Emil Truog 33 



water, adequate aeration is provided. It will be noted in Table I that 

 the composition of soil air as given is not markedly different from ordi- 

 nary atmospheric air. The main difference is that the content of carbon 

 dioxide may be 10 or 20 times greater in soil air. This is due to the rapid 

 production of carbon dioxide in the respiration of soil organisms and 

 plant roots, and its delayed escape to the air above because diffusion 

 and other movement is greatly impeded in the fine pores of the soil. 

 Under waterlogged conditions, the escape of carbon dioxide and en- 

 trance of fresh air almost ceases, and, thus, biological activity changes 

 to an undesirable type in which toxic concentrations of methane, hydro- 

 gen sulfide, and other substances develop. Nearly everyone has noted 

 how quickly an excess of water in soils, causing a lack of aeration, 

 brings about the yellowing and death of plants. This emphasizes the 

 great importance of a proper regulation of the gaseous phase of soils. 



The "living phase" 



When soils are in proper condition for crop production, they con- 

 tain substantial amounts (often five tons or more per acre-plow-layer) 

 of living matter in the form of bacteria, fungi, and other organisms 

 listed in Table I. Matter animated with life attains special and unusual 

 properties, and, because of this and its great importance in soils, it may 

 well be given the special designation of the "living phase." Without 

 this "living phase" of soils, plant residues would not decompose, and 

 the resulting stagnation would in a relatively short time make crop pro- 

 duction in the usual manner impossible. The role of this "living phase" 

 in nutrient availability is discussed in detail in another paper. 



THE pH OF SOILS 



It is well known that the pH of systems which involve many chemical 

 reactions and, particularly, biological activity and life is tremendously 

 important. So without question, the most important single chemical 

 characteristic of a soil as regards its suitability for plant growth is its 

 reaction or pH status. Fortunately, this characteristic can be determined 

 easily and accurately by means of simple tests that can be operated, if 

 necessary, right in the field. 



As is also true with many biological systems, the pH of soils involves 



