234 



MICROBIOLOGY OF SOIL. 



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 inclination toward the sun's 

 rays as well as the drainage conditions. Tillage operations are of con- 

 siderable 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 of water. 

 Clay soils and clay loams are, on the other hand, late soils; it means, 

 therefore, that in the more open soils microbial activities become intense 

 earlier in the spring. Market gardeners usually attempt to improve 

 matters still further by the use of large quantities of readily fermentable 

 manure that develops enough heat to raise slightly the soil temperature. 



PRODUCTION AND ASSIMILATION OF PLANT FOOD. It was already 

 observed by Moller that slight amounts of carbon dioxide may be evolved 

 from frozen soil. Kostychev could detect a considerable production of 

 carbon dioxide at o-5. In a series of experiments carried out by Wollny 

 the amounts of carbon dioxide produced were as follows: 



COi in 1000 Vols. of Air. 



The increased production of carbon dioxide at the higher temperatures, 

 as shown in the above table, correspond with the observations that had 

 already been made by Ebermayer, Schloesing and others, that carbon 

 dioxide production in the soil is greater in summer than it is in winter. 

 These facts, taken together with the early observations of Forster on the 

 multiplication of photo-bacteria at o, and the more recent observations 



