Lawn Fertilizer Runoff Risk Calculator

How the runoff risk score works

This is a transparent screening index, not a calibrated runoff or nutrient-loss model. It follows the source-and-transport concept used in nutrient management: nutrients must be available at the surface, and runoff must have a path to carry them away.

Total score (0–100) = rate versus recommendation (0–15) + phosphate need (0–10) + pavement cleanup (0–15) + ground condition (0–15) + near-term water event (0–15) + slope/drainage (0–10) + turf cover (0–8) + connection to a drain or water (0–12).

Scores are grouped as low (0–19), moderate (20–39), high (40–59), or very high (60–100). Frozen or snow-covered ground, saturated ground with near-term water, or fertilizer left on pavement before near-term water can raise the result to a very-high minimum. These conservative floors reflect conditions in which normal soil and vegetation retention may be bypassed.

The nutrient math is separate from the risk index:

nutrient applied = product amount × label percentage ÷ 100
elemental phosphorus = label P₂O₅ amount × 0.4364

Important limit: the calculator does not turn the score into pounds or kilograms lost. Actual loss depends on fertilizer chemistry, rainfall intensity and duration, infiltration, antecedent moisture, microtopography, plant uptake, erosion, drainage networks, and maintenance.

A practical pre-application check

1. Read the label. Confirm the analysis, rate, watering directions, spreader setting, and restrictions.
2. Check need and timing. Use current soil-test and local turf guidance; avoid applying before a forecast storm or to frozen, snow-covered, saturated, or ponded soil.
3. Break the transport path. Sweep hard surfaces, keep fertilizer away from drains and water, maintain dense turf or a vegetated buffer, and prevent irrigation from running off.
4. Recheck after changes. Adjust the calculator inputs to see which feasible change most reduces the screening score.

Sources, assumptions, and review

Calculator version: 1.0. Implemented by: Starlight Robotics. Last technical content review: August 3, 2026. No field calibration, laboratory validation, or site-specific professional review is claimed.

• US EPA, What You Can Do: In Your Yard — recommended amount, rain timing, avoiding waterways, careful irrigation, native planting, and buffer practices.
• US EPA, Sources and Solutions: Stormwater — how stormwater transports nitrogen and phosphorus from developed landscapes.
• USDA NRCS, Nutrient Management (Code 590) — rate, source, placement, timing, runoff-risk conditions, soil, slope, cover, and setback considerations.
• USDA NRCS, National Agronomy Manual — the fertilizer conversion P₂O₅ × 0.4364 = elemental P.
• Penn State Extension, Turfgrass Fertilization — fertilizer label analysis, nutrient-rate calculations, soil testing, and phosphorus runoff context.
• University of Minnesota Extension, Caring for Shoreland Lawns and Gardens — sweeping spills, frozen-ground avoidance, surface-water caution, and natural filter strips.

The point weights and category boundaries are Starlight Tools screening assumptions built from the cited factor groups; they are not an EPA, NRCS, or extension-service rating system. Local laws and product labels may be stricter.