the tropics, ammonia is being steadily converted into nitrates in the 

 soil, and that plants avail themselves of these soluble salts for their 

 supply of nitrogenous nourishment It is therefore necessary in con- 

 sidering the manurial effect of sulphate of ammonia to go beyond the 

 stage ot its absorption by the soil and to study the conditions under 

 which the nitrogen of the ammonia is converted into nitrate. This lat- 

 ter process is known as iiitriHcation, and although its practical work- 

 ing has been recognised for centuries in the manufacture of nitre, it 

 was only by the assistance of modern bacteriology that the exact cause 

 and mechanism of the process have become clear and intelligible 



The change of ammonia into nitrates in the soil earl 3' attracted the 

 attention of chemists. Boussingault studied this change as occurring 

 in cultivated soil, and found the amount of nitrate present at any one 

 time to be dependent on the rainfall. After heavy rains the nitrates 

 were washed out, while during d'y weather an accumulation again 

 took plnce. In 1846 Dumas concluded from bis experiments that 

 nitrification was a purely chemical process of oxidation, and regarded 

 nitrate as the direct outcome of the combined action of chalk, oxygen 

 and ammonia. Other chemists ascribed the action to the porous or- 

 ganic matter in the soil which was supposed to have the same powers 

 of oxidation as that possessed by the aerated pores of charcoal. 



In 1862 Pasteur suggested that this change was analagims to that 

 of the souring of beer or the oxidation of alcohol to acetic acid by the 

 vinegar ferment, and that living organisms were involved in the pro- 

 duction of nitrates from ammonia. Experimental proof of this idea 

 was published hj Schloesing and Miintz in 1877, and from this date 

 all doubt ceased as to the biological nature of nitrification. 



Schloes'ng and Miintz showed 



(1) nitrification in a soil could be absolutely stopped by such an 



antiseptic as chloroform, which is known to have a powerful 

 effect on living organisms 



(2) a soil which has been deprived of its nitrifying properties by 

 treatment with chloroform recovered them again on beiug in- 

 oculated with a small fragment of ordinary soil. 



Warington extended these researches and published several valua- 

 ble memoirs on the conditions affecting nitrification and the distribu- 

 tion of the active nitrifying agency in soil and water. Attempts to 

 isolate specific organisms, however, were for many years abortive. 

 Munro showed in 1886 that nitrification could take place in the entire 

 absence of orgauic matter, and it gradually became apparent that the 

 failure of all the early attempts to isolate the organisms of nitrifica- 

 tion had been due to the use of nutritive media containing organic 

 matter which was then supposed to be an essential food-material of all 

 micro-organisms. 



At this juncture the brilliant Russian phj'siologist, Winogradsky, 

 undertook the investigation of the matter, and by new and ingenious 

 methods was successful in isolating the nitrifying organisms and in 

 cultivating them in the pure state. His medium for cultivation con- 

 sisted of pure silica jelly, which was solidified by the addition of a 

 minute proportion of the sulphates of potash, magnesia and ammonia 

 and carbonate of soda. The exclusion of organic matter and the use 

 of the solid jelly were the two secrets of his success in separating the 



