The form of nitrogen in your fertilizer is very important, Ammonium (NH4)is not able to be be used by your plants, it needs to be broken down to Nitrite (NO2) and then Nitrate (NO3), in the nitrate form the plant can easily take it up. What you have got to remember is the size of the elements that your dealing with, we are talking down to the atomic size (atoms and molecules), so it will absorb into the leaves. Just like pesticide can absorb into your skin.
Natural type fertilizers are always good to add to your fertilizing regime as they add vitamins, minerals and the normal fertilizer elements. Foliar feeding is very quick reacting but you can over do it. Balance it with a slow release fertilizer at the base of your plants.
Hope this helps
Regards
Wanderanwills