FACTORS AFFECTING RATE OF DECOMPOSITION 145 



cipal factors: First, the number of bacteria present; and 

 second, the kind of living matter undergoing decomposition. 



(a) Number of Bacteria in Soils. — As to the number 

 of bacteria in soils, it will vary between very wide limits. 

 When "number of bacteria" is mentioned it means the 

 number of colonies of bacteria that can be cultivated from 

 a given amount of soil in an artificial nutrient solution and 

 subsequently counted. The supposition is that each colony 

 is developed from a single bacterium. The numbers vary 

 from 100,000 to 50,000,000 or even 100,000,000 per gram 

 of soil. The average cultivated land probably contains 

 several million per gram. The more bacteria present, the 

 more decomposition takes place and hence a greater pro- 

 duction of those various compounds mentioned above, 

 namely, carbon dioxide, water, mineral salts, and nitrogenous 

 compounds, but ordinarily less humus. 



The number of bacteria is dependent on several factors, 

 the principal ones being: Temperature, moisture, food, and 

 reaction of the soil, whether acid or alkaline. 



(1) Temperature. — Bacteria thrive best between 15° and 

 25° C, although they do live from about 0° to 40° C. Near 

 0° bacterial life is inactive although not dead. Above 40° 

 the bacteria begin to die; at 100° C. most bacteria are killed, 

 although the spores are not. 



(2) Moisture. — It is claimed that the best moisture con- 

 ditions for bacteria vary from 8 to 10 per cent, in sandy 

 soils to 20 per cent, and even more in heavy clays. As 

 soils dry out the bacteria for the most part merely become 

 dormant, although some are killed outright. Excessive 

 water tends to kill off the aerobic bacteria but encourages 

 the growth of the anaerobes. Since for the most part the 

 beneficial bacteria are aerobes, aeration is essential to 

 optimum soil conditions. 



(3) Food. — This includes of course organic matter from 

 which most bacteria derive their sustenance. Being plants, 

 they must have inorganic salts as well, and there are neces- 

 sary such salts as sulphates, phosphates, lime, and potash 

 compounds, derived either from the organic remains or from 

 mineral particles in the soil. 



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