SOIL STRUCTURE AND ITS MODIFICATION 143 



localized, the internal strains and pressures are unequal and 

 a tendency arises for the mass to divide along lines of weak- 

 ness into groups of particles. The binding capacity of col- 

 loidal material, as well as of salts deposited from the soil 

 solution, tends to make such a crumb structure more or less 

 permanent. The moisture content most favorable for granu- 

 lation seems to be that which is optimum for plant growth. 1 



78. — Forces facilitating granulation. 2 — Granulation is 

 nothing more or less than a favorable condition brought 

 about by the force exerted by a variable water film and the 

 pulling and binding capacities of colloidal material, operating 

 at numberless localized foci. It is evident that any influence 

 or change in the soil which will cause a greater localization 

 of these operative forces will promote the aggregation of the 

 particles. The addition of materials from extraneous sources 

 is also a practice that may tend to develop lines of weakness 

 and thus cause a more intense activity of the forces at work. 



The conditions, additions, and practices tending to develop 

 or facilitate a granular structure in soils may be listed under 

 six heads: (1) wetting and drying of the soil, (2) freezing 

 and thrawing, (3) addition of organic matter, (4) action of 

 roots and animals, (5) addition of lime and (6) tillage. Only 

 the last two need additional consideration. 



79. Granulating influence of lime. 3 — One of the effects 

 of lime in the soil, especially of the oxide and hydroxide forms, 



1 Cameron, F. K., and Gallagher, F. E., Moisture Content and Physical 

 Condition of Soils; U. S. Dept. Agr., Bur. Soils, Bui. 50, p. 8, 1908. 



2 Fippin, E. O., Some Causes of Soil Granulation; Trans. Amer. Soc. 

 Agron., Vol. 2, pp. 106-121, 1910. Czermak, W., Ein Beitrag zur Erkent- 

 uis der Veranderungen der Sog physikalischen Bodeneig enshaften durch 

 Frost, Hitze, und die Beigabe einiger Salze; Landw. Ver. Stat., Band 

 76, Heft 1-2, Seite 73-116, 1912. Also, Ehrenberg, P., und Romberg, 

 G. F. von, Zur FrostwirJcung auf den Erdboden; Jour. f. Landw. 

 Band 61, Heft 1, Seite 73-86, 1913. 



3 Lime in a strictly chemical sense refers only to calcium oxide (CaO). 

 The term is used here with an agricultural meaning, including all cal- 

 cium and magnesium compounds which are ordinarily added to the soil 

 to correct acidity, thus including not only calcium oxide but calcium 

 hydroxide and calcium carbonate [Ca(OH) 2 and CaC0 3 ] as well. 



