Abstract
The microbiology of a Fibrisol (little decomposed), a Mesisol, and a Humisol (highly fermented) were compared to a depth of 1.25 m. Counts of up to 2 million psychrophilic bacteria per gram oven-dry weight were common for the shallow fibric horizons, whilst the deeper and more humified layers generally had much higher counts of 3–10 million. Mesophilic bacteria were about one-fifth more abundant, but thermophiles averaged less than 1 million. The influence of aeration in the top 15 cm increased counts noticeably. From 1 to 6 million fungi were present in the top 25 cm but numbers dropped sharply around this depth. The abundance of Cytophaga and Chromobacterium species in the Humisol, and in the humic horizons of Mesisol, is interesting and its significance is discussed. Physiological studies concentrated on the nitrogen cycle, with determinations of nitrification, denitrification, ammonification and nitrogen fixation being made. The only significant conversion was that of proteins to ammonia, and "most probable number" studies showed maximum activity in August, with 30–70 million ammonifying bacteria to a depth of 1 m.
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