SOIL STRUCTURE AND ITS MODIFICATION 141 



due to the presence of large amounts of complex hydrated 

 aluminum silicates in a colloidal condition. The more plastic 

 a soil becomes, the more likely it is to puddle, 1 especially if 

 worked when wet. Moreover, a soil of high plasticity is prone 

 to become hard and cloddy when dry, due to the cohesive ten- 

 dencies of the small particles. Heavy soils must, therefore, 

 be treated very carefully, especially in tillage operations. If 

 plowed too wet, puddling occurs, the aggregation of particles 

 is broken down, and an unfavorable structure is sure to re- 

 sult. If plowed too dry, great lumps are turned up which 

 are difficult to work down into a good seed-bed. In a sandy 

 soil, no such difficulties are encountered. 2 



Granulation or the production of a compound-grain struc- 

 ture is the only means of correcting the physical condition of 

 a heavy fine-grained soil. In this process the small particles 

 are drawn towards innumerable suitable nucleii and a porous 

 structure is developed. The size of the individual pore spaces 

 is thereby increased and air and water drainage is facilitated. 

 The structural condition in reality simulates a single-grain 

 state with this important difference, however: the particles 

 are porous and not solid. Unless a heavy soil possesses at least 

 some granulation, it is more or less unfit for agricultural 

 operations. (See Fig. 24.) 



77. Granulation. — "While it is possible to list the factors 



auf dem Gebeite des Pflanzenbaues; Band I, Seite 22, 1875. Puchner, 

 H., Untersuchungen uber die Eohareszenz der Bodenarten; Forsch. a. d. 

 Gebiete d. Agri.-Physik., Band 12, Seite 195-241, 1889. Atterberg, A., 

 Die Konsistenz und die Bindiglceit der Boden; Internat. Mitt. f. Boden- 

 kunde, Band II, Heft 2-3, Seite 149-189, 1912. Cameron, F. K., and 

 Gallagher, F. E., Moisture Content and Physical Condition of Soils; 

 U. S. Dept. Agr., Bur. Soils, Bui. 50, 1908. 



a When a soil in a plastic condition has been kneaded until its pore 

 space is much reduced and it has become practically impervious to air 

 and water, it is said to be puddled. The development of gelatinous 

 and viscous colloidal materials seems to be the controlling factor in 

 such a condition, the pore space of a puddled soil being largely filled 

 with such material. When a soil in this condition dries, it becomes hard 

 and dense. 



2 Sandy soils are often plowed rather wet in order to render them 

 more compact than they normally would be. 



