1 78 CLYDE C. HAMILTON. 



amount of heat used in the evaporation of water. Thus con- 

 siderable heat can be used in the evaporation of moisture from 

 wet or moist soil and the temperature of the soil itself will remain 

 almost unchanged. On the other hand, in dry compact soil 

 most of the heat is absorbed and the temperature of the soil 

 is raised accordingly. 



The moisture in the soil is derived largely from precipitation, 

 but part of it may come from the adjacent soils or rock. The 

 amount which a soil will retain is dependent upon the porosity, 

 the physical structure, the litter covering the soil, and the growth 

 of vegetation. A soil with a living vegetative cover loses 

 moisture, both through direct evaporation and absorption by its 

 vegetation, much faster than bare moist soil. The organic debris 

 on the surface also conserves the moisture by the formation of a 

 blanket or mulch. Evaporation is further retarded by the use 

 of windbreaks or hindering the movement of the air over the 

 soil, as has been shown by numerous experiments with wind- 

 breaks. Adams ('15) in discussing the soil moisture says: 

 "McNutt and Fuller ('12) have made a study of soil moisture at 

 3 inches (7.5 cm.) and at 10 inches (25 cm.) below the surface in 

 an oak-hickory forest, at Palos Park, Illinois. They found that 

 the percentage of water to the dry weight of the soil at the 3-inch 

 level averaged 18.9 per cent, and at 10 inches was 12.5 per cent, 

 of the dry weight of the soil. The greater moisture near the 

 surface is due to the humus present in this layer." 



The ventilation and amount of air in the soil is dependent upon 

 its physical properties, temperature and moisture. Dry soil, 

 according to Hilgard ('06) contains from 35 to 50 per cent, its 

 volume of air, and in moist or wet soil this space is replaced by 

 water. Thus the conditions influencing the amount of water 

 present have a very important influence upon aeration. The 

 rapidity with which ventilation occurs is dependent upon the 

 porosity and temperature; the greater the porosity and the higher 

 the temperature, the more rapid the change. The amount of 

 carbon dioxide in the soil is partly dependent upon the ventila- 

 tion, the moisture, and the amount of decaying organic material. 

 Since carbon dioxide is very soluble in water, it is found in the 

 soil moisture in much larger proportions than in the atmosphere. 



