192 SOILS PHYSICS AND MANAGEMENT 



has been based on the air-dry soil, and while this is somewhat better 

 than the former, yet it is subject to the same objection, since the 

 air-dry soil varies with the temperature and humidity of the air. 

 Without doubt the most satisfactory base for expressing the per cent 

 is the weight of water-free soil obtained by drying in an oven at 

 100 to 110 degrees C. Hilgard has used the temperature of 200, 

 others 140 degrees C., for obtaining the water-free soil, but 

 in general practice a temperature of 100 degrees C. is easier to 

 maintain and just as satisfactory. Probably none of these is the 

 exact point at which all of the hygroscopic moisture is driven off. 

 The thing desired is a uniform standard. 



(b) Cubic Inches or Per Cent of Volume. Expressing the 

 water content in cubic inches or per cent of volume may have its 

 advantage in case of certain soils, such as peats, or mucks, which 

 are very light, or sands which are heavy. A peat soil with 50 per 

 cent of moisture may contain no more than a silt loam with 20 per 

 cent. A cubic foot of peat, water-free, weighs about 30 pounds, and 

 50 per cent of moisture would mean 15 pounds per cubic foot, while 

 the silt loam, weighing 75 pounds per cubic foot with 20 per cent, 

 would have the same amount. Expressed in per cent of volume, the 

 amount would be 24 in each case. To determine the per cent of 

 volume necessitates the finding of the per cent of moisture based 

 on the water-free soil. 



(c) Acre-inches. It is often desirable to express the water 

 content of soils for convenient comparison with the rainfall. This 

 may be done in square-foot-inches or in acre-inches, the depth of 

 water in inches over a square foot or an acre. To do this it is neces- 

 sary to determine the weight of water in the soil per square foot to 

 the depth desired. The weight of water in a cubic foot in pounds 

 divided by 5.2, the weight of a square-foot-inch of water, will give 

 the depth in inches. 



QUESTIONS 



1. What amount of water is required by plants? 



2. Give the effects of temperature changes on water. 



3. What is latent heat? 



4. Give comparison of metals and water as to the amount of heat required 



to change the condition or state. 



5. Give differences between the calorie and English heat unit. 

 f>. Define specific heat. 



7. How does water compare with other substances? . 



8. What factors modify viscosity? 

 0. Give uses of water in soils. 



10. How much water is required for a pound of dry matter? 



