620 PRINCIPLES OF SOIL MICROBIOLOGY 



of these nutrients as well as their utilization will be greatly influenced 

 not only by the (1) physical and chemical composition of the solid 

 part of the soil, but also by the (2) composition of the soil atmosphere, 

 (3) composition of the soil solution, (4) reaction of the soil and (5) 

 soil temperature. When any one of these factors is changed there is 

 a correlated change in the biological composition of the soil; i.e., any 

 modification in the physical, chemical or physico-chemical conditions 

 of the soil will greatly affect the biological flora and its activities. 2 

 We possess only fragmentary information concerning the biological 

 responses to modifications of these soil conditions. 



Soil composition and microbiological activities. The solid, liquid, and 

 gaseous phases of the soil influence individually and collectively the 

 distribution and activities of microorganisms. The mineral frame- 

 work is of prime importance. It is made up of (1) mineral matter 

 derived from the disintegration of rock materials; (2) calcium carbo- 

 nate, calcium phosphate, and organic matter deposited in the soil by 

 the various marine organisms during soil formation; and (3) the mineral 

 substances (zeolites) present in a colloidal state. The organic soil 

 constituents contributing to the soil as a medium for the growth of 

 microorganisms are (1) undecomposed plant and animal residues of 

 recent origin; (2) various intermediary substances, various more or 

 less inert constituents of the original organic matter added to the soil, 

 and final products of decomposition of plant and animal substances; and 

 (3) the various dead and living cells of microorganisms and the products 

 resulting from their decomposition. 



The mineral framework influences the activities of microorganisms 

 by modifying the mechanical condition of the medium, by forming sub- 

 stances (such as the zeolitic silicates) which, either themselves or by 

 interaction with certain of the organic compounds, form the colloidal 

 film in which most of the microbial transformations take place, by offer- 

 ing direct mineral nutrients to the microorganisms, and by combining 

 with various products of the metabolism of microorganisms, such 

 as the organic and inorganic acids. 



The inorganic as well as the organic soil colloids give to the soil such 

 desirable properties as its capacity for absorption and retention of 

 water and bases and its buffering action, which regulates changes in 



2 Lantsch, K. Bacillus amylobacter A. M. et Bred, und seine Beziehungen zu den 

 Kolloiden. Centrbl. Bakt. II, 54: 1-12. 1921; Konig, J., and Hasenbaumer, J. 

 Die Bedeutung neuer Bodenforschungen fur die Landwirtschaft. Landw. 

 Jahrb., 55: 185-252. 1920. 



