DISINTEGRATION OF ROCKS 73 



Air is able to circulate or move among the soil particles, if the 

 soil is in good physical condition. For the best growth of crops 

 the soil must allow the entrance of fresh air to supply oxygen. 

 A fresh supply of atmospheric nitrogen to be worked upon by 

 nitrogen-fixing bacteria is brought into the soil through the venti- 

 lating process. Air which has once been used must be removed; 

 thus an excess of carbon dioxide may be liberated from the soil 

 by ventilation. 



There are a number of forces constantly active which tend to 

 aid the movements of air in soils. Each change of temperature 

 is helpful. Some days the air is heavy and other days it is light. 

 These changes will condense or expand the soil air. The action 

 of wind, particularly on a hillside, has a strong influence. As rain 

 water enters the soil it may not only carry air in solution but also 

 draw air downward through the openings left behind it. Under- 

 drains may take away surplus soil water and leave pores to be 

 filled with air. Ventilation often follows the channels left by de- 

 caying roots. 



If a field becomes hard and dry the crust may be broken by a 

 cultivator, harrow or other implement. This will allow air, as 

 well as rain water, to enter more readily. Deep tillage will also 

 greatly increase the amount of air space in the soil and aid in its 

 ventilation. 



Soil Grains. The finest particles of those portions of the soil 

 formed from rocks are called clay. The next finer are called silt, 

 and those coarser than silt are fine sand or coarse sand. Gravel 

 may also be present in the soil. These particles of rock are larger 

 than sand and are usually not considered a part of the soil at all. 

 Following are the sizes of such soil particles as are used in soil 

 analyses (mm. indicates milimeters in diameter) : 



Clay 0.005 to 0.0001 mm. 



Silt 0.05 to 0.005 mm. 



Very fine sand 0.1 to 0.05 mm. 



Fine sand 0.25 to 0.1 mm. 



Medium sand 0.5 to 0.25 mm. 



Coarse sand .1 to 0.5 mm. 



Fine gravel 2 to 1 mm. 



Disintegration of Rocks. There a number of forces or agencies 

 that work in nature tending to disintegrate both large and small 

 rocks and make them into finer particles which may finally be- 

 come useful in helping to form soils. Among these agencies com- 

 monly seen about us are the action of running water; the freez- 



