38 PHYSICAL CONSTITUTION OF SOIL 



power, and if accomplished would be but a poor imitation 

 of what actually takes place in nature. The favourable 

 texture of soil of which we are speaking may be brought 

 about without any use of implements it is due to the 

 formation of compound particles in the soil. 



In an ordinary fertile surface soil the particles are to a 

 large extent associated in groups ; the atoms are, so to speak, 

 built up into molecules ; the fine constituents do not behave 

 as separate entities, they form part of larger compound 

 particles. This condition is highly favourable to fertility. 

 In a soil consisting solely of uniform particles, all of one size 

 and substance, the formation of compound particles could 

 not arise, the force of cohesion being in every direction the 

 same. Compound particles also do not arise in soils composed 

 wholly of coarse sand, consisting of particles with a diameter 

 not less than 0-2 mm., the cohesion being in this case too 

 small. Compound particles are produced in soils composed 

 of mixed particles, including a considerable proportion of 

 the finer sizes, and the formation is greatly aided by the 

 presence of colloid constituents capable of acting the part 

 of cement. 



A soil may, however, have a constitution very favourable 

 to the formation of compound particles, and yet none may 

 exist ; they are formed under some conditions, and destroyed 

 by others. Let us take as an example a good friable loam. 

 If a spadeful of this is taken when very wet, placed on 

 a hearth or paving stone, and thoroughly beaten, we obtain 

 a cake of very stiff mud, utterly unsuitable for the purposes 

 of vegetation, and which will dry into a hard stony mass. 

 By this treatment the compound particles have been shattered 

 and destroyed, and the soil transformed into an aggregation 



