Unfortunately the USDA Nutrient Database does not track Nitrates/Nitrites, so we had to look to other sources.
The content of nitrates and nitrites in fruits, vegetables and other foodstuffs
lettuce, frozen spinach, fennel, radishes, parsley: >1000 mg of KNO3/kg
lettuce: 3500 mg/kg
carrots, celery, leeks and frozen French bean: 24 - 800 mg KNO3/kg
strawberries 58.7 mg KNO3/kg
fruit-vegetable juice: 355.30 to 584.53 mg KNO3/kg
The exact numbers were highly variable between samples, often with an order of magnitude difference. The soil conditions, fertilizer, maturity of the plant, etc. affect the levels.
Meanwhile, cured meats are federally limited to 200 ppm of nitrite (same as mg/kg in this context)
So when you turn down that bacon in favor of a "healthier" salad, you're likely consuming seventeen times as much nitrogenous compounds.
Now there is an issue that we're talking nitrates and nitrites, but nitrates do convert to nitrite both in storage and in the body. "Nitrate free" cured meats work by converting the natural nitrates in celery juice or other sources into nitrate, and the same can occur in your own body.
Meanwhile, the nitrite in those cured meats makes them resistant to breakdown by bacteria and resistant to contamination by the deadly botulism toxin, making your meat safer as well as more flavorful.
Yes, excessive Nitrites and Nitrates are toxic, yes, under some conditions they can turn into carcinogenic compounds in your body, but they are a normal part of natural food. They have a very long standing history as a food additive, much longer than most of the additives in your average can/box from the store. The amounts included in cured meats are nothing to worry about.