Organic Matter & Nitrogen Mineralization

Amendment & Soil Conditions

Carbon : Nitrogen ratio 20:1

Soil temperature 20 °C

Soil moisture (WFPS) 60%

Nitrogen Cycle

Net N rate

0.0 kg/ha/day

Days elapsed

0.0

Organic C remaining

100%

Microbial biomass C

0 kg/ha

Available soil N (NH₄⁺+NO₃⁻)

15.0 kg/ha

How it works

Fresh organic matter (compost, crop residue, cover crop) decomposes at a rate set by first-order kinetics, sped up by warmth and moisture:

dC/dt = -k·C
k = k_ref · Q10^((T-20)/10) · f(WFPS)

Soil microbes consume the released carbon for energy and growth. Whether that decomposition frees nitrogen for plants or locks it up in microbial bodies depends entirely on the material's C:N ratio, compared against the microbial community's own C:N ratio (~8:1) and growth efficiency (~40%):

N released  = ΔC / CN_material
N demanded  = (ΔC · efficiency) / CN_microbe
Net N       = N released − N demanded

Working the numbers gives a critical C:N ratio of about 20–25:1 — the textbook threshold real agronomists use. Below it (compost, legume residue, young cover crops) microbes release more N than they need: net mineralization, shown as cyan ions drifting up to the root zone. Above it (straw, mature cover crop, sawdust) microbes scavenge extra N from the soil to balance their own tissue: net immobilization, which can temporarily starve a crop of nitrogen even while total soil organic matter is rising.

C:N ratio — the amendment's quality; compost/manure ≈ 12–20:1, cover-crop residue ≈ 15–40:1, cereal straw ≈ 60–100:1.
Temperature & moisture — both throttle microbial activity; decomposition nearly stops in cold or waterlogged (anaerobic) soil, exactly as in a real field.
Apply fresh batch — resets the organic carbon pool at the current C:N ratio, so you can compare amendments back to back.

This mineralization–immobilization turnover (MIT) model is the reason organic farms build fertility with compost and legume cover crops instead of soluble synthetic fertilizer: nitrogen supply is biologically metered rather than applied on demand.

Organic matter (compost) Microbial biomass Mineral nitrogen (N)

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