Studies in Soil Physics, V. 18§ 



on them than do bodies of lighter hue, and soils behave like 

 other substances in this respect. The darker the soil the more 

 radiation it will absorb and, other things equal, the warmer 

 it will become. However, the light colored soil by no means 

 fails to absorb radiant energy altogether. It simply 

 absorbs a little less and indeed, there is seldom sufficient color 

 diflference between ordinary soils to make any very great dif- 

 ference in the thermal absorption. Nevertheless color is of 

 some importance, and it is the only soil property which mater- 

 ially affects the gains and losses of heat. From the practical 

 viewpoint the variations in exposure (dependent upon topo- 

 graphy, vegetal cover, etc.) have much more effect upon the 

 supply of radiant energy, but these matters are so obvious 

 and well known that their discussion here is hardly necessary. 



The really important way in which the character of the 

 soil influences its temperature is connected with the second 

 question of rapidity of temperature change rather than with 

 the gains and losses of heat. The amount of water which will 

 be normally held by a soil depends upon its physical nature, * 

 and it is obvious from the above that the more water retained 

 by, and present in, the soil the higher will be the total specific 

 heat of the system, and the more slowly will the soil be warmed. 

 Thus a clay which holds thirty percent of water will warm much 

 more slowly than a sand which holds only five percent. To 

 this factor must be ascribed the well known tendency of sandy 

 soils to warm early in the spring and their proverbial utility 

 for the ' ' early ' ' vegetables. 



So important is this matter of water content that it far 

 outweighs color and any ordinary differences in exposure. Of 

 two soils, one sandy, well drained and light in color; the other 

 heavy and wet but very dark, the sandy soil will warm much 

 the faster in spite of the apparent advantage given to the other 

 by its darker color. Exactly this case is common in the truck 

 regions along the Atlantic seaboard where the light colored 

 sandy soils of the rises alternate with the much darker heavier 

 soils of the slough bottoms. The light colored sandy soils are 

 so much earlier that it would be quite impossible to convince 



*As discussed in the second paper of this series Plant World 14, No. 3. (1911) 



