136 



NATURE AND PROPERTIES OF SOILS 



ment of colloidal matter may seriously interfere with aeration 

 and capillary movement of water. The loosening of a clay 

 soil tends to ameliorate such conditions and to counteract 



1000 Z000 5000 4000 5000 C.C. 



Fig. 23. — Curves showing the absorption of P0 4 in parts per million by 

 various soils from a solution of mono-calcium phosphate containing 

 200 parts to the million of P0 4 . The volume of the percolate is 

 used as the abscissas. Such absorption is a rough measure of the 

 colloidal content of a soil. 



the unfavorable influence of the colloidal condition of the 

 soil. Such a structural condition is largely ascribed to the 

 plasticity and cohesion 1 of the soil, which are in turn, of 



1 Any material which allows a change of form without rupture and 

 which will retain this form when the pressure is removed, is said to be 

 plastic. Putty with a proper admixture of oil is a very good example 

 of a plastic body. As is well known, various materials differ "in 

 plasticity. 



Very closely correlated with plasticity, but not in exact similarity, is 

 cohesion. By the cohesion of a soil is meant the tendency that its 

 particles exhibit in sticking together and in conserving the mass intact. 



