Persistence of Herbicides in Soil

Meaning: Persistence refers to the period a herbicide remains in the soil in an active form that can influence crops, weeds, or soil microorganisms.

Fate of herbicides in soil (how persistence is determined):

1. Adsorption & Desorption: Herbicide molecules attach to soil particles (especially clay & organic matter). Strong adsorption → less available for degradation → more persistence.
2. Leaching & Volatilization: Some herbicides move downward with water (leaching), reducing persistence but risking groundwater contamination. Volatile herbicides (e.g., EPTC) may evaporate → less persistence.
3. Microbial Degradation: Most herbicides are broken down by soil microbes (bacteria, fungi). Warm, moist, well-aerated soils = fast microbial activity = reduced persistence.
4. Chemical Degradation: Hydrolysis, oxidation, and reduction can break down herbicides. Soil pH often plays a role (e.g., sulfonylureas degrade faster in acidic soils).
5. Photodecomposition: Herbicides exposed on the soil surface may degrade due to sunlight (e.g., trifluralin).

Persistence Categories in Soil

• Non-persistent (up to 4 weeks): Paraquat, 2,4-D, Glyphosate
• Moderately persistent (1–3 months): Atrazine, Alachlor, Butachlor
• Persistent (3–12 months): Simazine, Pendimethalin
• Very persistent (1–2 years or more): Picloram, Tebuthiuron

Carryover effect: Highly persistent herbicides may damage succeeding crops. Example: Atrazine applied in maize can harm legumes planted next season if residues remain.

Persistence of Herbicides in Plants; Meaning: Persistence in plants refers to how long herbicide residues remain within plant tissues (roots, leaves, stems, fruits, seeds) after application.

Fate of herbicides in plants

1. Absorption: Through leaves, stems, or roots. Contact herbicides → stay localized (less persistence). Systemic herbicides → translocate and persist longer.
2. Metabolism & Detoxification: Plants can detoxify herbicides through oxidation, conjugation, or compartmentalization. Resistant crops often metabolize herbicides faster → lower persistence.
3. Dilution & Growth: As plants grow, herbicide residues may dilute within new tissues. Fast-growing crops generally have lower persistence.
4. Environmental Influence: Rainfall, sunlight, and temperature accelerate breakdown in plant tissues. Shady or cooler conditions can slow degradation.

Persistence Categories in Plants

• Rapid degradation (days): Paraquat, Glufosinate
• Moderate persistence (weeks): Glyphosate, 2,4-D
• Long persistence (months, residues in harvest): Atrazine, Simazine, Sulfonylureas

Residue hazards: If herbicides persist in edible parts beyond Maximum Residue Limits (MRLs), they can pose health risks to humans and livestock.

Practical Implications of Persistence

Soil Persistence
Advantage: Long-term weed control
Disadvantage: Carryover injury to next crop, soil/water pollution

Plant Persistence
Advantage: Sustained control of internal weed growth
Disadvantage: Residue hazards in food/fodder, export rejection of crops