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How do Soil Applied Herbicides Work? How do they Control Weeds? (05/23/19)

Soil applied herbicides are applied and incorporated into the soil before planting (called preplant incorporated or ppi) or applied after planting and before crop emerges from the soil (called preemergence).

Soil applied herbicides are applied and incorporated into the soil before planting (called preplant incorporated or ppi) or applied after planting and before crop emerges from the soil (called preemergence). Weed scientists like myself have been promoting the features and benefits of soil-applied herbicides as a strategy to manage resistant weeds. Today I will write and describe exactly how soil applied herbicides control weeds.

To be clear, soil-applied herbicides do not prevent weed seed germination. Weed seeds germinate and herbicide is absorbed by the root or shoot of the developing seedling. Thus, soil-applied herbicide needs to be ‘available’ or in the right place for uptake in the soil solution or in vapor phase (i.e. an available form) for herbicide to be absorbed by weed seedlings.

Many weed species germinate near the soil surface. The top 1 to 2 inches of soil is the primary zone of weed seed germination and should be the target area for herbicide placement. How is ‘available’ achieved? Shallow incorporation can be achieved by mechanical methods such as tillage or carried into the soil by rainfall. Which of these two methods is more consistent? Rainfall provides for uniform incorporation, but mechanical incorporation reduces dependence on receiving timely rainfall. How much rainfall is needed and how soon after application rainfall should be received for optimal herbicide performance depends on many factors, but generally 1/2 to 3/4 inch of rainfall within 10 to 14 days after application is enough.

Full disclosure, there can be losses with soil-applied herbicides and why are soil-applied herbicides applied at a range of rates? It is because soil-applied herbicides are affected by many factors including volatility (evaporation), adsorption, photolysis (degradation due to absorption of sunlight), soil moisture, and even tillage equipment. Volatility potential is determined by soil properties and properties of the herbicide formulation. Most soil-applied herbicides used by farmers today have a medium or low vapor pressure meaning they generally will not be lost during warm and dry conditions. Adsorption is binding of herbicides to soil colloids or organic matter. Herbicides must be bound to soils or they would easily leach away. Conversely, herbicides bound to organic matter or clay soils need to be applied at higher rates to ensure there is sufficient herbicide available in solution or available for controlling weeds. Finally, herbicide remaining on the soil surface due to insufficient mechanical incorporation or herbicide placed too deep in the soil may not be intercepted by roots and shoots of emerging weed seedlings.

I get it, application timing for soil-applied herbicides couldn't be worse, especially in a late spring. It is true, soil-applied herbicides must be applied during the prime planting window. However, once activated, soil-applied herbicides reduce weed species mixtures in fields and increase the simplicity and effectiveness of POST herbicides.

Tom Peters

Extension Sugarbeet Agronomist

NDSU & U of MN

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