38 CHEMISTRY OF FARM PRACTICE] 



ing very marked properties. Nitrogen, in the free state, 

 forms about four-fifths by volume of the atmosphere; 

 it exists in combination with other elements in important 

 compounds, such as ammonium hydroxide (NH40H), one 

 of the alkali bases; in nitric acid (HNOs), one of the strongest 

 acids; and in the ammonium form (NfLi), as a salt of many 

 acids. It is present in combination in animal and in plant 

 tissues. Nitrogen is available as plant or animal food 

 only when it enters into some combination. It is the most 

 expensive and at the same time the most elusive element 

 with which the farmer has to deal. 



While four-fifths, by volume, of the atmosphere is 

 nitrogen, most plants are powerless to extract it from 

 the air. A small amount of ammonia gas is formed in 

 the atmosphere by electrical discharges and washed to 

 the earth by rain water; in a similar way some of the 

 oxides of nitrogen are formed. Certain bacteria that exist 

 on decaying organic matter have the power of " fixing " 

 atmospheric nitrogen in such combination that it will 

 become available to the plants. A large amount of atmos- 

 pheric nitrogen is " fixed " by means of bacteria that 

 exist in so-called symbiotic union with leguminous plants. 

 These bacteria form nodules on the roots of the plants 

 which they infest, as shown in Fig. 9, the plants furnishing 

 food for the bacteria, while the bacteria take nitrogen 

 from the atmosphere and convert it into such a form that 

 the plants can use it for the elaboration of their tissues. 

 Peas, beans, vetches, clovers, alfalfa, peanuts, and beggar- 

 weeds are examples of legumes. 



33. Phosphorus. Phosphorus is very easily oxidized, 

 and, therefore, exists in nature in compounds only. It 

 is quite widely distributed in combination with oxygen 

 and calcium, as phosphate rock, which is largely calcium 

 phosphate Ca3(PO4)2. Phosphorus is often deficient in 

 soils, and, as it is used rather plentifully for the develop- 

 ment of both plants and animals, it is very often necessary 



