*3o THE TEMPERATURE OF THE SOIL [chap. 



slowly after the heat of the summer. On clay soils 

 growth will be noticed to continue later into the autumn 

 than on the lighter lands. 



Heat required for Evaporation, 



The coldness of a wet and undrained soil is due, not 

 only to its high specific heat, but to the fact that so much 

 of the heat it receives is spent in evaporating some of its 

 retained water, without causing any rise in temperature. 

 The evaporation of I lb. of water at 62 F., z>., its con- 

 version into water vapour at the same temperature, 

 requires as much heat as would raise the temperature of 

 1050 lbs. of water by i°F., and, if there be no source of 

 external heat bringing about the evaporation, the sub- 

 stance from which the water is evaporated must become 

 cooled to a corresponding extent The cooling effect of 

 evaporation is well known, but its application to the 

 soil is not always realised ; clays and even more so un- 

 drained soils are cold and late, not only because of their 

 high specific heat, but because they retain so much 

 water which can be evaporated. The drying winds of 

 early spring exercise a great cooling effect whenever 

 the soil moisture is allowed to evaporate freely, hence 

 the importance of establishing a loose tilth, if the 

 seed bed is to warm up the temperatures requisite for 

 germination. 



Anything providing a little shelter to check evapora- 

 tion and break the force of the wind in the spring will 

 have a considerable effect in raising the soil temperature. 

 The dotted curve in Fig. 10 shows the effect of enclosing 

 a plot of the same land with a slight hedge made of 

 spruce fir boughs about 2 feet high. In the morning 

 the temperature of the sheltered plot was below that of 

 the open ground because of the shading from the direct 

 rays of the sun, but as soon as this effect was over 



