Production of vegetables in the greenhouse, the warming due to artificially create the conditions for insulation, can avoid severe weather, the vegetables precocious high yield. However, the shed greenhouse has a large amount of fertilizer, is tightly closed, and has weak light. The nitrate reductase activity in vegetables is low, and it is easy to accumulate nitrate. Thus, to reduce the content of nitrate in greenhouse vegetable, there are many issues that need attention during the winter and spring fertilization.
   First, increase the application of organic fertilizer
   The best is the administration more fiber content of organic fertilizer, can greatly enhance the nutrient buffering capacity of the soil to prevent the accumulation of salts, anti-soil salinization process, but must decomposition of organic fertilizer, to prevent bacteria and insects eggs passed vegetables. In addition, if the organic fertilizer is re-cooked in the greenhouse, ammonia will be produced and the "burn" seedlings will be produced.
   Second, the application of nitrate nitrogen fertilizer is prohibited
   The nitrate nitrogen fertilizer includes ammonium nitrate, potassium nitrate and a compound fertilizer containing nitrate nitrogen. The application of these fertilizers tends to accumulate nitrates in vegetables. In addition, it is not suitable to apply a chlorine-containing chemical fertilizer such as potassium chloride or ammonium chloride in the greenhouse. This is because the chloride ion can be reduced in vegetables and starch sugar, reduced quality of vegetables, and the chlorine ion content in the soil is too high, the calcium is also extremely easy to soil compaction. Magnesium sulfate, ammonium sulfate fertilizer should not be administered in large indoor shed, because the fertilizer is buried in the soil decompose the sulfate ion, which ions are not absorbed by vegetables, when it is too hazardous to accumulate vegetable growth. It is forbidden to apply ammonium bicarbonate. Because ammonium bicarbonate is volatile, a large amount of ammonia gas causes ammonia damage to greenhouse vegetables.
   Third, control the amount of nitrogen fertilizer application
   The application of nitrogen fertilizer in greenhouse vegetables is conducive to the growth of vegetables, but not excessive. Generally , 10 to 12 kg of pure nitrogen per mu (1 mu = 1 / 15 hectares ) is suitable. Nitrogen deeper facilities and phosphorus and potassium should be administered with, or administered NPK, mulch promptly after administration. It is best to use the fertilizer intubation or combined with the drip irrigation under the membrane to apply roots to reduce the loss. However, the top dressing should be stopped 20 days before the harvest . Because of the good degree of airtightness, the greenhouse can easily fertilize the fertilizer by spreading or shallow fertilization, which not only reduces the fertilizer efficiency but also easily causes the ammonia gas hazard. Must banding top dressing or fertilizing, appropriate depth of 5-6 cm.
   Fourth, the implementation of formula fertilization
   The period when the different vegetable varieties require the most fertilizer is also different. For example, 30 to 52 days after transplanting, 30 to 60 days after transplanting, and about 75 days after transplanting . In addition, the demand for each vegetable three elements N, P, have a suitable ratio, such as cucumbers 1: 0.67: 1.83, tomato 1: 0.38: 1.75, sweet pepper is 1: 0.38: 1 . Although the demand for vegetables trace elements rare, but indispensable. The concentration and standard should be mastered when using micro-fertilizer foliar spray. For example, ferrous sulfate is 0.1 % to 0.3 %, zinc sulfate is 0.05 % to 0.2 %, borax is 0.3 % to 0.5 %, and copper sulfate is 0.02 % to 0.04 %. Note that leafy vegetables are best not to foliar spray, so as not to increase nitrate content. Vegetables generally require a lot of nitrogen and potassium, phosphorus need less, not more phosphorus diammonium. Potassium is required before and after flowering, and gradually decreases afterwards.
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