VVATEK IN SOILS. 99 



sandy soils, while permitting drainage to take 

 place more freely, retain less water than clayey 

 soils, and therefore require rain more frequent- 

 ly than clays, or the crops growing on them 

 would suffer from drought. Illustrations of 

 these differences, drawn from his own neigh- 

 bourhood, will probably occur to the reader. 



2. The explanation of this retention of 

 water by soil is to be found in its physical 

 structure. There are spaces between the 

 small particles of soil, through which the 

 water passes. Usually these spaces are 

 filled with air, but when heavy rain comes the 

 air is largely replaced by water,' returning 

 when the water drains away. The better the 

 tilth of the soil the larger will be the number of 

 these fine air-spaces, which are necessary for 

 the maintenance of vigorous plant growth, 

 (roots needing air as well as moisture). As 

 we shall have occasion to seo later, important 

 changes, requiring free access of air, are going 

 on in every fertile soil. When water drains 

 away, the draining is never complete, for soil, 

 after it has been wetted, always retains some 

 moisture however thoroughly it is drained. 



3. This water is retained by 'capillary 

 attraction ,' which is the power that causes 

 water to flow into any very small cavities and 



