690 PRINCIPLES OF SOIL MICROBIOLOGY 



with alkaline earths ("humates"), removing the excess of acid by wash- 

 ing with water, then adding ammonia to the soil and allowing this to 

 stand a few hours; this results in a dark brown solution of the "humus" 

 or "matiere noire." An aliquot portion of this solution was used for 

 the determination of the "humus." This method served as a basis 

 for all modern methods, with various modifications. 



Some investigators differentiated between the total alkaline solution 

 and the hydrochloric acid precipitate. Schreiner and Shorey, 53 for 

 example, referred to a 2 per cent NaOH solution as the "humus" ex- 

 tract and to the precipitate obtained by adding acid to this solution as 

 "humic acid." However, it has been shown 54 that not only does a 4 

 per cent sodium hydroxide solution not extract the same quantity and 

 quality of substances as a 4 per cent ammonium hydroxide solution, but 

 that the very "humus" extract of soils is not a typical soil product. It 

 was also shown that the organic matter extracted by an alkaline solution 

 consists of a black pigment, which contains a relatively small propor- 

 tion of the nitrogen, and of colorless substances. It was frequently 

 concluded, therefore, that the determination of "humus" by alkaline 

 extraction is wholly without scientific justification. Other methods for 

 determining "humus" in the soil are based upon the color of the alkaline 

 extract, 55 or upon extraction of the soil with pyridine. 56 However, none 

 of these methods were found to be free from criticism. 



When a soil is treated with an equal volume of 5 per cent solution of sodium 

 hydroxide (or 2 consecutive portions of 2.5 per cent solution), either for 48 hours 

 in the cold or for 30 minutes at 15 pounds pressure, a dark extract is obtained; an 

 equal volume of water is added immediately to the mixture of soil and alkali and 

 the solution filtered through paper; the soil is again treated with an equal volume 

 of 2 per cent NaOH solution and the second filtrate and the washings of the soil 

 with distilled water are added to the first. The combined solution (I) is treated 

 with warm hydrochloric acid (1:1) until a flocculant precipitate is formed, adding 

 a few cc. of acid in excess and shaking the flask. The precipitate (a) is filtered 



" Schreiner, O., and Shorey, E. C. The isolation of harmful organic sub- 

 stances from soils. U. S. Dept. Agr. Bur. Soils Bui. 53. 1909. 



M Gortner, R. A. The organic matter of the soil. I. Some data on humus, 

 humus carbon, and humus nitrogen. Soil Sci., 2: 395-441; 539-548. 1916. 



" Eden, T. A note on the colorimetric estimation of humic matter in mineral 

 soils. Jour. Agr. Sci., 14: 469-472. 1924; Oden, 1919 (p. 671); A detailed review 

 of the investigations dealing with the origin and nature of "humus" in the soil 

 is given by Baumann and Gully, 1910 (p. 644); Oden, 1919 (p. 671); and Waks- 

 man, 1926 (p. 447). 



" Piettre, 1923 (p. 672); Page, H. J. The part played by organic matter in 

 the soil system. Trans. Faraday Soc, 17: 272-287. 1921. 



