KELATION OF SOIL TO HEAT. 21 



Evaporation is greatest when the soil is occupied by 

 a crop, and will be in proportion to the activity of its growth 

 and the extent of its root development. On uncropped soil 

 evaporation is greatest when the soil is consolidated, and 

 the conditions are consequently favourable to capillary 

 action. The presence of stones on the surface tends to 

 diminish evaporation. Evaporation is least when the 

 surface soil has been broken up by tillage, as the subsoil 

 water cannot then reach the surface by capillary attraction. 

 The amount of water draining through a soil depends on 

 the amount of the evaporation, and is most simply ex- 

 pressed as equal to the rainfall, minus the quantity 

 evaporated. 



3. Relation to Heat. A. soil shaded by a crop is far 

 cooler than a bare soil. Dark-coloured soils absorb the 

 greatest amount of heat from the sun's rays, and light- 

 coloured soils least. The presence of humus is thus 

 favourable to soil warmth. Quartz sand is an excellent 

 conductor of heat ; chalk is a bad conductor. A soil rich 

 in sand will thus be warmed or cooled more rapidly, and 

 to a greater depth, than a soil containing but little sand. 

 AVater has a very considerable effect in cooling a soil, 

 partly from its high specific heat, and partly from the 

 immense consumption of heat during its evaporation. 

 During spring and summer a wet soil is always colder 

 than a dry one. The drainage of wet land will thus 

 result in a greater warmth of the surface soil, and con- 

 sequently an earlier growth in spring. 



4. Plant Food in Soil. The proportion of plant food 

 present in soils is very small, even when the soil is ex- 



