132 BACTERIA IN THE SOIL 



perpetually responded to.* We know that a virgin soil cropped for 

 several years loses its productive powers, and without artificial aid 

 becomes unfertile. For example, through this exhaustion forty 

 bushels of wheat per acre have dwindled to seven. Eotation of crops 

 is an attempt to meet the problem, and the four-course rotation of 

 turnips, barley, clover, and wheat witnesses to the fact that practice 

 has been ahead of science in this matter. It is unnecessary to add 

 that rotation of crops and the use of the Leguminosse does not absolve 

 the agriculturist from maintaining the land in ripe condition by 

 manuring and ordinary tillage. 



The store of nitrogen in the atmosphere is practically unlimited, 

 but it is fixed and rendered assimilable only by organic processes of 

 extreme slowness. We may shortly glance at these, for it is upon 

 these processes, plus a return to the soil of sewage, that we must 

 depend in the future for storing nitrogen as nitrates. 



1. Some combined nitrogen is absorbed by the soil or plant from 

 the air, for example, fungi, lichens, and some algse, and the absorption 

 is in the form of ammonia and nitric acid. This is admittedly a 

 small quantity. 



2. Some free nitrogen is fixed within the soil by the agency of 

 porous and alkaline bodies. 



3. Some, again, may be assimilated by the higher chlorophyllous 

 plants themselves, independently of bacteria (Frank). 



4. Electricity fixes, and may in the future be made to fix more, 

 nitrogen. If a strong inducive current be passed between terminals, 

 the nitrogen from the air enters into combination with the oxygen, 

 producing nitrous and nitric acids. 



5. Abundant evidence has now been produced in support of the 

 fact that there is considerable fixation by means of bacteria. 



Bacterial life in several ways is able to reclaim from the atmo- 

 sphere this free nitrogen, which would otherwise be lost. The first 

 method to which reference may be made is that involving 

 symMosis. This term signifies " a living together " of two difierent 

 forms of life, generally for a specific purpose. Marshall Ward has 

 recently defined it as the co-operation of two associated organisms to 

 their mutual advantage, each symbiont being incapable of carrying 

 on alone the work which the symbiotic association is able to per- 

 form.f It is convenient to restrict the term symbiosis to comple- 

 mentary partnerships such as exist between algoid and fungoid 

 elements in lichens, or between unicellular algse and Eadiolarians,| 



* Sir John Lawes and Sir Henry Gilbert {Times, 2nd December 1898) have 

 pointed out that the addition of nitrates only would be of no permanent use to the 

 wheat crop. They rely upon thorough tillage and proper rotation of crops as the 

 means of improving the nitrogen value of the soil. 



t British Association for the Advancement of Science Report, 1899, p. 693. 



J Geddes, Nature, xxv., 1882. 



