Taming a Fumigant for Squash Production
Imagine pouring precious water into a bucket full of holes. For decades, Florida farmers faced a similar challenge with soil fumigants—applying chemicals to control pests only to watch them vanish into thin air before doing their job. This is the story of 1,3-dichloropropene (1,3-D), a critical soil fumigant, and the scientific quest to understand its behavior in Florida's unique agricultural environment.
The phase-out of the ozone-depleting methyl bromide created an urgent need for alternatives, with 1,3-D emerging as a promising candidate. However, its tendency to volatilize—escape from soil as gas—posed significant challenges for farmers specializing in high-value crops like fall squash.
Through innovative research, scientists have uncovered how to work with Florida's sandy soils and climate to make 1,3-D an effective tool for protecting both crops and the environment.
Soil fumigation serves as a reset button for agricultural land. Intensive farming practices allow soil-borne pests including root-knot nematodes, fungal pathogens like Fusarium oxysporum, and aggressive weeds to accumulate, threatening crop yields and quality.
Fumigants are volatile chemicals that diffuse through soil as gases, reaching and eliminating these pests to create a clean slate for the next crop. Soil fumigants must achieve what scientists call effective exposure—maintaining sufficient concentration for enough time throughout the treatment zone.
For decades, methyl bromide was the gold standard—a broad-spectrum fumigant effective against nearly all soil pests and less affected by environmental conditions. Its phase-out under the Montreal Protocol due to ozone depletion created a agricultural crisis, particularly for Florida vegetable production where pests thrive in the warm, sandy soils.
No single chemical has proven to be a perfect replacement for methyl bromide's broad-spectrum efficacy. As research revealed, "Alternatives that have acceptable efficacy under favorable weather may fail in other years when the weather is less favorable" 1 . This inconsistency has driven scientists to study combination approaches and application methods that can compensate for individual limitations.
Florida's agricultural environment presents particular challenges for soil fumigants. The state's predominant sandy soils have larger particles with more space between them compared to clay-rich soils. While this allows for excellent drainage, it also creates easy pathways for fumigant gases to escape upward into the atmosphere rather than diffusing evenly through the soil profile.
The problem doesn't end with reduced efficacy. When fumigants volatilize from soil, they can contribute to air quality concerns and expose agricultural workers and nearby communities. For farmers, this means wasted chemical inputs and inadequate pest control—a lose-lose scenario.
Compounding the soil texture challenge is Florida's climate, where sudden rainfall can quickly alter soil moisture conditions. As researchers discovered, "fumigation efficacy is sensitive to weather conditions" with both soil temperature and rainfall significantly impacting results 1 . This variability explains why a fumigant that works well one season might disappoint the next.
To understand and manage 1,3-D volatilization, researchers conducted meticulous experiments using microplots of Florida's characteristic Arredondo fine sand. They injected 1,3-D at 30 cm depth—typical agricultural practice—then tracked the movement of its Z- and E-isomers under different conditions 2 .
Three regimes: air-dry, near-field-capacity, and near-water-saturated
Yard waste compost amendment
Virtually impermeable film (VIF)
The research revealed dramatic differences in 1,3-D behavior across moisture conditions. In air-dry soil, both isomers raced upward, reaching 10 cm depth in less than 5 hours and rapidly volatilizing into the atmosphere. In stark contrast, diffusion was "negligible in near-water-saturated soil, even 101 hours after injection" 2 .
The Z-isomer of 1,3-D consistently moved faster than the E-isomer, though this difference diminished over time. This finding has practical importance since both forms contribute to pest control but may distribute differently through the soil profile.
Perhaps most importantly, the research demonstrated that VIF cover greatly reduced emissions and helped retain fumigant vapors in shallow subsurface areas where many pests reside. Similarly, organic matter amendments slowed diffusion, though sometimes to the point of potentially reducing efficacy against deeper pests.
| Soil Moisture Condition | Time for Z-1,3-D to Reach 10 cm | Time for E-1,3-D to Reach 10 cm |
|---|---|---|
| Air-dry soil | <5 hours | <5 hours |
| Near-field-capacity | Intermediate rate | Intermediate rate |
| Near-water-saturated | Negligible movement after 101 hours | Negligible movement after 101 hours |
The research points to a critical conclusion: effective 1,3-D application requires precise soil moisture management. As the study authors summarized, "adequate soil water in this sandy soil is needed to prevent rapid emissions, but excess soil water slows diffusion of (Z)- and (E)-1,3-D" 2 .
