Optimization of the Phosphorus and Potassium Ratio in Tobacco Special Fertilizer in Longyan

To determine the appropriate ratio of phosphorus and potassium application for tobacco soils with high available phosphorus content in the Longyan tobacco area, a field experiment was conducted including 3 levels of phosphorus fertilizer (P9, P7, P5, indicating P₂O₅ application rate of 7.65, 5.95, 4.25 kg/666.7 m², respectively) and 3 levels of potassium fertilizer (K21, K18, K15, indicating K₂O application rate of 17.85, 15.30, 12.75 kg/666.7 m², respectively). The nitrogen fertilizer was applied at 8.5 kg/666.7 m² in all treatments, and P9K21 was treated as the control. The results showed that P7K18 and P7K21 had better comprehensive agronomy traits, of which the plant height were significantly higher than that of P9K18, P7K15, P5K15 treatment, and the effective leaf number of P7K21 was significantly higher than that of P9K18, P5K18, P5K15. There was no significant difference in the average price and the proportion of middle and upper grade tobacco among all treatments, but the yield and output value of P7K18 and P7K21 were the highest. Furthermore, the output value of P9 level was significantly lower and the upper grade tobacco proportion of P5 level was relatively higher among three phosphorus fertilizer treatments. The total nitrogen content and nitrogen–alkaloid ratio were relatively high in lower tobacco leaves, and the content of total sugar and total potassium were appropriate in the upper, middle and lower tobacco leaves. For P7K18 treatment, the nicotine, total sugar and reducing sugar contents of the upper leaves were in the suitable range, and the sugar-alkaloid ratio and nitrogen-alkaloid ratio were well coordinated. In conclusion, as the ratio of N, P₂O₅, and K₂O in tobacco special fertilizer was adjusted from 10:9:21 to 10:7:18, indicating P₂O₅ application reduced from 7.65 kg/666.7 m² to 5.95 kg/666.7 m², and K₂O application reduced from 17.85 kg/666.7 m² to 15.30 kg/666.7 m²with the nitrogen fertilizer applied at 8.5 kg/666.7 m², the tobacco in Longyan area could get better tobacco plant growth, more harmonious chemical composition and the optimal economic character.