5 oo THE POPULAR SCIENCE MONTHLY. 



The rnicro-organisms that are found in great variety in soils 

 must have an important influence on the processes of metabolism 

 that are constantly taking place in the soil itself ; and the results 

 of their activities, which are not limited to processes of putrefac- 

 tion and nitrification, can not be measured solely by the amount 

 of nutritive materials appropriated. In my own experiments with 

 soil-microbes they have proved their ability to take their required 

 supplies of lime and potash from solid fragments of gypsum and 

 feldspar, and even from the glass tubes in which cultures were 

 made, which were deeply etched by their action. 



The roots of plants undoubtedly aid in determining condi- 

 tions of the soil that favor the vital activities of certain microbes, 

 and interfere with the well-being of others of different habits ; 

 and the plants, in their turn, are presumably benefited by the 

 activities of the microbes best adapted to the prescribed conditions. 

 In the struggle for existence the dominance of these favored 

 forms can not, however, be indefinitely maintained. The roots 

 of one species of plant and their associated microbes, in appropri- 

 ating their required supplies of nutritive materials, induce a 

 metabolism of the soil that, sooner or later, renders it better fitted 

 for other species of plants and other microbe associates; and 

 these, in their turn, prepare the way for species of still different 

 requirements in their processes of nutrition. 



Soil metabolism, and the involved liberation or elaboration of 

 plant food, will thus be promoted by a succession of plants of 

 different habits of growth, each with its associated microbes ; and 

 the elements of fertility stored in, or permeating the soil, must, 

 under such conditions, be more completely utilized. 



It is practically misleading and inaccurate to say that legu- 

 minous plants appropriate the free nitrogen of the atmosjmere. 

 The evidence clearly shows that the soil-microbes which find 

 favorable conditions for the exercise of their vital activities in 

 the vicinity of, or in contact with, the roots of leguminous plants, 

 are able to make use of the free nitrogen that permeates the soil, 

 and that it is thus made available as combined nitrogen in the 

 nutrition of the higher chlorophyl-bearing leguminous plants. 

 The latest investigations are, therefore, strictly in accordance with 

 the earlier experiments by Boussingault, and at Rothamsted, in 

 showing that the soil is the source of the nitrogen of plants, and 

 we must look to soil conditions as essential factors in determin- 

 ing the vital activities of the microbes that bring free nitrogen 

 into the combined form that is available for the nutrition of the 

 higher plants. 



It must be admitted that red clover appropriates nitrogen that 

 has been prepared for it from the free nitrogen of the soil through 

 the agency of its symbiont microbes, but it is well known that it 



