Phosphorus Fixation & Feed the Crop, Not the Soil
Models sorption
High-Fe/Al soils can fix 70%+ of broadcast P before the crop ever sees it. Enter your soil group, pH and applied P₂O₅ to see the fixed-versus-available split, the rate that overcomes fixation, and how much banding and a pH tweak save.
Your soil & P plan
Next: band rather than broadcast — banding raises the available fraction to 36% and cuts the rate to hit 45 kg available from 213 down to 126 kg P₂O₅/ha. Liming toward pH 6.5 would lift availability about 18.5%. Iron/aluminium oxides fix P strongly in acid soils; band, lime toward 6, build P over time.
Available fraction = soil-group base × pH-factor (U-shaped, peak pH 6.5), clamped 8–92%; banding multiplies it. Basis: Brady & Weil; extension P-sorption/fixation bulletins; Randall & Hoeft (1988) banding response. P₂O₅ basis throughout. Real fixation also depends on soil-test P level, organic matter, time and P source — calibrate to a local soil-test correlation.
Phosphorus fixation — key facts
- Available fraction
- soil-group base × pH factor
- Fixed fraction
- 1 − available fraction
- Min fixation pH
- ≈ 6.5 (U-shaped curve)
- Effective rate
- target available ÷ available fraction
- High-Fe/Al soils
- fix 75–90% of broadcast P
- Banding
- raises available fraction 1.2–1.7×
- Unit basis
- P₂O₅ throughout
- Source
- Brady & Weil; P-sorption bulletins; Randall & Hoeft 1988
- Privacy
- Runs in your browser; nothing uploaded
Phosphorus fixation by soil group
| Soil group | Available at pH 6.5 (broadcast) | P-sorption index | Banding gain | Fixing class |
|---|---|---|---|---|
| Sandy / low-CEC | 70% | Low (P-sorption index < 150 mg/kg) | 1.2× | low |
| Loam / silt (temperate mineral) | 50% | Moderate (P-sorption index 150–500 mg/kg) | 1.45× | moderate |
| Calcareous / high-lime | 40% | Moderate–high (Ca-P precipitation) | 1.5× | high |
| Oxisol / Ultisol (high Fe/Al) | 25% | High–very high (P-sorption index > 1000 mg/kg) | 1.7× | very high |
| Andisol / volcanic (allophane) | 18% | Very high (allophane, P-sorption index > 1500 mg/kg) | 1.7× | very high |
Representative first-season available fractions for broadcast water-soluble P at the pH of minimum fixation. Source: Brady & Weil, "The Nature and Properties of Soils"; extension P-sorption/fixation bulletins; Randall & Hoeft (1988).
Why applied P is not available P
Soil is a phosphorus sponge. The moment water-soluble P dissolves, iron and aluminium oxides (in acid soils), free lime (in calcareous soils) and allophane (in volcanic soils) bind and precipitate it into forms the crop can barely reach. On a heavy-fixing soil, most of a broadcast application is locked away within weeks — which is why the soil test can climb while yield response stays flat. Treating applied P as if it were all available is the single most expensive mistake in P management.
This tool draws the available-versus-applied curve that bends away from the 1:1 line as fixation rises, names the fixing class, and tells you the effective rate — broadcast and banded — needed to actually deliver your target available P. Band on high-fixing soils, lime acid soils toward pH 6.5, and build soil-test P over years rather than over-broadcasting each season. Pair it with the Phosphorus Buildup and Broadcast vs Band tools.
How to use it in five steps
- 1
Pick your soil group
Sandy, loam, calcareous, oxisol/ultisol or andisol — this sets the base fixation level.
- 2
Enter soil pH
Fixation is lowest near pH 6.5; acid and alkaline extremes both raise it.
- 3
Enter the applied rate and target
Type the P₂O₅ you plan to apply and the available P₂O₅ the crop needs.
- 4
Read the fixed split
See what fraction stays available, what is fixed, 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
How much of my applied phosphorus will my soil fix?+
It depends on the soil. On a sandy low-CEC soil near pH 6.5 maybe 30% is fixed and 70% stays available; on a temperate loam about half is fixed; on a calcareous high-lime soil 55–60%; and on a high-Fe/Al oxisol or an allophanic andisol 75–90% of broadcast P can be fixed in the first season. Enter your soil group, pH and rate and the tool returns the exact fixed-versus-available split for your situation.
