144 THE TEMPERATURE OF THE SOIL [chap. 



very much upon colour : with black soils the absorption 

 is almost complete ; it is greater for red than for yellow 

 soils, least of all for those which look distinctly white 

 or light coloured. It has already been shown that 

 the colour of soils depends mainly upon humus and 

 hydrated ferric oxide, the latter accounts for all the 

 red, yellow, and brown shades, the former for the black 

 coloration of the soil. Deep-seated clays are often 

 blue or green, due to various ferrous silicates or to 

 finely divided iron pyrites, which afterwards oxidise 

 to brown ferric oxide. The more finely grained a 

 soil is the more surface it possesses, and the greater 

 amount of colouring matter that is required to pro- 

 duce a given colour ; a coarse sand is often quite 

 black though it contains but a small percentage of 

 humus. 



Though the colour of a soil affects the rate at which 

 it absorbs heat, it does not follow that the dark soils 

 will lose with a corresponding rapidity when radiation 

 is taking place at night ; the emissive power of the 

 substance for rays of low refrangibility, such as are 

 emitted at ordinary temperatures, is not affected by 

 colour. Hence, the extra heat gained by a dark soil 

 is retained and not lost by a corresponding increase in 

 its radiating power. 



The curves in the accompanying diagram (Fig. u) 

 show the temperatures of the soil at a depth of 6 

 inches during an April day with a bright sun and a 

 strong drying wind. The land was a light loam of 

 a grey-brown colour when dry ; it had been culti- 

 vated, rolled, and the surface hoed over before the 

 thermometers were inserted ; on plot I the bare 

 ground was left untouched, plot 2 received a dressing 

 of soot until the surface was black, plot 3 was 

 similarly whitened over with lime. It will be seen 



