748 PRINCIPLES OF SOIL MICROBIOLOGY 



This favorable influence of heating was found to be due not to the 

 destruction of toxins or protozoa, but to a chemical modification of 

 the peat. A stimulative effect of steam treatment of soil upon the 

 numbers of bacteria in the soil has also been observed. 30 The results 

 of heating (65°C. for one hour) of a soil upon the numbers of fungi and 

 bacteria, evolution of carbon dioxide and accumulation of available 

 nitrogen (ammonia + nitrates) are given in figure 65. Heating a soil 

 even to low temperatures seems to improve it as a medium for bacterial 

 growth. It is interesting to mention, in this connection, the change 

 brought about by heat in the physical and chemical condition of the 

 soil. While treatment of soil at temperatures lower than 100°C. 

 rendered the soil more fertile, temperatures higher than 100° rendered 

 it less fertile. 31 The lower temperatures bring about an increase in 

 the soluble organic matter 32 in the soil; treatment of soil at higher 

 temperatures (especially above 10Q°C.) results in the formation of 

 substances toxic to the growth of higher plants: among those guanine, 

 arginine, dihydroxystearic acid have been recorded. There is also an 

 increase in acidity, or rather ability of soil to neutralize bases. 33 The 

 toxic substances are unstable, gradually disappearing in the course 

 of time, if the soil is kept moist and aerated. This is probably due to 

 oxidation and other activities of microorganisms. 



The increase in soluble organic matter, as a result of heating, varies 

 directly with the temperature to which the soil is subjected, particularly 

 in the case of carbohydrates. It has also been observed that heating 

 of soil brings about a greater solubility of the phosphorus and nitrogen 

 compounds in the soil. 34,35 Similar results were obtained by a number 



30 Osmun, A. V. A comparison of the numbers of bacteria in sterilized and 

 unsterilized soils. Mass. Agr. Exp. Sta. Rpt. 1905, 146-148. 



31 Pickering, S. U. The activities of heat and antiseptics on soils. Jour. 

 Agr. Sci., 3: 32-54, 258-276. 1910. 



32 Schreiner, O., and Lathrop, E. C. The chemistry of steam heated soils. 

 U. S. Dept. Agr. Bur. Soils Bui. 89. 1912; Jour. Amer. Chem. Soc, 34: 1142- 

 1159. 1912. 



33 Robinson, R. H. Concerning the effect of heat on the reaction between 

 limewater and acid soils. Soil Sci., 9: 151-157. 1920. 



34 Liebscher, G. Welche Kulturpflanzen vermehren den Stickstoffvorrat der 

 Wirtschaft. II. Die Stickstoffsammlung der Kulturpflanzen. Deut. Landw. 

 Presse., 20: 975-976. 1893. 



36 Kruger, W., and Schneidewind, W. Ursache und Bedeutung der Salpeter- 

 zersetzung im Boden. Landw. Jahrb., 28: 217-252. 1899. 



