THE SOIL IN RELATION TO PLANT GROWTH 123 



wide difference in chemical composition. Kraus, therefore, argues that 

 the true chalk plants inhabit chalk soils not because they need much 

 calcium carbonate, but because they find there the general physical and 

 chemical conditions they require. 



The fact that a calcareous rock lies beneath is no proof that the 

 soil itself is calcareous : on the contrary the soil may often contain 

 practically no calcium carbonate, either because it has become decalci- 

 fied by rain, or because it really represents some deposit of wholly ex- 

 traneous origin. 



The agricultural value of chalk soils depends very largely on their 

 depth, and is much greater in valleys where the soil and water collect 

 than on the higher ground where the soil is thinner. The two defects 

 most in need of remedy are the lack of organic matter and the tendency 

 to become light : these are met by additions of dung or other organic 

 manures, by rolling and cultivating with heavy instruments, and above 

 all by feeding animals on the land with the crops actually growing there 

 and with purchased food, a process known as " folding ". Heavy 

 wooden ploughs are still in use, and until recently were worked in 

 many places by large teams of heavy oxen. Sheep are by far the 

 most suitable animals to be fed on the land, and they form the centre 

 round which the husbandry of chalk districts has developed, indeed 

 so important are they that each chalk region has evolved its own breed 

 of sheep South Downs, Hampshires, etc. As fertilisers potassic 

 manures, especially kainit, are generally profitable, superphosphate is 

 needed for turnips, and in wet districts basic slag is useful on the grass 

 land. Skilful cultivation is always necessary, or the soil dries into hard, 

 steely lumps that will not break down. And, lastly, the pre-eminent 

 suitability of the chalk to plant and animal life has its disadvantages ; 

 no soils are more prone to carry weeds, turnip "fly," or wireworm. 

 Skilful management is the keynote of success and it generally obtains, 

 the bad farmers not usually surviving many seasons. 



Black Soils or Humus Soils. In these the organic matter domi- 

 nates all other factors, but the case is more complex than the preceding, 

 because several varieties of organic matter occur, giving rise to several 

 types of soil. A sufficiently full description having already been given 

 (pp. 65 et seq.) it is only necessary to mention the chief agricultural char- 

 acteristics. Peat soils generally need drainage and addition of calcium 

 carbonate and potassium salts ; their agricultural possibilities are much 

 investigated in Prussia where several million acres of moorland occur. 



Fen soils, on the other hand, stand more in need of phosphates and 

 respond well to superphosphates : they do not require lime. Potatoes 



9* 



