148 THE TEMPERATURE OF THE SOIL [chap. 



The evaporation of i lb. of water at 62 F., i.e., 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. 1 1 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 

 it will be seen that the wind break, by checking evapora- 

 tion, maintained the soil temperature more than 2 

 above that of the open ground. Sufficient attention is 

 not given in practice to the value of even slight wind 

 breaks for checking evaporation and so raising the 

 temperature of the soil in early spring. The raisers of 

 specially early vegetables, radishes in particular, on a 

 strip of light land close to the sea in Kent are, however, in 

 the habit of breaking the sweep of wind across their fields 

 by erecting temporary fences of lightly thatched hurdles. 