What rate of P overcomes fixation to feed the crop?+
The effective rate is the target available P divided by the available fraction. If your soil keeps only 25% of broadcast P available and the crop needs 40 kg P₂O₅/ha available, you would have to broadcast 40 ÷ 0.25 = 160 kg P₂O₅/ha. Banding raises the available fraction, so the same 40 kg available might need only 95 kg banded. The tool shows both rates side by side.
Why does banding phosphorus beat broadcasting?+
Fixation happens where fertilizer P contacts the reactive soil. Broadcasting spreads P through the whole soil volume, maximising contact with Fe/Al oxides, lime or allophane, so most of it is fixed. Banding concentrates P in a narrow zone, so far less soil surface can react with it and a much larger fraction stays available to roots. On high-fixing soils banding can deliver the same available P from 40–55% less fertilizer.
What soil pH minimises phosphorus fixation?+
Phosphorus availability peaks around pH 6 to 7, with the practical minimum-fixation point near pH 6.5. Below that, iron and aluminium oxides fix P strongly (worse the more acid the soil); above pH 7.5, free calcium carbonate precipitates calcium phosphates. The tool's pH factor follows this classic U-shaped curve, so liming an acid high-fixing soil toward 6.5 can lift availability noticeably.
Does liming reduce phosphorus fixation?+
On acid, Fe/Al-dominated soils, yes — raising pH toward 6.5 reduces the activity of the iron and aluminium that fix P, so a larger share of applied and native P becomes available. The tool estimates the percentage availability gain from moving your pH to the minimum-fixation point. On calcareous soils the opposite is true: you would not raise pH further, and acidulated or banded P sources help instead.
What is the difference between P sorption and P fixation?+
Sorption is the initial, partly reversible binding of phosphate to soil surfaces; fixation is the slower conversion into more strongly held or precipitated forms (Fe/Al phosphates in acid soils, Ca phosphates in calcareous soils) that are much less available. In practice they grade into each other, and both reduce the share of applied P a crop can use this season. The tool's available fraction captures the net first-season effect.
Which soils fix the most phosphorus?+
Highly weathered tropical Oxisols and Ultisols rich in iron and aluminium oxides, and volcanic Andisols dominated by allophane, fix the most — P-sorption indices above 1000–1500 mg/kg. Calcareous soils fix moderately through calcium-phosphate precipitation. Sandy, low-CEC soils fix the least but leach P instead. The soil-group selector sets the base fixation level accordingly.
Is the result in P or P₂O₅?+
All applied and available figures here are on a P₂O₅ basis, the unit fertilizer grades and most recommendations use. If your soil-test or crop-removal figures are in elemental P, multiply P by 2.29 to get P₂O₅ before entering them, or divide P₂O₅ by 2.29 to go the other way. Keep the units consistent so the available fraction applies correctly.
Does a high-fixing soil mean I should apply more every year?+
Not blindly. On very-high-fixing soils the smarter strategy is to band the crop's needs each year AND build soil-test P gradually over several seasons, because once the high-affinity sites are satisfied, fixation of new P declines. Over-broadcasting every year wastes fertilizer into the fixed pool. Use the tool to size the banded rate, and a P-buildup tool to plan the multi-year build.
How accurate are these fixation fractions?+
They are sound planning estimates from soil group, pH and placement, drawn from extension P-sorption literature. Real fixation also varies with your current soil-test P level, organic matter, the P source and time since application, so calibrate against a local soil-test P 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 broadcast P seem to disappear?+
Because on a fixing soil most of it never reaches the crop in an available form — it is sorbed onto Fe/Al oxides or precipitated as Ca phosphate within weeks. Soil tests can even rise while crop response stays flat, because much of the measured P is in slowly available pools. Banding and pH management put more of each kilo where roots can get it, which is exactly what this tool quantifies.
Can I use this for starter or pop-up phosphorus decisions?+
Yes — the banded scenario is essentially the starter/band placement case. Because banded P resists fixation far better than broadcast, a modest banded starter often out-yields a much larger broadcast rate on high-fixing soils, especially in cold early-season soils where availability is already low. The tool's banded rate and saving show the size of that advantage for your soil group and pH.