SUPPLY OF CALCIUM CAKBONATE 287 



the carbonate of lime, instead of the zeolites, as soon as the 

 former was applied to the soil. 



That the use of ammonium-salts as manure does directly 

 cause the removal of carbonate of lime from the soil may be 

 learnt from a detailed examination of the amounts present in 

 the soils taken from the Rothamsted plots at successive dates 

 during their history. At Rothamsted the carbonate of lime 

 in the soil, the amount of which varies from about 2 to about 

 5 per cent, in different fields, is all of artificial origin; for 

 though the "red clay with flints," the drift formation out of 

 which has arisen the soil at Rothamsted, rests on the solid 

 chalk rock at a depth of 10 to 15 feet below, yet both soil and 

 subsoil in a natural state are almost wholly lacking in carbonate 

 of lime. Such natural soil may be found on the neighbouring 

 Harpenden Common, which has never been cultivated nor 

 subject to any improvements ; and again on the grass land 

 and a few of the other arable fields on the estate ; in all these 

 cases analysis shows only about one-tenth per cent, of car- 

 bonate of lime in either soil or subsoil until the underlying 

 chalk rock is reached. It is, however, on record that up to the 

 early years of the nineteenth century it was a regular custom 

 in Hertfordshire farming on this hill land to sink pits through 

 the clay into the chalk, extract the chalk, and spread a layer 

 representing six to ten tons per acre over the arable land, the 

 process being repeated at intervals of a few years. The 

 " dells " or hollows in the fields, which represent the fallen-in 

 pits, are evidence to-day of the old practice, and much of the 

 friability and dryness of this heavy land, through which alone 

 it has been possible to keep it under the plough, is due to the 

 work thus done for the present generation of farmers during 

 the seventeenth and eighteenth centuries or even earlier. As 

 a consequence, the surface soil of the Rothamsted arable 

 fields now contains 2 per cent, or more of carbonate of lime, 

 visible to some extent as tiny rounded fragments from the 

 size of a pin-head to that of a pea, but mainly present in 

 particles too small to be seen ; the subsoil, however, contains 



