144 THE PRINCIPLES OF SOIL MANAGEMENT 



Per cent of hygroscopic 

 water at 21 C. 



Very fine sand 1.8 



Silt 7.3 



Clay 16.5 



Muck 48.0 



The above soils were pure separates derived by 

 mechanical analysis. These figures serve to show the 

 direct relation between the (1) surface area exhibited 

 by soil particles and the hygroscopic moisture retained. 

 The hygroscopic moisture content of a soil depends 

 also on the (2) temperature, and the (3) humidity 

 of the atmosphere. The hygroscopic moisture decreases 

 with increase in temperature. It varies directly as 

 the relative humidity of the atmosphere with which 

 the soil is in contact. Consequently, in the air-dried 

 condition, while a soil always retains some moisture, 

 it seldom exhibits its maximum hygroscopic capacity. 

 Under average conditions of humidity, a light sand 

 may retain from .5 to 1 per cent, a silt loam from 2 

 to 4 per cent and a clay from 8 to 12 per cent. This 

 is, of course, unavailable for the use of plants. 



71. Capillary water. The capillary water capacity 

 is much larger than the hygroscopic capacity. Its 

 amount is determined by three things: (1) Texture, 

 (2) structure; (3) content of organic matter. 



72. Texture. Texture is well known to be the great- 

 est determining factor in the water-holding capacity 

 of soils, due to its control of the internal surface, 

 and this is particularly true with reference to the 

 capillary form. The following table illustrates this 

 effect of texture. 



