CEMENTING MATERIALS IN SOIL 25 



chalk, and humus is greatest when they hold sufficient water 

 to fill up all the finer spaces between their particles. As 

 these substances dry, they lose to a great extent the coherence 

 they possessed when wet. Clay, on the other hand, increases 

 greatly in coherence as it dries, and finally becomes a hard, 

 solid substance. These are fundamental facts to bear in mind 

 when conducting tillage operations on a farm. 



Cementing Materials. We have seen that as the particles 

 diminish in size, and their total surface increases, there is 

 a marked increase in the cohesion of the mass. The difference 

 in the size of the particles is not, however, the only circum- 

 stance which determines the different degrees of cohesion 

 observed in different soils. Soils, in fact, contain various 

 cementing materials, the presence of which has a marked 

 influence on their tenacity. The principal of these cementing 

 materials is clay. 



We have already mentioned (p. 16) that ordinary clay 

 consists of extremely fine particles held together by a small 

 proportion of a colloid body. Its constitution thus resembles 

 that of putty, in which the particles of whiting are united by 

 means of linseed oil. In typical clay (kaolin, A1 2 O 3 2SiO 2 

 2tL,0), derived from the decomposition of felspar, the whole 

 substance has practically the same chemical composition, the 

 various sediments into which it may be divided all containing 

 the same percentages of silica and alumina. The small 

 portion of the clay which possesses a colloid character is 

 probably, however, more hydrated than the rest. The clays 

 ordinarily met with in soil are not, however, chemically homo- 

 geneous, the fine sand which they contain commonly consists 

 of quartz particles, though it may at times have a different 

 origin. Thus in marls we have a clay in which the sandy 



