Answer: Based on our knowledge of these products, the seaweed, insecticidal soap and Bacillus thuringiensis (Bt) probably can be mixed together as long as your water isn't hard, says Bill Wolf, owner of Necessary Trading Company, a gardening supply company in New Castle, Virginia, that specializes in organic fertilizers and pesticides. Hard water ties up the fatty acids in the soap and renders it useless, he explains. However, copper fungicide should be sprayed alone because its low pH causes the Bt's toxin to break down prematurely, says Wolf. Even if an individual substance is fairly safe, there could be a problem if you combine them. You might experiment with a mixture that seems useful to you on a few weeds or sample plants. If you get the desired effect, then use it on the other plants, says Wolf.
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