io6 SOIL CONDITIONS AND PLANT GROWTH 



because this gas diffuses out more slowly than water vapour, oxygen, 

 or nitrogen, but the percentage volumes of oxygen and nitrogen are 

 nearly the same as those of the atmosphere. Of course, if the air sup- 

 ply is cut off by an accumulation of water on the surface, the oxygen 

 may fall considerably in volume, but this case is exceptional. At still 

 lower depths the volume of carbon dioxide may rise above i per cent.* 

 As might be expected, the carbon dioxide is higher in amount in 

 summer than in winter, and higher in grass land than in arable. It 

 may rise considerably in grass land, or in land recently dunged. 



The Temperature of the Soil. 



The temperature of the surface layer of soil, which in turn deter- 

 mines the temperature of the lower layers, is the resultant of several 

 different effects. The actual amount of heat reaching the surface is that 

 portion of the sun's rays that passes unabsorbed through the atmosphere, 

 and is therefore dependent on the climate. The intensity of distribu- 

 tion of the heat over the surface depends on the slope of the land, and 

 is greater the more nearly the land lies at right angles to the mid- 

 day rays : thus, in our latitudes a south slope is warmer than a north 

 slope, so much as often to produce marked vegetation differences. 

 Many of the rays may be intercepted by vegetation, consequently 

 land densely covered by plants is cooler and moister than bare land ; 

 advantage is often taken of this fact in tropical countries to protect soil 

 from intense evaporation by the growth of " shade " crops. Of the 

 rays that do finally reach the surface not all are absorbed, an un- 

 known fraction being reflected back again into space: although no 

 actual measurements have been made, the loss from this cause is prob- 

 ably greater on a white chalky soil than on a black humus soil. 



The extent to which a given quantity of absorbed heat raises the 

 temperature of a soil depends on its specific heat and this again on its 

 water content. Dry soil has a specific heat of o-2, while wet soil has a 

 specific heat approximating more closely to i, hence a dry soil will 

 attain a higher temperature than a moist one. It commonly happens 

 that the surface layer of the soil is hotter than the air, especially on a 

 sunny day. 



As a result of the interaction of these factors marked temperature 

 variations occur over comparatively small areas of soil, being produced 

 by differences in aspect, moistness, vegetation cover, looseness and so 

 on. Illustrations are afforded in Table XLV. These variations are, 



' Pettenkofer's determinations at Munich at depths 1J-4 m. below the surface are published 

 in N. Rep. Pharm., 1873, xxi., 677-70Z, and abstracted in yourn. Chem. Soc, 1873, 361, and 

 1874, 36. 



