82 



THE TEXTURE OF THE SOIL 



[chap. 



seen when organic manures are applied late in the 

 season and dry hot weather succeeds j the soil water at 

 the top in contact with the manure has a lower surface 

 tension and consequently less lifting power for the 

 subsoil water ; hence any shallow rooted crop is deprived 

 of some of the subsoil water which would have otherwise 

 been lifted to it. A rise of temperature diminishes the 

 surface tension of water, and therefore lessens the 

 sustaining power of the film ; as it also lessens the 

 viscosity of water, it will often cause percolation to 

 begin afresh from soil that had apparently ceased to 

 yield any more drainage. This effect may sometimes 

 be seen in variations in the flow of land drains or in the 

 level of water in shallow wells. The following table 

 shows the comparative surface tension of water and 

 various solutions: — 



Cohesion caused by Surface Tension. 



In certain cases the stretched film surrounding soil 

 particles will give them an apparent cohesion by 

 enclosing them and drawing them together. A handful 

 of wet sand can be moulded into shape, but falls in 

 pieces as soon as it is dry : just as in a camel-hair 

 pencil the bristles, which stand apart when dry or 

 wholly immersed in water, are drawn together to a 



