156 THE TEMPERATURE OF THE SOIL [chap. 



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 

 temperatures and preventing severe frosts ; in the 

 British Islands, #or example, the earliest potatoes are 

 grown in Jersey, near Penzance, and on other light land 

 along the southern coast of Cornwall, and again, a little 

 later near the sea in Ayrshire. Light land rouna the 

 coasts of Kent and Essex, which borders, and in some 

 cases is almost surrounded by the sea, is also specially 

 valued for the growth of early vegetables. 



The soils naturally retentive of water are late, 

 both because they dry slowly and are rarely fit to work 

 early in the year, and because the high water content 

 keeps their temperature down. Except in long-con- 

 tinued droughts they maintain a supply of water to the 

 plant, their high specific heat keeps them at a com- 

 paratively equable temperature and prevents them from 

 cooling down so soon when the summer heats are 

 past. In consequence, the crop is neither forced early 



