Key Microorganism: Bacteria
Under aerobic conditions, bacteria are single-celled organisms that are responsible for a significant portion of nutrient cycling in living soil. Plants manufacture different kinds of exudates to support a vast array of bacteria (and fungi) for the purpose of obtaining nutrition. Decomposer bacteria mine and absorb nutrients from minerals and organic matter through enzymatic digestion, dividing and creating soil structure along the way (microaggregates).
Root nodules on the roots of Trifolium repens (White Clover). The pink colour is leghaemoglobin and the bacteria within are Rhizobium species. By Rosser1954 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=39603602
Once these nutrients are within the bacterial biomass they remain there until bacteria are consumed by one of their respective micrograzers (protozoa or bacterial-feeding nematodes) who excrete the nutrients for root absorption. For this reason, we often refer to soil bacteria as the ‘fertilizer bags’ in the soil.
If soil contains a balance of bacterial biomass and diversity, and we have high enough micrograzer populations, we have the foundation of biological nutrient cycling. The required bacterial biomass depends on the crop grown and where that crop is found in natural ecosystem succession. The minimum requirement is higher for earlier succession plants, such as vegetables, and less for late successional tree crops.
Recommended levels
Bacterial biomass should always be at or above 300 micrograms per gram of soil (ug/g) and below approximately 3,000 ug/g.
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