242 NATURE AND PROPERTIES OP SOILS 



April that an nndraiiied soil in "Wisconsin ranged from 2.5°P. 

 to 12.5° P. lower than one of the same type well drained. 

 Parks ^ reports data of the same order from England. Drained 

 and iindrained soil held in trays at Urbana, Illinois,^ showed 

 maximum differences of 13.7° P., 9.0° P., and 6.2° P. at 

 depths of 1, 2, and 4 inches, respectively. The differences 

 were greatest in the day, Wollny considers that the depres- 

 sion of temperature due to evaporation is roughly propor- 

 tioned to the moisture present. Texture, structure, and or- 

 ganic matter influence the cooling action of evaporation, since 

 they exert such a marked effect on water capacity and capil- 

 lary movement. The practical importance of evaporation 

 study lies in the fact that it can be controlled to such a 

 marked extent in the field. Such is not true of radiation and 

 conduction. Windbreaks and shelters have been shown by 

 King ^ to reduce evaporation over short distances as much as 

 25 per cent. This means a conservation of soil energy for 

 the time being. Thorough under-drainage not only checks 

 evaporation losses but lowers the specific heat of the soil, 

 retards its radiation and facilitates convection. This means 

 a faster warming up, especially of the root zone. Optimum 

 moisture encourages optimum heat conditions as well as other 

 favorable phenomena. Drainage, tillage, and organic matter 

 are the dominant factors in this moisture control. 



127. Soil temperature and its variations. — The tempera- 

 ture of the soil at any time depends on the ratio of the energy 

 absorbed and the heat being lost. The constant change in 

 this coordination is reflected in the seasonal, monthly, and 

 daily soil temperatures. The following data ^ are representa- 



^ Parks, J.J On the Influence of Water on the Temperature of Soils; 

 Jour. E07. Agr. Soc. Eng., Vol. 5, pp. 119 146, 1845. 



^Hosier, J. G-., and Gustafson, A. R, Soil Physics and Management; 

 p. 302; PMladelpMa, 1917. 



^King, F, H., The Soil, p. 189; New York, 1906. 



*Swezey, G. D., Soil Temperatures of Lincoln, NedrasJca; Nebr. Agr. 

 Exp. Sta., 16th Ann. Rep., pp. 95-102, 1903. 



