SOILS AND FERTILIZERS 527 



some other manure must be used containing directly available 

 nitrogen. Manures containing directly available nitrogen are very 

 expensive. The best of these is nitrate of soda, and at the present 

 rate of use the known supply will be exhausted in less than fifty 

 years. (Y. B. 1906; Y. B. 1909; Bu. Pit. Ind. B. 101; Del. Col. 

 B. 40.) 



The fertility of the soil is dependent largely upon the avail- 

 ability of the plant food contained in the soil. In some soils the 

 plant food is not available because it has not been set loose by the 

 action of bacteria, by cultivation, or by some other process. 



The supply of nitrogen in the soil being exhausted by the 

 growing of crops it must be restored by some means. The prin- 

 cipal means for restoring nitrogen to depleted soils are: (1) Ap- 

 plication of barnyard manure; (2) use of commercial fertilizers; 

 (3) growing of leguminous crops which have been infected with 

 nitre-fixing bacteria and which take nitrogen from the air and 

 store it within the plants. The supply of barnyard manure is in- 

 sufficient to make up for the losses of nitrogen occasioned by ordi- 

 nary cropping, and other means of supply must be resorted to. 



Ever since the importance of increasing the combined nitrogen 

 supply has been realized, men of science have naturally turned to 

 the atmosphere as being the most promising field for experiment 

 and the one most likely to eventually solve the whole problem. 

 When it is remembered that nearly eight-tenths of the air about us 

 is nitrogen, and that plants are able to obtain their entire source of 

 carbon from a gas which is present in the comparatively small pro- 

 portion of one-tenth of 1 per cent, it seems almost incredible that 

 there should be any more difficulty about a plant's nitrogenous food 

 than about its supply of carbon dioxid. Since it seemed so well set- 

 tled, however, that plants could not use nitrogen as a gas, the chem- 

 ists and physicists have made every effort to devise some mechanical 

 means of making this element available in a combined form. It 

 has been known that discharges of lightning passing through the 

 air are able to fix free nitrogen, and, beginning with this as a basis, 

 some very satisfactory results have been obtained by the use of 

 electricity. With a power sufficiently cheap and with perfect ma- 

 chinery, there seems good reason to believe that in the near future 

 it will be possible to place upon the market a manufactured nitrate 

 of soda or nitrate of potash that will be superior in quality to the 

 deposits found in South America, and that will also be reasonable 

 enough in price to compete with the natural product. (Y. B. 

 1902.) 



Bacteria the Agents of Supply. The organic matters which 

 are added to the soils in manures and in vegetable and animal re- 

 mains must go through certain processes of decay before the plant 

 foods they contain become available to plants. Bacteria and fungi 

 are the active agents which bring about these changes. Decay is 

 not a simple process, the same in all places and under all conditions. 

 The process varies when the same materials under the same condi- 



