Potassium Fixation & Feed the Crop, Not the Clay
Models clay fixation
High-charge vermiculitic soils can fix 65%+ of broadcast potash in non-exchangeable interlayer positions before the crop ever sees it. Enter your clay mineralogy, soil-test K and applied K₂O to see the fixed-versus-available split, the rate that overcomes fixation, and how much banding saves.
Your soil & potash plan
Next: band rather than broadcast — banding lifts the available fraction to 37% and cuts the rate to hit 60 kg available from 261 down to 163 kg K₂O/ha. Apply to moist soil — hard drying collapses the 2:1 layers and fixes even more. High-charge vermiculite collapses around K+ and fixes most broadcast potash — band heavily, build K over years.
Available fraction = clay-class base × soil-test-K factor (low-K soils fix more) × moisture factor (drying fixes more), clamped 10–95%; banding multiplies it. Some fixed K is slowly plant-available over later seasons, so 'fixed' = unavailable to the current crop. Basis: Brady & Weil; Sparks & Huang (1985); extension K-fixation bulletins; Randall & Hoeft (1988) banding. K₂O basis throughout — calibrate to a local soil-test K correlation.
Potassium fixation — key facts
- Available fraction
- clay base × soil-test-K × moisture
- Fixed fraction
- 1 − available fraction
- Worst fixer
- high-charge vermiculite (2:1)
- Effective rate
- target available ÷ available fraction
- Vermiculitic soils
- fix 60–70% of broadcast K
- Low soil-test K
- fixes a larger share of new K
- Hard drying
- collapses 2:1 layers, fixes more
- Unit basis
- K₂O throughout
- Source
- Brady & Weil; Sparks & Huang 1985; K-fixation bulletins
- Privacy
- Runs in your browser; nothing uploaded
Potassium fixation by clay mineralogy
| Clay class | Mineralogy | Available (broadcast, medium K) | Banding gain | Fixing class |
|---|---|---|---|---|
| Kaolinitic / sandy (1:1) | Kaolinite, oxides; few 2:1 layers | 88% | 1.05× | low |
| Smectitic (montmorillonite) | Expanding 2:1 smectite | 70% | 1.2× | moderate |
| Illitic / micaceous | Illite (hydrous mica), 2:1 non-expanding | 55% | 1.35× | high |
| Vermiculitic | Vermiculite, high-charge 2:1 | 35% | 1.6× | very high |
| Hydroxy-interlayered / weathered mica | HIV/HIM weathered 2:1 with Al interlayers | 45% | 1.45× | high |
Representative current-crop available fractions for broadcast water-soluble K (KCl/MOP) at medium soil-test K. Low-K soils fix more of a fresh addition; hard drying fixes more. Source: Brady & Weil, "The Nature and Properties of Soils"; Sparks & Huang (1985); extension K-fixation bulletins; Randall & Hoeft (1988).
Why applied K is not available K
Soils built on 2:1 clays are potassium traps. The moment muriate of potash dissolves, the interlayer spaces of vermiculite, illite and weathered micas — sized close to the K⁺ ion — collapse around the potassium and lock it into non-exchangeable positions the crop cannot reach this season. On a vermiculitic soil, most of a broadcast application can be fixed within weeks, which is why the routine soil test can stay flat while you keep applying. Treating applied K as if it were all available is an expensive mistake on these soils.
This tool draws the available-versus-applied curve that bends away from the 1:1 line as fixation rises, names the fixing class from your clay mineralogy, and tells you the effective rate — broadcast and banded — needed to actually deliver your target available K. Band on high-fixing soils, apply to moist soil to avoid drying-driven fixation, and build soil-test K over years rather than over-broadcasting each season. Pair it with the Phosphorus Fixation and Broadcast vs Band tools.
How to use it in five steps
- 1
Pick your clay mineralogy
Kaolinitic/sandy, smectitic, illitic, vermiculitic or hydroxy-interlayered — this sets the base fixation level.
- 2
Set soil-test K and moisture
Low-K soils fix more of a fresh addition; soils that dry hard fix more than soils that stay moist.
- 3
Enter the applied rate and target
Type the K₂O you plan to apply and the available K₂O the crop needs.
- 4
Read the fixed split
See what fraction stays available, what is fixed in interlayers, and the kilos tied up.
- 5
Choose placement
Compare the broadcast and banded rates to hit your target and the banding saving.
