---
title: "Herbicide Carryover Calculator: Use Soil Temperature and Rainfall to Find the Actual Plant-Back Risk"
canonical: "https://theyieldgrid.com/herbicide-carryover-calculator/"
model_id: "tyg-663"
model_version: "1.0.0"
last_reviewed: "2026-08-25T01:18:05"
reviewed_by: "Umer Hayiat"
---

# Herbicide Carryover Calculator: Use Soil Temperature and Rainfall to Find the Actual Plant-Back Risk

> Canonical calculator: [https://theyieldgrid.com/herbicide-carryover-calculator/](https://theyieldgrid.com/herbicide-carryover-calculator/)

## What this calculator does

Home - Free Gardening Calculators & Tools - Herbicide Carryover Calculator: Use Soil Temperature and Rainfall to Find the Actual Plant-Back Risk Herbicide persistence is not a fixed number on a label. The half-life printed in a product’s use guide assumes adequate soil moisture and microbial activity near 77°F. When soil temperatures drop below 50°F or rainfall is scarce for months, the bacteria responsible for breaking down herbicide residue essentially stop working. The chemical half-life pauses. What the label says should be gone by spring may still be present at phytotoxic concentrations when your next crop germinates.

## Inputs

| Input | ID | Type | Unit | Range or choices | Required |
|---|---|---|---|---|---|
| Herbicide Active Ingredient | `hlhbr_ai` | select |  | — Choose herbicide — = ``; Atrazine = `atrazine`; Mesotrione (Callisto) = `mesotrione`; Chlorsulfuron (Glean) = `chlorsulfuron`; Clopyralid (Stinger) = `clopyralid`; Dicamba (Banvel / XtendiMax) = `dicamba`; Imazapyr (Arsenal) = `imazapyr`; Glyphosate (Roundup) = `glyphosate` | Yes |
| Initial Application Rate | `hlhbr_rate` | number | oz | 0.01 to 9999 | Yes |
| Days Since Application | `hlhbr_days` | number | Days | 0 to 3650 | Yes |
| Average Soil Temperature (°F) | `hlhbr_temp` | number | °F | 32 to 120 | Yes |
| Rainfall Since Application (inches) | `hlhbr_rain` | number | inches | 0 to 99 | Yes |
| Planned Next Crop | `hlhbr_crop` | select |  | — Choose crop — = ``; Soybeans = `soybeans`; Corn = `corn`; Wheat = `wheat`; Sorghum / Milo = `sorghum`; Cotton = `cotton`; Sunflower = `sunflower`; Alfalfa = `alfalfa`; Vegetables (sensitive) = `vegetables` | Yes |

## Outputs

| Output ID | Default state |
|---|---|
| `hlhbr_results` | Results will appear here after you click Calculate Carryover. |

