286 



SOIL CONDITIONS AND PLANT 



bacterial numbers. Untreated soils were maintained at 10, 

 20, 30 C., etc., in a well-moistened aerated condition, and 

 periodical counts were made of the numbers of bacteria per 

 grm. Rise in temperature rarely caused any increase in 

 bacterial numbers ; sometimes it had no action, often it caused 

 a fall. But after the soil was partially sterilised the bacterial 

 numbers showed the normal increase with increasing tempera- 

 tures. Similar results were obtained by varying the amount 



TABLE LXXVII. EFFECT OF TEMPERATURE OF STORAGE ON BACTERIAL 

 NUMBERS IN SOILS, MILLIONS PER GRAM. 



of moisture but keeping the temperature constant (20 C.). 

 The bacterial numbers in untreated soil behave erratically and 

 tended rather to fall than to rise when the conditions were 

 made more favourable to trophic life ; on the other hand, in 

 partially sterilised soil, the bacterial numbers steadily increased 

 with increasing moisture content. Again, when untreated 

 soils are stored in the laboratory or glass-house under varying 

 conditions of temperature and of moisture content the bacterial 

 numbers fluctuate erratically; when partially sterilised soils 

 are thus stored the fluctuations are regular. 



(7) When the curves obtained in (6) are examined it be- 

 comes evident that the limiting factor in the untreated soils is 

 not the lack of anything l but the presence of something 

 active. 



(8) This factor, as already shown, is put out of action by 

 antiseptics and by heating the soil to 60 C., and once out of 



1 The soils included fertile loams well supplied with organic matter, calcium 

 carbonate, phosphates, etc. 



