Answer: Degradation of herbicides depends upon a lot of things so I can't really answer your question. How long a herbicide persists is dependent on several factors including light, temperature, and soil moisture. Herbicides dissipate via several pathways including: photodegradation, chemical degradation, microbial degradation, leaching, and volatilization. Photodegradation occurs when ultraviolet (UV) light breaks chemical bonds of the herbicide?s active ingredient.
Microbial degradation occurs when soil microorganisms use the herbicides as a food source. Virtually all pesticides are organic compounds comprised mostly of carbon, hydrogen, nitrogen, and sulfur. These compounds are a food source for microorganisms. When the same chemical is applied repeatedly, microorganisms that preferentially feed on the chemical increase in number, consuming the chemical and thus reducing its efficacy and longevity of control. This is particularly important with preemergence herbicides which are designed to have longevity in soil.
How long will a herbicide persist in the environment depends on a lot of factors, but there is a gauge by which we can predict herbicide persistence. Herbicide half-life is a measure of how long it takes for 50% of a chemical to degrade. For example, oxadiazon has a half-life of 60 days. So 60 days after application, it will have degraded to ? of the amount applied. After 120 days, the concentration will have decreased by 50% again so that only ? of the applied amount remains. Assuming a 240 day growing season with conditions optimum for herbicide breakdown, only 1/16 of the applied product will remain at the end of the season. While it will take a while, ventually the chemicals in your soil will degrade and disappear.
I applaud your desire to grow organically and you can start right now by using only natural products and only when necessary. Best wishes with your landscape!
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