4i6 SCIENCE PROGRESS 



2. Effect on the prevalence of plant diseases * ; 



3 . Effect on the competitive powers of different species 



of plants.^ 



B. The direct and specific influence of soil acidity on plant 

 growth : 



1 . Effect on the supply of available calcium needed by 



plants as direct plant food material ;i.e. the effect 

 on the " lime requirements of the plant," which 

 are determined by : 



(a) Lime content of the plant ; 



(b) Rate of growth of the plant ; 



(c) Feeding power of the plant for lime, which 



depends on : 

 (i) Extent of the root system ; 

 (ii) Character of the root system ; 

 (iii) Internal acidity of the roots ; 

 (iv) Excretion of carbonic acid ; 



2. Effect on the symbiotic nitrogen-fixing bacteria of 



legumes ; 



3. Effect on the root tissues of plants. 



Considerations such as these bring out the importance of 

 distinguishing clearly between the " acidity " of a soil and its 

 lime requirements. The latter is a more comprehensive term 

 than the former, and has a larger value in terms of lime per 

 acre. It is also what the farmer has in mind when he calls his 

 soil " sour." The usefulness of the ordinary laboratory methods 

 of determining " soil acidity " — as typified by the Hutchinson- 

 MacLennan method in general use in this country — is due to 

 the fact that the results obtained are always very much too 

 high when regarded as strict neutralisation values, so that a 

 farmer who limes his land according to the indications of such 

 a method will run no risk from underliming, although there 

 may in certain cases be danger of over-liming when calcium oxide 

 or hydroxide is the form employed. 



Possible Dangers of Over-liming 



Cases have frequently been recorded in the literature of the 

 addition of lime to acid soils retarding the growth of such 



* Cf. soft scab in potatoes, the prevalence of which appears to depend 

 on the reaction of the soil. 



2 This is possibly a factor in what is known as the calcifuge habit of many 

 plants. Another instance is the insensitiveness of rye as compared with 

 barley to an acid reaction. This difference has been traced by Hartwell and 

 Pember to the toxicity of Al-ion, brought into solution by the high hydrogen- 

 ion concentration. 



