THE SOIL IN RELATION TO PLANT GROWTH 121 



a sandy soil may, without any change in type, be a dry and barren 

 heath if underlain near the surface with rock or gravel, a highly fertile 

 fruit or market-garden soil if sufficiently deep, or a stagnant marsh 

 giving rise to peat if so situated that water accumulates and cannot 

 drain away. 



No sharp division can be drawn between the intrinsic and extrinsic 

 properties. The significant units in the soil that determine its intrinsic 

 properties are the compound particles made up of the ultimate mineral 

 particles — clay, silt, sand, etc. — together with calcium carbonate and 

 organic matter derived from plant remains. Now the nature and 

 amount of the organic matter are greatly influenced by the extrinsic 

 conditions — the temperature and water supply — that have obtained in 

 the past. Moisture, warmth and aeration favour the development of 

 a succulent vegetation which sheds easily decomposable leaves and 

 stems on to the soil ; earthworms and bacteria can now flourish and pro- 

 duce the normal decomposition products that go to make up " mild 

 humus " and a fertile soil. Dryness necessitates a narrow-leaved xero- 

 phytic vegetation, the leathery fragments of which mingle with the soil, 

 but afford a very indifferent medium for the growth of earthworms and 

 bacteria, so that little decomposition goes on and a barren sand results. 

 Wetness and lack of aeration necessitate special vegetation and decom- 

 position agents, and there may result a " mild humus " if sufficient cal- 

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

 humus " in the contrary event. 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 give rise to soils 

 wholly different in agricultural value and in natural flora. 



Further, the farmer has discovered how to build up these compound 

 particles by cultivation and thus change to a very great extent the re- 

 lation 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 divisions : clays, loams, sands, chalky soils 

 and soils rich in organic matter, all shading off into one another 

 and without sharp lines of demarcation, but representing classes of 



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