96 NITBIFYING- POWEE OF SOILS 



This complex compound has intense tinctorial power and is the 

 cause of the red colouration. 



Another reaction for nitrites, of considerable delicacy, though liable 

 to interference by the presence of organic matter, is the liberation of 

 iodine from hydriodic acid and its detection by the formation of the 

 blue colour with starch. 



Another good reaction for nitrites is the formation of a blue colour 

 with a solution in strong sulphuric acid of diphenylamine, (C 6 H 5 ) 2 NH. 



Chemical Methods for Measuring Bacterial Activity. It has 



been seen how dependent upon the activity of micro-organisms is 

 the rate at which the organic matter of a soil becomes available to 

 plants. It is thus obvious that the fertility of a soil, containing a good 

 supply of nitrogenous organic matter, will be largely influenced by its 

 bacteriological condition. To investigate, at any length, the number, 

 character and activity of the micro-organisms in a soil is a difficult 

 task and belongs to the domain of the bacteriologist rather than to 

 that of the chemist. 



Certain direct chemical methods of attacking the problem, however, 

 have been devised and are capable of yielding useful information. 

 Russell 1 has proposed to measure the total bacterial activity of a soil 

 by determining the rate at which oxygen is absorbed from a known 

 volume of air confined in contact with a known weight of the soil at 

 a definite temperature. 



Ashby ~ has described a method of estimating the comparative 

 nitrifying power of soils. Briefly it consists in " seeding " with 0*2 

 gramme of the soil, 100 cc. of a culture solution containing per 

 litre- 

 Potassium dihydrogen phosphate . . . . . 0'25 gramme 



Magnesium sulphate . 0-125 ,, 



Ammonium sulphate 0-5 ,, 



Ferrous sulphate 0-1 ,, 



Sodium chloride 0-5 ,, 



This can be most conveniently kept in a solution four times as 

 strong and diluted as required. 100 cc. of this sterilised solution are 

 placed in a clean flask, 0*2 gramme of the soil and 0-2 gramme of 

 sterilised precipitated calcium carbonate added, the flask closed by a 

 plug of cotton wool and incubated at 29 to 30 C. 



Periodically, a few drops of the liquid are taken from the flasks 

 and tested for nitrites by means of diphenylamine and strong sul- 

 phuric acid, or starch paste, potassium iodide and dilute sulphuric acid. 

 When most of the flasks show a distinct nitrite reaction or after, say, 

 thirty days' incubation, the amounts of nitrogen present in an aliquot 

 portion of the liquid as (1) nitrite, (2) nitrate, and (3) ammonia are 

 determined. In this way, the relative nitrifying powers of a series of 

 soils may be determined. Ashby states that he never found, in his 

 experiments with Rothamsted soils, any trace of nitrite or nitrate pro- 



1 Jour. Agric. Sci., 1905, 261. 



2 Jour. Chem. Soc., Trans., 1904, 85, 1158, and Jour. Agric. Sci., 1907, 2, 52. 



