ARTIFICIAL FERTILIZERS 



263 



247. Bacteria as Nitrogen Gatherers. Soil from which crops 

 are removed year after year usually becomes less fertile, but 

 the soil from which crops 

 of clover, peas, beans, or 

 alfalfa have been removed 

 are richer in nitrogen rather 

 than poorer. This is because 

 the roots of these plants 

 often have on them tiny 

 swellings, or tubercles, in 

 which millions of certain 

 bacteria live and multiply. 



FIG. 162. Roots of soy bean having tuber- 

 cle-bearing bacteria. 



These bacteria have the re- 

 markable power of taking 

 free nitrogen from the air in 



the soil and of combining it with other substances to form com- 

 pounds which plants can use. The bacteria-made compounds 

 dissolve in the soil water and are absorbed into the plant by the 

 roots. So much nitrogen-containing material is made by the 

 root bacteria of plants of the pea family that the soil in which 

 they grow becomes somewhat richer in nitrogen, and if plants 

 which cannot make nitrogen are subsequently planted in such 

 a soil, they find there a store of nitrogen. A crop of peas, 

 beans, or clover is equivalent to nitrogenous fertilizer and 

 helps to make ready the soil for other crops. 



248. Artificial Fertilizers. Plants need other foods besides 

 nitrogen, and they exhaust the soil not only of nitrogen, but 

 also of phosphorus and potash, since large quantities of these 

 are necessary for plant life. There are many other sub- 

 stances absorbed from the soil by the plant, namely, iron, 

 sodium, calcium, magnesium, but these are used in smaller 

 quantities and the supply in the soil does not readily become 

 exhausted, 



