24 PHYSICAL CONSTITUTION OF SOIL 



A farmer is accustomed to classify soils as light and heavy ; 

 this language is to be understood as referring to the resistance 

 which they offer to tillage. A heavy soil is one of great 

 tenacity, the particles of which are held together by a strong 

 cohesive force. When such a soil is dug, or ploughed, great 

 strength is required to perform the work, and the soil is 

 therefore said to behave as if it were heavy. This resistance 

 to the application of mechanical force is due to the cohesion 

 and not to the weight of the soil, the weight of a cubic foot 

 of wet clay is indeed much less than the weight of a cubic 

 foot of sand. 



A considerable degree of stability may be obtained when 

 the individual particles of a soil are very large and heavy, 

 as in the case of a coarse gravel ; in such cases the stability 

 is due to the weight of the particles, and not to any appreci- 

 able extent to the cohesion between them. 



As the particles diminish in size, they become more easily 

 moved, and we obtain blowing sands, such as are seen on our 

 own Norfolk coast, and which occur to a serious extent in 

 other countries. On the other hand, a decrease in the size 

 of particles is attended with a large increase in their total 

 surface, and sand exhibits a considerable degree of coherence 

 when wet, though scarcely any when in a dry state. 



When the particles become extremely fine, corresponding 

 in fact to the groups described as silt in the physical analyses 

 of soil already given, the cohesion of the mass when in a wet 

 state is very similar to that exhibited by clay ; the wet soil 

 is in fact a sticky mud. Soils of this character are often 

 spoken of as clay soils ; their true nature becomes revealed 

 when they dry, as they then easily fall to powder. 



It is important to note that the coherence of silt, sand, 



