342 ANNUAL REPORT OF THE Off. Doc. 



stores of nitrates, the bulk of which would be washed out of the 

 soil by the winter and early spring precipitation. 



The Presence of Moisture. 



Bacteria of all kinds, including the nitrifying organisms, grow 

 only in the presence of moisture. Soil waters, containing dissolved 

 mineral salts and ammonium compounds, are, therefore, the natural 

 media in which the nitrifying ferments develop. 



In a dry soil nitrification cannot take place; hence in periods of 

 drouth, where the superficial layers of the soil for a depth of several 

 inches become dry, nitrification is suspended. On the other hand, ex- 

 cess of water prevents nitrification by excluding air, hence water- 

 logged soils must first be drained before they become proper nitri- 

 fying beds. In a wet soil not only is nitrification inactive or en- 

 tirely suspended, but the opposite process of denitrificatlon takes 

 place with the loss of nitrogen in the free gaseous condition. 



A soil may be in good condition in its superficial layers, but have 

 an impervious clayey subsoil, which retains the downward drainage; 

 or a still deeper layer of clay may raise the level of ground water so 

 near the surface as to render the subsoil wet. In either case the 

 deeper zones of the soil become too damp for active nitrification, so 

 that this process instead of being carried on from the surface to a 

 depth of about 27-36 inches is confined to a more superficial zone; 

 the elaboration of much valuable plant food is thereby prevented. 



The functions of underdrains is thus not only to withdraw the 

 excess of subsoil water, but also by the downward movement of the 

 same to draw air into the soil and thus supply oxygen to the nitri- 

 fying ferments. 



The Reaction of the Soil. 



Nitrification can only take place in a feebly alkaline medium, but 

 the presence of anything beyond a small quantity of an alkaline salt 

 is a hinderance to the process, while a large amount will check it en- 

 tirely. Thus Warrington''^ showed that the presence of 0.032 per 

 cent, of bicarbonate of soda distinctly retarded nitrification, and 

 that with the presence of 0.09G per cent, nitrification was only barely 

 possible. The same author also showed that the presence of 0.0447 

 per cent, of ammonia in urine rendered it unnitrifiable. Dumont*^ 

 and Crochetelle showed that carbonate of potash added to soil at the 

 rate of from 1 to 2.5 grams per 1000 grams of soil markedly increased 

 nitrification, but that large applications of the salt progressively di- 

 minished the rate of nitrification, and that the addition of 8 grams 

 per 1000 grams of soil completely checked it. 



A heavy dose of lime by unduly inoreasing the alkalinity of the 



