SOIL TEMPERATURES. 245 



and the plant will speedily show when the amount required for 

 the absorptive activity of its roots is not furnished. Different 

 plants, however, require different amounts : thus aquatics and 

 marsh-plants do not need so much oxygen for their roots as 

 do plants which ordinarily grow in a porous soil. Partial ex- 

 clusion of oxygen from the roots of the latter by keeping the 

 soil saturated with water usually injures the plants in a short 

 time. 



It has been shown by Sachs and others that seedlings of many 

 plants normally growing in dryish soil will develop if treated as 

 aquatics ; better results are obtained, however, if air is occasion- 

 ally passed through the water. 



659. The temperature needed for the absorptive activity of 

 roots varies with different plants. It may be said, however, 

 that for any given plant the absorptive power increases with 

 increase of temperature. 



660. Different soils have very different relations to temper- 

 ature. Leaving out of account the small amount of warmth 

 derived from the chemical changes going on in the soil by which 

 heat is evolved, it may be said that the heat of the soil is derived 

 from the sun's rays. The angle at which these rays strike the 

 soil must have a great influence upon its temperature. Again, 

 there are various local causes, such as protecting or reflecting 

 walls, which may considerably modify the temperature in any 

 given case. The soil itself exerts a marked influence upon the 

 amount of heat which it can receive and retain. Dark soils ab- 

 sorb heat most readily ; but it has been shown that black soils 

 are less absorbent of heat-rays than are those which are* dark 

 gray. The radiating power of a soil depends upon the character 

 of its surface, being much greater in the case of fine mould than 

 in that of coarse, gravelly soils. 



661. It must be noted, however, that the heat-rays which fall 

 upon a given soil ma}- have different degrees of intensity. Some 

 bodies (e. g. lampblack), can absorb and give off b}- radiation 

 heat of high as well as that of low intensity ; while other bodies 

 (e. g. snow), absorb heat of low intensity only. Heat of high 

 intensity is converted into that of low intensity by the interpo- 

 sition of a black covering of an}- kind which can absorb it and 

 give it out below as heat of low intensity. 



662. At the depth of fifty feet the temperature of the soil in 

 the temperate zone varies within the limits of one degree, and 

 at a depth somewhat below this it is constant. The stationary 

 temperature at such a depth is the same as that of the mean 