## Formula and method

Show the calculation steps Step 1: Look up the base half-life. Each herbicide active ingredient has a field-median half-life at optimal microbial conditions (approximately 77°F soil temperature and adequate moisture). This is stored in the tool’s reference table. Step 2: Compute the temperature factor. Temperature Factor = 1 + max(0, (77 - SoilTemp_F) / 30) At 77°F the factor is 1.0 (no extension). At 47°F the factor is 2.0, meaning the half-life doubles. Below 32°F microbial activity approaches zero; the factor continues to grow, reflecting the slowdown. The factor does not decrease below 1.0 for soils warmer than 77°F because the simplified model does not account for accelerated high-heat degradation, which is a known limitation. Step 3: Compute the moisture factor. Moisture Factor = 1 + max(0, (3 - Rainfall_in) / 3) x 0.8 Soils that received 3 or more inches of rainfall receive a factor of 1.0 (no extension). At 0 inches the factor is 1.8, extending the half-life by 80%. The threshold of 3 inches is a simplified proxy for adequate soil moisture; this does not model soil type-specific field capacity. Step 4: Compute the adjusted half-life. Adjusted t½ = Base t½ x Temperature Factor x Moisture Factor Step 5: Apply exponential decay. Remaining (oz/ac) = Initial Rate x (0.5) ^ (Days / Adjusted t½) % Remaining = (Remaining / Initial Rate) x 100 Results are rounded to 4 decimal places for the primary output and 1 decimal place for percentage display. Step 6: Compare against crop tolerance threshold. Each crop-herbicide pairing has a threshold. If remaining exceeds the threshold, a danger warning is issued. If remaining is between 60% and 100% of the threshold, a caution warning is issued. Below 60% of the threshold is classified as safe. Assumptions and Limits Half-life values are field medians from published studies; individual field variation around the median is significant (typical range: 0.5x to 3x the median for the same herbicide) The temperature factor uses a linear model. Actual microbial response to temperature is non-linear and community-specific. Rainfall is used as a cumulative moisture proxy. Timing and intensity of rainfall events matters; 5 inches in a single event followed by dry conditions behaves differently than 5 inches distributed over 30 days. The model does not account for clay content, soil organic matter, or pH, which can substantially alter herbicide sorption and bioavailability. Volatilization and photolysis are not modeled. For volatile herbicides like dicamba, this can mean the tool slightly overestimates remaining residue. Tank mix interactions and synergistic effects between multiple active ingredients are not captured; run each AI separately. Application uniformity is assumed. Spray overlaps or skips will produce local concentrations that deviate from the field-average result. The tool produces an estimate, not a laboratory measurement. A soil bioassay or certified lab extraction is required for confirmed regulatory or insurance decisions.

## Verified worked examples

### Scenario 1: Atrazine on Corn Fields, Soybeans Planted the Following Year After a Dry, Cold Winter

Herbicide: Atrazine (base half-life: 60 days) Application Rate: 32 oz/acre Days Since Application: 300 Average Soil Temperature: 44°F (cold winter, no freeze-thaw recovery) Rainfall Since Application: 1 inch (drought conditions persisted) Planned Crop: Soybeans (injury threshold: 0.10 oz/acre) Temperature factor: 1 + (77 – 44) / 30 = 2.10 Moisture factor: 1 + ((3 – 1) / 3) x 0.8 = 1.53 Adjusted half-life: 60 x 2.10 x 1.53 = 193 days Remaining: 32 x 0.5^(300/193) = 32 x 0.5^1.554 = 32 x 0.341 = 10.91 oz/acre Result: DANGER. Estimated 10.91 oz/acre remaining, 109x above the soybean injury threshold of 0.10 oz/acre. This is the classic dry-winter carryover kill scenario. Even after 300 days, an Atrazine application under cold, dry conditions has degraded less than a single half-life’s worth. Soybeans planted in this field in spring would be expected to suffer severe chlorosis and death.

### Scenario 2: Chlorsulfuron Applied to Winter Wheat, Corn Rotated In the Same Season

Herbicide: Chlorsulfuron (base half-life: 40 days) Application Rate: 4 oz/acre Days Since Application: 90 Average Soil Temperature: 68°F (warm spring) Rainfall Since Application: 5 inches Planned Crop: Corn (injury threshold: 0.10 oz/acre) Temperature factor: 1 + (77 – 68) / 30 = 1.30 Moisture factor: 1.0 (rainfall above 3 inches, no moisture adjustment) Adjusted half-life: 40 x 1.30 = 52 days Remaining: 4 x 0.5^(90/52) = 4 x 0.5^1.731 = 4 x 0.301 = 1.21 oz/acre Result: DANGER. Estimated 1.21 oz/acre remaining, more than 12x above corn’s injury threshold of 0.10 oz/acre. Even under relatively favorable degradation conditions, a 90-day rotation to a sensitive crop is insufficient after a chlorsulfuron application. The label’s plant-back interval for corn from wheat exists precisely to prevent this situation, but growers sometimes attempt early rotations after wet, warm springs believing conditions will accelerate breakdown.

