NOTES ON THE SOIL, 493 



mile of English country. About half of this amount is lime- 

 stone. 



Since there is much diversity in the character and solubility of the 

 rock into which rain-water penetrates, varying as it does from granite 

 and gneiss to limestone and clay, it follows that the material dissolved 

 must also differ considerably in kind and quantity. Scotland, north 

 of a line joining the mouths of the Clyde arid Tay, consists chiefly of 

 hard crystalline rock, and the river w*ater of this region contains less 

 dissolved material than any other in the British Isles. Much of the 

 water of North Wales too is almost as free, for similar reasons. 

 England affords a great contrast. Its rounded limestone hills are 

 easily worn away by water, which is rendered acid by the carbon 

 dioxide it has previously dissolved. The gas expelled in bubbles when 

 ordinary water is heated consists partly of carbon dioxide, which was 

 dissolved out of the atmosphere. 



How Soil Temperature is Influenced. The 



specific heats of different kinds of soil differ considerably. 

 The following list gives the approximate number of heat- 

 units required to raise 100 Ibs. of dry soil from C, to 

 1 C. : Peat, 30 ; humus, 25 ; chalk, 20 ; sandy humus, loam, 

 pure clay and sand, each about 15. It is clear that much more 

 heat is required to raise the temperature of water through 1 

 than that of the same weight of dry soil ; hence a dry soil will 

 warm in the sunshine more rapidly than a moist soil. 



A dark-coloured soil is warmer than a light soil, and shows 

 a greater daily variation in temperature. The degree of in- 

 clination of the surface and the direction of the slope often 

 exert a marked influence on the temperature of the soil, and 

 especially on its daily range. The effect of a south exposure 

 is to make a difference of 2 or 3 C. in the first foot or more 

 at the surface. When the surface of the soil is very uneven, 

 its heat is lost more rapidly in warming the air above it. If 

 the soil is loose and. open in texture, the drier surface layer 

 loses heat to the air, while the poor conducting capacity of 

 the open soil prevents the heat from being conveyed deeply 

 below the surface, and a lower temperature results. If the 

 air is warmer than the soil, as often happens in spring, a 

 large amount of heat may be conveyed rapidly into the soil 

 with rain-water. 



Except direct sunshine on the one hand, and direct radia- 

 tion of heat away from the earth on the other, there is no 

 factor which exerts so strong an influence on the temperature 



