Fertilizers are used as plant protection products to provide the soil and thus the plant with nutrients and vital substances (micronutrients) – such as nitrogen, phosphorus, potassium – which are intended to promote growth, increase and secure the yield and improve the quality of value-giving ingredients.According to the origin, a distinction is made between economic and artificial fertilizers. The former is organic fertilizer produced on the farm, such as manure and slurry, as well as straw and plant residues. Artificial fertilizers are synthetically produced nitrogen, phosphorus and potassium compounds. If fertilizers are not used properly, such as with nitrogen fertilizers, the protein content increases, because nitrogen is a protein building block. In contrast, the content of essential amino acids in protein decreases. Too much of this fertilizer thus results in a decrease in the biological value of the protein, but also in an impairment of the taste and shelf life of plant foods. In addition, plants absorb only about 60% of the artificial fertilizer, with the rest ending up in the groundwater, leading to nitrate and phosphate contamination. According to the German Drinking Water Ordinance, the current limit for nitrate in drinking water is 50 mg/l and for nitrite 0.1 mg/l.If the label states “Suitable for the preparation of baby food,” the nitrate content may be a maximum of 10 mg/l and the nitrite content may be 0.02 mg/l. This means that the nitrate content in drinking water must not exceed the limit. An excessive amount of the most important plant nutrient nitrate – a component of nitrogen fertilizer – also has a damaging effect on the soil and on plant growth. Nitrate deposits in the soil deprive the roots of moisture. The roots dry out and can no longer supply the plant with sufficient water and nutrients and vital substances (macro- and micronutrients), which severely inhibits its growth. In addition, leaf lightening occurs in the case of deposits in the leaf tissue.The nitrate content varies from plant to plant. Some vegetable varieties are distinctly nitrophilous, which means they have the ability to store high amounts of nitrate from the soil in their tissues. The higher its input in the soil, the more can be absorbed by the plants. The following is an overview of average nitrate contents of various vegetable and lettuce varieties:
High nitrate contents (> 1,000 mg/kg): endive, lamb’s lettuce, kohlrabi, lettuce, cress, chard, purslane, radishes, radish, beet, celery.
Medium nitrate contents (> 500-1,000 mg/kg): Chinese cabbage, iceberg lettuce, fennel, kale, spinach, white cabbage, savoy cabbage
Low nitrate levels (< 500 mg/kg): eggplant, beans, cauliflower, broccoli, chicory, peas, cucumbers, potatoes, carrots, peppers, mushrooms, leeks, Brussels sprouts, asparagus, tomatoes, onions
Because lettuce is particularly high in nitrates, there are mandatory limits for it: In the summer months, lettuce may not contain more than 2,500 mg of nitrate per kg, and in winter (November to April) no more than 3,500 mg/kg. For spinach, lamb’s lettuce, radishes, radishes and beet, the following guideline values apply:
- Spinach 2,000 mg/kg
- Lamb’s lettuce 2,500 mg/kg
- Radish 3,000 mg/kg
- Radish 3,000 mg/kg
- Beet 3,000 mg/kg
Nitrate in itself is not toxic, but it can be converted in our body already in the mouth by saliva, as well as in the soil and in food by some bacteria into the toxic nitrite. In food, nitrite is found in cured sausages and meats, as well as in aged cheese. Nitrite can limit or block oxygen uptake by reacting with hemoglobin in erythrocytes (red blood cells). Infants are particularly at risk until about six months of age because their protective system, which can convert the blocked blood pigment back to the oxygen-carrying form, is not yet fully developed. Nitrite can react further with other endogenous substances – nitrogen compounds such as amines (contained in meat and sausage products and especially in cheese and fish) – and form so-called nitrosamines in the stomach. These are carcinogenic and the more nitrate a person ingests, the higher the risk of formation of the carcinogenic nitrosamines.Approximately 70% of nitrate intake is from vegetables, 20% from drinking water, and 10% from meat and meat products and fish (nitrate is used to preserve fish and meat and to improve their color and taste). The DGE (German Nutrition Society) and the WHO (World Health Organization) assume that an acceptable amount of nitrate is 220 mg/day. The use of phosphate fertilizers containing cadmium also contributes to the pollution of food substances and entails severe health consequences. High concentrations of cadmium are found, for example, in spinach, celery, and wheat. It is a trace metal that is toxic to humans and animals. If cadmium of increased concentration affects our organism, it can lead to coughing, headache, confusion and fever, and a short-term inhalation of a high dose, to not infrequently fatal fluid accumulation in the lung tissue.
