406 SOILS: PROPERTIES AND MANAGEMENT 



nitrogen and fifty times as much potassium as can be 

 extracted from the same volume of soil with pure water 

 or with water containing carbon dioxide. 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 rain in its descent through the atmosphere, 

 and further added to by the carbon dioxide and the or- 

 ganic and mineral substances obtained from the soil. It 

 has been held that the plant roots aid solution of mineral 

 matter by excretion of acids, which act effectively 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 investigators, grew 

 plants of various kinds in soil and other media in which 

 was placed a slab of polished marble or dolomite or cal- 

 cium phosphate, covered with a layer of washed sand. 

 After the plants had made sufficient growth the slabs were 

 removed, and on the surfaces were found corroded trac- 

 ings, corresponding to the lines of contact between the 

 rootlets and the minerals. 



322. Czapek's experiments. — In order to test this 

 theory, Czapek^ repeated the experiments of Sachs, 

 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 cor- 

 roded by the plant roots, plates of aluminium phosphate 



^ Sachs, J. Auflosung des Marmors dureh Mais-Wurzeln 

 Bot. Zeitung, 18 Jahrgang, Seite 117-119. 1860. 



2 Czapek, J. Zur Lehre von den WurzelausseheidTing. Jahrb. 

 f . Wiss. Bot., Band 29, Seite 321-390. 1896. 



