324 SOILS: PROPERTIES AND MANAGEMENT 



The daily and hourly temperature of the air and the 

 soil may be fairly constant or rather variable, according 

 to conditions. On days of sunshine, however, consis- 

 tent changes may be expected. The air temperature rises 

 from morning until about two o'clock, when the maxi- 

 mum is reached. It then falls rapidly. The soil, how- 

 ever, does not reach its maximum temperature until 

 later in the afternoon, due to the lagging so apparent in 

 soil temperature changes. This lagging is greater in 

 the lower layers than at the surface. The following data, 1 

 taken on a bright day on May 26 in Germany, illustrate 

 the ordinary differences that may be expected in soil 

 and air temperatures : 



HOURLY TEMPERATURES TAKEN IN GERMANY ON MAY 26, 1884, 

 ON A LOAM SOIL AT 4-lNCH DEPTH (IN DEGREES FAHRENHEIT) 



1 Wollny, E. Untersuchungen iiber den Einfluss der 

 Pflanzendecke und der Beschattimg auf die Physikolischen 

 Eigenschaften des Bodens. Forsch. a. d. Geb. d. Agri.-Physik. 

 Band 6, Seite 197-256. 1885. 



