490 NOTES ON THE BOIL, 



exerted by the root-hairs. The amount of water thus held by 

 the soil varies inversely as the size of the soil-particles, being 

 largest in fine-grained soils like clay and smallest in coarse 

 soils like sand and gravel. The greater part of the total 

 water-content is in any case available to plants, but it differs 

 for different kinds of plants and is diminished by excessive 

 amounts of salts in the soil and by low temperatures. 



The percentages of total water-content, available water, and non- 

 available water per 100 grammes of dry soil are given approximately 

 in the following table : 



Sand. Loam. Clay. 



Total Water-Content 18 '5 52 68 



Available Water 18 43 55 



Non-available Water 0*5 9 13 



How Water-Content is Influenced. The most 

 important factors which influence the water-content of a soil 

 are (1) the nature of the soil ; (2) the rainfall ; (3) the 

 humidity of the air ; (4) the physiography of the habitat. 



(1) The amount of the water-content of a soil depends 

 directly on its texture, which determines how much of the 

 water falling on the surface shall run off, and how much of 

 the entering water shall be held by the soil-particles, or 

 drain away under the action of gravity, or rise again from 

 the lower layers by capillarity and thus become available even 

 for roots growing just below the surface. 



The finer the texture of the soil i.e. the smaller the grains 

 and the narrower the spaces between them the greater the 

 depth from which water can be raised ; the capillary rise of 

 water is probably as much as 30 ft. in a fine-grained soil 

 e.g. clay loam. The amount of water evaporated from the 

 surface and of that raised by capillarity is greatly increased 

 by winds ; capillary action is stronger in moist than in 

 thoroughly dry soil, just as a cloth or sponge absorbs water 

 more slowly when dry than when wetted. The falling of rain 

 on a dry soil will therefore cause the lower parts to become 

 drier, while the surface layers will gain more water than that 

 represented by the rainfall. The capillary rise of soil 

 moisture is increased by rolling, which brings the upper soil 

 into closer contact with that below. 



The porosity of a soil is measured by the rate at which 

 water passes downwards through it. Since the capillarity of 



