14 



determined on the resulting- mixture by extraction with ammonia, 

 folio-wing a previous leaching with dilute acid, an. I the humus so 

 found was regarded as having been formed in the soil during the 

 preceding year. Unfortunately Snyder did not correct for am- 

 monia soluble organic matter in the mixtures at the beginning of 

 the experiment. The results of Fraps and Hamner (1910) and 

 Gortner (1917) show that ammonia dissolves a considerable por- 

 tion of material from unchanged organic compounds, so that the 

 humus gain at the end of the experiment was in all probability 

 actually a loss when compared with -the amount of ammonia soluble 

 materials at the beginning of the experiment. Fraps and Hamner 

 (1910) as well as Gortner (1917) report a series of experiments af- 

 ter the general plan adopted by Snyder, with the exception that the 

 ammonia soluble materials were determined both at the beginning 

 and at the end of the experiments, and in each instance the am- 

 monia soluble material was found to decrease. 



The experiments of Gortner (1917) furnish no evidence that a 

 specific "humification" of plant materials takes place in the soil 

 giving rise to an increased amount of "humus." He says: 



On the contrary, all of the evidence is directly opposed to such a con- 

 clusion, and it appears altogether probable that the maximum amount of am- 

 monia soluble material is present in a soil immediately after a green manuring 

 crop has been plowed under and before the 'humifying' bacteria or fungi 

 begin their work. 



Fraps and Hamner (1910) showed that the humus extract of 

 soil must contain substances from unchanged vegetable materials, 

 while Gortner (1916 a) pointed out that the extract must contain 

 substances from unchanged plant material, from bacteria and 

 protozoa. 



C. The Complexity of the Ammonia Soluble Material. 



With the development of chemistry the idea has been gradu- 

 ally abandoned, that the ammonia soluble compounds can contain 

 the whole of the organic matter which is responsible for the fer- 

 tility of the soil. The work of the U. S. Bureau of Soils in the 

 isolation of a large number of definite organic compounds from 

 the soil, has been a distinct contribution along this line. 



As has been suggested by Gortner (1916 a) 



If one speculates on the nature of the soil organic matter, it becomes obvi- 

 ous that the variety of compounds which are present in a soil is limited 

 only by those compounds which were present in the plants growing upon 

 the soil, plus those compounds which compose the bodies of bacteria and 

 protozoa, plus the compounds contained in the soil fungi, plus all the various 

 compounds which may be formed from the above sources by decay, oxidation, 

 and all the intricate chemical reactions which take place in converting dead 

 organic material, either into living protoplasm on the one hand, or into 

 water, carbon dioxide, and nitrogen on the other. Undoubtedly these organic 

 compounds are not the product of 'humification' but are derived from un- 

 changed plant material, from protozoa, or from bacteria. 



A part, or all, of these compounds would be found in the 

 "humus" extracted by Grandeau's method 



Inasmuch as the 'humus' extract of soils is undoubtedly a mixture of organic 

 compounds, many of which are colorless and in all probability are extracted 

 from unchanged plant or animal materials, and inasmuch as the soil pigment 

 present in this solution probably rarely exceeds 40 per cent of the 'humus', a 

 determination of 'humus', as ordinarily carried out, appears to be wholly 

 without scientific justification. (Gortner 1916 a.) 



