320 SOIL MICRO-ORGANISMS 



than calcium in high concentrations. Magnesium is also somewhat more essential in 

 the growth of certain organisms, like the fungi, than calcitjm. 



The presence of iron, even if only in very small quantities, is essential for the 

 growth of all micro-organisms. Some organisms refuse to grow when a medium having 

 an alkaline reaction is sterilized under pressure, due to the precipitation of the iron. 

 Iron, in the form of ferrous carbonate, can be used by certain specific bacteria as a 

 source of energy; the iron is thereby precipitated in the form as ferric hydroxide. 

 However, precipitation of iron may also take place as a result of the chemical inter- 

 action between the iron salt and products of microbial metabolism.^ 



SOIL MICROBIOLOGICAL ACTIVITIES AND SOIL FERTILITY 



Soil fertility or the ability of the soil to support the growth of higher plants de- 

 pends to a large extent upon the activities of micro-organisms. 



One of the primary functions of these organisms is the mineralization of the or- 

 ganic matter which is constantly added to the soil in the form of plant residues, in- 

 cluding roots, stubble, weeds, and the tree products, green manures, stable manures, 

 various organic fertilizers. These are decomposed, whereby most of the carbon is 

 given off as CO2, the minerals and nitrogen are made again available for plant growth 

 while a part of the organic matter which is more resistant to decomposition remains 

 in the soil, thus tending to improve the soil as a physical and chemical medium for 

 plant growth. When environmental conditions do not favor the activities of most of 

 the micro-organisms, as in the case of water-logged soil, the organic matter tends to 

 accumulate, giving rise to organic soils, such as peats. When conditions favor the 

 activities of micro-organisms, as in well-cultivated and limed soils, organic matter is 

 rapidly decomposed and the soil becomes depleted. This depletion is partly compen- 

 sated for by the residues of the crops grown on the soil. 



Fixation of nitrogen is carried on in the soil to a limited extent by chemical proc- 

 esses. The symbiotic bacteria, in the presence of the host plants, and the non- 

 symbiotic bacteria, in the presence of available sources of energy, bring about a con- 

 siderable fixation of atmospheric nitrogen. This valuable element, which is constant- 

 ly removed from the soil by the growing crops, is thus, at least partly, replaced in the 

 soil. 



In view of the fact that numerous micro-organisms carry out their varied activi- 

 ties in the soil simultaneously, stimulating or injuring the growth and activities of 

 one another, the resultant processes may often be unfavorable for plant growth. It 

 has been suggested that the limitation of soil fertility, or the inability of the soil to 

 support a better growth of cultivated plants, and frequently actual soil exhaustion, 

 are due to the destruction of one group of soil micro-organisms by another. This 

 theory was based on the following facts and assumptions: (i) Bacteria are largely 

 responsible for the liberation of plant nutrients from soil organic matter. (2) Protozoa 

 feed largely upon bacteria in the soil. (3) Treatment of soil with volatile antiseptics 

 or with steam or dry heat makes the soil considerably more fertile. (4) These treat- 

 ments result in an initial reduction in the numbers of bacteria followed by a very 

 considerable increase greatly in excess of the numbers present in untreated soil. This 



' See chap, xxiv in this volume by R. L. Starkey. 



