2S6 MATERIALS OF INDIRECT VALUE [CHAP. 



In dry seasons the clay will crack less and the crop 

 will keep on growing longer, because the improved 

 texture of the soil admits of a better supply of subsoil 

 water to the plant by surface tension. 



It is difficult to exaggerate the improvement that 

 lime effects in the dryness and workability of strong 

 soils, which in many cases would not be fit for arable 

 cultivation had they not been so treated. It has 

 already been mentioned that on the Rothamsted 

 estate the custom of chalking has added from 2 to 

 5 per cent, of carbonate of lime to the surface soil, 

 which is otherwise non-calcareous ; but on one of the 

 fields, formerly under experiment, the treatment had 

 never been carried out. This field, Geescroft, formerly 

 carried experimental crops of oats and beans, but during 

 the rainy seasons about 1879 the land lay so persis- 

 tently wet in the spring that on several occasions a 

 tilth could not be obtained in time for sowing, and the 

 land had to lie fallow, until at last cultivation was 

 abandoned and the field was allowed to fall down into 

 grass. Even now the herbage is very inferior and 

 shows the wet character of the soil by the prevalence 

 of Aira ccsspitosa ; yet in situation, drainage, and 

 mechanical composition this soil is in no respects 

 different from that of the other Rothamsted fields. 

 The essential factor which has caused all the difference 

 in the character of the two soils is the absence of calcium 

 carbonate from the Geescroft field, which for some reason 

 had escaped the chalking given to the other fields. The 

 physical improvement of a clay soil by lime is not appar- 

 ent at once but grows from year to year after the applica- 

 tion of the lime ; the flocculating action is really not due 

 to the lime itself but to the soluble calcium bicarbonate 

 which arises from the action of water and carbonic acid 

 upon the calcium carbonate formed from the lime. 



