Short answer: a hydroponic nutrient is designed to supply every element a plant needs from water alone. Most garden fertilizers are designed to top up soil, which already holds calcium, magnesium and trace elements. In a Kratky container there is no soil, so those gaps show up as poor growth.
Completeness
Plants need nitrogen, phosphorus and potassium, plus calcium, magnesium, sulfur and a set of micronutrients including iron, manganese, zinc, copper, boron and molybdenum. A hydroponic nutrient lists all of them. Many lawn and garden fertilizers list only N-P-K, because soil normally provides the rest.
The form of nitrogen
Some garden fertilizers supply much of their nitrogen as urea or ammonium. In soil, microbes convert these to nitrate over time. Hydroponic nutrients supply most of their nitrogen as nitrate, which roots take up directly from water.
Solubility
Hydroponic nutrients dissolve completely and stay dissolved. Slow-release granules and coated pellets are made to break down gradually in soil. In water they release unpredictably or leave residue in the container.
What this means for a Kratky grower
- Choose a product labelled for hydroponics.
- Check the label lists calcium, magnesium and micronutrients.
- Follow its rate for hydroponic or non-circulating systems (UF/IFAS).
An all-purpose water-soluble plant food can sometimes work if it is complete, but you take on the job of checking what it is missing. A product made for hydroponics removes that guesswork.
Related guide: Kratky nutrient solution basics.
Sources
- University of Florida IFAS Extension. Set It and Forget It Hydroponics (HS1488).
