298 NATURE AND PROPERTIES OF SOILS 



absorption of bases and rejection of the acid radicals of the 

 dissolved salts. It is conceivable that the root-hairs, by re- 

 moving bases from the solution existing between the cell-wall 

 and the colloidal covering of the soil particle, may cause 

 coagulation of the colloidal matter and thus liberate the nu- 

 trient materials held by absorption. The liberated material, 

 being of a readily soluble nature, would be taken up by the 

 solution between the rootlet and the soil particle, from which 

 the root-hair could readily absorb it. Such an hypothesis 

 would account for the ability of plants to obtain a quantity 

 of nutrients far in excess of that accounted for by the solvent 

 action of pure water, and even beyond what many investi- 

 gators are willing to attribute to the solvent action of water 

 charged with carbon dioxide. 



157. The present status of the question. — The available 

 evidence on excretion of acids other than carbonic by the 

 roots of plants does not admit of any very satisfactory conclu- 

 sion as to their relative importance in the acquisition of plant 

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

 dioxide resulting from root exudation and from decomposi- 

 tion of organic matter in the soil plays a very prominent part 

 in this operation. The very large quantity of carbon dioxide 

 in the soil, amounting in some cases 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 that is found where the root- 

 hair comes in contact with the soil particle, and that is sepa- 

 rated, in part at least, from the remainder of the soil-water, 

 must have a composition different from that found elsewhere 

 in the soil. Many plants grown in solutions of nutritive salts 

 have few or no root-hairs, but absorb through the epidermal 

 tissue of the roots. The special modification by which the 

 root-hairs come in intimate contact with the soil particle and 

 almost surround it, indcates a direct relation between the 



