286 



SOIL CONDITIONS AND PLANT GRO WTH 



bacterial numbers. Untreated soils were maintained at lo", 

 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 ^ 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 



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

 carbonate, phosphates, etc. 



