C 5-6 j 



rocks or soil particles, are immediately fixed by this absorp- 

 tive power. Those saline matters which are easily washed 

 away, e. g., Chlorides, Sulphates and Nitrates, are (with the 

 exception of nitrates), either required by plants in very 

 insignificant quantities, or are abundantly present in the soil, 

 or supplied to it without human aid, 



CHAPTER LXXXVllt. 



NITROGENOUS MANURES. 



C\V the four principal manurial constituents, nitrogen, 

 phosphorus, potassium and calcium, nitrogen is the 

 most important, and, on the whole, it may be said, the richer 

 a substance is in nitrogen the greater is its value as a 

 manure. Green plants are not able to make any use of 

 the free nitrogen of the air, but fungi can make some 

 use of nitrogen in this form. Minute fungi, called Bacteria, 

 having a tendency to accumulate nitrogenous organic com- 

 pounds at the roots of plants, chiefly of the leguminous 

 order, these plants derive benefit from the free N of the air 

 through the help of these bacteria. As nitrites also, plants are 

 not able to make use of N. The nitrates contained in irriga- 

 tion water coming in contact with sewage, become reduced to 

 nitrites, in which form the N is of no use to plants. Irrigation 

 with sewage water is therefore not a very effective means of 

 applying nitrogenous manure to land. Nitrogen is absorbed by 

 plants chiefly in the form of nitrates, ammonia salts, urea, uric 

 acid, and hippuric acid. Nitrates occur in nature as saltpetre 

 (KNOg), Chili saltpetre (Na NOa) and also as saline inflore- 

 scence on walls, which is calcium nitrate (Ca 2NO\ Ammo- 