Frequently Asked Questions
Will my soil fix the potassium I apply?+
It depends on the clay mineralogy. Kaolinitic and sandy soils fix little (≈ 85–95% of applied K stays available); smectitic soils fix a moderate share; illitic/micaceous and hydroxy-interlayered soils fix more; and high-charge vermiculitic soils can trap 60–70% of a broadcast addition in non-exchangeable interlayer positions. Enter your clay class, soil-test K level and moisture and the tool returns your exact fixed-versus-available split.
What rate of potash overcomes fixation to feed the crop?+
The effective rate is the target available K₂O divided by the available fraction. If your soil keeps only 35% of broadcast K available and the crop needs 60 kg K₂O/ha available, you must broadcast 60 ÷ 0.35 ≈ 171 kg K₂O/ha. Banding raises the available fraction, so the same 60 kg available might need far less banded. The tool shows both rates side by side.
Why do 2:1 clays fix potassium?+
Vermiculite, illite and weathered micas are 2:1 layer silicates whose interlayer spaces are sized close to the potassium ion. When K⁺ enters, the negatively charged layers collapse around it, trapping it in non-exchangeable positions the current crop cannot reach. Kaolinite is a 1:1 clay without these interlayer sites, so it fixes almost no K.
Does banding potassium beat broadcasting on fixing soils?+
Yes. Fixation happens where fertilizer K contacts reactive clay. Broadcasting spreads K through the whole soil volume, maximising clay contact, so much of it is fixed. Banding concentrates K in a narrow zone, so far less clay surface can react and a larger fraction stays exchangeable. On vermiculitic soils banding can deliver the same available K from much less fertilizer.
How does soil moisture affect potassium fixation?+
Wetting and drying cycles drive both fixation and release. On drying, 2:1 layers collapse around interlayer K⁺ and fix it; on rewetting, some of that K is released again. Hard, repeated drying fixes the most K, which is why applying potash to a moist soil — or banding it — reduces losses on a fixing soil. The tool's moisture setting adjusts the available fraction accordingly.
Do low-potassium soils fix more of what I apply?+
Yes. When a soil is low in exchangeable K, its high-affinity interlayer sites are unsatisfied, so they fix a larger share of any fresh addition before crop-available K builds up. As the soil-test K rises, those sites fill and a greater fraction of new K stays available. The tool's soil-test-K setting captures this, fixing more on low-K soils and less on high-K soils.
Is fixed potassium gone forever?+
No — fixation is partly reversible. Non-exchangeable interlayer K is released slowly over later seasons, especially on rewetting and as exchangeable K is drawn down by crops. So 'fixed' here means unavailable to the current crop, not permanently lost. On strongly fixing soils the strategy is to band each year's needs and build the slowly available reserve over time.
Is the result in K or K₂O?+
All applied and available figures are on a K₂O basis, the unit potash grades (e.g. MOP 0-0-60) and most recommendations use. If your soil-test or crop-removal figures are in elemental K, multiply K by 1.2046 to get K₂O, or divide K₂O by 1.2046 to go the other way. Keep the units consistent so the available fraction applies correctly.
Which soils fix the most potassium?+
High-charge vermiculitic soils fix the most, followed by illitic/micaceous and hydroxy-interlayered (Al-pillared) soils. Smectitic soils fix a moderate amount that largely cycles back with moisture, and kaolinitic or sandy soils fix the least but leach K instead. The clay-class selector sets the base fixation level from this ranking.
Should I just apply more potash every year on a fixing soil?+
Not blindly. On strongly fixing soils the smarter strategy is to band the crop's needs each year and build soil-test K gradually over several seasons, because once the high-affinity sites are satisfied, fixation of new K declines. Over-broadcasting every year wastes potash into the fixed pool. Use the tool to size the banded rate and a K-buildup plan for the multi-year build.
How accurate are these fixation fractions?+
They are sound planning estimates from clay mineralogy, soil-test K and moisture, drawn from soil-chemistry texts and extension K-fixation bulletins. Real fixation also varies with the specific mica/vermiculite content, temperature, the K source and time since application, so calibrate against a local soil-test K correlation where you have one. Treat the available-versus-fixed split and the band-versus-broadcast comparison as directionally reliable guidance, not a lab measurement.
Why does my soil-test K stay flat after I apply potash?+
Because on a fixing soil much of the broadcast K is converted to non-exchangeable interlayer forms that a routine exchangeable-K test does not capture. The K is in the soil but not in the pool the test reads or the crop draws on this season. Banding and applying to moist soil put more of each kilo into the exchangeable pool, which is exactly what this tool quantifies.