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326 HOW CROPS FEED. 
ing. Matters insoluble in water to-day become soluble 
to-morrow, and substances that to-morrow resist the action 
of water are taken up the day after. In this way there 
is no limit to the solution of the soil, and we cannot there- 
fore infer from what the soil yields to water 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 the roots of a crop 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- 
sented to vegetation by water in sufficient quantity, does 
not entirely satisfy us in reference to some of the needful 
elements of the plant, 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 which rocks are converted into soils, the 
action of the organic acids of the living plant has been 
mentioned. Since that chapter was written, further evi- 
dence has been obtained concerning the influence of the 
plant on the soil, which we now proceed to adduce. 
Sachs (Zixperimental Physiologie, 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 depth with fine quartz sand. 
Seeds of various plants were planted in the sand and kept 
