PHYSICAL PROPERTIES OF SOILS 17 



the soil and this is useful in many ways. Oxygen is necessary 

 for the sprouting of seeds and for the healthy growth of the 

 roots, and it aids in many chemical changes in the soil whereby 

 plant food is made ready. "Bacteria must have it for their 

 life and work. In any soil there are spaces without number 

 among the soil grains. These spaces are called pores. If 

 the grains are coarse, the pores are large; if fine, the pores 

 are small, but more numerous. When these spaces are not 

 filled with water they are occupied by air. This air is 

 called soil air. If it did not move it would become foul just 

 like the air in a closed room and just as injurious to the life 

 of the soil as the foul air of the room is to persons in it. The 

 movement of this air is brought about by the suction of the 

 winds which blow over the land. This draws out some of the 

 air in the soil and permits other air to enter. Where land is 

 drained by tile ditches there is a movement through the lines 

 of tile. When the land is soaked no air can circulate because 

 the pores are filled. Soil having large pores, and soil culti- 

 vated until it is loose, permit a freer circulation of air than 

 compact soil with small pores. A too free circulation is not 

 desirable, because it will dry the soil too much and will also 

 cause the vegetable matter to be burned out too fast. 



Water in the Soil. — The water in the soil has several im- 

 portant parts to play. 1. It keeps continually dissolving 

 small amounts of the soil grains. This dissolved material 

 furnishes the ash of plants. 2. By the constant evaporation 

 of water from the leaves the temperature of the plant is kept 

 from rising too high in hot weather. 3. By the movement of 

 water in the soil, of which we shall soon speak, the dissolved 

 material is brought in contact with the roots of the plant. 

 4. All the live cells of the plant are made up largely of water. 



