A poisonous heavy metal added to soil doesn't stay uniformly toxic — most of it binds to soil particles and organic matter, and only the dissolved fraction in the pore water is actually bioavailable to roots. The split is a linear sorption (distribution-coefficient) equilibrium:
Kd = Kd0 · 10^(k·(pH − 7)) · (1 + 0.15·OM%) [L/kg]
Cw = Ctot / (Kd + θ) [mg/L, pore water]
Cs = Kd · Cw [mg/kg, sorbed]
bioavailable % = θ / (Kd + θ) × 100
θ (soil moisture, L water per kg dry soil) is normally ≈0.3 in the field but is exposed here as its own slider — wetter soils dilute the pore water and free up more dissolved metal even at fixed Kd. Kd rises with pH and organic matter for cationic metals (Cd, Pb, Zn, Cu) — acidic, mineral soils mobilize far more metal than neutral, humus-rich ones, which is exactly why acid rain and acidic mine drainage increase heavy-metal poisoning risk in real ecosystems.
Roots draw metal from the pore water in rough proportion to a metal-specific bioconcentration factor (BCF), giving the crop tissue concentration and a hazard quotient against a food-safety limit (MRL):
Cplant = BCF · Cw HQ = Cplant / MRL (HQ ≥ 1 → unsafe)
- Metal — switches Kd0, the pH sensitivity, BCF and MRL to that element's real order-of-magnitude toxicology (Pb sorbs strongly and is poorly translocated; Cd is far more mobile and bioaccumulates readily).
- pH / organic matter — raise either one and watch the glowing "bioavailable" ions in the soil turn brown (bound) as fewer of them reach the roots.
- Soil moisture (θ) — a wetter, more dilute pore water pulls a larger share of the total metal into solution even without any chemistry change.
- Total contamination — the total mass of metal added to the soil, mg per kg dry soil.
- Glowing ions drift toward the nearest root and rise into the canopy when captured — a direct visual of the soil→root→shoot exposure pathway that environmental toxicologists model as the exposome. Drag the cross-section to pan and scroll/pinch to zoom.