"US: How to tackle "Brown Speckle” symptoms due to low pH"

On the website of the New England Greenhouse Updat, a reminder can be found on the subject of “Brown Speckle” symptoms due to low pH. "Zonal and seed geraniums, American marigold, and all types of impatiens are susceptible to iron/manganese toxicity, a nutritional disorder associated with low growing medium pH. This problem is sometimes called "bronze speckle" due to the appearance of numerous small brown spots on the edges of the leaves of geranium and marigold. The target pH range to prevent toxicity is 5.8-6.5. If the pH is lower than the target range the risk for toxicity increases because at low pH too much iron and manganese becomes available to these plants."