Standard diluted household ammonia. I use it on snails and slugs; messed them up big time. It degrades in the soil to nitrogen. You get the bonus of the critter decomposing.

Pickling strength vinegar (9 or 10% acidity) is a wonderful herbicide;
it's very strong, so use it on a windless day, wear protective clothing, & be careful not to get it on any desireable plants.

Mulch is a life-saver for the garden;
some cities give away the mulch that comes from tree trimmings, & often tree services will give you the shredded material rather than hauling it to the landfill & paying a fee to dump it.

Willow water is a natural root stimulator for cuttings & other plants;
cut willow twigs & toss into a pot of boiling water;
take the water off the heat & let it cool overnight.
Take out the twigs (throw 'em into the compost-see below) & water your plants & cuttings with the water.

Best & gentlest fertilizer I know of is compost & compost tea, & neither one need cost a dime.

Use newspaper, autumn leaves, torn-up or shredded junk mail & office paper provide carbon ("browns"), & food scraps, stale beer, coffee grounds & tea leaves, flat sodas, green weeds, &, best of all, cow, horse, rabbit, & chicken poop provide nitrogen ("greens").

Throw all of it into a pile, keep it about as moist as a wrung-out sponge, & you'll have your own purely natural & divinely rich compost.

You can pick up brown leaves curbside all autumn, & people are delighted to give you cow & horse & rabbit poop.

To make compost tea, shovel some compost into a big bucket or a garbage can, add water, & let it sit a few days.
Water your plants with the resulting "tea".

It's pure garden magic.

sylviatexas is correct. Compost tea is great.

Making a compost pile is also a good idea. Lots of kitchen scraps and stuff can go in there.

Other household items to use (most of these things I prefer to compost first)
coffee grounds
epsom salts
hair
egg shells
diluted urine. Yes, urine.

I agree with the compost idea in general. I tried egg shells in mine once and didn't like it later because you could still see white bits. I put any kinds of kitchen scraps in my compost pile. Common exceptions most people agree on are meat, bones, dairy, anything really salty, whole corn cobs, some weed roots, any weed seeds, any part of poison ivy oak or sumac, huge quantities of oak leaves, charcoal grill ashes, and sticks bigger around than your little finger. Composted organic material is the best fertilizer.

In_garden is correct--urine is an excellent high nitrogen fertilizer and the price is right. Dilute with water--some say 20/1, some say 10/1, but I don't measure.

No one mentioned grass clippings? As long as you haven't used any pesticides, it's great for getting a compost pile heated up quickly. I mix the usual stuff in my small bin, but in my big pile, it's mostly just grass clippings and leaves, which make an incredible compost. I have purposely created a traditional combination white clover/grass lawn. It feels great on the feet, stays green in drought, attracts pollinators and requires no fertilizers. The bonus is the clippings are very high in nitrogen for use in compost.

Ditto using human urine. Link below to a now 6 year old discussion on gardenweb. Amazing how it rapidly heats up compost and I've seen great results using the dilute for both foliar and root feeding.

There's a wealth of information on this. Google human urine garden for more.

Here is a link that might be useful: Use of human urine