SOIL HEAT 239 



the soil. Sucli a loss is of little moment, liowe-ver, compared to 

 that continually occurring through conduction and radiation. 



Some heat is carried downward into the soil by percolat- 

 ing water. This is a true convection activity. The impor- 

 tance of such a heat transfer is only conjectural. As percola- 

 tion is generally intermittent in a soil, it is probable that it 

 does not modify to any extent the influence exerted by con- 

 duction. 



125. Efifect of organic matter on soil temperature. — 

 Plants entrap a considerable amount of radiant energy from 

 the sun, part of which is utilized during the growth period. 

 The remainder exists as latent energy in the tissue. If any 

 amount of plant remains are incorporated in the soil and de- 

 cay proceeds, this heat is liberated. Thus a heat transfer is 

 similar in a way to convection, except that, in this case, the 

 transfer is by the movement of a solid and the energy is 

 latent. 



To what extent the decay of organic matter is effective in 

 bringing about any important modification of field soil, it is 

 difficult to say. In greenhouses and hotbeds perceptible in- 

 creases are obtained by the use of fresh manure. In the field, 

 however, where the absorption and loss of heat are very large 

 and where the organic matter makes up but a small portion 

 of the soil mass, it is doubtful whether any important heat 

 increase occurs. Georgeson,^ in Japan during the first twenty 

 days after an application of eighty tons of manure to the 

 acre, obtained an increase of only 3.4° F. over a soil un- 

 treated. Wagner 2 found an average increase of 1° F. from 

 the use of twenty tons of barnyard manure to the acre. Bou- 

 youcos ® has obtained the latest data on the subject. Under 



* Greorgeson, C. C, Influence of Manure on Soil Temperature; Agri. 

 Sei., Vol. 1, pp. 25-52, 1887. 



'Wagner, F., ner den Bmfluss der Bimgung mit OrganiscJien Sub- 

 stance auf die Bodentemperatur; Forseh. a. d. GeMete d. Agr.-Phys., 

 Band V, Seite 373-405, 1882. 



'Bouyoucos, Q, J,, An Investigation of Soil Temperature; Micli. Agr. 

 Exp. Sta., Tech. Bui. 17, pp. 180-190, 1913, 



