Organic Soils 35 



Calcareous soils accumulate "mild" as opposed to 

 acid humus. Calcareous loams and marls are very 

 favourable for agriculture; the lime is said to cause the 

 clay particles to flocculate and thus makes the soil lighter 

 and more open. 



Very shallow-soiled limestones possess a characteristic 

 dry soil, supporting a characteristic grass-vegetation. 



6. Organic sails derived from plant-remains are of 

 various types and differ very much in their effect on 

 vegetation : — 



(a) Mild humus such as is normally found on the 

 floor of woods, and in meadows, has a neutral or alkaline 

 reaction, and is well aerated by earthworms, etc. 



(b) 2Iihi peat. "\^Tien there is excess of water, as 

 on the margins of lakes, and a deficiency of oxygen, but 

 lime and other bases are present in quantity, the humus 

 accumulates and forms a purely organic soil or peat, 

 whose water gives an alkaline reaction. This peat may 

 reach a great depth and is the characteristic "black 

 soil" of the fens (see p. 211). 



(c) Acid humus is formed on soils deficient in lime, 

 where the surface is moderately damp and oxidation is 

 deficient. 



(d) Acid peat. Under certain conditions of moisture 

 and deficiency of oxygen, acid humus accumulates so that 

 a pure peat soil is produced, and if the conditions are 

 m.aintained this acid peat soil may reach a great depth, 

 giving the characteristic " moor " soil (in the narrower 

 sense, Chapter IX) and supporting the characteristic 

 moor plants '^. 



1 The distinction here given between " mild " and " acid " humus and 

 peat corresponds to a difference well known to agrionltuxalists, and haTing 

 a most important effect on vegetation. It is not by any means certain 

 however that the acid and alkaline reactions of the soil water which is 

 found in extreme cases is the really important differentiating factor of the 

 two classes of habitat. The researches of Van Bemmelen and the recent 



3—2 



