18 EXPLOITATION OF PLANTS 
respecting the influence of certain decomposition pro- 
ducts of organic matter upon the activities of soil 
bacteria, and Professor Oliver has asked me to give a 
short summary of the results obtained. 
Krzemieniewski and others demonstrated some ten 
years ago that soluble soil humates exercise a remarkable 
stimulating action on the fixation of nitrogen by azoto- 
bacter. As a result of a search made in 1912 to find a 
material rich in soluble humates to serve as a medium 
for the growth and distribution of these organisms, 
it was discovered that when peat is incubated with 
certain aerobic decomposition bacteria at a temperature 
of 26° C. for a fortnight, a large proportion of the 
humic acid present is converted into soluble humates, 
and this material after sterilisation forms an excellent 
nutrient medium for nitrogen-fixing bacteria. Tests 
as to the manurial value of this “ bacterised peat ’’ made 
at Kew Gardens and other places during 1913 showed 
. that it contained a substance or substances which stimu- 
lated plant growth to an extent which could not be 
accounted for by the mineral nutrients present. This 
suggested that the growth-stimulating properties of 
bacterised peat might be due to the presence of organic 
substances similar in nature to the accessory food bodies 
or “ vitamines ’ concerned in animal nutrition. 
As these accessory food substances are soluble in 
water and in alcohol, and are further precipitated by 
means of phosphotungstic acid, both aqueous and 
alcoholic extracts and also a phosphotungstic acid frac- 
tion were obtained from bacterised peat and were found 
to have a stimulating effect both on nitrogen-fixing 
bacteria and on plant seedlings grown in water cultures. 
