x.] CALCAREOUS SOILS 281 



Calcareous Soils. 



It is difficult to draw an exact line of demarcation 

 between the loams and calcareous soils, so variable is 

 the proportion of carbonate of lime, owing to its con- 

 tinual removal by the percolation of water containing 

 carbonic acid. Even on the chalk and limestone, where 

 the thickness of the soil layer is to be measured in 

 inches, the surface soil may have its calcium carbonate 

 almost wholly removed, and, again, where the deeper 

 soils of calcareous origin accumulate in the valleys, 

 there is nothing to distinguish them from ordinary 

 loams. However, the general aspect of the calcareous 

 soils containing from 5 to 60 per cent, of calcium 

 carbonate is characteristic, and the natural flora always 

 indicates the presence of much lime. The texture of 

 the calcareous soils may vary within any limits, accord- 

 ing to the formation from which they have originated. 

 On the one hand, extremely fine-textured heavy marls 

 exist; for example, the soils derived from the strata 

 at the base of the Chalk and upper beds of the Gault 

 in the south and east of England; on the contrary, 

 fairly coarse sand may form a considerable proportion 

 of the soil, rendering it light in texture, as is the case 

 with many of the soils resting on the chalk of the 

 North Downs. In all cases these calcareous soils are 

 typically sticky when wet, and easily cake on the surface 

 when dried. Such soils, again, lose their organic matter 

 very rapidly by decay ; in farming them it is desirable 

 to use every means to increase the proportion of humus 

 by adding farmyard manure, by folding roots on the 

 land, or by ploughing in green crops. Slowly acting 

 nitrogenous manures, like rape dust or shoddy, are 

 valuable ; again, there is always enough calcium 

 carbonate present naturally to render to sulphate of 



