FERTILIZERS 21 



NITRIFICATION. 



29. 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 carbon- 

 izing 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. These 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. (See 

 Figure 3.) 



NITROGEN FROM AIR. 



30. Certain plants of the leguminosae group have 

 power to accumulate nitrogen from the air in the 

 process of growth. Such plants are the lupins and 

 vetches, which with peas, clover, alfalfa and others, 

 when grown as catch or cover crops and ploughed 

 under, add to the store of nitrogen in soils. But in 

 this case, as with other organic substances, the two 

 processes of decay of tissue and nitrification, are nec- 

 essary before the nitrogen thus gathered becomes 



