THE ABSORPTIVE PROPERTIES OF SOILS 365 



silicates would be destroyed by the same treatment. 

 On the whole, the evidence appears to be in favor of 

 the dominance of colloidal absorption rather than 

 crystalloidal absorption by soils, with its important 

 function in conserving soluble fertilizers and retaining 

 a supply of plant nutrients in a more or less readily 

 available condition. 



267. Absorption by organic matter. — The partially 

 decomposed organic matter in soils, especially that part 

 which has undergone such transformations as to form 

 humus (par. 90), has an absorptive power. Soils rich 

 in humus, without doubt, owe much of their fertility 

 to the retention by that constituent of a large supply of 

 readily available plant-food material. Many prairie soils 

 that have been reduced in productiveness under culti- 

 vation respond to the application of organic matter in a 

 remarkable manner. Humus in these soils seems to be the 

 chief conserver of readily available plant-food materials. 



Van Bemmelen, 1 who has studied these compounds, 

 states that soils hold colloidal humous compounds con- 

 taining ammonia, potassium, sodium, and other sub- 

 stances, as well as iron oxide. A part is soluble, or forms 

 soluble compounds with alkalies, but the principal part 

 is insoluble. Some of these latter compounds are of a 

 colloidal nature and of changing composition. The soluble 

 matter is easily precipitated by a salt solution and carries 

 down with it bases from the solution. Absorption of bases 

 also takes place from solution, with substitution of one 

 base for another. Potassium is more strongly held in 

 combination than is calcium or magnesium. Bases are 

 removed, however, only from salts of the weaker acids. 



1 Van Bemmelen, J. M. Die Absorption, Seite 135-141. 

 Dresden, 1910. 



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