METHODS OF MECHANICAL ANALYSIS 5 



of a soil is of course increased when the particles composing 

 it are themselves porous. 



Many of the most important properties of soil depend on 

 the extent of the internal surface. We must always bear 

 in mind that the whole of the work done by the solid matter 

 of a soil takes place on the surface of its particles. The 

 retentive power of a soil for water is determined almost 

 entirely by the extent of its internal surface ; the retentive 

 power of a soil for manure is influenced also by the same 

 circumstance. With an increase in the internal surface the 

 feeding ground of the roots of plants is also plainly increased. 



An increase in the fineness of the particles beyond a certain 

 point is, however, attended with disadvantages. The cohesion 

 between the particles of a soil is enormously increased by an 

 increase in their surface. The resistance to tillage, and the 

 resistance to the passage of roots becomes greater, and the 

 soil is said to be ' heavy/ A soil composed of very fine 

 particles is also with difficulty permeable to either water or 

 air, owing to the immense friction which attends the move- 

 ment of any fluid through the minute interstices between 

 the particles. We shall return to this subject in detail 

 further on, at present we may just mention that the greatest 

 number of advantages are secured when the internal surface 

 is large, but the soil consists mainly of porous compound 

 particles, the size of which allows any excess of water to be 

 removed by drainage. 



Mechanical Analysis. The physical properties of a soil 

 are largely dependent on the size of the particles of which 

 it is composed. Every one recognizes at once the immense 

 difference there is between a coarse sand, a loam, and a clay ; 

 the differences between these soils, which so greatly influence 



