736 PRINCIPLES OF SOIL MICROBIOLOGY 



be used as a simple method of determining soil productivity, unless 

 the various factors are determined individually. 



OXIDIZING AND REDUCING POWER OF THE SOIL 



Oxidation consists in the addition of oxygen or subtraction of 

 hydrogen, the oxygen can be obtained either directly from the at- 

 mosphere or from a peroxide (see p. 520). Soil fertility and the rate 

 of oxidation were found 51 to be influenced by the same factors and to 

 the same extent, so that it was suggested that the latter could be used 

 as a measure of the former. Oxidation was found to be greater in 

 fertile than in unfertile soils, in surface soil than in subsoil. The oxi- 

 dizing power of soils can be determined 62 by shaking 5 grams of soil 

 with 10 cc. of an alcoholic solution of gum guaiac and then allowing the 

 soil to settle. The formation of a blue color indicates the degree of 

 oxidation. When the blue color fades, it can be brought back by the 

 addition of 0.5 cc. of a 2 per cent H 2 2 solution. Another method of 

 testing oxidation in soils consists in shaking 20 grams of soil with 50 cc. 

 of 0.125 per cent aqueous solution of aloin for one hour; the soil is 

 allowed to settle or the solution is centrifuged (if turbid, 50 cc. of 95 

 per cent alcohol is added to flocculate the soil and extract the oxidized 

 aloin) ; the clear solution is poured off and the depth of color is determined 

 by a colorimeter. It was found that soils known to be productive had 

 a strong oxidizing power, and that the poorer soils had little or no 

 oxidizing power; factors favoring oxidation also favor soil productivity. 

 However, agreement has not been obtained in all cases. 



Gerretsen 53 used, as an index of the oxidizing power of soils, the 

 amount of iodine liberated when 100 grams of soil are treated with a 

 dilute solution of potassium iodide acidified with sulfuric acid. The 

 method was carried out as follows. Two grams of soil were ground in a 

 mortar, then washed into an Erlenmeyer flask with water; 5 cc. of a 

 1 per cent solution of potassium iodide and 6 drops of a 1:1 solution of 

 sulfuric acid were then added to the soil suspension. After five minutes, 

 the suspension was centrifuged, filtered and titrated with 0.01 A r Na 2 S 2 3 . 

 When the moisture content of the soil is known, the oxidizing power of 

 100 grams of dry soil can be calculated. It was found that rich soils had 



51 Russell, 1905 (p. 683). 



62 Schreiner, O., and Sullivan, M. X. Studies in soil oxidation. Bur. of 

 Soils, U. S. Dept. Agr. Bui. 73. 1910. 



63 Gerretsen, F. C. Het oxydeerend vermogen van den bodem in verband met 

 het uitzuren. Meddl. Proefsta. Java. Suikerind., 5: 317-331. 1915; Meddel. 

 Proefsta. Java. Suik. No. 3. 1921. 



