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c. Escape of carbon dioxid as affecting composition (301). 



d. Effect of roots upon composition (302). 



III. Functions of the soil-air 437 



a. Oxygen (303). 



b. Carbon dioxid (304). 



IV. Movement of soil-air 439 



a. Diffusion of gases (305). 



b. Movement of water (306). 



c. Changes in atmospheric pressure (307). 



d. Changes in temperature (308). 



e. Suction produced by wind (309). 



V. Methods for modifying the volume and movement of soil- 443 

 air 



a. Tillage (310). 



b. Manures (311). 



c. Under-drainage (312). 



d. Irrigation (313). 



e. Cropping (314). 



F. Heat of the Soil 448 



I. Function of the heat of the soil in its relation to plant- 

 growth 448 



a. Biological (315). 



1. Germination (316). 



2. Growth and vegetation (317). 



3. Activity of the soil-organisms (318). 



b. Chemical changes (319). 



c. Physical changes (320). 



II. Sources of the heat of the soil 451 



a. Solar radiation (321). 



b. Conduction (322). 



c. Organic decay (323). 



III. Temperature of the soil 453 



a. Heat supply (324). 



b. Specific gravity and specific heat (325). 



c. Color of the soil (326). 



d. Slope of the soil (327). 



