| Onion is one of important foreign exchange earning vegetables, but there is rare research on fertilization and mineral nutrition absorption and distribution in onion. Applying fertilizer blindly to onion on production, influenced yield and quality and decreased the benefit of onion production greatly. The experiments were done with"99-3"onion. The uptake and distribution of nitrogen, phosphorus, potassium, the effects of nitrogen, phosphorus and potassium on growth, development, yield, quality, and physiological characteristics of onion were studied in this paper. Analyzed by computer, we determined an optimized fertilization scheme of onion which offered the scientific theoretical foundation for applying fertilizer rationally in onion. The main results were as follows:1. The effects of nitrogen, phosphorus and potassium on yield and quality parameters of onion (Allium cepa L) were studied with the design of three factors and quadratic saturation D-optimal regression (310). According to the data obtained, a mathematical model was established, in which N, P and K were independent variables and onion yield and quality parameters were dependent variables. The model analysis showed that N, P and K significantly influenced onion yield and quality parameters, with P having the biggest effect followed by N and K. When N, P and K reached 430.7, 449.5 and 1152.7 kg/hm2, the marginal yield was 0. The interactions of N, K and N, P for onion yield were significant, with the biggest interaction area being N at 240.0～720.0 kg/hm2, P2O5 at 250.0～500.0kg/hm2 and K2O at 600.0～1500.0 kg/hm2. In this experiment, when the fertilizer rates were N at 294.0～480.6 kg/hm2, P2O5 at 332.8～388.5kg/hm2 and K2O at 861.8～1119.8kg/hm2, yield reached 75t/hm2 and quality grade passed 90. The most suitable ratio of N: P2O5: K2O was 1:0.93:2.56.2. The growing characteristics of onion and its absorption and distribution rule to the nitrogen, phosphorus and potassium in autumn were studied in this paper. The results showed that onion grew very slow at the seedling stage, its accumulation of dry matter was very little and its absorption speed of the nitrogen, phosphorus and potassium was very low. It absorbed only about 4% of the whole nutrient in this stage. At plant vigorous growth stage, the uptake rate of onion to N, P2O5, K2O reached 22.03, 8.60 and 15.65 mg/(plant·d), which respectively accounted for 92.74%, 91.01%, 71.79% of whole growing stage. At the bulb expanding stage, the uptake rate of N and P2O5 decreased rapidly, but the rate of K2O was still kept at 7.23 mg/(plant·d). The proportion of onion absorption to N, P2O5 and K2O in these three stages was 1:0.40:0.92. As its growing, the absorption rate of phosphorus and potassium increased, and it reached 1:0.92:9.04 at the bulb expanding stage. Nitrogen, phosphorus and potassium were mainly distributed in leaf at the seedling stage, at the plant vigorous growth stage mostly in bulb and leaf, but at the bulb expanding stage primarily in bulb, and in this stage the distribution rate reached 75.88%, 87.7% and 71.81%. Every 1000kg bulb product, the demands for N, P2O5 and K2O were about 2.93, 1.16 and 2.69 kg, respectively.3. N, P and K significantly influenced growth and development of onion, with P having the biggest effect followed by N and K. The plant height and cauloid diameter of onion under P-deficiency treatment (T2) were 76.24% and 72.51% of which under balanced fertilization treatment (T4) respectively in 17th May. The fresh weights of root, cauloid leaf of T2 were 56.50% and 56.14%.Single bulb weight was 230g at harvest, which was 35.21% lower than T4. The single bulb weight under N-deficiency treatment (T1) and K-deficiency treatment (T3) were 12.96% and 15.21% lower than T4 respectively.4. N, P and K significantly influenced physiological characteristics, but the effects of which were various. N main influenced photosynthesis pigments content, and chlorophyll and carotenoid contents of onion under T1 were 9.95% and 6.23% lower than T4 in 17th May. P had great effects on root vitality and activity of nitratase, which under P-deficiency treatment were only 73.33% and 80.67% of T4 in 2th May. K influenced protection enzymes activity and light use efficiency, which both decreased significantly under K-deficiency treatment (T3), and were also lower than T1.
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