MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 



353 



It is obvious from the figures given here that, because of the im- 

 portant temperature variations of different soil regions, the micro- 

 biological activities must be profoundly modified. But apart from the 

 climatic variations already indicated there are seasonal variations in 

 any particular locality that are of great moment for soil microbiological 

 activities. Such differences are demonstrated by the temperatures 

 of 1898 and 1902, taken to a depth of 152 mm. (6 inches), at New 

 Brunswick, N. J. 



SOIL TEMPERATURES* 



In this instance, the season of 1898 was not only earlier, but the 

 temperatures of June to September were sufficiently higher to favor 

 more intense bacterial grow.th and activity. 



EARLY AND LATE SOILS. Under any given climatic conditions the 

 warming, up of soils in the spring will depend on their chemical and 

 mechanical composition, color, tillage and topography. Because of the 

 high specific heat of water, fine-grained soils containing a relatively 

 large amount of moisture will warm up more slowly than coarse-grained 

 soils containing a relatively small amount of moisture. The differences 

 in the specific heat of humus, sand, clay and chalk are less important, 

 yet they introduce appreciable variations in the soil temperature 

 according to the proportion of each present. The topography of the 

 soil introduces a factor of some importance for it affects the inclina- 

 tion toward the sun's rays as well as the drainage conditions. Tillage 

 operations are of considerable moment, since they influence the rate 

 of evaporation, that is, the rate at which heat is lost from the soil by 

 the transformation of liquid water into vapor. Finally the color of 

 soils exerts an influence on their temperature in that it affects the 

 absorption and reflection of heat. 



Taking all of the factors together, it is found that sandy soils and 

 sandy loams are early soils, because they part readily with their excess 



* Recorded in Fahrenheit scale. 

 23 



