410 SOILS: PROPERTIES AND MANAGEMENT 



the crop on the soil to which dicalcic phosphate had been 

 applied than did the soil not receiving carbon dioxide; 

 but the soil to which no phosphate was added yielded 

 equally well whether it received carbon dioxide or not. 

 The plants used were oats, peas, and lupines. These 

 investigators conclude that carbon dioxide is not a suffi- 

 cient solvent to account for the mineral nutrients obtained 

 from soils by plants. 



326. The present status of the question. — The avail- 

 able evidence on excretion of acids other than carbonic 

 by the roots of plants does not admit of any very satis- 

 factory 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 decomposition 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 from .5 to nearly 10 per cent 

 of the soil air, or several hundred times that of the at- 

 mospheric air, must aid greatly in dissolving the soil 

 particles. 



Whatever may be the concentration of the soil water, 

 it seems probable that the liquid which is 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. That portion of the 

 soil water immediately in contact with the soil grain is a 

 much stronger solution than the water farther from the 

 soil surfaces, because of the adsorptive action of the 

 particles. 



Many plants grown in solutions of nutritive salts have 

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



