Back in the day where I was basically in middle suburbia, with 250 neighbours all owning 2 acres and every last one of them had a well paid job in town, the prospect of obtaining organic fertiliser was a challenge. Initially I sourced literally truckloads of horse manure from stables in town which got me out of a hole so in year two, I found a nearby source…. only to lose my entire tomato crop for the year (about a 1/4 of my garden).  Further investigation uncovered a herbicide that was leaving residues in the hay that the horses were eating, and it was expelled by the horses, unaltered.   I had third-hand herbicide on my garden.  A hard lesson, but that was 10 years ago.
Now we’ve moved to 17 acres in a rural neighbourhood.  Fertiliser shortage is certainly not on my horizon.  I have a policy of never having bare land, so as soon as anything is harvested, something goes in its place.  I have areas where plants are allowed to run to seed, and even though I collect a process the seeds, there’s nearly always seedlings coming up when mother nature knows its the best time.  I have a large patch of comfrey that gets cut at random times as I see fit, my chickens have spent hay in their pen that gets cleaned out periodically, and should I feel the need for remediation on fruit trees for example, I take a couple of shovel-fulls of fresh cow manure and place it in a 20L bucket.  This pooh bucket is filled with rain-water and aerated with two fishtank aeration stones.  3-4 days of aeration and the resulting solution gets diluted down to 1:10 , poured through a fine mesh filter then distributed with a watering can where its needed.  I’ve found this to be a powerful soil conditioner, for virtually zero cost.
My other secret that bypasses the need for “fertiliser” as such, is to always have the soil covered with mulch.  I’m rather privileged to have access to unlimited volumes of spoiled hay, and with soil that is sandy with very little clay content ( so little that we don’t have rivers here, only underground water tables) I’ve found that laying hay down between plants, on paths and as boundaries to beds, I have worms proliferating in the top few inches of soil, which seems to reduce the need for watering during dry spells.