THE SOIL IN RELATION TO PLANT GROWTH 303 



€arth-worms and bacteria, so that little decomposition goes 

 on, and a barren sand results. Wetness and lack of aeration 

 necessitate special vegetation and decomposition agents, and 

 there may result a "mild humus" if sufficient calcium car- 

 bonate is present to determine a calcicolous flora, or an " acid 

 humus " in the contrary event. Even such small differences 

 as affect the liability to late frosts may affect the native 

 vegetation and therefore the humus : for example, bracken is 

 more readily killed than heather and is therefore less likely to 

 survive at levels where late frosts are common. 



Thus the soil is very much the result of circumstances ; its 

 character is determined in part by the rock from which it was 

 derived, and in part by subsequent events, particularly the 

 temperature and water supply it happened to obtain, in other 

 words, its climate. A given set of mineral particles may, 

 under different climatic conditions, give rise to soils wholly 

 different in agricultural value and in natural flora. This thesis 

 was first set up by Hilgard, but it has been greatly developed 

 by the Russian investigators and, indeed, has been one of their 

 most striking contributions to the study of the soil (85, 266). 



Further, the farmer has discovered how to build up these 

 compound particles by cultivation and thus change to a very 

 great extent the relation of the soil to the plant : the process 

 consists in adding dung, or ploughing in green crops, adding 

 lime, exposure to frost, and skilful (but wholly empirical) 

 cultivation, and, although not very rapid, it takes only a few 

 seasons to accomplish. 



But while the ultimate mineral particles do not entirely 

 control the relationships of the soil to vegetation they fix the 

 limits within which these relationships may vary and beyond 

 which they cannot pass Farmers recognise five great divi- 

 sions : clays, loams, sands, calcareous soils and soils rich in 

 organic matter, all shading off into one another and without 

 sharp lines of demarcation, but representing classes of soil 

 that cannot be changed one into the other by any cultivator's 

 artifice. 



Calcareous Soils. — The simplest case is presented by soils 



