102 THE SOIL AND ITS IMPROVEMENT 



low and dies. That is because the roots can no longer 

 get air. Low areas in fields that are flooded during the 

 summer show the same result. Trees along the sidewalk 

 often die because the pavement shuts off the air from the 

 roots. Certain trees, like willows, thrive beside streams, 

 because their roots can use some of the air in the running 

 water. 



Soil in a good state of cultivation has nearly half its 

 volume filled with air and water. Mixed with this " soil 

 air" there is usually more carbon dioxide than is found 

 in the air above the surface. This larger amount of car- 

 bon dioxide comes from the decay of vegetable matter in 

 the soil. 



61. Water in the Soil. Soil is not productive unless it con- 

 tains water. All other needful ingredients may be pres- 

 ent in proper quantities, and yet the soil will give small 

 yields if moisture is insufficient. There is an old saying 

 that soil, like a chain, is as weak as its weakest link, and 

 that the moisture is often the weakest link in soil fertility. 



Plants use an enormous quantity of water in their 

 growing season. An average crop of corn needs 8 inches 

 of rainfall to mature it, allowing nothing for evaporation 

 or percolation. It has been estimated that an acre of 

 cabbage draws from the soil, and throws off into the 

 air, 500,000 gallons of water in one summer, and that 

 an oak tree with 700,000 leaves transpires in this way 

 about 180 gallons of water daily. Plants must have this 

 vast amount of water for a number of reasons. For in- 

 stance, it is the water within them which transports the 

 food material from the soil to their leaves. In order 

 that a plant may get out of the soil something like an 

 ounce of earthy matter it must absorb nearly a barrel of 

 water for this purpose alone. 



Plant substance, too, like the human body, is largely 



