PLANT LIFE. OSMOSIS. FERTILIZERS 



411 



FIG. 101 



their growth, but peas, beans, clover, alfalfa and other leguminous 

 plants bear round their roots colonies of a special kind of bacteria 

 which has the power to bring free nitrogen into combination. 

 In these root nodules (Fig. 101) the bacteria first produce pro- 

 teins, which later decompose, and ultimately, by bacterial action, 

 yield nitric acid. In this way a crop of clover will 

 fertilize the soil, not only for itself, but also for the 

 following crop. The advantage of rotation of crops 

 is therefore explained. 



The beneficial action of bacteria upon plant growth 

 is not limited to this special case. The soil is crowded 

 with bacteria which assist in the decomposition or 

 rotting of vegetable and animal matter. The product, 

 the black, gummy stuff of a fertile soil, is called 

 humus. Certain varieties of bacteria convert insoluble 

 carbohydrates such as cellulose into simpler soluble mate- 

 rials, immediately available for plant use. Other species lib- 

 erate the nitrogen from proteins in the form of ammonia and 

 ammo-compounds (p. 353). Others, again, oxidize these com- 

 pounds to nitrous acid and nitrites. Still others, finally, oxidize 

 these products to nitric acid and nitrates. 



The actual assimilation of nitrogen by plants, hi whatever 

 form that element is originally present in or added to the soil, 

 takes place almost exclusively through soluble nitrates. The 

 co-ordinated team-work of all of the above classes of bacteria is 

 therefore an essential point in the efficient utilization of other 

 nitrogen-containing fertilizers. 

 



Q*hosph0rus) Natural calcium phosphate Ca 3 (PO 4 ) 2 is the 

 orthophosphate of calcium. It is found in considerable deposits 

 in S. Carolina, Florida, Tennessee, and several western states, 

 and in Algeria and Tunis. This insoluble salt, however, affords 

 only an exceedingly dilute phosphorus diet for plants. Hence, 

 a more soluble compound is to be preferred. This is found in 



