Here is
a link about myco. When a plant is in a situation where it needs something from the soil, it will trade roots sugars as food to attract soil biology for the purpose of gaining those nutrients. If it does not need anything it will choose not to trade its carbon. Bacteria can dissolve rock minerals with enzymes. when they die the minerals are left in a useable form near the root. Fungi also have incredible roots (mycorhizzae) that reach out deep into the soil for moisture, or that snare pathogens like nematodes. Some (endo) become an internal part of the root while others (ecto) stay just outside the root. Its a fascinating subject.
Free oxygen is available oxygen whereas chemically bound oxygen is not available for consumption and requires energy to be broken free such as in the electrical production of hydrogen from water. Oxygen in water is more of an emulsion even though oxygen also makes up a water molecule. One form is in the gaseous state, the other the liquid state. Both are obviously useful, but in different ways. Oxygen and water are the 2 primary nutrients before you get to the major nutrients of N-P-K.
The fertilizer is designed to a purpose, that being a close proximity to the root upon planting, with side dressing later in the season. Soaking it in water is system change, so the result will change. The product is designed for soil, not water. You also lose your ability to track fertilizer concentrations, which will become expensive quickly or you will fail to use enough. A better way to use water based fertilizer would be a dissolveable product like miracle grow, which is designed for small amounts of nutrient replenishment over a more frequent application schedule. The plants cannot use but so much at a time, and as they grow roots in search of nutrients, the roots grow large and healthy. Should one over-fertilize, then the root has no reason to grow large and becomes dependant upon you as well as less useful under stress conditions.