195 



from their normal medium, the soil. By placing a portion of the latter in an artificial 

 cult ure solution we create entirely new conditions for the growth of the bacteria. Free 

 play is iriv.-ii thereby to the establishment of new group relationships, and species 

 obscure in the soil itself may come to the front. Hence it may often happen, as it 

 often does happen, that the ammonification coefficients of two soils, as indicated by 

 experiments in solution, ' ' umsetzungsversuche, " as the Germans would call them, 

 do not at all correspond to actual conditions. We must therefore distinguish here 

 between ammonifying power as referring to the numbers and species of the bacteria 

 th.-ms.-lv.-. and what Fraps" has recently designated as ammonifying capacity, as 

 referring to the physical and chemical constitution of the soil as well as to the number 

 and species >f its bacteria. 



Th.' differentiation of the various bacteriological changes in the soil itself is not a 

 simple matt.-r. As already noted, the activities of the decay bacteria in the soil can 

 not be measured by estimating the quantity of ammonia formed under normal con- 

 ditions. Thr ammonia nitrogen does not accumulate, for reasons already noted. We 

 can, however, create conditions in the soil precluding the further oxidation of ammonia. 

 This may be accomplish. -<1 by the addition to the soil of a sufficient quantity of dextrose 

 or of other soluble carbohydrates, or salts of certain organic acids. The same purpose 

 may be achieved, iH>rhaps, by skillfully adjusting the reaction of the soil. By these 

 means the ammonifying bacteria are permitted to grow while the nitrifying bacteria 

 are suppressed. The ammonia accumulates in the soil and may be readily estimated. 



Th.- d.-f.-ct of this method lies in the fact that the formation of ammonia from the 

 soil humus is, analytically, a comparatively slow process. A further defect is due to a 

 probable rearrangement in the numbers and kinds of the decay bacteria, due to 

 th- materials added or the artificial conditions created. As will be seen presently, 

 i he first of these defects may be remedied without difficulty. The second can not be 

 .liminat-l so easily. \-t is not necessarily fatal to the successful application of the 

 method. 



Ammonia formation in the soil maybe greatly intensified and the simultaneous 

 suppression ..f nitrification effected in still another way. By mixing with the soil 

 certain quantities of peptone, of urea, or of other nitrogenous organic substances we 

 supply to the bacteria something from which ammonia may be produced readily and 

 in comparatively large quantities. At the same time, the presence of these substances 

 stops the growth of the nitrifying bacteria. In the practical application of this method 

 in our laboratory we thoroughly mix 0.5 gram of peptone or 0.25 gram of urea with 

 KM) grams of fresh noil, transfer the mixture into a beaker, adjust the moisture content 

 by the addition of sterile water, cover the beaker with a glass dish, and place it in the 

 incubator or closet. \\V usually sterilize the beaker, peptone, and urea before they 

 are brought in contact with the soil. The latter is drawn with the customary pre- 

 cautions a -a in- 1 -ross contamination. At the end of three or four days the contents 

 of the beaker are transferred to a 2-liter copper flask, about 150 cc of water added, and 

 a sufficient quantity of magnesium oxid. The distillation and titration of the ammonia 

 are performed in the customary manner. 



Similarly, we may study nitrate formation in the soil itself by placing weighed 

 quantities of the latter in beakers and maintaining suitable moisture and temperature 

 conditions. At the end of four weeks (or of longer intervals, if desired) the soil is 

 leached ami the nitrites and nitrates determined in the teachings. In order to in- 

 tensify the nitrification processes we may add to the soil weighed quantities of ammo- 

 nium salts or of organic nitrogenous substances. The quantities of nitrate, which is the 

 end product of various bacteriological activities, may serve to gauge the comparative 

 rate of oxidation of the organic matter. This method may be employed has, indeed, 



a Texas Agr. Exp. Sta., Bui. 106. 



