288 THE PRINCIPLES OF SOIL MANAGEMENT 



wall forms a contact so close as to practically make the 

 solution between the particle and the cell-wall distinct 

 from that between the soil-particles. 



There has been considerable difference of opinion 

 as to how 7 the plant can obtain its mineral nutrients 

 from a substance so difficultly soluble as the soil. It has, 

 of course, been recognized that the soil-water is aided 

 in its solvent action by a variety of substances that may 

 be normally present in solution, beginning with the 

 gases taken up by the rain in its descent through the 

 atmosphere, and further aided by the carbon dioxide 

 and organic and mineral substance obtained from the 

 soil. It has been held that the plant-roots aid solution 

 of mineral matter by excretion of acids, which act effec- 

 tively as solvents. The well-known root-tracings on 

 limestone and marble have been taken as proof of the 

 excretion of such acids. Sachs and, later, other investi- 

 gators grew plants of various kinds in soil and other 

 media in which was placed a slab of polished marble 

 or dolomite or calcium phosphate, covered with a layer 

 of washed sand. After the plants had made sufficient 

 grow r th, the slabs were removed and on the surfaces 

 were found corroded tracing, corresponding to the lines 

 of contact between the rootlets and the minerals. 



In order to test this theory, Czapek repeated the 

 experiments, using plates of gypsum mixed with the 

 ground mineral that he wished to test, and this mixture 

 he spread over a glass plate. Using these plates in the 

 same manner as previously described, Czapek found 

 that, while plates of calcium carbonate and of calcium 

 phosphate were corroded by the roots, plates of alumi- 



