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in volume when it becomes wet, and strong clay soils may con- 

 tract 8 to 10%. Light sandy soils with little humus undergo 

 little or no change in volume when wet. Humus soils ex- 

 pand up to 15% when wet, and more in frost. Clay soil also 

 expands in frost. This expansion often causes rupture of 

 roots of crops growing on these soils. Clay soils in drying 

 crack. These cracks also damage the roots of growing 

 crops. 



90. Colour. The colour of the soil somewhat affects its 

 temperature. Dark coloured bodies being more quickly heat- 

 ed than light coloured bodies, humus soils and dark basalt 

 soils are warmer than lime-stone soils and sandy soils. If 

 dark coloured shales are sprinkled over vineyards in cold 

 countries, ripening takes place quicker. Smooth and white 

 substances sprinkled over dark coloured soil would keep such 

 soil comparatively cool. As we are more interested in keep- 

 ing soils cool rather than warm, we might try the effect of 

 scattering white chips of stone or chalk on dark coloured 

 soils. For practical purposes the question of colour is not 

 of much importance in a country where coolness is best 

 secured by moisture which most soils are in need of at certain 

 critical periods. The question of temperature of the soil, 

 however, is of great importance, and we will treat of this 

 subject in the next Chapter. 



CHAPTER VIII. 



CONDITIONS AFFECTING FARMING. 



n^EMPERATURE. The mean temperature of the surface 



soil differs in different climates, but even in the same 



locality some soils are recognised as cold and others as warm. 



The heat of the soil is derived from three sources and it is dis- 



