136 THE TEMPERATURE OF THE SOIL [chap. 



at the same time the low water content gives rise to 

 less evaporation at the surface, and this great cause of 

 cooling is minimised. The dryness of the soil permits 

 of early cultivation, which, by cutting off the access of 

 subsoil water and diminishing the conduction of heat 

 from the surface, quickens the warming up of the 

 seed bed. The early aeration and warming of the soil 

 promotes the nitrification which is also necessary to 

 growth. If, further, the soil be stony, the conduction of 

 heat from the surface layer into the soil is more rapid, 

 solids being better conductors than powders. Such 

 soils, again, are generally dark coloured, because on the 

 comparatively small surface exposed by the coarse 

 grains the same proportion of humus has a greater 

 colouring effect. 



These conditions are generally fulfilled by the 

 alluvial soils bordering the larger rivers; in the 

 neighbourhood of large towns, which are generally 

 situated on a river, they form the typical market- 

 gardening soils, especially as their natural poverty can 

 be alleviated by the large supply of dung which is easily 

 obtainable from the town. 



At the same time these soils have their dis- 

 advantages ; from both their nature and their situation 

 they are subject to rapid changes of temperature ; they 

 suffer much from night frosts both in spring and 

 autumn, and dry out easily in the summer, so that 

 some crops do not come to their full growth. Autumn 

 planted vegetables grow away rapidly, and are apt to 

 become " winter proud " and killed by severe weather. 



Of course, to ensure the maximum of earliness and 

 freedom from spring frosts, the geographical situation 

 and the climate must be considered as well as the nature 

 of the soil. The neighbourhood of the sea or any large 

 body of water has a great effect in equalising the 



