It really depends on the soil. Many soils have phosphorous in the rock matrix. This is not highly available like triple superphosphate, but a healthy biological community can extract it in amounts much larger than corn and soybean farmers believe. If the soil is not plowed, if there is a healthy community of mycorrhizal symbionts, and if there are plants with
root systems that go much deeper than annuals like corn or soy, then phosphorous becomes much less of a concern (assuming it is present in a particular soil).
Also, there are ways to concentrate phosphorous. For example, bird droppings are a rather rich source. If a row of some berries is added (either just for the birds or for birds and humans), you can trade berries for guano. The same is true for those who can provide habitat for birds to nest in (and some species will control mosquitoes and capture nitrogen, phosphorous, etc). That may not be a comprehensive solution to declining phosphorous supplies, but it can be used by many to harvest one flow of that mineral.
The prairies were a highly productive ecosystem and nutrients were not a huge issue for massive herds of buffalo that survived on the prairie. Of course, the buffalo are an advanced species that do not feel compelled to move their
urine and manure into the rivers (resulting n both pollution and nutrient loss) ... but maybe we can learn from them.
All of these things are not to say we can indefinitely
feed an exponentially growing population ... human numbers and resource depletion are real concerns. Deposits of highly concentrated phosphate are becoming less common and more expensive.
My personal experiences? I live on a sand dune. I have applied very little fertilizer over the past 15 years, with mixed results. My citrus
trees do quite well - they have a deep root system that goes all the way down to the
water table, and are able to get most of what they need... though the grapefruit tree looks like it could use something with zinc in it. Tomatoes? Forget about it - you cannot grow tomatoes on sand without some input ... tried that, the leaves were quick to turn purple and display the signs of a phosphorous deficiency. A loquat (Japanese plum),
apple trees, and cacti all seem to have good nutrient status (though I am giving up on the apples - not cold
enough for the apples to fruit reliably). Strawberry guava is doing fine with little to no NPK input, although it has been too cold for them and frost damage the last two winters. Crowder peas (an annual) do well on the sandy soil, and New Guinea winged beans also seem fine without inputs. Figs were rather pathetic until I started pampering them with both organic matter (to ward off the nematodes) and a bit of 10-10-10. The row of pineapple guava (feijoa) I put in basically were in a holding pattern for 5-7 years, until I decided they would get regular watering and modest applications of water soluble fertilizer. Since deciding that, their growth has taken off. I am hoping that once they really get established, they will be able to thrive on less pampering ... but that remains to be seen. The seven olive trees that I put in over the last two years are being pampered - I look at them as an investment, I suspect that they will need minimal care once they are established. Bamboo seems to do fine without fertilizer, have given it a bit to help it establish. Bananas? They won't thrive here unless they are worshiped with nitrogen and other nutrients (and it's too cold - giving up on the fruiting variety). Meanwhile, I am adding more plants to the mix (especially legumes) and go up and down the street to collect bagged leaves to build the soil ... most of my neighbors still believe that leaves are a troublesome waste, not a valuable resource.
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