How to fertilize vegetables to avoid nitrate pollution

Fertilization Principles

For melon crops, including cucumber and watermelon, the main approach is to use organic fertilizers as the base, with a focus on balanced top-dressing. It's essential to limit nitrogen fertilizer application and avoid nitrate-based fertilizers. Soil testing should be conducted to maintain soil fertility balance. Base fertilizer application should include 4000-5000 kg of well-decomposed organic manure per 667 square meters, along with 30-50 kg of diammonium phosphate and 40-60 kg of potassium sulfate, or 100 kg of compound fertilizer per 667 square meters. Top-dressing can involve 1000 kg of human fecal urine or 10 kg of compound fertilizer (or urea) per 667 square meters. In protected areas, carbon dioxide gas fertilizers can also be used. However, no manure-based top-dressing should be applied during the harvesting period.

For solanaceous vegetables like tomatoes, peppers, and eggplants, the principle remains similar: prioritize organic fertilizers, apply sufficient base fertilizer, manage top-dressing carefully, and limit nitrogen use. Special and biological fertilizers are encouraged. The base fertilizer should consist of 4000-5000 kg of fully composted organic manure, 30-50 kg of DAP, and 40-60 kg of potassium sulfate, or 100 kg of compound fertilizer per 667 square meters. Again, no manure-based top-dressing should occur during harvest time.

Leafy vegetables such as cabbage, lettuce, celery, and leeks require a similar strategy: mainly organic fertilizers, proper base fertilizer, controlled top-dressing, and limited nitrogen. Nitrate-based fertilizers should be strictly avoided. Base fertilizer should include 4000-5000 kg of decomposed organic manure, 30-40 kg of DAP, 30-40 kg of potassium sulfate, or 80 kg of compound fertilizer per 667 square meters. Top-dressing may involve 1000 kg of fully decomposed human excrement and 50-100 kg of plant ash, or 10 kg of compound fertilizer. Later applications can include 20-30 kg of ammonium bicarbonate or 10 kg of urea per 667 square meters.

It's important to note that different vegetable types absorb nitrates differently. Fruiting and root vegetables like eggplant, pepper, carrot, tomato, and radish tend to accumulate less nitrate, so nitrate application should be stopped 20-30 days before harvest. Leafy greens like cabbage and lettuce accumulate more nitrates and should only receive small amounts. Vegetables such as cabbage, leeks, spinach, and mustard greens are prone to high nitrate accumulation and must not be given nitrate-based fertilizers. Spring vegetables generally need less nitrogen, as they are weak and prone to nitrate buildup, so nitrogen application should be reduced or avoided. Summer and autumn vegetables, growing in high temperatures, have lower nitrate levels and can benefit from some nitrogen. In low-fertilized fields, nitrates are less likely to accumulate, making nitrogen application safer. Soils rich in humus often lead to higher nitrate content in vegetables, so nitrogen should be banned. Additionally, nitrate levels vary depending on weather conditions—higher temperatures and strong light reduce nitrate accumulation, while low temperatures and weak light increase it. Therefore, when fertilizing, it’s crucial to consider the type of vegetable, planting season, and climate to ensure safe and clean produce.

Precautions:

Avoid using ammonium nitrate, potassium nitrate, calcium nitrate, or any compound fertilizers containing nitrate, as they can lead to excessive nitrate buildup in vegetables. These should never be applied in vegetable fields. Ammonium bicarbonate, ammonium sulfate, and ammonium acetate should be used sparingly and buried deep into the soil, about 15-18 cm, to slow nitrification and improve fertilizer efficiency, thus reducing nitrate accumulation in vegetables.

Control the amount of nitrogen fertilizer. As the amount of fertilizer increases, so does nitrate accumulation. Limit nitrogen application to 25 kg of standard nitrogen per 667 square meters, with 60%-70% used as base fertilizer and 30%-40% as seedling fertilizer, applied deeply for best results.

Apply nitrogen early. The best time to apply nitrogen is during the seedling stage, which helps plants grow quickly and reduces nitrate accumulation.

Use organic fertilizers regularly. Organic manure should be properly composted at high temperatures to eliminate pathogens and eggs before application. This prevents nitrate pollution and improves vegetable quality and storage life. Biogas residue liquid is highly efficient, reduces pests and diseases, and increases yield. Vegetables grown with biogas residue are considered the most pollution-free.

Do not apply fertilizer directly to the leaves. Foliar sprays can cause ammonium ions to convert into nitrates absorbed by the leaves, increasing nitrate levels and reducing storage life. Always apply fertilizers through the soil for better absorption and safety.

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