244 NATURE AND PROPERTIES OF SOILS 



two o'clock. It then falls rapidly. The soil, however, does 

 not reach its maximum temperature until later in the after- 

 noon, due to the usual soil lag. This retardation is greater 

 and the temperature change less as the depth increases. 1 The 

 substratum of a soil shows little daily, or even monthly, varia- 

 tion and is affected, if at all, by seasonal changes only. The 



JAN. FBSR. MXR. APR. MAY JUNE JULY AUG. SEPT. OCT. NO* DEC 



Fig. 45. — Curves showing the average monthly temperature readings at 

 various soil depths. Average of twelve years, Lincoln, Nebraska. 



curves in Fig. 46, comparing soil and air temperatures at 

 Munich 2 on a bright day in May, substantiates some of the 

 statements above : 



128. Control of soil temperature. — The most important 

 factor in the control of soil heat is obviously moisture. Good 



1 The following laws hold in a general way : 



1. The lag of the temperature wave is proportional to the depth. 



2. The diurnal amplitude of the temperature oscillation decreases in 

 geometric progression as the depth increases in arithmetic progression. 

 If the temperature variation at the surface was 24°F and at 6 inches 

 deep 12°F, according to this law the diurnal variation at 12 inches 

 would be 6°F and at 18 inches 3°F. 



a Wollny, E., Untersuchungen iiber den Einfluss der Pflanzendeclce und 

 der Beschattung auf die Physikalischen Eigenschaften des Boden; 

 Forsch. a. d. Gebiete d. Agr.-Physik., Band VI, Seite 197-256, 1885. 



