46 SOIL MOISTURE 



the soil temperature is as low as 51. Oats that will germinate 

 in two days at a temperature of 65.3 require seven days to make 

 the same growth when the temperature is as low as 41 F. 



We have already learned that the texture, slope of the land, and 

 the color of the soil all have a marked influence on the temperature 

 of the soil. 



Depletion of Soil Moisture. The store of soil moisture is de- 

 pleted in three principal ways: (1) percolation, (2) evaporation, 

 (3) transpiration. 



Percolation. The force of gravity tends to force the rain water 

 downward when it reaches the earth. When the soil is coarse 

 and full of gravel, this loss is considerable. Coarse soils should be 

 rolled and made compact in order to hold the water near the surface. 



Evaporation. If we spill water on the floor of a room in summer 

 time, it soon disappears. The same thing happens when we hang 

 wet clothes on the clothes line. The water that disappears goes 

 off in the form of vapor, and the process we call evaporation. 

 Much of the moisture brought to the surface of the soil by capil- 

 larity is removed by evaporation. Sunshine, warmth, and wind 

 all facilitate evaporation. Evaporation will also take place at 

 freezing temperatures, but less rapidlv. A dry, hot day soon 

 removes the surface moisture of the soil and parches vegetation. 

 In Nebraska, Kansas, and Texas hot winds have been known at 

 times to dry up and kill all growing crops in two days' time. 

 Fortunately such instances are rare. 



Transpiration. A great deal of water is removed from the soil 

 and given off to the air through the leaves of the plant. The leaves 

 correspond to the lungs of animals. This process of removing the 

 water is called transpiration, and it varies greatly in different plants. 

 Willows, poplars, cypress, and many other trees use up large quan- 

 tities of water and are often used for drying out wet pastures. 

 Transpiration is very rapid in warm, dry air and relatively slow in 

 cool, moist air. It is more active in sunlight than in the shade, 

 but it does not cease even at night. Evergreen plants transpire 

 less than other kinds of trees. An oak tree seventy feet high and 

 over eight feet in circumference has been found to transpire forty- 

 four hundred pounds of water on a single summer's day. Such a 

 tree, it is estimated, will transpire in the course of a single season 



