Answer: Nutrients need to be in a form that the plant roots can easily and quickly absorb. I don't know if human vitamins would break down readily in the soil or not. However, plant fertilizers, both organic and inorganic, provide exactly what is needed in a much cheaper form than vitamins, so I can't see any reason to add vitamins to the soil.
Here?s some basic info on fertilizer and nutrients that plants require. The 3 numbers on a fertilizer bag refer to the percentage of N (nitrogen), P (phosphorous), and K (potassium) in the bag. There are different formulations for different purposes. In general terms, nitrogen produces lush green growth, phosphorous helps strengthen stems and produce flowers, and potassium keeps the root system healthy. If you're applying fertilizer to fruiting (e.g., tomatoes) or flowering plants, you're not as interested in the plant developing leaves as you are in it flowers and fruit, so you'd use a formulation lower in nitrogen and higher in phosphorous, such as Miracle-Gro's Plant Food at 15-30-15. Bone meal is an organic source of phosphorous.
Organic sources of nutrients:
Nitrogen: alfalfa meal, blood meal, coffee grounds, cottonseed meal, fish emulsion, seabird guano.
Phosphorous: bone meal, rock phosphate
Potassium: greensand, seaweed, kelp
Other nutrients that plants need, but in lesser amounts, are referred to as micronutrients and include: calcium, magnesium, sulfur, boron, chlorine, copper, iron, molybdenum, and zinc. These are usually available in the soil in sufficient amounts.
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