above are those most generally used. The contents 

 as given are in terms of nitrogen and approximately 

 the maximum. 



NITRIFICATION. 



This is the process by which the nitrogen of 

 organic matter is changed into nitrates. The 

 ammonia and nitrogen of all fertilizers comes from 

 organic matter, and all organic materials contain 

 more or less of those substances in some form. 

 Nitrate of soda in the nitrified product of some 

 organic material, whether of seaweed or animals, is 

 not definitely known. Ammonium sulphate also has 

 an organic origin, being a by-product of carbonizing 

 works. 



Humus (which is decayed animal or vegetable 

 matter) is the main source of the plant's nitrogen. 

 When organic matter is applied to the soil it must 

 first decay and then nitrify before its nitrogen be- 

 comes available to the plant. Two processes are 

 necessary. The decay is produced by one set of 

 bacteria and their product is humus. Then the sub- 

 stance is attacked by another set of bacteria which 

 form nitrates. This latter process is nitrification. 

 The nitrates thus formed are water-soluble and can 

 be absorbed by root hairs into plant tissue. 



NITROGEN FROM AIR. 



Certain plants of the leguminosae group have 

 power to accumulate nitrogen from the air to the 



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