CHAPTER XXIV 



SOIL AIR AND AERATION 



EVERY individual who has grown crops knows that a soil must 

 contain air as well as water, and the amount of one will vary with 

 that of the other. In other words, the air of a soil occupies that 

 space not occupied by water, and when the proportion of the two 

 is about equal optimum conditions prevail. 



Use of Air in Soils. The most important element in soil 

 air is oxygen. It is necessary for the vital functions that take 

 place in plants, and in the case of water-logged soils, in which the 

 oxygen is reduced to a minimum, the effect can readily he seen. 

 Oxygen is necessary for root respiration. We find that there is an 

 interchange in the roots, the carbon dioxide being given off and the 

 oxygen taken in. Oxidation, with or without the agency of bacteria, 

 is necessary for furnishing available plant food for the crop. The 

 process that supplies available nitrates is known as nitrification, and 

 takes place through the agency of organisms. This is absolutely 

 necessary in soils, and if for any reason oxygen is prevented from 

 entering the soil, or if the supply becomes low, the lack of nitrates 

 is shown by the yellowish-green color that the plant soon assumes. 



A supply of oxygen is necessary in the soil for germination also. 

 Certain chemical processes take place in the seed for which oxygen 

 is necessary. In extremely wet soils seeds germinate very poorly. 

 Air is necessary in the soil for supplying nitrogen to the nitrogen- 

 fixing bacteria, both symbiotic and non-symbiotic. The 1 carbon 

 dioxide of the soil air is of importance because of its effect on min- 

 erals. These are slowly decomposed by the carbonic acid that is 

 formed, and plant food is liberated. 



Amount of Air in Soils. The amount of air in soil depends 

 upon the porosity, and this upon the texture. It would naturally 

 be supposed that the greatest amount of air would be in the soil 

 having the highest porosity. This may not always be true, since 

 soils with high porosity have also a high retentive capacity for 

 moisture, and it would not lie an unusual thing for a soil to retain 

 so much water that it would reduce the actual amount of air present 

 to a point less than that, held by sand. (See the table on composi- 

 tion of soil air, page 310.) 



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