ISS THE CHEMICAL ANALYSIS OF SOILS [chap. 



water does ; they diffuse through the cell wall because 

 the sap within is maintained in a less concentrated 

 state as far as they are concerned than the external 

 soil water, because they are constantly being withdrawn 

 from solution by the living protoplasm of the cells. 



It has been supposed that solvent action of the soil 

 water is also assisted by the cell sap of the root-hairs, which 

 is always distinctly acid in its reaction ; these root-hairs 

 are always very closely in contact with soil particles, 

 and some of the acid has been supposed to diffuse out- 

 wards through the cell wall. Sachs has shown that a 

 polished slab of marble is etched wherever the fine 

 roots of a plant came in contact with it, and on the 

 strength of this and similar experiments, the cell sap 

 has been regarded as a factor in bringing the minerals 

 of the soil into solution for the plant. All the solvent 

 actions, however attributed to the cell sap, can be 

 brought about by the carbon dioxide which is always 

 being excreted by the root, and more critical experiments 

 seem to negative the opinion that any fixed acids pass 

 outwards through the cell wall of a living plant, at any- 

 rate after it has passed the seedling stage. 



Whatever the theories which have been formed as 

 to the manner in which the mineral constituents of the 

 soil pass into solution for the plant, it is improbable that 

 the conditions can be reproduced in the laboratory, and 

 for the practical purposes of analysis the desideratum is 

 a solvent that will dissolve the class of material which 

 is found by experience to reach the immediate crop, but 

 which will not touch the same material should its state 

 of combination or physical condition be such as to render 

 it unavailable for the plant. Various solvents have been 

 proposed: for example, Deherain showed that dilute 

 acetic acid, while dissolving some phosphoric acid from 

 ordinary soils, was incapable of extracting any from a 



