n8 SOIL CONDITIONS AND PLANT GROWTH 



etc., but very different in agricultural value because of their 

 different content of calcium carbonate, one being readily 

 cultivated while the other is wet and sticky, and suitable only 

 for pasture land. 



"Sourness" of Soil: Its Relation to Acidity and Calcium 



Carbonate. 



Soils which are infertile from lack of calcium carbonate 

 are called "sour" by farmers. The older chemists substituted 

 the word "acid," but, as shown above, the effect is not 

 necessarily in all cases attributed to acids, and therefore it 

 seems desirable to retain the farmers' term as the broad one 

 and distinguish between " sourness " due 1 to acidity and that 

 due to lack of basicity. 1 



Whatever its cause in a given case sourness can be over- 

 come by addition of lime or calcium carbonate ; soils containing 

 calcium carbonate are never sour, and they are generally 

 fertile. 



It by no means follows, however, that soils devoid of 

 calcium carbonate are infertile. Hendrick and Ogg at Aberdeen 

 (132) and Robinson in N. Wales (240) have both described 

 soils free from calcium carbonate but fertile and indeed 

 neutral in reaction : Hoagland observes 2 that certain Cali- 

 fornian soils with low PH value, i.e. considerable intensity of 

 acidity, and large " lime requirements," are nevertheless able 

 to produce excellent crops of many types. 



In the present state of our knowledge it is hardly possible 

 to say from chemical examination of the soil alone whether 

 it is "sour" or not, i.e. whether it is or is not infertile through 

 lack of calcium carbonate. A combination of chemical analysis 

 and field observation, however, enables something to be done : 

 soils known by vegetation observations to be comparable can 



1 This seems preferable to the distinction made by some of the investigators 

 between "positive acidity" caused by actual acids, and "negative acidity" 

 caused by adsorption of base (cf. Lyon, Soils and Fertilisers, p. 112. See also 

 H. R. Christensen, Soil Sci., 1917, 4, 115, and C. J. Schollenberger, Soil Sci., 



> 2 79). 



2 Private communication to the author. 



