Atkins — Factors affecting Hydrogen Ion Concentration of Soil. 385 



concentration of the soil when submerged may so increase the solubility of 

 the iron salts or oxides present as to give the amount required. For, on 

 imitating the conditions of submergence on a laboratory scale, it was found 

 that the water standing over the soil in a test tube was at first as alkaline as 

 pH 9-0. Even though shut off from external supplies of carbon dioxide, the 

 pH value fell to 7'2 or 7'3 in less than six days, whilst in a day it was down 

 to 8'1 in certain cases; in samples of soil from the top six inches the mini- 

 mum values were reached more rapidly, as a rule, than in those from greater 

 depths, namely, six to twelve, and twelve to thirty-six inches. In addition 

 to this change in reaction, occasioned by bacteria, which are more numerous 

 near the surface, it must be noted that owing to submergence and the organic 

 matter present the soil is deprived of free oxygen, and so under the reducing 

 conditions iron compounds will tend to pass into the ferrous state, in which 

 they are more readily soluble. It is not, however, to be expected that the 

 reaction of the soil solution itself will change with as great rapidity, or, 

 perhaps, to as great an extent, for the calcium carbonate is continually beino- 

 attacked by the free carbonic acid, and so a new equilibrium is reached. 

 With excess of carbon dioxide there can, however, be no free carbonate in 

 solution, while there must certainly be free carbonic acid, so the soil reaction 

 must be, at least, no more alkaline than the bicarbonate stage. Bicarbonate 

 gives only a trace of colour with phenol phthalein in boiled distilled water, 

 ad this indicator shows a trace of colour at pH S'O with Clark and Lubs 

 standards, and a light pink at pH 8-2, the latter being the stage that would 

 ordinarily be observed. 



(f ) Effect of manuring upon soil reaction. — Another point to be considered 

 IS the effect of manuring upon soil reaction, in particular with regard to what 

 may be termed the unintentional alterations in reaction. Thus, while lime 

 obviously tends to reduce acidity, and sulphur to increase it, it is not at all 

 so obvious that ammonium sulphate and potassium sulphate should tend to 

 increase it also. Yet the prolonged experiments at Eothamsted, and work 

 elsewhere, have shown this to be so. How far these sulphates have affected 

 the highly calcareous silt of the Pusa manurial series during the fifteen 

 years it has been in existence is a matter for further research. It appears 

 as if the reaction in the plots so treated is about pH 0'2-0'4 less than in 

 other plots (Atkins, 1921) ; but as bacterial action in liberating carbon 

 dioxide in the soil solution may be more rapid in the presence of these salts, 

 it is uncertain to what extent the results are affected by this. At any rate, 

 the effective pH value — that which prevails in the moist soil in contact 

 with the plant roots — must be lower if bacterial action is more vigorous than 

 in the untreated plots. Sodium nitrate, in which the acid radicle is taken 



SfilENT. PKOC, K.D.S. VOL XVI, NO. XXX. 2 Y 



