rjo CAUSES OF FERTILITY AND STERILITY [chap. ix. 



even a hundred and fifty loads per acre were spread, 

 and from time to time the process was repeated. The 

 amount of chalk thus spread upon the surface was con- 

 siderable ; the surface soil of the arable fields on the 

 Rothamsted estate now contains from 3 to 5 per cent, 

 of carbonate of lime, which is equivalent to 30 to 50 tons 

 per acre ; and since none has been spread for the last 

 seventy years at least, and solution in the rain water has 

 constantly been going on, there must have been nearer 

 100 tons per acre at the beginning of the nineteenth 

 century. 



The result has been practically the creation of a soil 

 fit for arable farming, for some of the Rothamsted fields 

 which had never undergone the operation have had to 

 be laid down to grass, so difficult did their cultivation 

 prove in wet seasons. 



Chalk is perhaps more suited than lime to very 

 light sandy soils like the Lower Greensand or the 

 Bagshot beds, for on such dry hot soils the application 

 of quicklime is apt to result in too rapid a decay of the 

 organic reserves of the land ; on clay soils, however, 

 quicklime is preferable, as it is a much more effective 

 agent in coagulating and improving the texture of the 

 clay 



