CHAPTER XIII 



CHALK, LIMESTONE AND LIME 



Chalk is one of the oldest fertilisers in this country 

 and was used by the ancient Britons in much the same 

 way as is still done in parts of Hertfordshire to-day^. 



It has two main types of action: it supplies basic 

 material which is very necessary for the soil, and it 

 improves the physical condition. The necessity for a 

 base is very definitely marked: in its absence the soil 

 becomes "sour." Such soil is not well suited to plant 

 growth and will not carry luxuriant crops: certain 

 weeds, however, grow well, notably sorrel on heavy 

 land and spurry on light land. "Sourness" often arises 

 through neglect, though it also comes when land lies 

 waterlogged for a long time. The distinction between 

 "sour" soil and sweet soil is very great, for "sourness" 

 is not only inimical to plants but also to micro-organ- 

 isms, and the differences seen in vegetation are probably 

 no greater than those existing in the microscopic popu- 

 lation of the two soils. Just as certain weeds turn up 

 most commonly on "sour" soils so also do certain micro- 

 organisms, such as Plasmodiophora which causes finger 

 and toe in turnips and other plants of the Brassica tribe. 

 Wliatever the cause, the trouble can be put right by 

 suitable dressings of chalk or lime. 



The alteration in physical condition has been more 

 fully studied, but is still somewhat obscure. It is mainly 



1 See the author's Fertility of the Soil, Cambridge Manuals, and Journ. 

 Board of Agriculture, 1916, xxni. 625, for fuller details. 



