114 THE MOVEMENTS OF SOIL WATER [chap. 



a seed bed, for on that depends the future of the crop to 

 a far greater extent than on the inherent fertility of the 

 soil or the amount of manure it receives. 



The effect of the water-content of the soil upon its 

 temperature is one of the most important factors which 

 the cultivator of the soil has to bear in mind : it has 

 already been explained that the evaporation of water 

 results in a considerable lowering of temperature, so 

 that the heat of the spring sun may be largely spent in 

 drying a soil without raising its temperature at all. 

 The amount of heat necessary to raise the temperature 

 of a pound of water by one degree would heat a pound 

 of dry soil by about seven degrees, whereas it would 

 require about a thousand times as much heat to 

 evaporate a pound of water and even then cause no rise 

 of temperature. Growth does not take place below a 

 temperature of about 41" F., this being also the limit 

 below which most seeds do not start nor do the 

 roots of a plant function, hence before any growth can 

 take place in the spring, it is necessary to raise the soil's 

 temperature above this limit. Almost the only source of 

 heat is the radiation from the sun, which is absorbed by 

 the soil and converted into heat ; but to ensure that the 

 heat shall be effective in raising temperature, evaporation, 

 and therefore the access of the soil moisture to the 

 surface, must be checked by cultivation. Any method 

 of diminishing evaporation will, of course, aid in raising 

 the temperature of the soil ; for example, a rough surface 

 to the ground, wind breaks, hedges, or belts of trees, 

 which check the sweep of the wind over the ground, are 

 very effective in early spring towards getting the soil 

 quickly into condition for maintaining growth. In 

 many places the producers of early vegetables on light 

 land near the sea have learnt how desirable it is to 

 establish wind breaks by erecting lines of thatched 



