How exactly are nutrients absorbed through the rhizosphere? The rhizosphere refers to the area around the roots of a plant. Your roots are covered with hydrogen, a cation, which they exchange for other cations as well as attract anions. This is basic chemistry. Obviously if you just put a bunch of organic matter in your pot then the microbes will not have a chance to exude any cations or anions, thus the roots will have nothing to exchange for their hydrogen. If the clay and humus (sand is too large to carry anions or cations) in your soil has a sufficient amount of nutrition then it will exchange its cations for the hydrogen on the roots. This is how the plant gets it nutrients. The rate at which a plant can absorb nutrition is referred to as its CEC (cation exchange rate). The higher it is, the more nutrition your plant can absorb. However their is a limit to a good thing. You don't want your CEC to get too high because that will make it so your roots cannot get sufficient oxygen and/or water and your soil will also have very poor drainage. Balance is the key to a good soil as it is the key to good growing.
The way salt based nutrients (chemical ferts) give your plant nutrition is by skipping the whole process of microbes exuding food and going straight to the roots. Obviously what is not used by the plant is then left in your soil and acts as a build up that can be used later. Most of the time this salt build up is unwanted though and that's why flushing became common practice. Chemical ferts provide immediate, and most of the time, good results. However, the salt based nutrition coming from your chemical ferts results in the death of your microbes. Your soil will no longer be able to provide nutrition to your plants and you will rely solely on ferts to feed your plant. That's why as time goes on you need to add more and more ferts to your grow. There is nothing wrong with this at all, but if you plan on going the route of chemical ferts, don't waste your time with a soil recipe and "organic nutrition."