26 PHYSICAL CONSTITUTION OF SOIL 



element is largely composed of calcium carbonate. The true 

 colloid clay is always an aluminium silicate. 



The character of a natural clay is largely determined by 

 the size of the particles which form its chief bulk. Hilgard 

 remarks that the groups of coarser particles, numbered 1-9 

 in Table I, all tend to diminish the tenacity of a soil, while 

 Groups 11-13 increase it. The same amount of true clay will 

 thus generally produce a far more tenacious soil when 

 associated with fine particles than when associated with 

 coarse ones. Curious cases sometimes arise of ' putty soils/ 

 drying to extreme hardness, which contain much coarse sand, 

 while extremely little clay is shown by Hilgard's method 

 of analysis. In such soils there is always sufficient silt to 

 fit in between the coarse particles of sand. We must also 

 bear in mind that the cementing power of clay is confined 

 to the small amount of colloid matter present, and that the 

 proportion of this is not shown by the analysis. 



Though the tenacity of heavy soils is largely due to the 

 clay which they contain, it is by no means necessarily in 

 proportion to the amount of clay present. Not only has 

 the fineness or coarseness of the other soil constituents an 

 influence on the cohesion of the mass, the nature of these 

 constituents may also have a considerable effect. The presence 

 of lime and of humus in a clay soil may diminish its tenacity 

 very considerably ; ferric oxide, according to Hilgard, also 

 acts in the same direction. For these substances to affect 

 the tenacity of the soil they must be thoroughly distributed 

 throughout it. Aggregations of oxide of iron or of carbonate 

 of calcium occurring in a soil will scarcely affect its general 

 physical texture. We must always remember that the dis- 

 tribution or aggregation of the constituents in a soil is not 



