412 SOILS: PBOPERTIES AND MANAGEMENT 



an hypothesis would account for the ability of plants 

 to obtain a quantity of nutrient materials far in excess of 

 what can be accounted for by the solvent action of pure 

 water, and even beyond what many investigators are 

 willing to attribute to the solvent action of water charged 

 with carbon dioxide. 



328. Why crops vary in their absorptive powers.— 

 As has already been pointed out (pars. 331-336), crops of 

 different kinds vary greatly in their ability to draw 

 nourishment from the soil. The difference between the 

 nitrogen, phosphorus, and potassium taken up by a corn 

 crop of average size and a wheat crop of average size is 

 very striking. In the table on page 419 it is seen that 

 two tons of red clover contain three times as much potash, 

 nearly ten times as much lime, and somewhat more phos- 

 phoric acid than does a crop of thirty bushels of wheat 

 including the straw. 



The difference in absorbing power may be due to either 

 one or both of two causes : (1) a larger absorbing system ; 

 (2) a more active absorbing system. The former is deter- 

 mined by the extent of the root-hair surfaces ; the latter 

 by the intensity of the osmotic action. 



329. Extent of absorbing system. — Plants with large 

 root systems may be expected to absorb the larger amounts 

 of nutrients from the soil. Such is usually the case, 

 although the extent of the root system is not necessarily 

 proportional to the total area of the absorbing surfaces 

 of the root-hairs. 



330. Osmotic- activity. — The osmotic activity of a 

 plant under any given condition of soil and climate de- 

 pends on : (1) the rapidity and completeness with which 

 the plant elaborates the substances taken from the soil 

 into plant substance, or otherwise removes them from 



