60 SOILS AND MANURES 



Capacity for Water. The capacity of a soil for water 

 means the amount of water required to completely saturate 

 it, i.e., to fill up all the interspace. It can be measured 

 by direct experiment, but is more accurately determined 

 by calculation of the interstitial space (p. 54). 



The capacity of different kinds of soil for water is 

 approximately as follows : 



Per cent. 



Sandy soils (about) 40 



Loamy ,, x ....... 50 



Clay 60 



Humous 7080 



The capacity for water of an arable and an old pasture 

 soil from the same locality was found to be 53 and 64 per 

 cent, respectively. The difference was attributed to the 

 larger proportion of humus in the latter. The capacity for 

 water can be materially increased by addition of organic 

 matter to the soil. 



Retention of Water. Under natural conditions soils do 

 not long continue in a saturated state. A portion of the 

 water percolates downwards under the influence of gravity, 

 but some is retained. The retention of water by soils can 

 be explained by reference to the phenomena of surface 

 tension which cause suspended particles of liquids to 

 shrink to the spherical shape and exert a pressure towards 

 the centre. When a soil is moistened with water each 

 particle becomes enclosed in a film which forms an 

 elastic envelope about the particle, and is held in place by 

 the pressure due to the surface tension of the liquid. The 

 smaller the quantity of water the thinner will be the films 

 and the greater the pressure. When the quantity of water 

 is larger the films become thicker, the pressure is 

 relaxed, and some lodges in the interstices. But the force 

 of gravity increases in proportion to the mass of the water, 

 and a point is ultimately reached at which it is equal to 



