Phosphorus Risk & Score the Runoff
Rates source
Phosphorus reaches water only when there is both P to lose and water to carry it. The NRCS P-Index multiplies source by transport ratings to give a risk class, the limiting factor and whether you may apply P.
Rate your field & application
The P-Index multiplies your total source loading by your transport potential. Set each factor for this field below.
P allowed, but at or below crop-removal — watch placement & timing.
Weighted factor contributions
Pink = source, blue = transport. The tallest bar is your limiting factor — fix it first.
Next: Limit P to crop removal; incorporate; add a buffer where thin. Your single biggest contributor is Soil erosion (RUSLE) (3 pts) — address that before anything else.
USDA-NRCS Phosphorus Index (590 standard), multiplicative form: P-Index = (Σ weighted source ratings) × (Σ weighted transport ratings), mapped to Low ≤30, Medium ≤60, High ≤100, Very High > 100. Your state's official P-Index is the regulatory record — use this to plan, then confirm with your NRCS / extension agent.
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Phosphorus Index — key facts
- Formula
- P-Index = source × transport
- Loss ratings
- 0 / 1 / 2 / 4 / 8 per factor
- Strongest transport
- erosion (weight 1.5)
- Strongest source
- soil-test P (weight 1.0)
- Low class
- index ≤ 30 — agronomic P
- Very High class
- index > 100 — no P allowed
- Framework
- USDA-NRCS 590 P-Index
- Privacy
- Runs in your browser; nothing uploaded
The P-Index factor table
Every source and transport factor, its group, its weight and the 0–8 loss-rating options. The score is each factor's weight times its chosen rating, summed by group, then multiplied.
| Factor | Group | Weight | Rating options (loss 0–8) |
|---|---|---|---|
| Soil-test P (STP) | source | 1.0 | Low (< 25 ppm) (0); Medium (25–50 ppm) (1); High (50–100 ppm) (2); Very high (100–200 ppm) (4); Excessive (> 200 ppm) (8) |
| Fertilizer P rate | source | 0.2 | None (0); ≤ 30 kg/ha P2O5 (1); 30–60 kg/ha (2); 60–110 kg/ha (4); > 110 kg/ha (8) |
| Fertilizer application method | source | 0.2 | Injected / banded (0); Incorporated < 2 days (1); Incorporated within a week (2); Surface, not incorporated (cool) (4); Surface on frozen/saturated soil (8) |
| Manure / organic P rate | source | 0.4 | None (0); ≤ 30 kg/ha P (1); 30–50 kg/ha (2); 50–100 kg/ha (4); > 100 kg/ha (8) |
| Manure application timing/method | source | 0.4 | Injected / immediate incorporation (0); Incorporated within 3 days (1); Surface, growing season (2); Surface, autumn, no cover (4); Surface on frozen/snow-covered ground (8) |
| Soil erosion (RUSLE) | transport | 1.5 | < 2 t/ha/yr (0); 2–7 t/ha/yr (1); 7–12 t/ha/yr (2); 12–22 t/ha/yr (4); > 22 t/ha/yr (8) |
| Runoff class (soil + slope) | transport | 1.0 | Very low (flat, sandy) (0); Low (1); Medium (2); High (4); Very high (steep, clay/HSG D) (8) |
| Subsurface / leaching potential | transport | 0.5 | None / deep water table (0); Low (1); Medium (some tiles) (2); High (tile-drained / sandy) (4); Very high (karst / shallow water table) (8) |
| Distance to surface water | transport | 1.0 | > 300 m (0); 150–300 m (1); 60–150 m (2); 30–60 m (4); < 30 m / direct connection (8) |
| Riparian buffer / filter strip | transport | 0.8 | Wide buffer (> 15 m) (0); Good buffer (8–15 m) (1); Narrow buffer (3–8 m) (2); Minimal (< 3 m) (4); No buffer (8) |
| Index up to | Class | Verdict | Action |
|---|---|---|---|
| 30 | Low | P application allowed at agronomic (crop-removal) rates. | Apply N/P/K to soil-test recommendation; standard conservation. |
| 60 | Medium | P allowed, but at or below crop-removal — watch placement & timing. | Limit P to crop removal; incorporate; add a buffer where thin. |
| 100 | High | Restricted — P only up to crop removal; address the limiting factor first. | No P beyond removal; cut erosion/runoff; widen buffer; never on frozen ground. |
| over 100 | Very High | No P application allowed — high risk of off-site P loss to water. | Stop all P applications; implement erosion control, buffers, and reduce soil-test P over time. |
Sources: USDA-NRCS Conservation Practice Standard 590; Lemunyon & Gilbert (1993), JSWC 48:483-486; state NRCS Phosphorus Index technical notes.
