It's entirely up to you to decide which nutrition the marijuana plant gets via the feeding solution. Rock wool alone is useless for the plant. Rock wool is in fact an inert growing medium, which means that it doesn't contain any nutrition and that has a low buffering effect. This also means that rock wool can be quite unforgivable if you make mistakes when making your feeding water.
When growing out in the open, the soil and the age of it determine which elements can be included. Furthermore, soil has a certain buffer capacity. The marijuana plant picks up the substances it needs at that moment, the soil holds on to the rest. That kind of soil is meant to replenish the nutrients. That's the reason why growing on it is so much easier. With hydroponics you need to do everything yourself. This requires high accuracy, continuous monitoring and of course specific knowledge.
No fixed fertilizers are used in a hydroponics. Additionally, with growing on rock wool, the water and fertilizers have to be administered directly to the marijuana plant, which means it has to be quickly absorbable. Many fixed fertilizers are bad, or less well soluble and not available quickly enough for the plant. Moreover, poorly soluble fertilizer soon causes blockages in the water system. Download my free marijuana grow bible for more tips about nutrients and marijuana plant deficiencies.
Liquid fertilizers are generally purer; they contain exactly what is needed without any further additives. So you can quickly provide the plant with what it needs at that moment. Hydro and growing shops have an extensive range of fertilizers which can yield great results. Take a product like Marijuana Booster for instance; especially designed for hydro because it doesn't leave any residue behind, and the nutrition is easily soluble in water which then can be directly absorbed by your marijuana plants.
It's important to know how to handle fertilizers. Fertilizers are a mix of essential nutritional elements. They contain a dozen components, that when combined with oxygen, water and CO2, allow the plant to create cornerstones, hormones and vitamins to grow, flower and generate THC.
It is a misconception that an organic fertilizer is better, or healthier for the marijuana plant than a chemical fertilizer. Essentially all fertilizers do the same thing. These so-called biological fertilizers also contain a cocktail of organic compounds which have a chemical 'fingerprint' if you're looking from a molecular standpoint. So technically these are chemical as well. They are only bound a certain way to organic fertilizers, such as urea or to biological materials such as fish emulsion. The plant determines which substance it needs from this compound.
If you want to start growing, download my free grow guide and order some marijuana seeds. All top quality marijuana seeds are available in my marijuana seed shop. We ship seeds to the US, CA and many other countries. For any growing related question please visit the marijuana support page.