MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 297 



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 oJE 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 Jess 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 indina- 

 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 

 of water. Clay soils and clay loams are, on the other hand, late soils; 

 it means, therefore, that in the mdlre open soils microbial activities be- 

 come 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 pro- 

 duction of carbon dioxide at 0° to 5°. In a series of experiments carried 

 out by Wollny the amounts of carbon dioxide produced were as follows: 

 ' CO2 IN 1,000 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 



