https://extension.psu.edu/programs/nutrient-management/educational/soil-fertility/comparing-fertilizer-materials
plants consume cations and anions of nutrients, and turn them into sugars and carbohydrates, among many other interesting chemicals, and use them to feed, repair, produce and reproduce themselves. you cannot remove cations and anions once the plant absorbs them. they are no longer the original material you introduced into the medium, they've been broken down into their constituent parts, absorbed, and metabolized...
what is left behind in the soil are depleted salts, that are mostly inactive. they'll build up and start to burn your roots fairly quickly. you should be watering to run off in soil at least every third watering, every time in soilless medium...if you've been doing that, there should never be anything approaching a toxic level of depleted salts, so no reason to "flush".