266 



SOIL CONDITIONS AND PLANT GRO WTH 



to justify the assumption that the two phenomena are causally 

 connected. Two cases have, however, 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 effect some decomposition that does not give rise to 

 ammonia. (See p. 186.) The addition of certain organic 

 compounds to the soil has this effect (Table LXIX.). 



TABLE LXIX. 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 ammonia and nitrate also, but 

 it fails to do this after a certain quantity has accumulated 

 (Fig. 24). 



On looking over the figures in Tables LXVII. and LXVIII. 

 (p. 262) it is evident that the numbers of bacteria revealed by 

 this method bear no relationship to the amount of crop growth. 

 Other field experiments have given similar results. Yet in 

 laboratory and pot experiments bacterial counts have often 



See p. 212. 



