Actually, Chemical nutes DO rank pretty high up on the list. I had to do a paper on this very subject in school (College). Back in the early 1900's (1900-1930) in pursuit of ever greater crops, we deviated from the "Mom and Pop" family type farm and started huge farms that specialized in grains, mainly corn and wheat. Farms became so big, that the use of composts and manures to supplement the soil weren't feasible anymore, and the conventional tilling practices of smaller farms were not practical. We started deep tilling the earth with huge machinery, and farmers became dependent on the "New" breed of chemical fertilizers. We farmed the same crop over and over, and crop rotation became impractical as well. Farmers LOVED it, 'cuz they didn't have to rotate crops anymore or haul tons of manures in and let the fields lie fallow while the manures composted. Large machinery made tiling, planting and harvesting a lot easier.What we didn't realize was that between the deep tilling, and the build up of chemical salts in the soils, the soil microbes and fauna dies off, the soil became barren and compact. All of these factors combined at the very wrong time, when the drought of the '30s hit, the entire prarie regions of N America and Canada became what we called the "Dust Bowl" as the topsoil just blew away in the seasonal high winds. All because we didn't supplement the soil properly, and killed off a few (Gazillion) microbes.
While it certainly wasn't entirely due to chemical fertilizers, the role they played was pretty significant. Even today, although we have learned to use better farming techniques, the majority of all our soil is STILL depleted of nutritional value, and farmers are still dependent on chemical nutrients for their crops. Our only solution will be to start reverting back to building back the soil structure and re-generating the microbial life.
Nor am I bashing the chemical nutes. In the short term, their use in soil grows can be good for some people, but they would prohibit any kind of soil recycling or possibility of sustainability. For what we do, thats OK for some, but myself (and many others) are going for some kind of sustainability in our soils thru the use of organics and soil structural supplementation with BioChar, and manures.
As far as the microbiology goes, certainly there are many bacteria that still will be there, but the BENEFICIALS do get killed off, as does the fungi that are critical.
Organics work mainly because of the symbiotic relationship plants have with mycorrhizal fungi. The beneficial bacteria play their roles and are extremely important, but the mycos are the most crucial factor in organics. plants cannot take up manures, or any other organic compounds until they are broken down by the micro colonies. It is the presence of this micro colony that is the entire basis for organics. Certainly we can re-establish these critical elements, but that would first and foremost involve the cessation of the use of chemical ferts.
Obviously the soil is not sterile PER SE, but for all intents and purposes, it is. And chemical fertilizers have contributed in no small way to almost turning our vast "bread belt" into a waste land.
