BACTERIAL NUMBERS AND NITRATE PRODUCTION 123 



the soil ; in particular it is impossible to tell which of the forms de- 

 veloping on the plate are active and which are spores in the soil. No 

 account is taken of the kinds of bacteria on the plates ; in practice it 

 proves far too laborious to attempt any but the simplest identifications. 



This disregard of the nature of the bacteria constitutes a funda- 

 mental distinction from the method dependent on physiological group- 

 ing, and the two methods do not always give the same results. TJfe 

 counts show fairly correctly whether any given treatment of the soil 

 has raised or has lowered the number of bacteria, but unless the change 

 has been drastic they do not show whether all varieties have been 

 equally affected. Thus they have always to be combined with 

 determinations of the amounts of ammonia and nitrate in the soil. 



Chemical Analysis in Conjunction with Bacterial Counts. Increases 

 in bacterial numbers are often associated with increased production 

 of nitrate, but two cases have been studied where no such relationship 

 exists. 



(i) The soil treatment, while raising the total numbers, has either 

 acted differentially on the organisms and did not encourage the 

 ammonia producers to develop, or it has caused them to transfer their 

 energies to some decomposition that does not give rise to ammonia. 

 The addition of certain organic compounds to the soil has this effect 

 (Table XLIX.). 



TABLE XLIX. EFFECT OF CERTAIN ORGANIC SUBSTANCES ON BACTERIAL NUMBERS 

 AND ON NITRATE PRODUCTION. 



(2) Even when the ammonia-producing organisms are caused to 

 multiply they do not increase the stock of ammonia and nitrates in 

 the soil beyond a certain limiting amount. Thus partial sterilisation 

 increases bacterial numbers and usually increases the amount of am- 

 monia and nitrate also, but it fails to do this after a certain quantity 

 has accumulated (Fig. 6). 



On looking over the figures in Tables XLVII. and XLVIII. it is 

 evident that the numbers of bacteria revealed by this method bear 



