l62 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



tyrosin, asparagin, and hippuric acid can serve directly as sources 

 of nitrogen for cultivated plants. Hippuric acid is decomposed 

 into glycocoll and benzoic acid, the glycocoll taken up and the 

 benzoric acid left behind. It is also claimed that humic acid 1 

 can be absorbed. 



Humus. The term humus is applied by some chemists (prin- 

 cipally in European countries) to the entire quantity of organic 

 matter in the soil, in whatever form it may be present. In 

 America, it usually refers to the organic matter dissolved by am- 

 monia after the lime has been removed by acids. This ammonia- 

 soluble organic matter is supposed to be more valuable than that 

 not soluble in ammonia, though satisfactory evidence that such is 

 the case has not been presented. 



Ammonia-Soluble Organic Matter. In the preparation of the 

 ammonia-soluble organic matter, the lime is first extracted with 

 acid and the soil washed free from acid. The soil is then treated 

 with ammonia, but a quantity of clay is also suspended in the 

 liquid. The clay is precipitated by addition of ammonium sul- 

 phate, ammonium carbonate, or other salts, and the ammonia 

 neutralized. The precipitate is collected and washed thoroughly. 

 The precipitated substance is a black, amorphous body, slightly 

 soluble in water. It has acid properties. It decomposes calcium 

 carbonate, liberating carbon dioxide. A portion of it is soluble 

 in alcohol. 2 It is soluble in ammonia, and when the excess of 

 ammonia is evaporated, retains 4 to 6 per cent, nitrogen in the 

 form of an ammonium salt. It is precipitated by salts of lime, 

 barium, copper, zinc, lead, etc., forming salts of these metals. Its 

 combining weight varies from 228 to 327. The magnesium salt 

 is soluble in water, and also the sodium, potassium and ammonium 

 salts. When dissolved in ammonia, a portion of the humate 

 diffuses through parchment paper. 



Analysis of the substance prepared from several soils by 



1 Brial, Exp. Sta. Record 6, p. 484. 



2 Fraps and Hamner, Texas Bulletin 129. 



