88 ELEMENTS OF AGRICULTURE 



CHAPTEK XVII.— Water in Soils. 



90. Importance of Water in Soils. — Water and 

 nitrogen have both been mentioned as making np a part 

 of all fertile soils, but so far we have devoted little space 

 to them. These two plant foods are of so much impor- 

 tance that they each require a separate chapter for their 

 consideration, and we shall begin with water. In order 

 that any soil may grow plants a supply of water is 

 absolutely necessary; therefore it is of the first im- 

 portance. 



91. Film Moisture. — Soils are made up of a great 

 number of particles of various shapes and sizes piled 

 loosely together. A pile of broken rocks or stones may 

 serve to illustrate on a large scale how soils are made 

 up. The spaces between the soil particles are filled with 

 air or water, as the case may be. In dry soils most of 

 the spaces are filled with air; but when the soil becomes 

 wet the air is driven out, and they become filled with 

 water, and at the same time each soil particle becomes 

 surrounded with a film of water. Dip a marble in 

 water and notice how it comes out wet all over; it is 

 surrounded by a layer or film of water. When soil 

 becomes thoroughly wet each soil particle is surrounded 

 by a film of moisture, like the wet marble. The water 

 filling the air spaces soon sinks deep into the soil 

 and part of it drains off, leaving in the upper soil only 

 the moisture surrounding each particle of soil, or what 



