272 THE FERTIIJTY OF THE SOIL 



deficient. The remains of plants and animals and their 

 wastes have been used as fertilizers from time immemorial 

 and are naturally among the best, since they contain the 

 very elements which were removed from the soil. 



Certain deposits of rock are rich in phosphates which 

 can readily be made available for plant use. Cyanamid, 

 a waste product of certain manufactories, contains avail- 

 able nitrogen. Guano is the excrement of certain sea 

 birds which has been deposited for ages on some islands 

 off the Pacific shore of the rainless region of South 

 America. It is rich in nitrogen compounds. Sodium 

 nitrate, or Chili saltpeter, is mined in northern Chili and 

 shipped to Europe and the United States. Potash, com- 

 posed of potassium compounds, is extensively mined in 

 Germany and is obtained to a less extent in the western 

 United States. During the war it was secured from sea 

 weeds. Nitrogen compounds are now being manufac- 

 tured from the free nitrogen of the air by the use of 

 electricity. 



Animal Wastes. — In previous chapters it has been 

 shown that some of the carbohydrate foods are used in the 

 formation of protein by plants. The latter substances are 

 used chiefly to build new protoplasm or to repair the 

 wastage of mature cells. Once a part of the living ma- 

 chine, it is relatively fixed, but not absolutely so. In the 

 very process of living, protoplasm produces certain waste 

 compounds containing nitrogen, which are excreted from 

 the body in the urine. The last chapter described the 

 changes induced in these by bacteria, whereby they are 

 broken down to ammonia (NHs) compounds. 



Microorganisms. — There are many kinds of bacteria 

 (Fig. 63 j, fungi, and small animals in the soil. Some 

 bacteria are concerned with fermentations and putrefac- 

 tions, but several are directly concerned with changes in 

 the compounds of nitrogen and sulphur which have to do 

 with soil fertility. For present purposes, the following 

 will be discussed: (1) The nitrifying bacteria; (2) The 



