Objective The aims were to establish soil nutrient abundance and deficiency indices of sorghum - rape rotation in Chongqing, to determine soil nutrient classification and recommended fertilizer amount, and to provide scientific basis for rational fertilization of sorghum - rape rotation in Chongqing.

Method The "3414" field fertilizer efficiency experiment was carried out for two years to establish a function model between soil available nutrients and the relative yield and the optimal fertilizer amount of nitrogen, phosphorus and potassium in Chongqing sorghum-rape rotation area, and to determine the soil nutrient abundance and deficiency indices and the recommended fertilizer amount in different grading ranges. At the same time, validation tests were conducted on three different fertility plots to verify the rationality of the index system.

Result According to the relative yield of crops < 60% (low), 60% - 75% (low), 75% - 85% (medium), 85% - 95% (high) and ≥ 95% (high), the corresponding soil available nutrients were divided into 5 grades successively from low to high. The nutrient abundance and deficiency indices of sorghum were divided into alkaline hydrolyzed nitrogen < 105, 105 - 176, 176 - 225, 225 - 315 and ≥ 315 mg kg⁻¹. Available phosphorus < 2, 2 - 5, 5 - 10, 10 - 22 and ≥ 22 mg kg⁻¹; Available potassium < 10, 10 - 41, 41 - 109, 109 - 210 and ≥ 210 mg kg⁻¹. Soil nutrient abundance and deficiency indices in rape season were divided into alkaline hydrolyzed nitrogen < 63, 63 ~ 80, 80 ~ 94, 94 ~ 110 and ≥ 110 mg kg⁻¹. Available phosphorus < 2, 2 - 8, 8 - 21, 21 - 53 and ≥ 53 mg kg⁻¹; Available potassium < 18, 18 - 56, 56 - 120, 120 - 250 and ≥ 250 mg kg⁻¹. The test results showed that the amount of nitrogen applied by formula fertilization decreased by 33.33% compared with farmers' customary fertilization, the average yield of sorghum increased by 10.42%, and the average harvest index increased by 2.71%. The average increase of rape season was 9.36%, and the average increase of harvest index was 9.76%.

Conclusion According to the soil fertility grade from low to high, the corresponding fertilization amounts of sorghum were N ≥ 210, 165 - 210, 150 - 165, 120 - 165 and 0 - 120 kg hm⁻², respectively. P₂O₅ ≥ 198, 160 - 198, 124 - 160, 90 - 124 and 0 - 90 kg hm⁻²; K₂O ≥ 290, 224 - 290, 178 - 224, 148 - 178 and 0 - 178 kg hm⁻²; The recommended fertilizer amounts for rapeseed were N ≥ 165, 135 - 165, 120 - 135, 90 - 120 and 0 - 90 kg hm⁻², respectively. P₂O₅ ≥ 140, 90 - 140, 53 - 90, 18 - 53 and 0 - 18 kg hm⁻²; K₂O ≥ 100, 75 - 100, 59 - 75, 43 - 59 and 0 - 43 kg hm⁻². By establishing the sorghum - rape rotation fertilization index system in Chongqing, the yield and harvest index of sorghum - rape rotation were increased, the input of nitrogen fertilizer was reduced, and the theoretical basis for rational fertilization in southwest China was provided.