Conditions ?
Calculating pH needs a complete, charge-balanced analysis, which soil solution data rarely is. Fixed at the measured value is right for most work.
Components ?
Add a group of the components usually measured in soil solution, or search the whole database.
Soil mode is on: choose for each component whether you are entering its reactive soil content (mg/kg, e.g. 0.43 M HNO3-extractable), so the model partitions it between solid and solution, or its measured dissolved concentration, which is then held fixed.
| Component | Basis | Concentration | mol/L |
|---|
Dissolved organic matter ?
| DOC pool | DOC (mg C/L) | Fulvic acid (% of DOC) | Humic acid (% of DOC) |
|---|
NICA-Donnan settings
Only the humic part of DOC binds metals in the NICA-Donnan model. Measured by fractionation (for example the batch DAX-8 method of Van Zomeren & Comans 2007), enter it as fulvic and humic acid, each as % of DOC. Without a measurement the soil pool defaults to 65 % fulvic acid, an effective value for copper: measured humics are only about 40 % of soil DOC (Groenenberg et al. 2010), but Cu also binds to non-humic DOC, and 65 % reproduces measured free Cu2+ (Weng et al. 2002). Visual MINTEQ’s 82.5 % (DOM:DOC 1.65) is a surface-water value.
Biowaste DOM (compost, biosolids, manure, digestate) varies too much for any default, so a biowaste pool needs its measured fractions; if you measured only the humic fraction, raise it for copper work, since Cu also binds to the non-humic part of DOC (Vulkan et al. 2002).
Soil mode ?
Off, KiwiSpec speciates a solution. On, it also partitions each reactive component between the soil and the solution — solid-phase humic and fulvic acid by NICA-Donnan, iron and aluminium oxides as hydrous ferric oxide — and reports the dissolved concentration and Kd. It is an equilibrium partition: ageing, occlusion and kinetics are not modelled.
The water held per kg of dry soil: the gravimetric water content (0.27 for pore water at 27 % w/w) or the extraction ratio (10 for a 1:10 extract).
Soil organic matter
| Pool | SOC (% of soil) | Reactive humics (% of SOC) | Humic acid (% of humics) |
|---|
Only part of soil organic carbon binds metals. Without a measurement, 31 % of SOC is taken as reactive humic substances (the CEC-based estimate of Weng et al. 2001, within the 50 ± 23 % of Wiersma et al. 2025), 74 % of them humic acid. At 50 % KiwiSpec over-binds Cu and Cd against measured free ions; this is the soil input predictions are most sensitive to. Organic matter is taken as 50 % C.
Iron and aluminium oxides
From oxalate and dithionite extractions
Amorphous oxide from the oxalate extraction plus one sixth of the crystalline part (dithionite minus oxalate), as mol Fe + Al, at 89 g/mol (Wiersma et al. 2025 after Van Eynde et al.).
Dzombak & Morel (1990) two-layer model, 0.2 weak and 0.005 strong sites per mol Fe. Oxides matter most for phosphate, arsenate and molybdate, and for Zn in soils above pH 6.5; for Cu and Cd in acid to neutral soils organic matter dominates. Log K values are for 25 °C.
Clay
Clay as a Donnan exchanger with a fixed negative charge and a gel volume of 1 L/kg (Weng et al. 2001): cations are held electrostatically, without specific edge binding. 0.25 mol/kg is illite (range 0.1–0.4); smectites carry about 0.8–1.2. In acid to neutral soils clay usually holds only a few per cent of the trace metals.
Biowaste application ?
| Component | In the biowaste (mg/kg DM) | Reactive (% of that) | Soil after (mg/kg) |
|---|
The biowaste is mixed evenly into the soil layer. Its reactive metal joins the soil’s reactive pool and its reactive organic matter becomes a second organic-matter pool; the unamended soil stays as entered above, so the application can be switched off or swept. Add the biowaste’s dissolved organic carbon as a DOC pool on the Solution tab. The reactive humic fraction of a biowaste has no default: it must be measured.
Soil air ?
Soil air is enriched in CO2 by root and microbial respiration: typically 10−2.5 to 10−1.5 atm, against 10−3.4 in the free atmosphere. It sets carbonate speciation and much of the pH buffering.
| Gas | log P (atm) |
|---|
Mineral phases
| Mineral | Role |
|---|
Recommended whenever there is organic matter: Fe3+ and Al3+ above their solubility are not real solution species, and left in solution they crowd Cu, Pb and other metals off the organic sites.
May precipitate lets a mineral form only if the solution becomes supersaturated — usually the honest choice. At equilibrium holds it saturated, letting it dissolve or grow without limit. Saturation indices for every mineral are reported whether or not you add any here.
Electron activity ?
Well-drained soil pe 8 to 12; waterlogged pe −2 to 2. pe only takes effect when a couple is switched on below.
Redox couples
Switching on a couple links the two oxidation states through the electron activity, so the element is distributed between them rather than treated as two separate components.
Activity corrections
Davies is reliable to about I = 0.5 mol/L, which covers nearly all soil solutions. Use WATEQ for saline soils.
Ionic strength
Database
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Where each element sits, as a share of its total: the free ion, inorganic and small-ligand complexes, and dissolved organic matter (bound to sites plus held in the Donnan gel). Free-ion activity is what governs uptake and toxicity. ?
Soil mode: the bar starts with the solid phase — soil organic matter and iron oxide and clay — and totals are per litre of solution. For dissolved-basis components the solid share is what the soil holds in equilibrium with the measured solution.
Sweep one input
Disclaimer. KiwiSpec is an independent JavaScript implementation based on the thermodynamic equilibrium concepts and database conventions of the MINTEQ model lineage developed by the US EPA and Battelle. It is not affiliated with or endorsed by the US EPA or Visual MINTEQ.