The main source of human nitrates intake is 81.2% from vegetables. High levels of nitrate in vegetables may cause harm to the human body, and even more serious is that nitrates can easily be reduced to nitrite under the influence of microorganisms. Nitrite can be formed from nitrites and amines in foods. Nitrosamines can induce gastric cancer, colon cancer, and the like. The excessive levels of nitrate and nitrite in vegetables are mainly caused by the unreasonable use of fertilizers by vegetable farmers in pursuit of yield. Therefore, the following fertilization principles should be followed in the production process:
(1) As far as possible without chemical fertilizers, it is advisable to apply formula fertilizer if it is necessary to apply it in limited and selective manner. The use of nitrate-nitrogen fertilizers is prohibited; in principle, no more than 25 kilograms per 667 square meters; chemical fertilizers must be applied in conjunction with organic fertilizers, and foliar fertilizers should be used less. In other words, the production of pollution-free vegetables is dominated by organic fertilizers, supplemented by other fertilizers; mainly by multiple compound fertilizers, supplemented by single-element fertilizers; mainly by base fertilizers, supplemented by top dressing.
2. Early application. Early application of chemical fertilizers is beneficial to the early growth of plants, prolonging the effect of fertilizer and reducing the accumulation of nitrate. In particular, the use of chemical fertilizers should be avoided before vegetables are harvested. Generally, the top-dressing of chemical fertilizers should be carried out 30 days before harvest.
3. Deeply. Deep application of chemical fertilizers can reduce nitrogen volatilization, extend fertilizer supply time, and increase nitrogen utilization.
In addition, vegetables should be marketed immediately after they are harvested. With the extension of storage and transportation time, the nitrate content in vegetables will also increase.
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