954 EXPERIMENT STATION RECORD. 



tLe soil tciiipcratnre. It bas been found, bowever, tbat witb the incor- 

 poration of organic manure into cultivated soils tbe rise in temperature, 

 other things being equal, is [)roportioual to the quantity of manure and 

 (within certain limits) the amount of heat produced by the decomposi- 

 tion of the manure is proportional to tbe initial temperature and humid- 

 ity of the soil. Tbe greater the chemical tension and the larger the 

 amount of easily (iecomposable compounds (especially those containing 

 nitrogen) present in tbe material applied, the greater the amount of the 

 heat produced, if the porosity and tem perature (over 10° 0.) are favorable. 

 For tbese reasons the most energetic liberation of heat takes place, as a 

 rule, immediately after the introduction of stable manure or undecom- 

 posed vegetable matter. This persists for a period dependent upon the 

 favorableness or unfavorableness of the external conditions and the 

 quantity and nature of the organic material, and diminishes by degrees 

 until the temperature of the manured and unmanured soil has become 

 the same. 



The warming eftect of the above-mentioned manures lasts from 4 to 

 12 weeks and amounts to 0.1 to 0.4° on an average, though only when 

 comparatively large quantities of readily decomposable material are 

 used and the circumstances are favorable to decomposition. From these 

 data, and also from the fact that in most cases the amount of heat pro- 

 duced by the quantities of manure ordinarily used or by the decompo- 

 sition of the humus already present in the soil is scarcely appreciable, 

 it may be assumed that the source of heat in question is of subordinate 

 significance.' 



Of still less importance is the liberation of heat on the addition of 

 water, or the absorption of gases and water vapor by the soil, inasmuch 

 as the effect thus produced is of only short duration and seldom occurs 

 under natural conditions, since the conditions are not often favorable to 

 condensation.^ 



Very little of the internal heat of the earth is communicated to the 

 soil, owing to the poor conductivity of the earth's crust, and the effect 

 is approximately the same for all soils. 



From the above facts it follows that the marked differences observed 

 in the temperature of the soil are brought about only by the heat of the 

 sun, since other sources of heat than those already mentioned do not 

 exist. The effects of the sun's heat are modified principally by (1) the 

 physical structure of the soil, (2) the covering, (3) the position of the 

 soil, and (4) the course of the meteorological elements. 



INFLUENCE OF THE PHYSICAL STRUCTURE OF THE SOIL. 



For a proper understanding of the rather complicated phenomena 

 connected with the warming of the soil it is necessary to consider sep- 

 arately the different factors concerned. 



'F. Wagner, Forsch. Geb. agr. Phys., 5, p. 373. 



«A. Stellwaag, Ibid., p. 210. J. Breitenloliner, Ibid., 7, p. 208. 



