Soil Food Web: How Protozoa Release Plant Nitrogen

How does the soil food web turn nutrients into plant-available nitrogen? Dr Mary Cole explains the overlooked relationship between soil bacteria, protozoa and plant nutrition.
Learn why microbial grazing, nutrient timing and soil disturbance matter—and how farmers can support a more functional soil biology.

At Agpath Laboratories( https://agpath.com.au/), Tim Thompson speaks with plant pathologist and soil microbiologist Dr Mary Cole about the trophic structure of the soil food web.

Mary explains how bacteria and saprophytic fungi process organic matter, why bacteria retain nutrients within their cells, and how protozoa grazing on those bacteria can release excess nitrogen back into the soil in a plant-available form.

The discussion then explores what can happen when nutrients are applied beyond the immediate requirements of a crop. This includes nutrient lock-up, leaching and volatilisation, along with the potential effects of excess inputs and soil disturbance on microbial activity.

Dr Cole also explains the concept of suppressive soil: a biologically diverse soil community in which organisms remain sufficiently balanced that no single group becomes dominant. She then outlines her preferred approach to rebuilding soil function through reduced disturbance, greater organic matter, feeding existing soil biology and adding microbial diversity where appropriate.

This conversation is useful for farmers, growers, landholders, advisers and agricultural students who want to understand how soil microbes cycle nutrients, why biological diversity matters and how management decisions can influence soil function.

Topics covered include the soil food web, soil bacteria and protozoa, nitrogen mineralisation, plant-available nitrogen, bacterially dominant soils, suppressive soil, soil microbial diversity, regenerative agriculture, biological farming, compost extracts, compost tea, organic matter, minimal tillage, nutrient efficiency and fertiliser-loss pathways.

Watch next on Farm Learning:
Lime, soil biology and the next Dr Mary Cole discussion
Compost, compost extracts and biological inputs
Regenerative agriculture case studies showing soil change in the paddock
Practical explanations of nutrient cycling and fertiliser efficiency

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What have you observed in your own soil? Have cultivation, fertiliser use, organic matter or biological inputs changed how your soil performs? Add your experience or questions in the comments.

Details

Filmed at: Apath Laboratories
Guest: Dr Mary Cole, founder of Apath Laboratories
Featured system: The soil food web and microbial nutrient cycling
Key processes: Bacterial nutrient retention, protozoan grazing, nitrogen mineralisation, nutrient-loss pathways and suppressive soil
Relevant practices: Reduced soil disturbance, compost, compost extracts, organic matter and feeding soil biology

#SoilFoodWeb #SoilBiology #RegenerativeAgriculture #BiologicalFarming #SoilHealth

00:00 Why soil microbes control plant nutrition
00:58 How the soil food web begins
01:16 Bacteria, fungi and organic matter breakdown
02:19 Protozoa: the missing nitrogen link
03:12 How microbial grazing releases plant nitrogen
04:10 What excess nutrient applications can disrupt
05:03 Locked up, leached or volatilised
06:16 How suppressive soils keep biology balanced
07:40 Three steps to rebuild soil function
08:35 Compost, extracts and feeding soil biology
09:25 The practical lesson for lower-input farming
09:45 Next: the controversial question about lime