SOIL-FOEMING PEOCESSES 23 



One peculiarity of pure water is that its maximum density 

 occurs at 39.2 deg. F. From this point the volume increases 

 as the temperature is lowered. Ice, which forms at 32 deg. F., 

 thus occupies a greater space than the water from which it 

 was derived. The force developed by freezing is equivalent 

 to about 150 tons to the square foot or a pressure of 141 

 atmospheres. The cracks and crevices of surface rocks in 

 humid regions are from time to time filled with moisture. 

 Eocks below the surface contain water continuously. The 

 change of this water from a liquid to a solid always produces 

 marked disintegration. Mountain-top rubble, talus slopes, 

 alluvial fans, and similar formations are evidences of such 

 action. The load of sediment carried by streams is often due 

 to the prying action of temperature change, especially where 

 crevice water is present. 



This action of temperature is by no means ended when a 

 soil is produced. Freezing and thawing is of tremendous im- 

 portance in bettering the physical condition, especially of 

 heavy soils. It is to such forces that the farmer owes the 

 good tilth of his land. In addition it must be noted that the 

 rapidity of chemical change is largely a function of temper- 

 ature. The concentration of the soil solution and the avail- 

 ability of the nutrient elements thus come under the influence 

 of this apparently simple force. 



15. Plants and animals. — ^While plants and animals unite 

 their activities with the processes already mentioned, their 

 influence is confined largely to the soil and the soil material. 

 Simple plants such as mosses and lichens grow upon exposed 

 rock, there to catch dust and dirt until a thin film of highly 

 organic material accumulates. Higher plants sometimes exert 

 a prying effect on rock, which results in some distintegration. 

 Such influences, however, are of but little import in soil for- 

 mation compared to the drastic activities of water, wind, ice 

 and temperature change. 



In the soil, roots by their ramifications promote aeration 



