12 THE BIOLOGY OF FLOWERING PLANTS 



any case, the action of humus compounds is the chief cause 

 of soil acidity. Soils that contain large amounts of unde- 

 composed humus — the most extreme is peat — are remarkable 

 for their very acid properties. The various factors, there- 

 fore, which lead to an accumulation of humus are indirectly 

 responsible for increase in soil acidity. 



Many investigations have been made on the hydrogen 

 ion concentration of various natural and agricultural soils. 

 The relation to carbonate content is well shown in an 

 investigation by Salisbury (1921). On a calcareous heath, 

 on a steep gradient, in Hertfordshire the exposed top of the 

 slope, subject to extreme leaching of soluble constituents, 

 showed a p^ value of 5 •1-5 '4, with a carbonate content of 

 0*02 per cent. ; halfway down the slope the carbonate 

 content was o"68-i'o per cent, and the^'^ value 7*3 ; near 

 the base the carbonates reached 30 per cent., and the p^ 

 value 7*6. 



The effect of humus may be illustrated by the example 

 of the different layers of soil in a birch wood in Epping 

 Forest summarised in Table III. 



TABLE III 

 Hydrogen Ion Concentration and Humus Content 



In the different types of soil in the same locality, bearing 

 different types of vegetation, the acidity may be markedly 

 different, though in each soil a moderately wide range may 

 exist. Thus the surface soil in a beech wood growing on 

 chalk showed p^ values ranging from 6"i to 7*4. In an 

 extensive investigation of Danish soils, Olsen (1923) found 

 an extreme range horn, p^ 3*5 on moorland peat to ^" 7^9 

 on calcareous soils. 



