RHYTHM OF THE SOIL 153 



and for the most part, surface organisms — of tlie eggs and 

 larvae; (5) the appearance of swarms of medusae and other 

 coelenterates in the summer; (6) the reappearance of diatoms 

 in the late summer and autumn; and (7) the scarcity of the 

 plankton as the winter begins. Throughout all these changes 

 a number of forms of life remain as permanent inhabitants 

 of the surface area all the year round. 



The temperature of the soil also undergoes a daily and 

 yearly rhythmic change, at any rate within certain limits of 

 depth from the surface. At the surface there is a daily rise 

 and fall of temperature, and this temperature wave is propa- 

 gated into the soil and as a result the temperature at any 

 given depth shows a fluctuation which is a kind of reduced 

 image of that of the soil surface. The time between the maxima 

 and minima of temperature remains unaltered. But the 

 temperature range becomes diminished, and there is a "time- 

 lag" which at a depth of six inches may amount to some hours. 

 A rise of temperature which begins at the surface soon after 

 daybreak is not apparent at a depth of six inches until about 

 9 a.m., after which it becomes warmer and warmer till about 

 3.30 p.m., and then remains constant for an hour or there- 

 about. About 5.30 p.m. it begins to fall slowly and con- 

 tinuously throughout the next sixteen hours, that is to say, 

 till about 9 a.m. the following day. 



Soil warms up more quickly than it cools. It attains its 

 maximum temperature in seven hours, and it remains con- 

 stant for about an hour and takes about sixteen hours to cool. 

 These figures, however, vary to some extent at different 

 seasons and the daily rise is slower after rain, thougli wind 

 appears to influence it but slightly ; and there are other factors 

 which slightly interfere with the figures. Roughly speaking, 

 the soil population enjoys a warmer climate than that of the 

 atmosphere, and a moister one. The minimum temperature 

 of the soil is in the summer from six to eight degrees Centi- 

 grade above the minimum temperature of the air, and in 

 winter some three degrees higher. Further, the minimum 

 temperature in the soil is attained early in the day during 

 the summer, somewhere about 7.45 a.m., and in the winter 

 when it is coldest about 10.30 a.m. The air on the other hand 



