60 PRODUCTIVE FARMING 



also prevent the rise of temperature. Fine film moisture 

 probably does not aid plants in their growth, but will help 

 to maintain life at the time of great drought. Slow-growing 

 plants, such as those found in deserts, may be kept alive by 

 the presence of fine film moisture. 



Soil Moisture Used by Plants. — Plants use moisture 

 while growing. One ton of a dry corn crop may use up 300 

 to 600 tons of soil moisture during its growth. The moisture 

 while in the soil dissolves plant-food which it finds there. 

 The plants take up this food in the water they drink through 

 the roots. This all passes up to the leaves of the plant and 

 the surplus water is lost by evaporation into the air. 



How Capillary Moisture Moves. — ^When grains of soil 

 are wet with capillary moisture, each grain is covered all 

 over with a thin layer of water. Deep down in the soil near 

 where the free water is found the layers of water around the 

 grains are quite thick. Higher up, the films are less thick. 

 The films about the grains are as thin as the films of soap 

 bubbles. The grains next to the air become as dry as the 

 air itself. 



But the moisture is constantly moving from the lower 

 depths toward the surface. The power by which this move- 

 ment takes place is called capillary action. It is in the same 

 way that oil will rise in the wick of a lamp. 



The moisture moves upward in fine-grained soils such as 

 clay more readily than in coarse sand. The openings be- 

 tween soil grains are called 'pore spaces. If these are rather 

 large the capillary movement of water is slow. Packing the 

 soil grains closer together, as with a roller, will greatly in- 

 crease the rise of water in the soil. This is often done when 

 small grain or grass seeds are sown, to hasten the sprouting 

 of the seeds. 



Water Holding Power of Soils. — One hundred pounds of 

 each of the following soils, when saturated, held the following 

 amounts of water in capillary form: 



