MICROORGAOTSMS AS A FACTOR IN SOIL FERTILITY 295 



The Mineralization of Organic Matter. — Conditions that favor 

 the. intense activities of decay bacteria lead to a relatively rapid restora- 

 tion of the phosphorus, sulphur, calcium, magnesium and potassium 

 that had been made fast in plant tissues, to the stock of available plant 

 food in the soil; indeed, in extremely well-aerated soils the decomposition 

 of organic matter and its ultimate mineraUzation proceed too fast. It 

 often happens that the farmer is unable to maintain a proper supply 

 of humus in these soils because of their openness and is forced to adopt 

 measures that will retard soil aeration. He resorts therefore, to rolling, 

 marUng, manuring and green manuring. 



On the other hand, heavy, fine-grained soils are not sufficiently well 

 aerated to allow a rapid mineralization of the organic matter. Under 

 extreme conditions the decomposition processes do not keep pace with 

 the process making toward the accumulation of organic matter, and a 

 more or less considerable increase in the amount of the latter takes 

 place. This occurs in low lying meadows, and, more particularly, in 

 bogs and swamps. Hence the farmer attempts to intensify aeration 

 and the resulting mineralization of the humus by more thorough 

 tillage, drainage, liming and manuring. 



Temperature 



Influence of Climate and Season. — An illustration of the differ- 

 ences that may exist in the soil temperatures of different regions is given 

 by a comparison of the mean temperatures of 1901 recorded at Moscow, 

 Idaho, and New Brunswick, New Jersey. The soil temperatures were 

 taken to a depth of 152 mm. (6 inches). 



Soil Temperature,* 1901 



' Recorded in Fahrenheit scale. 



