CONSTITUTION OF THE SOIL 247 



sideration. Unlike the coarser fractions which are composed of small frag- 

 ments of unmodified rock minerals such as quartz, feldspar, and mica, the 

 clay portion of soils is made up almost entirely of the products of chemical 

 weathering of the rock minerals and hence differs not only in its physical 

 state, but in its chemical composition from the particles in the coarser frac- 

 tions. Most of the particles in the clay fraction are of colloidal dimensions 

 and exhibit the characteristic properties of colloidal systems. The particles of 

 the clay complex, like those of many other colloidal systems, retain water by 

 imbibition, i.e. within the structure of the particle. On the contrary the sand 

 and silt particles of the soil retain water only on their surfaces. The presence 

 of a considerable proportion of clay in a soil therefore endows it with a high 

 water retaining capacity. Changes in the water content of colloidal clay 

 result in marked changes in its volume. One result of such changes in volume 

 is the commonly observed cracking of a soil rich in clay upon drying. The 

 plasticity and cohesiveness of soils are also due very largely to the colloidal 

 clay present. 



Like other colloidal systems colloidal clay is markedly sensitive to the 

 influence of electrolytes. The micelles of colloidal clay usually bear a nega- 

 tive charge when in contact with water. In the presence of calcium ions the 

 individual clay particles are more or less completely flocculated into compound 

 particles. Much of the colloidal clay in most soils exists as enveloping films 

 around the larger soil particles and is also closely associated with organic 

 material in the soil. Flocculation of the clay particles therefore usually results 

 in the formation of compound granules including sand and silt particles and 

 organic matter in addition to the clay. These are called soil crumbs. For 

 agricultural purposes a well developed granular structure of the soil is highly 

 desirable since such a structure favors both a high moisture-retaining capacity 

 and a good aeration of the soil. "Calcium soils," that is, those with a high 

 calcium content, are in general the most suitable and most valuable for agri- 

 cultural purposes partly because calcium favors the development of a crumb 

 structure. In the presence of an excess of univalent cations (Na+, etc.), on 

 the other hand, a greater proportion of the clay fraction of the soil disperses 

 into its ultimate particles, and the soil has a single grain structure. In this 

 condition the soil is in the least desirable structural condition for agricultural 

 purposes. Many soils contain hydrogen ions in excess. Such soils often de- 

 velop a good crumb structure, but the crumbs are less stable than those de- 

 veloped in a calcium soil. The addition of lime improves the physical struc- 

 ture of such soils. The crumb structure of a soil, especially in its surface 

 layers, can also be destroyed by purely mechanical effects, such as trampling, 

 or beating by heavy rains. 