### Scenario 3: Mesotrione on Corn, Wheat Planted 270 Days Later Under Favorable Conditions

Herbicide: Mesotrione (base half-life: 30 days) Application Rate: 12 oz/acre Days Since Application: 270 Average Soil Temperature: 70°F Rainfall Since Application: 7 inches Planned Crop: Wheat (injury threshold: 0.08 oz/acre) Temperature factor: 1 + (77 – 70) / 30 = 1.23 Moisture factor: 1.0 (rainfall above 3 inches) Adjusted half-life: 30 x 1.23 = 37 days Remaining: 12 x 0.5^(270/37) = 12 x 0.5^7.30 = 12 x 0.00630 = 0.076 oz/acre Result: Safe. Estimated 0.076 oz/acre remaining, below wheat’s injury threshold of 0.08 oz/acre at this time point. At 270 days under warm, adequately moist conditions, mesotrione has completed enough half-lives to fall just below the wheat injury threshold. Planting 2 to 3 weeks earlier under these same conditions would have returned a borderline or caution result, which illustrates why counting calendar days from the label’s minimum interval is insufficient without verifying environmental conditions.

## Assumptions

Show the calculation steps Step 1: Look up the base half-life. Each herbicide active ingredient has a field-median half-life at optimal microbial conditions (approximately 77°F soil temperature and adequate moisture). This is stored in the tool’s reference table. Step 2: Compute the temperature factor. Temperature Factor = 1 + max(0, (77 - SoilTemp_F) / 30) At 77°F the factor is 1.0 (no extension). At 47°F the factor is 2.0, meaning the half-life doubles. Below 32°F microbial activity approaches zero; the factor continues to grow, reflecting the slowdown. The factor does not decrease below 1.0 for soils warmer than 77°F because the simplified model does not account for accelerated high-heat degradation, which is a known limitation. Step 3: Compute the moisture factor. Moisture Factor = 1 + max(0, (3 - Rainfall_in) / 3) x 0.8 Soils that received 3 or more inches of rainfall receive a factor of 1.0 (no extension). At 0 inches the factor is 1.8, extending the half-life by 80%. The threshold of 3 inches is a simplified proxy for adequate soil moisture; this does not model soil type-specific field capacity. Step 4: Compute the adjusted half-life. Adjusted t½ = Base t½ x Temperature Factor x Moisture Factor Step 5: Apply exponential decay. Remaining (oz/ac) = Initial Rate x (0.5) ^ (Days / Adjusted t½) % Remaining = (Remaining / Initial Rate) x 100 Results are rounded to 4 decimal places for the primary output and 1 decimal place for percentage display. Step 6: Compare against crop tolerance threshold. Each crop-herbicide pairing has a threshold. If remaining exceeds the threshold, a danger warning is issued. If remaining is between 60% and 100% of the threshold, a caution warning is issued. Below 60% of the threshold is classified as safe. Assumptions and Limits Half-life values are field medians from published studies; individual field variation around the median is significant (typical range: 0.5x to 3x the median for the same herbicide) The temperature factor uses a linear model. Actual microbial response to temperature is non-linear and community-specific. Rainfall is used as a cumulative moisture proxy. Timing and intensity of rainfall events matters; 5 inches in a single event followed by dry conditions behaves differently than 5 inches distributed over 30 days. The model does not account for clay content, soil organic matter, or pH, which can substantially alter herbicide sorption and bioavailability. Volatilization and photolysis are not modeled. For volatile herbicides like dicamba, this can mean the tool slightly overestimates remaining residue. Tank mix interactions and synergistic effects between multiple active ingredients are not captured; run each AI separately. Application uniformity is assumed. Spray overlaps or skips will produce local concentrations that deviate from the field-average result. The tool produces an estimate, not a laboratory measurement. A soil bioassay or certified lab extraction is required for confirmed regulatory or insurance decisions. Half-life values are field medians from published studies; individual field variation around the median is significant (typical range: 0.5x to 3x the median for the same herbicide) The temperature factor uses a linear model. Actual microbial response to temperature is non-linear and community-specific. Rainfall is used as a cumulative moisture proxy. Timing and intensity of rainfall events matters; 5 inches in a single event followed by dry conditions behaves differently than 5 inches distributed over 30 days. The model does not account for clay content, soil organic matter, or pH, which can substantially alter herbicide sorption and bioavailability. Volatilization and photolysis are not modeled. For volatile herbicides like dicamba, this can mean the tool slightly overestimates remaining residue. Tank mix interactions and synergistic effects between multiple active ingredients are not captured; run each AI separately. Application uniformity is assumed. Spray overlaps or skips will produce local concentrations that deviate from the field-average result. The tool produces an estimate, not