SOIL WATER 79 



1. Enlargement in the supply and movement of soil air. 



2. Improvement in tilth. 



3. More available water throughout the growing season. 



4. Longer growing season. 



96. Soil air. — Drainage increases the supply and move- 

 ment of soil air by allowing the gravitational water to run 

 off and thus to be replaced by air. With each fall of rain 

 there is a movement of air through the soil. The increased 

 air supply is of benefit in the following ways : 



1. It furnishes air to roots which require it for the proper 

 performance of their functions. 



2. It facilitates the decomposition of organic matter of 

 all kinds, thus disposing of the vegetable matter incorporated 

 with the soil, and permitting the most beneficial kind of de- 

 composition (see §§ 59, 60). 



3. It furnishes the conditions necessary for the trans- 

 formations of nitrogen in the soil which prepare that sub- 

 stance to be used by plants (see §§ 116-168). 



97. Soil tilth. — Alternate drying and wetting of soil is 

 one of the processes that causes the formation of granular 

 structure and consequent improvement of tilth. A soil 

 that is constantly saturated or very wet when worked in 

 the spring assumes a compact condition. The larger air 

 space reduces heaving by allowing expansion of freezing 

 water within the soil, and diminishes the tendency to erosion, 

 by allowing water to sink quickly into the soil, instead of 

 running over the surface. 



98. Available water during the growing season. — A soil 

 in need of drainage is often in need of moisture in midsummer, 

 because when it does dry out its water-holding capacity 

 is low, on account of its compact condition. Furthermore, 

 plants form shallow roots in a saturated soil, and if the 

 weather becomes dry later in the season, the roots do not 

 then go to the depth necessary to reach the water supply. 



