NUMBERS OF BACTERIA IN THE SOIL 121 



figure, the normal soil bacteria finally getting the upper 

 hand in the struggle which ensues. 



No doubt the food material suited to B. colt and 

 other allied putrefactive forms soon diminishes, and 

 there is probably also an accumulation of organic acids 

 and other compounds inimical to the growth of this class 

 of bacteria. 



The ploughing in of weeds, stubble, and " green 

 manures," which contain organic nitrogenous and carbon 

 compounds, provides pabulum for soil organisms and 

 assists their development. 



Certain forms of Streptothrix and many moulds are 

 increased by the addition of straw, and probably the 

 organisms which " fix " free nitrogen are increased also 

 by straw residue. 



Ammonium sulphate and nitrate of soda can be 

 utilized by many soil bacteria for nutritive purposes, 

 and potash phosphates and lime have been found to 

 assist in the development of the nitrogen assimilating 

 group of bacteria. 



The growth of certain crops favour the development 

 of bacteria in the soil more than others. Stohlau and 

 Ernest in one case found where clover had been grown 

 seven or eight millions present in a gram of the soil, 

 but only one to two millions per gram of soil from an 

 adjoining plot after barley. 



Ex. 68. Prepare the following " Heyden-Agar " medium : 

 Agar, . .12.5 gr. 

 Albumose, . 7.5 gr. 



(Heyden) 



Distilled water, . i litre. 



Dig up a block of soil with a spade ; break the block, and from 

 the centre take out a small sample and place it in a sterilized bottle. 



