82 SOILS AND MANUKES 



water vapour by the hygroscopic and deliquescent action 

 of the soil is too small to be of any practical importance. 



The oxidation of organic matter as a source of heat has 

 already been briefly referred to (p. 11). A heap of fresh 

 stable manure is usually warm and steamy, and when 

 'partially dried by its own heat may become so hot as to 

 catch fire. It is possible therefore to obtain almost any 

 temperature desired by this means. In the hotbeds for 

 forcing plants in gardens, layers 2 or 3 feet deep, or even 

 more, are often used simply for the purpose of generating 

 heat. This method is generally preferred to hot pipes or 

 other devices because it heats the soil from below and has 

 not the same tendency to dry the atmosphere and promote 

 undue evaporation from the plants. The manure cannot 

 bo profitably employed on the farm for this purpose all 

 that is produced must be distributed over a much larger 

 area to fertilise the soil but when large quantities 

 are incorporated with the soil the temperature of the 

 latter is sensibly elevated. The maximum rise of tem- 

 perature due to admixtura of 10 tons of fresh manure was 

 2 C., and this disappeared entirely in less than three 

 weeks' time. Under the ordinary conditions of agriculture 

 the effect of the heat produced by the oxidation of organic 

 matter on the temperature of the soil is small and 

 transient. 



The Temperature. The mean temperature of the soil 

 at the surface generally follows that of the air very closely, 

 but the range of temperature is smaller. Thus, when the 

 monthly mean temperature of the air was 57'1 F. that of the 

 soil was 56*7 F. The mean daily range of temperature of the 

 air was 22 F., and that of the soil was only 15 F. Beneath 

 the surface the daily range of temperature rapidly diminishes 

 as the depth increases. At a depth of 6 inches the range 

 of temperature was only 4'6F., and at 24 inches only 0'5 F. 

 The daily variations generally cease to be noticeable at a 



