Herbicide residues from previous crops can damage horticultural crops. Yes, we know this can be a problem after triazine herbicides, and we have recently been seeing damage from new herbicides known as Group 2 herbicides (mostly ALS inhibitors like Pursuit and Broadstrike products). But what does this have to do with soil pH?
The soil pH will dramatically affect how quickly herbicides are broken down by natural processes in the soil. In fact, if the soil pH is out of the proper range, there may be no breakdown 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 allow the herbicide to breakdown at a normal rate after the pH rises above 6.0 (which takes several months after application).
We also know that the new postemergent herbicides Classic and Peak 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.
Observations in sensitive crops like sugar beets has shown that low pH circles occur in many fields, including sand, loam, and clay fields. Composite soil samples (ie 1 sample for 10 or 20 acres) may not indicate these low pH circles exist. Areas of high pH are also spotty across fields, and some areas of the province are generally high in soil pH.
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.