How pesticides degrade in soil


Drought this year has not only hindered yields on the Prairies, but it could also directly affect next year’s crop by not allowing for pesticide breakdown in soil.

There are several different ways pesticides can break down in the soil: by volatilization after application; through photodegradation on the soil surface (sunlight); through hydrolysis in soil; oxidation-reduction reactions; and, through one of the most important turnover mechanisms, soil microbial processes.

Even areas that have all the conditions stacked in favour of microbial processes — such as the moist Black or Grey soil zones on the Prairies  — are at risk of herbicide carryover, according to precipitation data collected from Environment Canada, for the current growing season, as per an urgent notice recently released by BASF for imidazolinone herbicides.

Soil texture

To understand what conditions help in chemical residue breakdown, first it’s helpful to know how soil texture will affect pesticide breakdown, from the Brown all the way to the Grey soil zones.

Herbicide residue can bind more tightly to organic matter, or adsorb to clay colloids, meaning less residual pesticide is “free-floating” in the soil.

Sandy soils with low organic matter are at higher risk of herbicide carryover, due to the herbicide’s ability to leach further down into the rooting zone. Clay textured soils with higher organic matter, and generally higher abundance of soil microbes, tend to be lower risk.

Moisture drives it all

Soil microbes require soil moisture to perform basic processes such as enzyme production, which cleave molecules off the chemical residue compound, allowing bacteria and other microbes, to use them as a carbon source or food.

Organic matter and clay content can also provide more surface area for hydrolysis reactions to occur — the other very important process for breakdown, that also requires moisture.

The extreme heat and dry conditions will have altered microbial community functions throughout the growing season, but keep in mind that the soil microbes won’t die in a drought year like this, but instead will go dormant.

The same temperatures that support crop growth — between 20 and 30 degrees C — are the same temperatures that support that soil microbial growth. As the temperature gets cooler going into fall, their processes will slow down to a full stop as winter hits, and resume again in spring.

In addition to temperature, the same moisture that drives crop growth, also activates imidazolinone herbicides, or others that have residual action. If the moisture isn’t there to “kick” the herbicide residue off of organic matter or clay colloids into suspension, the herbicide can’t be broken down through hydrolysis, or microbial means — that moisture threshold varies product to product, and soil zone to soil zone.

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