THE PHYSICAL PROPEKTIES OF SOILS 87 



that alkaline substances, such as wood ashes, should not be 

 applied to them. As the soil dries, the cementing power of 

 the colloidal substance increases and forms hard brick- 

 like lumps which are not easily disintegrated by tillage. The 

 hard lumps of soil, however, retain the power of reabsorbing 

 water, and, when moistened, resume the plastic condition. 



When the colloidal clay is kept in the floculent condition, 

 the wet soil becomes much more porous and less tenacious ; 

 also, the lumps formed when the soil dries are not nearly 

 so hard and are easily reduced to that finely pul- 

 verent condition known as tilth, which is essential in the 

 seed beds. Lime possesses this floculating property in 

 pre-eminent degree, and its well-known ameliorating effects 

 on clay soils are largely due to this cause. According to 

 Hilgard, the addition of J per cent, of lime to a sticky 

 clay soil will almost entirely change its character in 

 that respect. Much less than this will produce a marked 

 improvement, and even the relatively small quantity in an 

 ordinary dressing of basic slag may have a certain bene- 

 ficial effect. Acid substances like superphosphate act to 

 some extent in a similar manner. 



The cementing power of colloidal clay appears the more 

 remarkable in view of the small quantity of it found even in 

 soils of the most strongly plastic character. The propor- 

 tion probably never exceeds .2 or 3 per cent., but its in- 

 fluence on the tenacity is, of course, much greater when the 

 other particles of the soil are also small. 



The colligating power of humic acid is probably much 

 greater than that of the colloidal hydrates of alumina, 

 silica, etc. Schloesing found that 1 per cent, of humic 

 acid, in the form of freshly precipitated calcium humate, 

 had the same cementing power as 11 per cent, of plastic 

 clay. Like colloidal clay, all these substances produce a 

 much greater degree of tenacity when they are mixed with 

 smaller particles. They do not remain permanently su$- 



