The production of vegetables in greenhouses can artificially create warming and heat preservation conditions to avoid severe weather and increase the productivity of vegetables. However, greenhouses have a large amount of fertilizer, are tightly closed, and have weak light. The activity of nitrate reductase in vegetables is low, and nitrates are easily accumulated. Therefore, in order to reduce the nitrate content in shed vegetables, the following problems should be paid attention to when fertilizing in winter and spring.
First, increase organic fertilizer
It is advisable to apply organic fertilizer with high cellulose content, which can greatly enhance the nutrient buffering capacity of the soil, prevent salt accumulation, and delay the process of soil salinization. However, organic fertilizers must be decomposed to prevent transmission of pathogens and eggs to vegetables. In addition, if organic fertilizers are decomposed in the greenhouse, they will produce ammonia and “burn†the seedlings.
Second, prohibit the application of nitrate nitrogen fertilizer
Nitrate nitrogen fertilizers include ammonium nitrate, potassium nitrate, and nitrate-containing compound fertilizers. After applying these fertilizers, it is easy to accumulate nitrate in vegetables. In addition, chlorine-containing fertilizers such as potassium chloride, ammonium chloride, etc. should not be used in greenhouses. This is because the chloride ion can reduce the content of starch and sugar in vegetables and reduce the quality of vegetables. When the chloride ion content in the soil is too high, it can easily make the soil lack calcium and harden. Magnesium sulphate, ammonium sulfate and other fertilizers should not be used in large quantities in greenhouses, because these fertilizers will decompose sulfate ions into the soil. This kind of ions will not be absorbed by vegetables, and when it accumulates too much, it will harm the growth of vegetables. Ammonium bicarbonate is prohibited from being used because ammonium bicarbonate can volatilize large amounts of ammonia, causing ammonia harm to greenhouse vegetables.
Third, control the amount of nitrogen fertilizer application
The application of nitrogenous fertilizer to greenhouse vegetables is conducive to the growth of vegetables, but not excessive. Usually 10-12 kg of pure nitrogen per acre is appropriate. Nitrogen fertilizer should be applied deep and should be applied in combination with phosphorus and potassium fertilizers, or ternary compound fertilizer should be applied, and soil should be covered in time after application. It is best to use chemical fertilizer infusion or drip irrigation under the membrane to apply to the roots to reduce the loss. However, top dressing should be stopped 20 days before harvest. Because of the tight confinement of greenhouses, applicator or shallow fertilization, it is easy to volatilize the fertilizer, which not only reduces fertilizer efficiency but also easily produces ammonia hazards. When applying topdressing, it is necessary to apply acupuncture points or strips. The depth should be 5-6 cm.
Fourth, the implementation of formula fertilization
The period when the different vegetable varieties need the most fertilizer is not the same. For example, 30 to 52 days after the transplanting of the tomato, 30 to 60 days after the transplanting of the pepper, about 75 days after the transplanting of the cucumber. In addition, each vegetable has a suitable ratio for the three elements of NPK needed, such as cucumber 1:0.67:1.83, tomato 1:0.38:1.75, and sweet pepper 1:0.38:1 . Although the demand for micronutrients in vegetables is extremely small, it is indispensable. When using foliar spray, concentration and standards should be mastered. For example, 0.1-0.3% of ferrous sulfate, 0.05-0.2% of zinc sulfate, 0.3-0.5% of borax, and 0.02-0.04% of copper sulfate. Pay attention to the best leaf vegetables do not spray fertilizer, so as not to increase the nitrate content. In general, vegetables need a large amount of nitrogen and potassium, and less phosphorus is needed. Therefore, it is not appropriate to use more diammonium phosphate. Potassium is required most before and after flowering and gradually decreases thereafter.
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