312 CAUSES OF FERTILITY AND STERILITY [chap. 



The custom of "chalking" was very extensively 

 practised during the seventeenth and eighteenth centuries 

 in Hertfordshire on the high plateau land on which the 

 Rothamsted estate is situated. There the "clay with 

 flints" and the "boulder clay," though not, as a rule, 

 more than 10 to 12 feet thick, and resting on the chalk 

 rock from which they have largely been derived, have 

 been completely decalcified by the solvent action of the 

 rain water, and no longer contain more than a trace of 

 carbonate of lime. It was customary to sink bell pits 

 through the clay until the chalk was reached ; this was 

 then dug out, hauled to the surface in baskets, and dragged 

 out on to the fields in sledges. Sixty to a hundred or 

 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. 



A similar process, with like results, has been applied 

 to the "red clay with flints" which occurs on the top of 

 the chalk plateau in Kent, Surrey, Hants, and Dorset, as 

 well as in the Chilterns and Herts. In ^several cases 

 where advice has been sought as to the reasons for the 

 infertility or bad working quality of a particular field, it 



