370 NORMAN 



that gaseous diffusion is greatly hindered. Whereas air contains about 21 

 per cent oxygen, no adverse effects due to oxygen deficiency seem to develop 

 in most species until the oxygen content of the root atmosphere is reduced 

 to 5 per cent or less, if the carbon dioxide content is not excessive. Soil 

 physicists have given much thought to the problem of expressing the capacity 

 of a soil to supply oxygen to plant roots. The total pore space alone is not 

 a satisfactory measure because the size and distribution of the pores determine 

 the degree of "aeration" of a soil at different moisture levels. A fine-textured 

 soil will have a greater pore space than a coarse sandy soil, but would not 

 necessarily provide a better root environment in so far as oxygen supply is 

 concerned. In the field aeration is largely determined more by the degree 

 of aggregation of the individual soil particles into larger structural units than 

 by the size distribution of the component soil particles. Much of the interest in 

 soil conditioners arises from the improved aeration that can result if the 

 extent of aggregation of fine-textured soils is increased. 



Micro-organisms in soil. If, then, the soil environment is one from which 

 the plant derives nutrient ions, water, and oxygen, how does it differ from an 

 aerated water culture in which are present soluble salts supplying all the 

 essential nutrient ions that ideally should meet plant requirements? The 

 answer to this question could lead into a discussion of soil chemistry and the 

 forms in which the essential nutrient ions are presented to plant roots. How- 

 ever, it is pertinent at this stage to point out that any definition of soil should 

 include the statement that it is characteristically the habitat for organisms 

 other than higher plants and that soils ordinarily contain extensive and 

 diverse populations of micro-organisms. The soil organisms are indeed potent 

 as soil-forming agents, and higher plants are interlopers in an intensely active 

 and competitive microbiological world. The living component of soils, its 

 microflora and microfauna, constitutes a metabolic pool of nutrients. To a 

 degree higher plants may be said to be dependent on some of the by-products 

 and end products of the activities of micro-organisms in the soil. Plant and 

 animal residues, in or on the soil, are decomposed by micro-organisms. The 

 nutrients which they contain are released for re-use. In nitrogen transforma- 

 tions the soil population plays a vital part, inasmuch as the only reserve of 

 this element in the soil is in the organic form, unavailable to plants until 

 mineralized. 



Much more attention has been given to bacteria, fungi, and actinomycetes 

 in soils than to protozoa and the other components of the soil fauna. Quanti- 

 tative studies of the distribution of bacteria show that millions of organisms 

 are present per gram of top soil. Such figures are often baffling unless it is 

 recognized that a million bacterial cells, l/x by O.Sfi in size, would have a 

 volume of only 0.0002 mm'' and that a gram of soil may have a volume of 

 0.8 cc. and a surface greater than 1 m-. More important, however, is the 

 fact that the microbial cells are concentrated where there is food. In the 



