THE COLLOIDAL MATTEB OF THE SOIL 137 



course, governed by the amount and the quality of colloidal 

 matter present.^ 



In general it is found that, other conditions being equal, 

 an increase of certain types of colloidal matter increases plas- 

 ticity; in other words, the ease with which a soil may be 

 worked into a puddled condition becomes greater. This is a 

 rather undesirable quality when too pronounced, and in clays, 

 in which it is most likely to be developed because of the pres- 

 ence of large amounts of mineral colloids, some means of 

 decreasing the colloidal influence is advisable. This great 

 plasticity is developed because the colloids, especially those 

 of a gelatinous and viscous nature, facilitate the ease with 

 which the particles may move over one another and yet cohere 

 sufficiently to prevent disruption of the mass. In general, 

 also, the greater the plasticity of a soil, the greater is the 

 cohesion when dry. In soils, then, in which certain kinds of 

 colloidal materials are very high, clodding may occur if the 

 soil is tilled too dry because of the great tendency of the par- 

 ticles to cohere. Cohesion and plasticity, as factors in soil 

 structure, soil granulation, and tilth will receive further atten- 

 tion later. 



It must not be inferred from the preceding discussion that 

 the generation of colloidal matter is always detrimental to 

 soil conditions. In sandy soils the presence of such material 

 is extremely beneficial as it tends to bind the soil together, 

 promotes granulation, and prevents loss of plant nutrients by 

 leaching. It is only in heavy soils in which excessive amounts 

 of mineral colloids may develop that a detrimental condition 

 is likely to exist. This occurs because of a high cohesion and 

 plasticity, because of the absorption of plant nutrients and 

 because of tendencies toward acidity. The addition of organic 



=^ Davis, ISr. B., Tlie Flasticity of Clay; Trans. Amer. Cer. Soc, Vol. 

 16, pp. 65-79, 1914. Cushman, A. S., The Colloid Theory of Flastioitp; 

 Trans. Amer. Cer. Soc, Vol. 6, pp. 65-78, 1904, Also, Ashley, H. B„ 

 The CoUo%d Matter of Clay and Its Measurement; U. S. Greol. Survey, 

 Bui. 388, 1909. 



