386 Scientific Proceedings, Royal Duhlin Society. 



up, leaving sodium carbonate in the soil, tends to decrease acidity, but the 

 writer has had no opportunity of measuring this effect. 



In general, it is found that a garden soil supplied with stable manure in 

 a limestone region in the British Isles gives a reaction of pH 7'0-7'6, though 

 very rich in calcium carbonate, as shown by effervescence with acid. The 

 same soil unmanured varies from pH 8'0 to as much as 9'0 in quarry 

 subsoil. Dressing with seaweed probably tends to give an alkaline reaction 

 on account of the surface contamination with sea salt and salty sand. 

 Another method whereby the alkalinity may be increased or acidity reduced 

 is by burning the surface growth. This well-known device leaves an ash of 

 oxides and carbonates of the plant bases. In one instance examined, the 

 burnt patch of grass in a field gave a good deep pink when tested with 

 cresol phthalein, thus denoting a reaction of about pH'9, whereas the soil 

 around the roots of the unburnt grass was at pH 6-5. The soluble ash 

 alkalis are, of course, washed down by rain, and the bases again become 

 available for plant growth. 



(g) Acid soils. — So far, with the exception of an account of the existence 

 of free sulphuric acid in soils, soil alkalinity has been discussed ; that is to 

 say, the soils with pH values of 7'0 or over. The exact neutral point varies 

 somewhat with temperature, being 7-10 at 16°, 7"00 at 22", and 671 at 

 40°. Oden (1921) has discussed soil acidity in special relation to tlie 

 question of the existence of humic acid. He distinguishes between this 

 problem and that of the cause of the acidity actually found, and has shown 

 that preparations of well-washed humic acid give no greater acidity than 

 pH i'S, owing to the slight solubility of the acid. Since higher acidities 

 may be met with in peat, it is obvious that other acids must be concerned in 

 its production ; in the case he gives the acidity of the peat was pH 4'09. 

 Indeed, he quotes various workers to the effect that the extra acidity is due 

 to such acids as sulphuric, phosphoric, acetic, malic, propionic, and others in 

 very minute amounts. Just as calcium carbonate acts as a buffer in alkaline 

 soils, and prevents the reaction being altered to acidity till it is all 

 neutralized, so does humic acid act in acid soils. Od^n illustrates this by 

 giving a titration with sodium hydroxide of a peat suspension, noting the pH 

 values after various additions. Unlike the calcium salts got from limestone, 

 which are soluble and are washed out, the humates are relatively 

 insoluble, and remain in the soil as regulators of its reaction. 



Thus, to sum up, limestone soils are alkaline, the buffer action depending 

 upon the percentage of carbonate and bicarbonate ; peaty soils are acid, the 

 buffer action being proportional to the amount of the humus acids of low 

 solubility. There remain the sandy and clay soils, and these, if free from 



