162 



THE PRINCIPLES OF SOIL MANAGEMENT 



One effect of moisture on porosity is to be noted 

 here. When a dry soil imbibes water it expands, so that, 

 when the porosity is determined in a wet soil, it is always 

 found to be larger than in the same soil when dry. 

 This expansion is greatest in the fine-textured soil, 

 and in muck it is at the maximum. There are several 

 factors which enter into this result, one of which is 

 the tendency of the soil moisture to float the particles, 

 so that they rest together with less force than when 

 the soil is dry. Gallagher has shown that a sample of 

 muck soil having a hygroscopic capacity of above 40 

 per cent lost 29.2 per cent of its original volume in dry- 

 ing from a moisture content of 210 per cent to one 

 of about 80 per cent. 



In Table XXIII on page 154 is given the maximum 

 capacity and approximate wilting point of the soils 



Table XXVII 



