SOIL AIR AND AERATION 313 



(e) Tillage is the most effective method for producing a change 

 of soil air. The best implement for accomplishing this purpose is 

 the plow. When the furrow slice is turned over the shearing pro- 

 duced pulverizes the soil and brings about a complete change of air 

 in all except the granules, and breaking the soil up brings al>out a 

 much better chance for a change in these. Any form of tillage, 

 however, will materially aid aeration. Plowing cloddy ground ac- 

 complishes the least. When these clods are thoroughly pulverized 

 much better interchange takes place, and this is one of the groat 

 advantages of thorough pulverization of the soil. 



(f) Wind Movement. The wind as a general rule moves in 

 gusts, and these passing over a field have a tendency to draw out 

 the air from the soil and aid aeration to some extent in this way. 

 Any exact determination of this effect of wind would be very diffi- 

 cult, yet it is probable that on soils having large air spaces, such 

 as cloddy or sandy ones, this plays quite an important part in 

 aeration. 



Water-logged Soil. Many soils which have imperfect drain- 

 age due to a high water table or an impervious stratum may con- 

 tain such a large amount of water as to exclude the air, resulting 

 in a very serious condition, so far as the vital soil activities arc con- 

 cerned. The remedy, of course, is drainage, and the drainage 

 should be sufficiently complete so that a heavy rain fall will not 

 saturate the soil for any length of time. If the water table is two 

 or three feet from the surface, a heavy rain may raise this sufficiently 

 to injure the crop unless the soil is thoroughly drained. Many 

 systems of drainage have not been sufficient to lower the water table 

 rapidly and the result is that in wet seasons the crop is badly 

 damaged. Even in moderately wet seasons the crop in the lower 

 places where the water table is near the surface will assume a 

 yellowish-green color, indicating that injury is being done by lark 

 of aeration. 



Running Together. Soils that are deficient in organic matter 

 are in condition to be easily puddled, especially the fine and medium 

 grained ones. A heavy rain may be sufficient to do this. The 

 beating of the rain drops breaks the granules into individual par- 

 ticles that render the surface impervious both to air and water, thus 

 cutting off the supply of air. If this condition continues for any 

 length of time, the crop may be retarded in its growth and be- 

 come of a greenish-yellow color, indicating nitrogen starvation. 

 The remedy, of course, is tillage for breaking the crust and aerating 