This creates a narrow optimal window for soil moisture—too dry and the fumigant escapes; too wet and it cannot move through the soil to reach pests. For squash farmers, this means careful irrigation management before fumigation is as important as the fumigant application itself.
The moisture effect occurs because water displaces air in soil pores. In dry soils, large, interconnected air spaces create express routes to the surface. As moisture increases, water blocks these pathways, forcing the fumigant to take slower, more tortuous routes or dissolve into soil water, from which it gradually releases.
Finding the optimal soil moisture window is critical for effective fumigation
| Management Factor | Effect on 1,3-D Diffusion | Effect on 1,3-D Emissions | Overall Efficacy |
|---|---|---|---|
| Air-dry soil | Very rapid | Very high | Poor |
| Optimal moisture | Moderate | Reduced | Good |
| Saturated soil | Very slow | Very low | Poor |
| VIF cover | Slightly reduced | Greatly reduced | Improved |
| Organic amendment | Slowed | Reduced | Variable |
Research conducted in China on cucumber production demonstrates that 1,3-D combinations with other fumigants can broaden the spectrum of pest control. Scientists found that "the combined use of 1,3-D and MNa [metam sodium] exhibited a higher control efficacy than when 1,3-D or MNa was utilized alone" 3 . The combination effectively controlled root-knot nematodes, Fusarium wilt, and weeds while maintaining cucumber yields comparable to methyl bromide treatments.
This combination approach addresses a fundamental limitation: while 1,3-D excels against nematodes, it's less effective against fungi and weeds. Partner chemicals fill these gaps, creating a more comprehensive pest management solution.
However, simply mixing fumigants isn't always optimal. Studies note that "simultaneous application of 1,3-D and MNa accelerated the degradation of 1,3-D, reducing its availability in the soil" 3 . This has led researchers to recommend sequential applications or applying the chemicals at different depths to maximize their individual strengths while minimizing interactions that reduce efficacy.
For Florida squash producers, this research translates to specific management recommendations:
Before fumigation is essential—soil should be at field capacity, neither excessively dry nor saturated
High-quality VIF films significantly improve retention and should be considered standard practice
With complementary fumigants may provide more consistent results than 1,3-D alone
Soil temperature and rainfall patterns after application significantly influence outcomes
These research findings help farmers make informed decisions about fumigation timing, product selection, and application methods to maximize efficacy while minimizing environmental impact.
The volatility of 1,3-D in sandy soils has both economic and environmental consequences. When the fumigant escapes, farmers pay for chemicals that never control pests, reducing profitability. Simultaneously, emissions may contribute to air quality issues, creating regulatory concerns.
Research analyzing the economics of fumigation in tomato production found that "although 1,3-D:Pic:Kpam outperforms methyl bromide over the experiment period, methyl bromide is still the best treatment when accounting for the impact of weather variability" 1 . This underscores that consistency across variable conditions remains a challenge for alternatives.
| Research Tool | Function/Purpose |
|---|---|
| Virtually Impermeable Film (VIF) | Special plastic tarps that dramatically reduce fumigant emissions from soil surface |
| Soil Moisture Sensors | Precisely monitor soil water content to determine optimal application timing |
| Microplot Systems | Controlled experimental setups for tracking fumigant movement under different conditions |
| Air Sampling Equipment | Measure fumigant concentrations in atmosphere to quantify emissions |
| Thermal Desorption Tubes | Capture and analyze fumigant vapors from soil pore air at various depths |
| Gas Chromatography | Highly sensitive detection and quantification of 1,3-D isomers in soil and air samples |
The journey to understand and optimize 1,3-dichloropropene in Florida's agricultural systems exemplifies how modern science tackles complex farming challenges. By unraveling the intricate dance between soil properties, environmental conditions, and chemical behavior, researchers have transformed a problematic fumigant into a manageable tool.
The key insight—that careful management of soil moisture and barrier films can dramatically reduce volatilization while maintaining efficacy—empowers squash producers to use 1,3-D more effectively. This knowledge represents more than just better pest control; it signifies progress toward sustainable agricultural systems that protect both crop yields and environmental quality.
As research continues to refine combination approaches and application techniques, the future of soil fumigation looks increasingly precise—a world where chemicals hit their targets with minimal waste and environmental impact. For now, Florida farmers can add another tool to their pest management toolbox, one grounded in a deep understanding of the invisible battles waged beneath our feet.