if you are interested in only collecting a certain part of a plant (like the leaves or roots) could you give them a higher dose of a nutrient that relates to that bit? or would to much cause other problems? or would doing that sort of thing even have an effect? such as giving Valerian or Licorice more Phosphorus so as to get a bigger and better root, or giving plants your only conserned with the leaves less nitrogen(for more consentraited oils) and/or more sulfer to increes its fragrence?
If a nutrient is deficient supplying an adequate amount would be expected to result in improved growth of the plant, so theoretically the development and condition of the useful parts would improve also. But tweaking specific parts with specific nutrients may be hard to accomplish or not even possible. Look for in-depth discussions of growing the specific plants you are interested in to see if anything comes up. And supplying nutrients above the levels required could have deleterious effects, if nothing else being a waste of time and money. Phosphorus in particular is often over-applied and can produce a toxicity. Some commonly used products contain whopping amounts of it.
