ASSIMILATION OF NITROGEN 67 



colorless and has lost all the readily recognizable characteristics of manure 

 extract. 



More exact studies show that not all compounds are thus retained by the 

 soil; while ammonium salts are absorbed, nitrates easily pass through. This 

 characteristic of nitrates, their ability to be washed out of soils, explains the 

 small nitrate content of the soil. All of the nitrates not absorbed by plants are 

 washed down by the rain into the deeper soil layers. Of all the nitrogenous 

 substances occurring in the soil, the organic materials and ammonium salts 

 form, so to speak, the nitrogen stock of the soil. These are firmly held and so 

 act as a constant source of nitrates, which may be absorbed by plant roots. 



The investigations of Kostychev 1 have shown that organic nitrogenous 

 compounds of humus do not consist solely of decomposition products of plant 

 and animal substances but are mainly proteins, such as are the constituents of 

 living organisms. In the leaf-mould formed by oak leaves that had been de- 

 composing for twelve months the nitrogen content was 2.98 per cent., of which 

 2.73 per cent, was protein nitrogen and only 0.25 per cent, was made up of 

 simpler nitrogenous compounds. These experiments constitute a new proof 

 that the processes going on in the soil are not exclusively chemical, without 

 the intervention of living cells, but are also physiological in their nature, being 

 connected with the life-processes of organisms. The same author has shown 

 that the phosphorus of the soil appears mainly in complex organic compounds 

 such as are constituents of the lowest organisms. By virtue of its abundant 

 bacterial life, the soil is practically a living mass.' 1 



§3. Nitrification in Soils. — The ability of the soil to produce nitric acid 

 or nitrates from various more complex nitrogenous compounds depends upon 

 various conditions. One of these, according to Schlosing, is free access of 

 oxygen. Equal amounts of the same soil were confined in five vessels, and a 

 current of gas was passed through each vessel. The gas passed through the 

 first vessel was pure nitrogen, so that this soil was without oxygen. The other 

 vessels, II, III, IV and V, received mixtures of nitrogen and oxygen containing 

 6, 11, 16, and 21 per cent, of the latter gas, respectively. The amount of 

 nitrate present in the soil was determined for each vessel at the beginning and 

 end of the experiment. The results of these determinations, expressed as nitric 



1 Kostytschew, P., Ueber die Mikroorganismen des Bodens. Kurlandische Land- und Forstwirtsch . 

 Zeitg. (Riga) 5: 13-14- 1890. 



h On the nature of the organic matter of the soil see the following : Schreiner, Oswald, 

 and Shorey, Edmund C, The isolation of harmful organic substances from soils. U. S. 

 Dept. Agric, Bur. Soils, Bull. 53. 53 p. Washington, 1909. Idem, Chemical nature of 

 soil organic matter. Ibid. Bull. 74, 48 p. Washington, 1910. Schreiner, Oswald, and 

 Skinner, J. J., Nitrogenous soil constituents and their bearing on soil fertility. Ibid. Bull. 

 87. 84 p. Washington, 191 2. Trusov, A., The formation of humus by means of vegeta- 

 ble substances. [Russian.] Selskoie khoziaistvo i liesovodstvo (Economie agricole et syl- 

 viculture) Petrograd 246: 233-245. 1914. Rev. in: Month, bull, agric. intell. and pi. 

 diseases 6: 540-541. 1915. Also rev. in: Exp. sta. rec. 34: 619. 1916. Idem, same title. 

 [Russian.] Selskoie khoziaistvo i liesovodstvo (Economic agricole et sylviculture) Petro- 

 grad 248: 409-437. 1915. Rev. in: Month, bull, agric. intell. and pi. diseases 7 '• 46~47- 

 1916. Also rev. in: Exp. sta. rec. 34: 516. 1916. — Ed. 



