294 THE NITRIFYING BACTERIA 



are fixed by the soil and protected from wasteful lixiviation. the nitrifying 

 ba teria then oxidising the ammonia and supplying the plant with nitrates 

 according to its requirements. 



So far as plantation-saltpetre is concerned, the external conditions favouring 

 the rapid formation of this compound have been gradually ascertained by means 

 of tentative experiments. A pyramidal heap, resting on an impervious clay 

 foundation, is prepared by mixing chalky soil with various kinds of organic 

 matter, and is frequently watered with liquid manure, an admixture of brush- 

 wood in the heap imparting porosity and facilitating aeration. The nitrates, 

 <fec., formed in the interior appear like wall-saltpetre on the surface of the 

 mass, and gradually increase to form a crust which is richer in nitrates than the 

 interior of the heap. The crude lye obtained therefrom by lixiviation is treated 

 by adding a potassium salt in order to convert the nitrates of calcium, mag- 

 nesium, and sodium into potassium nitrate, the crude saltpetre thus produced 

 being then purified in refineries. 



The elucidation of the optimum external conditions for influencing nitrification 

 has been attempted by numerous investigators, and a few of their results will 

 now be given. J. DUMONT and J. CROCHETELLE (II. and III.) found that the 

 chlorides of potassium injuriously affect nitrification, whereas the carbonates of 

 these metals, and also potassium sulphate, act beneficially. From what has 

 already been stated it will be evident that the merely faint (or altogether 

 inoperative) activity of the nitrifying bacteria in soils poor in calcium carbonate 

 (e.g. sour meadow-land) can be stimulated by the addition of the said 

 carbonate. On this point a few experiments have been made by J. DUMONT 

 and J. CROCHETELLE (I.). The kind of acid with which the ammonia is combined 

 must not be regarded as unimportant, Hueppe and Winogradsky having noticed 

 that as afterwards shown by the special experiments of 0. LOEW (VI.) 

 nitrifying bacteria do not attack ammonium formate at all, and that the oxalate 

 is acted upon only very imperfectly, and with great difficulty. 



