134 NATURE AND PROPERTIES OF SOILS 



probable fate of the silica as the process is strongly one of 

 hydration. 

 74. Influence of colloidal material * on soil properties. — 



*The amount of matter in a colloidal state in soils is extremely 

 variable, ranging from almost nothing in sand to a very large percentage 

 in heavy plastic clays. There is no satisfactory means of finding the 

 amount of colloidal material in soil. All of the available methods depend 

 for their expression on the intensity of certain qualities, supposed to 

 be developed by colloid content. This indicates that the methods are 

 largely comparative rather than exact or strictly analytical in nature. 



Ashley 's method depends on the absorption of certain dyes to indicate 

 the relative amount of material in a colloidal state. The difficulty in this 

 method, however, lies in choosing the most effective dye and regulating 

 its concentration. Moreover, different colloids vary so much in absorp- 

 tive capacity for the same dye, that only roughly comparative results 

 have thus far been possible. 



Mitscherlich uses the absorptive capacity of the soil for water vapor 

 as a colloidal index. In this method the air-dry soil in a thin layer is 

 brought to absolute dryness over phosphorus pentoxide. It is then 

 placed in a desiccator over a 10 per cent, solution of sulfuric acid and 

 the condensation is hastened by a partial vacuum. The sulfuric acid 

 is used in order to prevent the deposition of dew on the soil. After 

 exposure for about twenty-four hours, the soils are found to have taken 

 up their maximum moisture of condensation, which is called the hygro- 

 scopic water. The soil is then weighed, and the increase, figured to a 

 percentage based on dry soil, is taken as a measure of colloidal content. 

 The reverse process may also be followed, by exposing air-dry soil in a 

 saturated atmosphere and afterwards drying over phosphorus pentoxide. 

 The hygroscopicity of the soil, or its hygroscopic coefficient, is thus the 

 basis for colloidal comparison. 



Ashley, H. E., The Colloid Matter of Clay and Its Measurement; 

 U. S. Geol. Survey, Bui. 388, 1909. 



Rodewald, H., und Mitscherlich, A. E., Die Bestimmung der Hygro- 

 skopizitat; Landw. Ver. Stat., Band 59, Seite 433-441, 1903. Also, 

 Mitscherlich, E. A., und Floess, R., Ein Beitrage zur Bestimmung der 

 Hygroskopizitat und zur Bewertung der physikolischen Bodenanalyse ; 

 Internat. Mitt. f. Bodenkunde, Band 1, Heft 5, Seite 463-480, 1912. 



Ehrenberg, P., und Pick, H., Beitrage zur Physikolischen Bodenunter- 

 suchung; Zeit. f. Forst- und Jagdwesen, Band 43, Seite 35-47, 1911. 

 Also, Vageler, P., Die Bodewald-Mitscherlichsche Theorie der Hygro- 

 skopizitat vom Standpunkte der Colloidchemie und ihr Wert zur Beur- 

 teitung der Boden; Fuhling's Landw. Zeit., Band 61, Heft 3, Seite 73-83, 

 1912. 



Stremme, H., and Aarnio, B., Die Bestimmung des Gehaltes anorgan- 

 ischer Kolloide in Zersetzten Gesteinen und deren tonigen Vnlagerungs- 

 produkten; Zeitsch. f. Prak. Geol., Band 19, Seite 329-349, 1911. 



Tempany, H. A., Shrinkage in Soils; Jour. Agr. Sci., Vol. VIII, 

 Pt. 3, pp. 312-330, June, 1917. 



Beaumont, A. B., Studies in the Beversibility of the Colloidal Condi- 

 tion of Soils; Cornell Agr. Exp. Sta., Memoir 21, Apr., 1919. 



