I am mostly pointing this out because this is a wine forum, but the "myco-" in "mycorrhiza" means that this is a fungus, not a bacterium. You know, from the same Kingdom (fungi) as our beloved yeasts (genus Saccharomyces)? The last -myces in Saccharomyces has the same origin, the Greek word for mushroom.
Good catch; dunno where my head was.
There's actually two 'families' or what-have-you (always seem to reply to this thread when im not fully awake lol), of mycorrhizae.. Endo- & Ecto- ; basically meaning 'inside' and 'outside' the root, some have the ability to enter/grow through the root while others just grow around.
Grapeman beat me to the punch of the 'virgin ground'; I'd be highly surprised if theres very many large tracts of land in the US that are still completely untouched, as far as agriculture goes.
Pumpkinman was on-point, with building up the soils slowly.
The object in all of this, is to build humus (HUE-mus), which is the final stage of composted organic material. When humus is attained in the garden/plot, because it wont break down any further, it's said that as long as environmental factors don't change drastically, that the humus should last longer than we will (centuries or more).
It's role in the garden, is a bit of almost everything - it retains micro-films of moisture for the bacteria/fungi - even when the soil appears rather 'dry', enlarges the soil CEC - basically expanding the nutrient carrying capacity of the soil... It just doesnt really provide aeration to the soil, thats about all it's missing.
You may notice (if you havent already, next time you look around) that Mother Nature doesn't plan empty dirt into what she's doing. To keep soil alive, to keep the bacteria and fungi going, they need plant roots to feed them; the soil also needs a break from the intensity of the sun because *most* of the life found in the soil is in the top 6-inches or so and if this dries out its like starting all over, almost.
Plant roots push out exudates - mostly sugars - that attract different soil bacteria and fungi; the plants can actually control the populations of the various soil bacteria/fungi by manipulating the amounts of the types of exudates it produces. This is how a plant feeds naturally. These bacteria and fungi will build up, based on the available exudates, then when predators come along, they simply lived out their life-cycle, or the plant switches the amounts of available foods, those larger, now-unsustainable populations will begin to decline. In this decline is decomposition, and this decomposition provides the plant-available nutrients.
If, however, theres no roots, then theres nothing for which the soil life can feed on & it simply moves away (if it can) or dies off. Some of these bacteria and fungi will 'cyst' up, until the environmental conditions are more favorable and they'll become active again. Repeated die-offs/cystings can throw unbalance into the populations of various micro-organisms, causing a further unbalance in plant-available nutrients, and the process involved with making the missing nutrients more prevalent (hard to do when those micro-organisms died off in large numbers).
This is why, a few pages back, I suggested to James that he may have better luck trying to establish some fruit trees alongside his cover crops or other gardening ventures. Texas heat can fry just about anything, I'm sure, and most plants we consider valuable, wont be able to stand it. Trees are built to take abuse though, as rarely is there anything between them and the harshness of the sun. Anything less than a 'full-sun' plant, would do better in Texas, under some shade, as anything over 90-95F tends to shut-down the plants ability to breathe and grow.
These trees would also supply a long-term source/home for beneficial bacteria and fungi, refuges of a sort. With the trees providing filtered sunlight to the plants below, this would enable those plants to be more vigorous and provide more for eating/composting/ground cover, but also larger root systems to spread-out the soil life, nutrients, and eventually aeration when those stronger roots decay.
I could keep babbling but this probably already looks like a short essay..
