628 MISCELLANEOUS FARM SUBJECTS 



tions are acted upon by different organisms or groups of bacteria 

 which produce it. 



In general, organic materials contain three classes of com- 

 pounds: (1) The nitrogenous or albuminoid compounds like flesh 

 and blood of animals and the protoplasm of plant cells; and (2) 

 the non-nitrogenous compounds or the carbohydrates (such aa 

 sugars, starches and cellulose) ; and (3) the hydro-carbons or fats. 

 The first class contains the nitrogen formerly taken from the soil as 

 nitrates by some plant, but which in its highly organized form is 

 unavailable for crops until converted into ammonia or nitrates again 

 by certain bacteria. The compounds of the second class serve as 

 food for certain bacteria which are enabled to obtain nitrogen from 

 the air. 



Fortunately, there are still other means by which nitrogen gas 

 may be made available for plant food, and that, too, without re- 

 quiring the introduction of a commercial product, whicji must al- 

 ways be rather expensive, whatever degree of perfection may be 

 reached in the mechanical operation of the process. Ever since the 

 earliest days of agricultural science it has been noticed that certain 

 land, if allowed to stand fallow for a considerable length of time, 

 would gain in nitrates without any visible addition having been 

 made. It is now known that one of the principal means of this in- 

 crease in nitrogen content is due to a few forms of soil bacteria 

 which have the power of fixing the free nitrogen from the air and 

 rendering it available for plant food. These organisms have been 

 isolated and cultivated artificially, and great hopes were held at one 

 time that it would be possible to inoculate land with these cultures 

 and thus bring about a large increase in the nitrogenous salts with- 

 out the aid of any manure or mineral fertilizer. Under certain con- 

 ditions these bacteria seemed able to do a large amount of work, 

 and there are experiments on record where the crops raised from 

 plots inoculated with nitrogen-fixing organisms were much greater 

 than crops from uninoculated land. 



Ever since the time of Pliny and other early writers upon agri- 

 cultural topics, it has been known that certain leguminous crops, 

 such as clover, beans, peas, etc., did not require the same amount 

 of fertilizer as other plants, and indeed it seemed as though they 

 actually benefited the soil instead of being a detriment. Various 

 theories have been advanced to account for this effect, perhaps the 

 most widespread opinion being that members of this family, owing 

 to the unusual length and strength of their root system, were able 

 to draw upon a store of food that was not available to wheat and corn 

 and other crops not belonging to the pod-bearing group. It is only 

 within a comparatively recent time that the real cause of the bene- 

 ficial effect of these legumes has been fully understood, and it seems 

 that here again the bacteria are responsible for the nitrogen-gather- 

 ing power; for it is because these plants are able to fix and use the 

 free nitrogen of the air that they are of such benefit in rotation and 

 in reviving poor and exhausted land. The immense yields of 

 wheat following alfalfa or clover are easily understood when it is 



