38 SOILS 



like plowing and harrowing, promotes a better aera- 

 tion, or ventilation, of the soil. Plowing under farm 

 manure, green manure or stubble also has the same 

 desirable effect, since the humus thus produced 

 separates the particles of soil and renders it more 

 porous, hence more open to the downward passage 

 of air. Under-draining, however, is the chief 

 means of ventilating a heavy soil. Remove the 

 water and the air will rush in. When the water 

 table is lowered two or three feet, as it may be by 

 under- draining, the roots of plants grow deeper; 

 when they decay, they leave little channels in the 

 soil and through these air penetrates. Earthworms 

 and ants still further deepen and aerate the soil 

 by following these channels. 



When land is tile-drained, the tiles themselves 

 provide a system of underground ventilation of far 

 reaching influence. The soil of a tile-drained 

 field is ventilated much more thoroughly than the 

 soil of another field of the same character in which 

 the water table stands naturally at the same height. 

 The air in tile drains is largely surface air. 



The roots of most farm crops deepen and aerate 

 the soil, but the roots of leguminous plants, espe- 

 cially of clover and alfalfa, are particularly useful 

 in soil ventilation. This is partly because clover 

 roots are large and bore straight down into the sub- 

 soil for several feet, leaving much larger and more 

 effective channels for the passage of air and water 

 than the roots of grains ; and also, in a very slight 

 measure, because these plants absorb nitrogen 

 from the soil air, thus making it necessary for more 

 surface air to be forced into the soil to replace that 

 which is lost. 



Fortunately for the farmer, most soils are able 

 to absorb various gases, notably ammonia, which 



