Vegetables become contaminated with nitrates when crops take up more than they can use for growth. Nitrates are the main form in which the essential plant nutrient, nitrogen, is absorbed by the plant from the soil. Microorganisms in the soil break down organic compounds, converting them to nitrates which are readily taken up by the roots. Fertilisers are added to the soil to increase plant growth. Organic fertilisers are subject to the same processes as organic matter in the soil but mineral fertilisers already contain nitrates and can be used directly by plants.
Nitrates occur in many foods and in drinking water but vegetables are by far the main source.
More than three quarters of our average nitrate intake is from vegetables. Certain types of vegetables tend to accumulate nitrate - spinach, lettuce, broccoli, cabbage, celery, radish, and beetroot. Other vegetables do not accumulate as much nitrate - carrots, cauliflower, French beans, parsnips, peas and potatoes. Some plant-foods such as cereal grains contain no nitrates. The reason for this wide range is not just the amount of fertiliser used, although that is important.