a laboratory measurement. A soil bioassay or certified lab extraction is required for confirmed regulatory or insurance decisions. Critical Warnings The difference between a total crop loss and a healthy harvest often depends on invisible soil degradation rates. The Dry Winter Carryover Kill: This is the failure mode that destroys fields every year. A grower applies Atrazine to corn in June. The following May, soybeans are planted. The beans emerge, turn white, and die. The label’s plant-back interval assumed normal microbial activity. A cold, dry winter stopped bacterial degradation for 4 to 6 months. The chemical half-life paused. This tool is specifically designed to detect that pause. If your soil temperature average was below 50°F and rainfall was below 2 inches over the winter period, your adjusted half-life may be 2 to 3 times the base value, and a nominal “safe” interval becomes a kill zone. Understanding how herbicide residue interacts with soil chemistry broadly is related to work done with the soil leaching requirement calculator , which addresses how water movement affects chemical persistence in different soil profiles. Crop Sensitivity Is Not Uniformly Distributed: Most growers know soybeans are sensitive to atrazine carryover. Fewer know that chlorsulfuron injures corn at concentrations as low as 0.03 oz/acre, and that imazapyr has an injury threshold for all common crops below 0.02 oz/acre at a base half-life of 90 days. Using this tool only when planting sensitive soybeans misses the risk entirely when rotating to other supposedly tolerant species. Always run the specific crop you are planting, not a general “is it gone yet” check. Minimum Standards The result from this tool should never be the sole basis for a planting decision when residue is within 2x of a crop tolerance threshold. A soil bioassay from a certified lab provides direct confirmation that no simplified decay model can replicate. If the tool returns a danger result, do not assume that waiting a few more weeks resolves the problem without rerunning the calculation with updated elapsed days. The adjusted half-life determines how fast that danger drops, and it may be much slower than expected. Soil pH is a compounding variable: the soil pH sulfur calculator is relevant because high-pH soils slow breakdown of certain ALS inhibitors like chlorsulfuron dramatically more than the base half-life suggests. Competitor Trap: Many herbicide persistence tools on the internet ask only for the chemical name and number of days, then return a fixed percentage remaining based on the base half-life alone. This approach produces a systematically optimistic result in any year with below-normal precipitation or a cold dormant season. A calculator that ignores soil temperature and rainfall is not modeling biological degradation; it is modeling time, which is an entirely different thing. The biological decay rate that governs plant-back safety is driven by microbial activity, and microbes respond to temperature and moisture, not the calendar. Always use a tool that adjusts for both environmental variables, and always be skeptical of any “percent degraded” claim that does not ask you for weather conditions. For context on how application rates and surfactant ratios interact with herbicide performance at the spray event itself, the non-ionic surfactant ratio calculator addresses the application side of the equation. The result from this tool should never be the sole basis for a planting decision when residue is within 2x of a crop tolerance threshold. A soil bioassay from a certified lab provides direct confirmation that no simplified decay model can replicate. If the tool returns a danger result, do not assume that waiting a few more weeks resolves the problem without rerunning the calculation with updated elapsed days. The adjusted half-life determines how fast that danger drops, and it may be much slower than expected. Soil pH is a compounding variable: the soil pH sulfur calculator is relevant because high-pH soils slow breakdown of certain ALS inhibitors like chlorsulfuron dramatically more than the base half-life suggests. Competitor Trap: Many herbicide persistence tools on the internet ask only for the chemical name and number of days, then return a fixed percentage remaining based on the base half-life alone. This approach produces a systematically optimistic result in any year with below-normal precipitation or a cold dormant season. A calculator that ignores soil temperature and rainfall is not modeling biological degradation; it is modeling time, which is an entirely different thing. The biological decay rate that governs plant-back safety is driven by microbial activity, and microbes respond to temperature and moisture, not the calendar. Always use a tool that adjusts for both environmental variables, and always be skeptical of any “percent degraded” claim that does not ask you for weather conditions. For context on how application rates and surfactant ratios interact with herbicide performance at the spray event itself, the non-ionic surfactant ratio calculator addresses the application side of the equation.

