LAND AND OTHER AGENTS OF PRODUCTION 167 



normally rises as the water increases up to a certain point, it then 

 falls off, because the excess of water reduces the air supply for the 

 roots. Changes in the amount of water in the soil would alone lead 

 to a renewal of the air supply in the soil, but other factors diffusion, 

 changes in pressure, air movements, etc. come in, making the gaseous 

 interchange still more complete. 



The temperature of the surface layer of soil, which in turn deter- 

 mines the temperatures of the lower layers, is the resultant of several 

 different effects. The actual amount of heat reaching the surface is 

 that portion of the sun's rays that passes unabsorbed through the 

 atmosphere, and is therefore dependent on the climate. The inten- 

 sity of distribution of the heat over the surface depends on the slope 

 of the land, and is greater the more nearly the land lies at right angles 

 to the midday rays: thus, in our latitudes a south slope is warmer 

 than a north slope, so much as often to produce marked vegetation 

 differences. Many of the rays may be intercepted by vegetation; 

 consequently land densely covered by plants is cooler and moister 

 than bare land. Of the rays that do finally reach the surface not all 

 are absorbed, an unknown fraction being reflected back again into 

 space. Although no actual measurements have been made, the loss 

 from this cause is probably greater on a white chalky soil than on 

 a black humous soil. 



The extent to which a given quantity of absorbed heat raises the 

 temperature of a soil depends on its specific heat, and this again on 

 its water content. A dry soil will attain a higher temperature than 

 a moist one. It commonly happens that the surface layer of the soil 

 is hotter than the air, especially on a sunny day. The passage of 

 heat through the soil is slow and consequently fluctuations in tem- 

 perature at a depth of 3 inches are less marked than at the surface, 

 especially in dry, loose soils. The thermal conductivity of a soil is 

 increased by moistening and by compacting. 



46. BACTERIA AND SOIL FERTILITY 1 

 BY P. E. BROWN 



The factors which bring about the change of insoluble substances 

 into soluble in the soil may be grouped into three classes physical, 

 chemical, and bacteriological. The bacteriological factors have come 



1 Adapted from "Bacteria and Soil Fertility," Circular No. 7, Agricultural 

 Experiment Station of Iowa State College, pp. 3-15. 



