, then add some caustic 

 soda and warm. The smell of ammonia will soon be 

 quite distinct, showing that nitrate of soda also con- 

 tains nitrogen. 



Nitrate of soda and sulphate of ammonia are 

 both very soluble in water (as we can easily see by 



