soil erosion is not exclusively, but very commonly, a side effect of industrial ag practises such as scorched-earth farming (burning, poisoning or scraping off all surface vegetation prior to planting the cash crop), over irrigation (washing topsoil -- degraded and polluted with poisons, excess nitrates and heavy metals) away into streams and lakes; compacting of soil by the passage of heavy mechanised equipment, thus preventing water absorption and increasing runoff and the formation of permanent hardpan (desertification in other words).
that land has become nonproductive not because it inevitable "gets worn out" -- there are fields in Asia that have been farmed for 3000 years and are still productive -- it has become nonproductive because it has been vandalised by stupid, profit-driven rather than food-driven or survival-oriented farming methods.
oh dear oh dear, I need to go calm down. I'll be typing in all caps next :-) gonna go take a break...
seriously, all, do some reading... the literature on sustainable ag is huge and growing daily, and the living success stories are many and also growing daily. it's not a question of "could our agriculture be more productive?" it's a question of "why in hell are we still doing this all wrong?" and the answer is, basically, because of inertia and because a very powerful elite make a lot of money by doing things wrong and wiping out all competing knowledge and models. [takes deep breath] gonna go rack some home made wine and enjoy the fact that I cannot (yet!) be arrested for making it. The difference between theory and practise in practise ...
I assume you are also right that we could feed sustainably modern world population, but as you guessed, I am not familiar enough with sustainable agriculture and the earth carrying capacity to say so and, as you noted, it would also demand much more than our optimizing agricultural techniques for resource conservation to get there. In fact, nothing short of a radical transformation would probably suffice. Consumption in developped countries would have to be cut back to sustainable levels, efficiency would have to increase across the board, and waste eliminated. It'd probably entail a total restructuration of our economies on principles much broader than financial profit, and necessitate favoring local economies to minimize the energy expenditure of long distance trading, the full accounting of externalities, major investments in RD and infrastructure to enable the all of it.
Unfortunately, the identification of the root cause of problems and their solutions by few individuals has never guaranteed the ability to gather the political will to implement systemic change or even simple remediations. The limits to growth were already mostly identifyied nearly 40 years ago and nothing has been done to begin addressing any of it, au contraire. The almost total lack of consciousness of the importance of soil conservation in the crop sustainability equation, even among people aware of the wall we seem to be approaching at ever greater speed, as well as the willingness to perpetuate productivism by reform minded elites point to the need for multiple levels of discourse (is there a problem? what's the cause? what are the solutions?). It thus seems to me that it is also worthwhile to spend a little time simply but clearly and rigorously identifying what is irremediably unsustainable about intensive agriculture and how it will fail to provide for our future needs.
