232 



SOIL CONDITIONS AND PLANT GRO WTH 



(5) The top 6 inches of soil has a higher mean tempera- 

 ture than the air both in summer and in winter. At 6 inches 

 the warmer part of the day centres round 5.30 p.m., and the 

 cooler part round 9^30 a.m. 



(6) The warming of the soil in spring is facilitated by 

 drying ; the cooling in autumn is increased by clear nights 

 and diminished by rain (Keen and Russell, 146^). 



TABLE LX. TEMPERATURES OF SOIL AT DIFFERENT DEPTHS UNDER 

 VARYING CONDITIONS. RUSSELL. 



Effect of Weather. 



Effect of Vegetation. 



Records extending over periods of some months have been 

 published by Wollny (318) and by Thiele (281). British data 

 generally refer only to 6 inch or 1 2 inch readings ; they have 

 been collected and worked up by Mawley, by Mellish, 1 and by 

 Franklin. 2 Systematic readings are taken at the Radcliffe 

 Observatory, Oxford, 3 at Kew, and also at Rothamsted, where 

 a continuous self-recording thermometer is installed. Detailed 



1 See Quart. Journ. Roy. Meteor. Soc., 1899, xxv., 238-265. 

 iProc. Roy. Soc., Edin., 1920, 40, 10, 56. 



3 See A. A. Rambaut, Radcliffe Observations, 1901, 48, 1-245, and 1911, 51, 

 103-204; also Phil. Trans., 1901, 195 A, 235-8. 



