In this photo taken on Saturday, Feb. 9, 2013, instructor Ted Smith uses a slide to demonstrate different types of damage to marijuana plants during class at THC University in the Tivoli in Denver. Chemical fertilizers can be finely crafted to target a specific deficiency in the plants, whether they’re lacking phosphorus, evident by telltale “purple vein,” or potassium, a likely culprit when leaves show brown spots between the veins. (AP Photo/Ed Andrieski)

Associated Press
In this photo taken on Saturday, Feb. 9, 2013, instructor Ted Smith uses a slide to demonstrate different types of damage to marijuana plants during class at THC University in the Tivoli in Denver. Chemical fertilizers can be finely crafted to target a specific deficiency in the plants, whether they’re lacking phosphorus, evident by telltale “purple vein,” or potassium, a likely culprit when leaves show brown spots between the veins. (AP Photo/Ed Andrieski)
In this photo taken on Saturday, Feb. 9, 2013, instructor Ted Smith uses a slide to demonstrate different types of damage to marijuana plants during class at THC University in the Tivoli in Denver. Chemical fertilizers can be finely crafted to target a specific deficiency in the plants, whether they’re lacking phosphorus, evident by telltale “purple vein,” or potassium, a likely culprit when leaves show brown spots between the veins. (AP Photo/Ed Andrieski)
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