Herbicide Residues and Soil pH

Herbicide residues from previous crops can damage horticultural crops. Most growers know about this problem, but may not realize the effect of soil pH on herbicide breakdown, which can extend the problem for many years after the herbicide application.

For many years, triazine herbicides have caused damage to subsequent horticultural crops, and more recently, Group 2 herbicides (mostly ALS inhibitors like Pursuit and Broadstrike products) have caused damage to rotational horticultural crops. But what does this have to do with soil pH?

The soil pH dramatically affects how quickly herbicides are broken down by natural processes in the soil, especially by soil microbial breakdown. In fact, if the soil pH is out of the proper range, herbicide breakdown may be totally stopped for many years.

We know that imazethapyr (the active ingredient in Pursuit) and flumetsalem (the active ingredient in Broadstrike) break down extremely slowly when the soil pH falls below 6.0. Applications of lime will slowly raise the pH above 6.0, and only after the pH moves above 6.0 will the herbicide breakdown at a normal rate. Remember that it may take several months for lime to raise the soil pH.

We also know that the postemergent herbicides, Classic and Peakplus break down very slowly when the pH is above 7.5. Atrazine breakdown is also very slow at high pH levels above 7.5. Sulfur applications may help, but it is difficult to lower soil pH significantly with sulfur.

In sensitive crops like sugar beets, cole crops, grapes, strawberries and tomatoes, we have seen both small and large circles of herbicide injury in many fields and soil types, including sand, loam, and clay.

The challenge has been to detect these areas before growing sensitive crops. Remember that other crops will grow normally in soils with pH slightly lower or higher than the recommended range; however, once the pH drops below 6.0 or rises above 7.5, herbicide breakdown virtually stops. If you are doing composite soil samples (ie 1 sample for 10 or 20 acres) these low pH circles may not be detected.

Areas of high soil pH can also be spotty across fields, and some areas of the province are generally high in soil pH.

Another concern is the dry conditions over the past several years that may have decreased soil microbial and chemical activity, leading to less herbicide breakdown.

So what's a grower to do? Continue to soil test regularly, and especially to test areas showing problems. Apply lime where indicated to bring pH above 6.0: for sensitive crops like sugar beets, tomatoes, and cole crops, this application should be done several years in advance of planting. Maintain a history of herbicide use in your fields over several years. And if circles of poor plant growth appear in sensitive crops, determine if factors other than herbicides may cause those symptoms.

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