326 TiOW CROPS FEED. 



ing. Matters insoluLle in water to-day become soluble 

 to-morrow, and substances that to-morrow icsist the action 

 of water are taken up the day after. In this way tliere 

 is no limit to the solution of the soil, and we cannot there- 

 fore infer from what the soil yields to Avater at any given 

 moment nor from what is taken out of it by any given 

 amount of water, the real extent to which aqueous action 

 operates, during the long period of vegetable growth, to 

 present to tlie roots of a croj) the indispensable ingredi^ 

 ents of its food. 



The discussion of the question as to the capacity of 

 water to dissolve from the soil enough of the various in* 

 gredients to feed crops, while satisfactorily establishing 

 this capacity in case of rich soils, and making evident 

 that in poor soils most of the inorganic matters are pre- 

 isented to vegetation by water in sufficient quantity, does 

 not entirely satisfy us in reference to some of the needful 

 elements of the ]ilant, especially phosphoric acid. 



It is therefore appropriate, in this place, to pursue fur- 

 ther inquiries into the mode by which vegetation acquires 

 food from the soil, although to do so will somewhat inter- 

 rupt the general plan of our chapter. 



Direct action of Roots upon the Soil. — In noticing 



the means by whicli rocks are converted into soils, the 

 action of the organic acids of the living plant has been 

 mentioned. Since that chapter was written, fuitlier evi- 

 dence has been obtained concerning the influence of the 

 plant on the soil, whicli we now proceed to adduce, 



Sachs [Experimental Physlologie^ 189) gives an ac- 

 count of observations made by him on the action of roots 

 on marble, dolomite (carbonate of lime and magnesia), 

 magnesite (carbonate of magnesia), osteolite (phosphate 

 of lime), gypsum, and glass. Polished plates of these 

 substances were placed at the bottom of suitable vessels 

 and covered several inches in dept'.i with fine quartz sand. 

 Seeds of various plants were planted in the sand and kepi 



