Atkins — Factors affecting Hydrogen Ion Concentration of tSoil. 383 



records soil derived from limestone as giving pH 8'5, and that from dolomitie 

 limestone pH 9'0, as determined colorimetrically. 



(d) Soluhility of jptiosphates and occurrence of siclphur acids. — An other point 

 worthy of consideration is the effect of soil reaction upon the solubility of 

 calcium phosphate. It appears probable that in the presence of calcium 

 carbonate, bicarbonate, and sulphate the influence of the common calcium ion 

 is such as to lessen the solubility of the phosphate. It is well known that 

 the ordinary methods of estimation show low values for " available phosphate " 

 in soils rich in calcium carbonate. Conversely, the action of an acid soil is 

 to increase the available phosphate. Thus in Assam the soils of tea and 

 indigo estates and jungle land have been shown by Davis (1918) to be rich in 

 the latter. Samples, for which the writer is indebted to Mr. W. A. Davis, 

 were found to be as acid as pH 5'4. Jn this case the acidity was clearly due 

 to the oxidation of sulphur derived from iron pyrites, the latter being found 

 in small grains in ten out of eleven samples ; the pyrites had apparently all 

 disintegrated in one surface sample. The possibility of the acidity being 

 derived from organic sources is, it appears, excluded by the fact that the soil 

 samples were not at all of a peaty nature, but of an open sandy texture, some 

 sub-soils being just coarse sand, rurthermore, pyrites alone in water gives an 

 acid reaction pH 4 to 5. Hall (1910) mentions cases where iron pyrites has 

 beeir the cause of soil sterility, and states that such soils show an acid reaction. 

 Alway (1920, 1), too, mentions a toxic layer of this nature under certain peat 

 lands. Within the last few years much attention has been given to the 

 action of sulphur bacteria in producing acid in the soil. The studies of 

 Gillespie and Hurst and of Martin have already been mentioned in connexion 

 with the control of potato scab. Further, Lipman, M'Lean, and Lint (1916) 

 showed that the biological oxidation of sulphur in the soil produced soluble 

 phosphate. Organic matter favours this, doubtless by providing additional 

 acid and food for the bacteria. Brown and "Warner (1917) demonstrated the 

 production of soluble phosphate from rock phosphate by composting with 

 sulphur and manure, Ames and Eichniond (1918) andShedd (1919) studied 

 in addition the effect of nitrification. Tottingham and Hart (1921) found 

 acidity as high as pH 2-9 and 3-1 with sulphur composts, and pH 3-4 and 3-7 

 with sulphur and rock phosphates. The general relations of inoculated 

 sulphur as a plant-food solvent liave been studied by Lipman, Blair, Martin, 

 and Beckwith (1921). It is not, however, only by organisms that sulphur is 

 oxidised. Thus Maclntire, Gray, and Shaw (1921) have shown that in sterile 

 quartz media elemental sulphur is oxidised with production of acid. For its 

 production from iron pyrites they give the following equation : — 

 FeSi + 36*2 ^'t FeSO^ + SO^. 



