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The chlorophyll meter was used to determine the SPAD value of the leaves. During the entire growth and development of big green vegetables, the chlorophyll index was measured five times from the beginning of colonization to three stages before harvest, namely, the early growth period (September 23) and the middle growth period (October). On the 24th and November 6th) and late growth period (December 17th and 25th), the chlorophyll meter readings were measured using the middle of the leaves of the outer leaves of the Chinese cabbage to avoid the veins and damaged parts, and randomly selected in each plot. About 10 pieces of leaves with the same growth tendency, no appearance damage, the measurement result was taken as the average value; the output was measured with an electronic scale. Nitrate nitrogen content determination: At harvest (December 25th), the leaves with SPAD values ​​were determined. For each nitrogen fertilizer treatment, 2 samples were selected and a total of 10 known SPAD leaves were weighed after deferred veins and put. Into a stoppered 25 ml test tube, immersed in 1 mol/L hydrochloric acid for 24 h until the volume is set, and then sucked in 5 ml of the extract to a volume of 100 ml, determined by UV spectrophotometry (determined wavelength of 220 and 275 nm). At the same time do blank and standard curves.
Determination of chlorophyll SPAD values ​​of Chinese cabbage under the condition of the control facility by the chlorophyll meter for the rapid diagnosis of nitrogenous fertilizer application, yield, and nitrate nitrogen content in the leaves, and people's input (nitrogen use amount), output (yield) and quality (nitrogen The comparative analysis of the three requirements for the contents, etc. showed that the nitrate nitrogen content of the cabbage leaves treated with SPAD values ​​was the lowest, but the yield was the lowest, and the N100 kg/hm2 yield was too low. Both are not desirable. The nitrate nitrogen content was highest at N600kg/hm2, while the yield was less than N450kg/hm2. The comprehensive analysis of the three methods, applying N450kg/hm2, was the best recommended amount of fertilizer. Due to the limited treatment of different nitrogen fertilizers and the analysis of only five representative SPAD values ​​at different growth stages, specific prediction methods and nitrogen diagnostic results still need to be further improved and verified.
Nitrogen nutrition is the main judgement of crop nutrient diagnosis, and there is an inseparable link between the nitrogen nutrition status of the crop and its yield and quality. The conventional experimental test method has a very high accuracy and reproducibility. However, there are also some shortcomings, such as poor timeliness, human and material resources, and so on. In recent years, chlorophyll meter has achieved good results in indirect rapid measurement of crop leaf nutrient. The SPAD value measured by chlorophyll meter can indirectly reflect the chlorophyll content and nitrogen content of crop leaves, and can further predict crop yield. And used to guide fertilizer.