How did our ancestors do it? Well, for one thing, they used copious amounts of manure. Manure is great for the garden, but most of our urban/suburban gardeners do not have our own cows/goats/sheep/chickens/rabbits... and so we do not have access to manure.
At this point in time (with no emergency looming) we can access manure from area farmers or from stores. But in a real emergency, we will have to fertilize without it or be prepared to trade for it.
Here is the non-chemical fertilizing plan:
1. Build your soil now. If you are currently composting your yard/garden/kitchen waste, you can continuously add that to your garden to improve the soil. You can also be adding manure or composting manure while it is still available. You can also add leaf mould.
2. Stop using chemical fertilizers now. According to Teaming with Microbes, our soil microorganisms either die or "get lazy" with the application of chemical fertilizers. If you are going to count on them to keep your plants healthy in the future, you have to start working with them today.
3. Gather supplies now. Start a compost pile. Stockpile manure to rot and leaves to mold. Obtain mulches.
4. Mulch your beds. Any organic mulch you use today can be turned into the soil after the season is ended to add organic material to your soil.
5. Keep all organic materials on your property. This is a tenet of permaculture. Do not let any organic material leave. Compost your kitchen scraps, fallen leaves, grass clippings, pet hair!, shredded paper, cardboard...everything organic and return it to the soil. Try to keep a closed loop system and you will lose less soil and fertility than if you continuously remove contents from your garden.
5. Grow your own compost. Find space to grow "green manure" like buckwheat, vetch, clovers, and other legumes. Turn them into the soil when they are finished growing to trap the carbon and nitrogen they pulled from the soil.
6. Grow more Perennials. When perennials die each fall, their fallen leaves contribute to soil fertility. Also, since their roots stay rooted in the ground, they are place holders for fungus and bacteria that inhabit roots keeping those essential microorganisms alive in your garden. They most likely also attract beneficial insects and pollinators, so they add to the garden productivity in more ways than one.
7. Rotate your plants. Rotate your plants to ensure that each plant is able to use the nutrients left behind by the last resident and to limit diseases. Long tap rooted plants can pull up nutrients from deep in the soil and also aerate and break up hard soils. Legumes fix nitrogen. "Weeds" often pull up nutrients that other plants can not access.
8. Get creative. Plants need nitrogen, phosphorus and potassium. They also need trace minerals and micronutrients. There are other sources beyond the "usual suspects." Urine is full of ammonia (nitrogen!) Wood ashes contain potassium, magnesium, and other minerals. Egg shells contain calcium. Pond scum contains nitrogen, possible some phosphorus and other trace minerals. Plus pond water can be a watering source if you do not have access to city water or a well.
Dead fish are an amazing fertilizer. If you eat fish, do not waste the heads and tails. First of all, you might want to make soup with the heads and/or eat the eyes, but either way, you can add them to the garden when you are finished. Don't forget crab and lobster shells, sea shells, clam shells, seaweed (if you have access to it) - all contain beneficial nutrients for soil.
Hair contains nitrogen (cat hair, dog hair, human hair, use them all!) When you cook down your bones to make bone broth, let them air dry then crush them up and add them the garden for a boost of phosphorus. Add the blood from any animals that you've eaten.
Save the shells from nuts and add them to your compost. Add newspapers, old phone books (non-glossy pages), cardboard, shredded office paper, leaves, pine needles, shredded twigs, shredded bark, food scraps, and if you are able to compost it correctly, you can even add feces. It's called "dark soil" and in a true emergency, you will want to find a way to recycle that organic matter that is currently being wasted.
Having any other suggestions? Add them in the comment section below.