Wood ash may be mixed into the soil around tomato plants as a source of additional potassium. Oregon State University cautions that fertilizers with ammonium should not be used after ash, as this lessens the nitrogen from ammonia.
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Marie Notnats enjoys learning about new topics and teaching people new skills. She has been writing professionally since 2008. She is a professional horticulturist with a Bachelor of Science degree in horticulture from Brigham Young University. She writes for About.com, Examiner.com, eHow, and assorted gardening blogs.