250 SOILS AND SOIL WATER RELATIONS 



solutes too numerous to mention may also occur in the soil solution but are 

 usually present only in very low concentrations. Soil water relations receive 

 a more detailed discussion later in this chapter, while the soil solution is dis- 

 cussed more fully in Chap. XXIV. 



4. The Soil Atmosphere. — The irregularity of the soil particles in size, 

 shape, and arrangement insures the existence of a certain amount of space 

 between them, even in the most tightly packed soils. This is termed the 

 pore space of a soil. The pore space of soils varies from approximately 30 

 per cent of the volume of the soil in sandy soils to about 50 per cent of the 

 volume in clay soils, or even higher in soils which are very rich in organic 

 matter. The pore space of any given soil depends upon the physical and 

 chemical conditions to which the soil is subjected. Conditions favoring a 

 crumb structure of a soil, for example, usually result in an increase in its pore 

 space. The interstitial spaces of a soil may be occupied entirely by air, as in 

 desiccated soils, entirely by water, as in saturated soils, or, as is most usually 

 true, partly by water and partly by air. The relative proportions of water 

 and air present in any given soil vary depending upon the water content of 

 the soil. 



The air present in the soil is generally referred to as the soil atmosphere. 

 The soil air usually contains a higher concentration of carbon dioxide and a 

 lower concentration of oxygen than the above ground atmosphere. Concen- 

 trations of carbon dioxide as high as 5 per cent have been recorded for the 

 soil atmosphere; such values are far in excess of the average value of 0.03 

 per cent for the air. The accumulation of carbon dioxide in soils is due more 

 to the metabolic activities of micro-organisms than to respiration of soil animals 

 and the underground portions of vascular plants. Except in very dry soils 

 the soil atmosphere is usually saturated or nearly so with water-vapor. 



5. Soil Organisms. — The soil flora includes bacteria, fungi, and algae. 

 The bacteria are generally the most abundant of all the living organisms 

 present in any soil. Among them are the nitrifying, sulfofying, nitrogen 

 fixing, ammonifying, and cellulose decomposing bacteria. The bacteria ac- 

 complishing the oxidative decomposition of cellulose and similar compounds 

 are the most important agents in the production of humus. The numbers of 

 bacteria present vary greatly from soil to soil, and in any one soil vary with 

 seasonal and other fluctuations in soil conditions. Most soils contain between 

 two million and two hundred million individual bacteria per gram of soil. 

 The number of bacteria decreases rapidly with increasing depth, subsoils being 

 sterile or practically so. In general an abundant representation of most species 

 of bacteria is favored in soils by warm temperatures (35 - 40° C), good 

 aeration, and a good but not superabundant water supply. A high calcium 