What the Phosphorus Index measures
Phosphorus is the nutrient that triggers algal blooms in lakes and rivers, and most of it that reaches water comes from a small fraction of fields where high phosphorus and an easy water path coincide. The Phosphorus Index, introduced by Lemunyon and Gilbert in 1993 and adopted into the NRCS 590 Nutrient Management Standard, separates the two questions: how much phosphorus is available to be lost (the source) and how readily water and sediment can carry it off the field (the transport). Because both must be present, the index multiplies the two, so a field high on only one axis stays low risk.
This tool scores each factor on the canonical 0–8 loss-rating scale, weights it, sums the source and transport groups, and multiplies them into a P-Index with a Low, Medium, High or Very High class. It then names the limiting factor — the single change that lowers your risk most — and states the allowed phosphorus application. Use it alongside the Crop Nutrient Removal and Manure Application Rate tools to set a rate that feeds the crop without feeding the lake.
Find the real risk
Multiply source by transport to see whether P actually reaches water.
Target one fix
The limiting factor names the cheapest change for the biggest drop.
Know the limit
Get whether P is allowed at crop removal, restricted, or banned.
Plan BMPs
Test buffers, incorporation and erosion control before you spend.
Frequently Asked Questions
How is the Phosphorus Index calculated?+
This calculator uses the multiplicative NRCS form: each factor is rated on the 0/1/2/4/8 loss-rating scale, multiplied by its weight, then summed within two groups. The source score adds up soil-test P, fertilizer and manure factors; the transport score adds up erosion, runoff, leaching, distance and buffer factors. The P-Index is source score × transport score, and that product maps to a Low, Medium, High or Very High class.
Why multiply source by transport instead of adding them?+
Phosphorus only reaches water when there is both P available (source) and water to move it (transport). A field with very high soil-test P but no runoff path, or a steep eroding field with almost no P, are both low risk — multiplying captures that, because either factor near zero drives the product down. Adding would wrongly flag a field that is high on only one axis.
What do the Low, Medium, High and Very High classes mean?+
Low (index up to 30) lets you apply P at agronomic, crop-removal rates. Medium (up to 60) allows P but only at or below crop removal. High (up to 100) restricts P to crop removal and tells you to fix the limiting factor first. Very High (above 100) means no P application is allowed because the risk of loss to water is too great.
What is the limiting factor the tool reports?+
It is the single weighted factor contributing the most to your score — the one cheapest to fix for the biggest reduction. If erosion is the limiting factor, residue and cover crops help most; if it is manure timing, switching from surface-autumn to incorporated application helps most. The tool surfaces it so you target the right practice instead of guessing.
Which factors carry the most weight?+
Erosion has the highest transport weight (1.5) because sediment carries particulate phosphorus and is the strongest delivery path. Soil-test P carries the highest source weight (1.0) because it is the reserve at risk. Runoff class and distance to water each carry 1.0 on transport. The weighted table on this page lists every factor's weight and its rating options.
Does incorporating fertilizer or manure really lower the risk?+
Yes — strongly. Surface-applied phosphorus on frozen or saturated ground rates 8 (very high), while injected or banded placement rates 0. Because the source factors are summed and then multiplied by transport, moving manure from surface-autumn to immediate incorporation can cut the whole index by a large margin without changing the amount of P you apply.
How does a buffer strip change the score?+
A vegetated riparian buffer intercepts runoff and sediment, so a wide buffer over 15 m rates 0 while no buffer rates 8 on the transport side. Adding or widening a buffer lowers the transport score, which multiplies through the whole index. It is one of the most effective single changes for a field close to surface water.
What soil-test P level becomes a problem?+
On the rating scale here, low is under 25 ppm (rating 0), medium 25–50 ppm (1), high 50–100 ppm (2), very high 100–200 ppm (4) and excessive over 200 ppm (8). Above the agronomic optimum the crop gains nothing from more P, but the loss risk keeps rising, which is why high-testing fields are restricted even when transport is moderate.
Can I apply manure if the field is High risk?+
On a High-risk field you may apply phosphorus only up to crop removal, and you should address the limiting factor before applying — never on frozen ground, and not until erosion, runoff or buffer issues are improved. On a Very High field, no P application is allowed until the index is brought down. The verdict line states exactly what is permitted for your result.
Is the Phosphorus Index a regulatory tool?+
Many U.S. states adopt a P-Index within the NRCS 590 Nutrient Management Standard to decide nutrient-application limits, and exact weights and thresholds vary by state. This calculator uses the canonical multiplicative framework for planning and education; check your state technical note for the official version before using a result for compliance.
How can I lower a high P-Index score?+
Target the limiting factor first, then the next highest. Cut erosion with residue, cover crops and conservation tillage; reduce runoff with contouring and improved infiltration; widen the riparian buffer; incorporate or inject nutrients instead of surface-applying; and over years, draw down excessive soil-test P by holding P below crop removal. Each change lowers a weighted factor, and the multiplicative index falls quickly.