## Limitations and safety

Half-life values are field medians from published studies; individual field variation around the median is significant (typical range: 0.5x to 3x the median for the same herbicide) The temperature factor uses a linear model. Actual microbial response to temperature is non-linear and community-specific. Rainfall is used as a cumulative moisture proxy. Timing and intensity of rainfall events matters; 5 inches in a single event followed by dry conditions behaves differently than 5 inches distributed over 30 days. The model does not account for clay content, soil organic matter, or pH, which can substantially alter herbicide sorption and bioavailability. Volatilization and photolysis are not modeled. For volatile herbicides like dicamba, this can mean the tool slightly overestimates remaining residue. Tank mix interactions and synergistic effects between multiple active ingredients are not captured; run each AI separately. Application uniformity is assumed. Spray overlaps or skips will produce local concentrations that deviate from the field-average result. The tool produces an estimate, not a laboratory measurement. A soil bioassay or certified lab extraction is required for confirmed regulatory or insurance decisions. Critical Warnings The difference between a total crop loss and a healthy harvest often depends on invisible soil degradation rates. The Dry Winter Carryover Kill: This is the failure mode that destroys fields every year. A grower applies Atrazine to corn in June. The following May, soybeans are planted. The beans emerge, turn white, and die. The label’s plant-back interval assumed normal microbial activity. A cold, dry winter stopped bacterial degradation for 4 to 6 months. The chemical half-life paused. This tool is specifically designed to detect that pause. If your soil temperature average was below 50°F and rainfall was below 2 inches over the winter period, your adjusted half-life may be 2 to 3 times the base value, and a nominal “safe” interval becomes a kill zone. Understanding how herbicide residue interacts with soil chemistry broadly is related to work done with the soil leaching requirement calculator , which addresses how water movement affects chemical persistence in different soil profiles. Crop Sensitivity Is Not Uniformly Distributed: Most growers know soybeans are sensitive to atrazine carryover. Fewer know that chlorsulfuron injures corn at concentrations as low as 0.03 oz/acre, and that imazapyr has an injury threshold for all common crops below 0.02 oz/acre at a base half-life of 90 days. Using this tool only when planting sensitive soybeans misses the risk entirely when rotating to other supposedly tolerant species. Always run the specific crop you are planting, not a general “is it gone yet” check. Minimum Standards The result from this tool should never be the sole basis for a planting decision when residue is within 2x of a crop tolerance threshold. A soil bioassay from a certified lab provides direct confirmation that no simplified decay model can replicate. If the tool returns a danger result, do not assume that waiting a few more weeks resolves the problem without rerunning the calculation with updated elapsed days. The adjusted half-life determines how fast that danger drops, and it may be much slower than expected. Soil pH is a compounding variable: the soil pH sulfur calculator is relevant because high-pH soils slow breakdown of certain ALS inhibitors like chlorsulfuron dramatically more than the base half-life suggests. Competitor Trap: Many herbicide persistence tools on the internet ask only for the chemical name and number of days, then return a fixed percentage remaining based on the base half-life alone. This approach produces a systematically optimistic result in any year with below-normal precipitation or a cold dormant season. A calculator that ignores soil temperature and rainfall is not modeling biological degradation; it is modeling time, which is an entirely different thing. The biological decay rate that governs plant-back safety is driven by microbial activity, and microbes respond to temperature and moisture, not the calendar. Always use a tool that adjusts for both environmental variables, and always be skeptical of any “percent degraded” claim that does not ask you for weather conditions. For context on how application rates and surfactant ratios interact with herbicide performance at the spray event itself, the non-ionic surfactant ratio calculator addresses the application side of the equation.

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## Provenance

- Model ID: `tyg-663`
- Model version: `1.0.0`
- Reviewed by: Umer Hayiat
- Page modified: 2026-08-25T01:18:05
- Runtime SHA-256: `47b32d35f800cd0711f771eba51c24164030f7100dbe296ca79783e9bbe403f8`

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