SOIL MOISTURE 15 



greater in thin layers of soils ; it is not sharply separated 

 from the gravitational water. 



The importance from the botanist's point of view of any 

 classification of soil moisture into different fractions lies 

 not in the objective reality of these, but in the practical use 

 to which it may be put in studying the relation of the water 

 supply to the plant in different soils, and at different degrees 

 of moisture. This relation we shall consider later in the 

 present chapter. 



Relation oi Soil Constitution to Retention of Water. — 

 The amount of water which can be retained is very different 

 in different soils ; it is a complex function of the soil 

 constitution. It is clear that the greater the proportion of 

 fine particles in the soil, the greater will be the total surface 

 and the greater the amount of water retained as surface film- 

 Again, the greater the proportion of colloidal constituents, 

 clay and humus, the more imbibition water will there be. 

 This relation between soil constitution and water content 

 has been long known. Pfeffer quotes experiments by 

 Meister in 1859 which showed that a sandy soil could 

 absorb 30*4 parts of water, and a peaty soil 105 "2 parts per 

 100 parts dry weight. Later investigators have extended 

 such observations to other soils. In a series of determina- 

 tions on American soils, Briggs and Shantz (i9i2«) found 

 the moisture retained by fine sandy soils to range from 

 4*7 to 67 per cent,, by sandy loams from 97 to 11 '9 per 

 cent., by loams from i8"9 to 27 per cent., and by clays 

 from 27*4 to 30*2 per cent. 



Attempts have been made in recent years to correlate the 

 amount of water retained with one or all of the constituents 

 of the soil. Kraus (191 1), in an intensive investigation of 

 the soils near Carlstadt, found that the water content was 

 inversely proportional to the amount of *' soil skeleton," by 

 which is understood the coarser particles which cannot 

 pass through a sieve with half-millimetre meshes. Thus 

 two basalt soils with 41*08 and 8o'i per cent, of coarse 

 particles contained respectively 13*09 and 6*39 per cent, 

 of water. This means, of course, that the amount of water 



