80% of our nitrate consumption comes from vegetables. We consume more nitrates from vegetables than cured meats.
Some plants are high in nitrates about 1.5%. Celery, beets, parsley, endive, leaks, cabbage, sorghum, fennel, johnson grass, pigweed. Almost all plants contain nitrates.
You can grow plants in your garden to be used just for fertilizer, dry cabbage leaves & parsley makes a good nitrogen fertilizer. Sprinkle dry ground up plant material around your garden plants for fertilizer work it into the soil.
You can extract nitrates with water. Put 100 gallons of water in a child swimming pool then add, cabbage, endive, parsley, johnson grass, pigweed, leaks, celery, endive, 10 gallons of wood ash, stir well. After 1 week in hot summer weather plant material can be removed and water can be used as fertilizer. If you want concentrated nitrogen fertilizer evaporate most of the water in a tarp pond on your patio or driveway when there is only a few gallons of water left you will began to see 100% potassium Nitrate KNO3 crystals forming in the water. Yard grass clipping are low in nitrogen about 1/2% nitrates.
Cured meats are 4% salt & 1/2% sodium nitrate for a preservative then after aging 4 months sodium nitrate converts to sodium nitrite. Celery juice is used for a preservative in meat it adds flavor too. Recipe for making your own hard salami calls for 3% celery juice.
I would be very careful with pigweed it is loaded with seeds and a nightmare to get ride of in the garden.
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All green vegetative material has plenty of nitrates. Too much, in fact, which is why you compost that material before applying it to plants. If you dig in uncomposted vegetation, your plants can suffer nitrogen burn. Even dried greens, which are brown, will do this. Fallen leaves are an exception, because before they fall, the tree sucks most of the nitrogen back in to the stems. So digging in uncomposted fallen leaves is a safe thing to do.