THE COMPOSITION OF THE SOIL 117 



TABLE XXXI. EFFECT OF CALCIUM CARBONATE ON THE TEXTURE OF SOILS. 



The present position may be summarised as follows : 



The existence of a hydrogen ion concentration greater 

 than that of water is satisfactory evidence of the presence of 

 true acids in soils. The measurements at present throw no 

 light on the nature or quantity of the acids. The acids may 

 be 



0) Organic, 



(b) Siliceous, 



(<:) Formed by hydrolysis of iron or aluminium salts. 

 They may adversely affect plants or micro-organisms by 

 reason of their strength or their quantity. On the other hand, 

 the acids themselves may be without serious action (see p. 81). 

 The special properties of an " acid soil " may result from a 

 lack of basicity, whereby 



1. Special absorption relationships appear; 



2. Lack of calcium may affect plants or micro-organisms ; 



3. Certain toxic substances, e.g. metallic salts, may remain 

 effective, which in presence of lime would be thrown out of 

 action ; 



4. The clay may become deflocculated and therefore as- 

 sume a sticky condition unfavourable to plant growth. 



Table XXXI. shows pairs of soils similar in constitution 

 and in general external conditions, temperature, water-supply, 



