290 THE PRINCIPLES OF SOIL MANAGEMENT 



The present status of experimental evidence on excre- 

 tion of acids other than carbonic by the roots of plants 

 does not admit of any very satisfactory conclusion as to 

 their relative importance in the acquisition of plant-food 

 materials. There can be no doubt, however, that carbon 

 dioxide, resulting from root exudation, and from decom- 

 position of organic matter in the soil, plays a very promi- 

 nent part in this operation. The very large quantity 

 of carbon dioxide in the soil, amounting in some cases 

 to from 5 to nearly 10 per cent of the soil air, or several 

 hundred times that of the atmospheric air, must aid 

 greatly in dissolving the soil-particles. 



Whatever may be the concentration of the soil-water, 

 it seems probable that the liquid to be found where the 

 root-hair comes in contact with the soil-particle, and 

 which is separated, in part at least, from the remainder 

 of the soil-water, must have a density much greater than 

 that found elsewhere in the soil. The comparatively 

 rich juices of the plant separated from the soil water 

 only by the delicate cell-walls of the root-hair insures 

 a copious transfer of the constituents of these juices 

 into the intervening water, thus bringing into contact 

 with the soil mineral salts, of which some are doubtless 

 acid salts and also mineral salts of organic acids, and, 

 possibly, some free organic acids. That portion of the 

 soil-water immediately in contact with the soil grain is 

 a much stronger solution than the water further from 

 the soil surfaces on account of the absorptive action of 

 the particles. These solutions, coming in contact with 

 the surface of the soil-particles already subjected to the 

 bacterial and other disintegrating agents of the soil, 



