THE NATURE OF SOIL 37 



THE VENTILATION OF THE SOIL 



The spaces between the soil grains are filled 

 either with water, or air, or both. This soil air is 

 somewhat different from the free air above the sur- 

 face, containing less oxygen, more carbonic acid gas 

 and more ammonia gas. Part of its oxygen is used 

 by the plant roots; the other gases are absorbed 

 from the vegetable matter decaying in the soil. 



Practically all of our farm crops need a well 

 ventilated soil. The roots of plants, except certain 

 bog, marsh and water plants, must have air to 

 breathe. If it is denied them, because the inter- 

 spaces of the soil are filled with water, the plants 

 will die. Corn is "drowned out" in low, wet 

 places, chiefly because the roots cannot breathe. 

 Furthermore, air is needed in the soil to make more 

 plant food. The air penetrates deeply into the soil 

 and its oxygen, carbonic acid and ammonia dissolve 

 the minerals arid make the soil more fertile. The 

 nitrogen of this air may be used as a food by certain 

 plants '{See Chapter XII), The oxygen of the 

 soil air combines with the nitric acid produced by 

 the decay of plants, making it a nitrate, which is 

 a plant food. Manure which is piled loosely, so 

 that air penetrates it readily, heats quicker and 

 stronger than tightly packed manure; likewise a 

 soil that is well drained and open, so that air passes 

 into it freely, has more life, fermentation and 

 fertility in it than a close-grained, air-tight soil. 

 Air may penetrate the soil to a depth of many feet, 

 depending upon its openness. Soil air changes in 

 temperature like surface air, and continually passes 

 up and down in currents. 



Methods of Improving the Ventilation of Soil. 

 Any kind of tillage which stirs and loosens the soil, 



