ORGANIC MATTER AND NITROGEN 207 



THE FIXATION OF FREE NITROGEN 



As already stated, the nitrogen naturally in the soil is contained 

 essentially in the organic matter. Any process which tends to 

 decompose or destroy this organic matter, such as nitrification or 

 other forms of oxidation, will also tend to reduce the total stock of 

 nitrogen in the soil, whether removed by cropping or lost by leach- 

 ing. Because of this fact, the matter of restoring nitrogen to the 

 soil becomes of very great importance. Of course, a part of the 

 nitrogen removed in crops may be returned in the manure produced 

 on the farm; and nitrogen may also be bought in the markets in 

 such forms as dried blood (14 per cent), sodium nitrate (15^ per 

 cent) , and ammonium sulfate (20 per cent) ; but when we bear in 

 mind that such commercial nitrogen costs from 15 to 20 cents a 

 pound, and that one bushel of corn contains about one pound of 

 nitrogen, it will be seen at once that the purchase of nitrogen 

 cannot be considered practicable in general farming, although in 

 market gardening, and in some other kinds of intensive agriculture, 

 commercial nitrogen can often be used with very marked profit. 



Considering all of these facts, and the additional facts that there 

 are about seventy-five million pounds of atmospheric nitrogen 

 resting upon every acre of land, and that it is possible to obtain 

 unlimited quantities of nitrogen from the air for the use of farm 

 crops, and at small cost, the inevitable conclusion is, that the inex- 

 haustible supply of nitrogen in the air is the store from which we 

 must draw to maintain a sufficient amount of this element in the 

 soil for the most profitable crop yields. 



. It is often stated that legume plants, such as clover, have power 

 to obtain free nitrogen from the air. This is not strictly true. 

 Red clover, for example, has no power in itself to get nitrogen from 

 the air. It is true, however, that certain microscopic organisms 1 

 which commonly live in tubercles upon the roots of the clover plant 

 do have the power to take up free nitrogen and cause it to unite 

 with other elements to form compounds suitable for plant food. 



1 Among the scientists who were prominent in making these discoveries regard- 

 ing the action of bacteria in the fixation of free nitrogen were Hellriegel, Willfarth, 

 and Nobbe in Germany, Atwater in America, Lawes and Gilbert in England, and 

 Boussingault and Ville in France. 



