A'JKINS — Factors affecting Il/jdrogcn Ion Concentration of Soil. 377 



grown on Caribou loam of pH 4-8 were unaffected, whereas in the less acid 

 Washburn loam much damage resulted. They draw attention to the fact that 

 absolute neutrality is not necessarily the most siiitable reaction for a crop. 

 Lipman (1919) has accordingly suggested that the reaction of the soil should 

 be altered to suit the crop, this being effected on the acid side by the addition 

 of sulphur, which is oxidized both by air alone and by bacteria in the soil. This 

 condition is favourable for potatoes. By suitable liming it is then possible 

 to neutralize the soil, thus rendering it fit for the leguminous crops of the 

 rotation, such as clover and alfalfa. This is obviously only possible with 

 certain types of soil. 



Working on these lines, Martin (1920) has shown by a most convincing 

 quantitative study how the potato scab, Actinomyces chromogenus, Gasperini. 

 may be checked on the Irish Cobbler potato. The soil at the start was 

 at pH 6-03, but treatment with 400 and 600 pounds of sulphur per acre 

 altered the acidities to pH 5'20 and 5'07 respectively, the percentage unsale- 

 able through scab decreasing from 42-9 in the untreated soil to 8-9 and 7"5 

 respectively in the treated, the total crops being closely the same. 



It has, moreover, been shown that there is an intimate relation between 

 the growth of nitrifying organisms and soil reaction. Thus Hesselman 

 (1917-1919) found that both nitrate formation and denitrification are stopped 

 by the acid soil in the mossy type of conifer forest in Sweden. Arrhenius 

 (1920) studied the relation between the pH value of the soil and the organically 

 combined nitrogen, nitrate nitrogen, and nitrogen as ammonium compounds. 

 The maximum for nitrate nitrogen was found at about pH G^S, after which it 

 decreased rapidly, being reduced to one-sixth at pH 5-5. Nitrogen as 

 ammonia increased sharply from pH 6'.3 to 5'8, being nearly twice as great 

 at the latter, but after a fiat portion the curve falls, the value at pH 4'8 

 being identical with that at pH 6'3. Throughout these changes the total 

 nitrogen was much the same. 



In direct work on the organism, Gainey (1918) proved the extreme 

 sensitiveness of Azotobacter to slight changes in reaction, pH 6-0 being given 

 as its upper limit. Fred and Davenport (1918) ascertained the critical pH 

 values for the root nodule bacteria of various legumes. For example, the 

 bacteria of alfalfa and sweet clover were unable to stand acidity greater than 

 pH 4'9, whereas those of lupines could withstand up to pH 3-15, other types 

 being ranged intermediately. The alkaline limit appears to be the same for 

 all. Growth is, of course, hindered in the neighbourhood of the limiting 

 concentrations. 



Furthermore, the oxidation of various carbohydrates is more rapid in 

 alkaline than in acid .solution or suspension. Accordingly, quite apart from 



