CHAPTER III 



THE RELATION OF SOILS TO MOISTURE AND AIR * 



THE roots of plants, if we except the aquatic plants 

 of the swamps and marshes, do not need water 

 in the soil, for standing water in the soil shuts 

 out the air which is essential to the oxidation of plant food, 

 and keeps the soil cold. What we need in the soil is 

 moist air. We have seen in our first lesson that the per- 

 centage of moisture in the soil will depend largely on the 

 size of the soil particles, each surrounded by its film of 

 water. A soil that parts with this moisture rapidly by 

 evaporation into the atmosphere will always be a colder 

 soil than one which holds the moisture and evaporates it 

 more slowly. So standing water is harmful, though 

 moisture is necessary. It is essential that the soil be 

 penetrated by air that the oxidation of matter in the soil 

 may take place, and it is necessary that there be a due 

 amount of moisture so that plant food may be dissolved 

 for the roots since, as we shall see, no plant food is taken 

 up until completely dissolved in the soil moisture. 



The rise of water in the soil to supply 

 Supply of that taken by plant roots and by the evapo- 



Moisture by ^^^^^^ .^^^^ ^^^ ^j^. j^ ^^^^^^ ^ ^h^t is 

 Capillary . 



Attraction known as capillary attraction. Flace a 



thick towel with one end in a pail of water 

 and the other end resting on the ground and you will see 

 that the water is taken up from the pail and transferred 



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