That explains what happens when I use ferrous sulfate in the fertilizer mixes instead of the chelated micronutrient blends. My usual rate is up to 16 oz per 1000 in 5 gallons. Many weeds are scorched. This is on bermuda, St Augustine, or zoysia. One note, if the grass was rotary mowed, weed whacked, or the reel was dull/out of adjustment, there is an obvious burning and discoloration on the damaged ends of the grass. If someone else does the mowing and trimming, I do well to wait at least a day before spraying.
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Benjamin Franklin Poor Richard's Almanac1738