EELATIONS OF SOIL TO HEAT 29 



Soils composed of silt, or extremely fine sand, are 

 those which yield water most effectually to a growing 

 crop. 



4. Belations to Heat. — The sources of heat to a soil 

 are solar radiation and chemical action. The oxida- 

 tion of organic matter in a soil will undoubtedly raise 

 its temperature, but the effect is generally too small to 

 be appreciable. At Tokio 40 tons of farmyard manure 

 per acre were incorporated with the soil to a depth of 

 one foot. During the next twenty days the average 

 temperature of this soil was 2°'3 higher than that of 

 unmanured soil ; during the next five days the excess 

 of temperature was only 0^*8. Chemical action is 

 most vigorous during the summer months. 



Both the amount of heat received from the sun and 

 the amount of heat which the soil loses by radiation 

 are largely influenced by the degree of transparency of 

 the atmosphere. The greatest extremes both of heat 

 and cold occur with a clear sky and dry air; in a 

 cloudy, moist climate, the variations in temperature 

 are comparatively small. 



The heating effect of the sun is largely determined 

 by the angle at which its rays strike the earth ; it is 

 greatest when these rays are perpendicular to the 

 surface. The different power of the sun's rays at 

 sunrise and at mid-day is familiar to all. At sunrise 

 the solar radiation is weakened by diffusion over a 

 wide area, and its intensity is further diminished by 

 excessive atmospheric absorption. At mid-day the 

 illummation has reached its maximum, and the sun's 

 rays also pass through a minimum thickness of the 

 atmosphere. The difference in the angle of incidence 



