It's not the same. It's much easier to do dosing with water since they're soluable and can be evenly dispersed. The different in medium with dirt makes this much hard. There will also be build up of unused nutrients that throw off the proportions. The build up of nutrients and non-uniform distribution can make the crops vulnerable to various side effects including nutrient burn. This build up in hydroponics is mitigated by periodically flushing and replacing the water system.