104 NATURE TEACHING 



Water in Soils. 



Sandy or light soils differ in a marked degree from 

 clayey or heavy soils as regards their relation to water. 

 Water drains through sand with ease, while it passes 

 through clay soils with difficulty. When water falls, or 

 is poured upon soil, which is then allowed to drain, a 

 certain quantity of the water is retained by the soil, and 

 does not drain out. Sandy soils retain only a small 

 amount of water, and clayey soils a great deal. Thus 

 sandy soils, while permitting drainage to take place 

 more freely, retain less water than clayey soils, and 

 therefore require rain more frequently than clays, or the 

 crops growing on them would suffer from drought. 

 Illustrations of these differences, drawn from his own 

 neighbourhood, will probably occur to the reader. 



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 

 see 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. 



