The process by which herbicides enter a plant via the roots, shoots, or foliage, depending upon the herbicide applied. An herbicide must first get into the plant then reach its site of action to control sensitive weed species.
In general, herbicide rates are often based upon soil type/OM with heavier soils (more clay) with higher OM requiring higher use rates. Soil pH can influence the soil adsorption of some herbicides such as the triazines (AAtrex) and sulfonylureas (Classic). Soil/OM adsorption can also greatly influence herbicide persistence (carryover).
Simply put, energy from the sun breaks down chemical bonds (the glue that holds herbicide molecules together). The “yellow” herbicides (Prowl, Sonalan, and Treflan) are particularly sensitive to photodecomposition.
Hopefully, all readers have attended the mandatory 2019 auxin trainings (2,4-D/dicamba) somewhere and know much more about these already. As a quick reminder, particle drift occurs at the time of spraying, usually when small spray droplets move off-target under windy conditions. Volatility occurs sometime after an application when an herbicide enters a gaseous phase and moves off-target when nobody is looking (can’t really see this anyway). Understanding volatility is more complicated because it is influenced by many factors such as active ingredient, formulation, temperature, moisture, and where the herbicide is actually applied (bare soil, dead plants, or large live green plants).
Runoff and erosion can remove herbicides laterally from the soil when conditions are too wet or too dry/windy. Herbicides applied to water saturated soils can easily move off-target if followed by additional rainfall events. On several occasions, I have observed Valor injury on plants from windblown soil.
Uncountable (with fingers and toes) micro-organisms are in the soil that can degrade herbicides and other pesticides. Temperature and moisture are the 2 most important factors that can influence microbial degradation. In general, too cold, too hot, and too dry conditions decrease microbial degradation.
The soil is basically a big organic chemistry laboratory. Lots of carbon, hydrogen, and oxygen around. One of the most common ways herbicides are degraded is thru the process of hydrolysis (breakdown via the addition of water, good ole’ H2O).
The downward movement of herbicides in the soil is influenced by soil texture, herbicide solubility (how easily it dissolves in water), and herbicide adsorption properties. Herbicide leaching is more of a concern in sandier soils. Herbicides with high water solubility and/or low adsorption properties are more prone to leaching.
While you were looking at your smart phone in your GPS-guided, starship-like sprayer, you might not have realized all of this stuff was going on. Kind of cool though! Without a doubt, a better understanding of the fate of herbicides in the environment is important for good stewardship, improved performance, and my ultimate goal of helping you become the best herbicide applicator possible. As always, good weed hunting!