HUMUS THE USUAL SOURCE OP NITRATES. 47 



hamate, are not soluble in water, but form a usually black precipitate, 

 Avhich gives a dark color to the soil. 



To extract its humus a soil is first washed with dilute acid, by 

 which the lime, magnesium, or other humus-precipitating substance is 

 dissolved and leached away. The humus itself is then removed from 

 the soil by long-continued washing with a weak solution, commonly 

 4 per cent, of ammonia. Upon the application of this treatment to 

 kalmia peat an inky-black extract is secured. When this is evap- 

 orated to dryness the residue is a black substance which when scraped 

 from the dish resembles coal dust or, even more closely, burned sugar. 

 This substance is one of the forms of humus. It absorbs water read- 

 11}^, assuming the texture of thin jelly. It has a somewhat sooty odor 

 and taste. It dissolves in water, the solution being acid in reaction. 

 A liter of water in which had been dissolved a gram of humus ex- 

 tracted from kalmia peat showed Avhen tested a 0.002 normal acidit}". 

 Such a solution is black unless viewed in a thin layer, and when 

 diluted to 10,000 c. c. it has a brown color similar to that of ordinary 

 cider vinegar. If lime is added to the solution the humus unites with 

 it and is thrown down as a black precipitate, leaving the liquid clear. 

 As stated in the preceding paragraph, it is in such a precipitated and 

 neutral or alkaline form that humus ordinarily occurs. The charac- 

 teristic brown color of the water in bogs indicates an acid condition, 

 the presence of humus in solution, and the absence of soluble lime. 



The process of decomposition by which cellular matter is trans- 

 formed into humus, in which the cellular structure has entirely dis- 

 appeared, is known as humification. 



Humns contains nitrogen, but the nitrogen is not in the form of 

 nitrates and therefore can not be assimilated by ordinary plants. 

 The transformation of humus nitrogen into nitrates occurs during a 

 further process of decomposition known as nitrification. 



The nitrification of humus is brought about by certain bacteria 

 which, growing in the humus-laden soil under suitable conditions, 

 produce first ammonia, then nitrites, and then nitrates. In artificial 

 cultures, in addition to proper conditions of temperature, moisture, 

 and good aeration, these nitrifying bacteria require for vigorous 

 growth a neutral or slightly alkaline medium. In a distinctly acid 

 medium the nitrifying bacteria grow little or not at all. 



In order to ascertain the degree of nitrification, if any, taking place 

 in kalmia peat, a series of nitrification tests of this material was made 

 by Mr. Karl F. Kellerman. These tests showed that neither in fresh 

 peat nor in peat rotted for three months was nitrification in progress, 

 but when the acidity of the peat was neutralized by the addition of 

 lime niti'ification began. 



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