Objectives Balanced supply of mineral elements is critical to ensure high quality and yield of citrus. Combined application of nitrogen (N), phosphorus (P) and potassium (K) rationally is significant in improving the citrus yield and quality.

Methods The ‘Newhall’ navel orange (Citrus sinensis Osbeck) used in this study was trifoliate rootstock, the orange trees were 18–20 years old. The field experiment was conducted in Zigui County, Hubei Province since 2007. P and K fertilizer treatments were designed with “3414” pattern, as PK₂, P₁K₂, P₂K₂, P₃K₂, P₂K₀, P₂K₁ and P₂K₃. The respective four P and K rates included P₂O₅ 0, 0.20, 0.40, 0.60 kg/tree and K₂O 0, 0.25, 0.50, 0.75 kg/tree each year, along with N 0.80 kg/tree each year. The fruit yield was investigated and the fruit quality was analyzed in 2012 .

Results 1) The fruit yield in treatment P₂K₂ was 38.34 kg/tree, significantly higher by 44.6% than that in treatment P₀K₂ (P < 0.05). The total soluble solid, Vc, glucose and sucrose contents of citrus fruit in P₂K₂ treatment were significantly increased by 3.7%, 14.6%, 20.8% and 12.5%, respectively, over the treatment of P₀K₂. P fertilizer application did not significantly affect fruit fructose contents. In P₁K₂, P₂K₂ and P₃K₂, the titratable acid and citric acid contents of citrus were lower than P₀K₂. Compared to P₀K₂, the titratable acid contents of P₁K₂ and P₃K₂ were significantly decreased by were 18.8% and 14.7%, respectively, while the citric acid contents of P₂K₂ and P₃K₂ significantly decreased by 18.8% and 14.7%. 2) The fruit yield of P₂K₃ was significantly increased by 24.6%, compared to P₂K₀. As the application rate of K fertilizer increased, the total soluble solid and Vc contents increased initially and then decreased. P₂K₂ treatment resulted in the highest total soluble solid and Vc contents, which were 13.1% and 20.3% higher than those of P₂K₀, respectively. Within the tested K levels, application of K fertilizer could increase the content of soluble sugar content in fruit. Fructose, glucose and sucrose contents in P₂K₂ were significantly increased by 23.8%, 33.2% and 25.1%, respectively, compared to P₂K₀. In P₂K₁ treatment, the fruit titratable acid and citric acid content of citrus were decreased compared to P₂K₀, while titratable acid of P₂K₂ treatment increased by 21.8% compared to P₂K₀. There was no significant difference in fruit citric acid contents.

Conclusions The optimum P and K application rates were P₂O₅ 0.41–0.46 kg/(tree·a) and K₂O 0.45–0.50 kg/(tree·a) along with N 0.80 kg/(tree·a) with N∶P₂O₅∶K₂O ratio of 1∶0.51–0.58∶0.56–0.63 for high yield and high quality of ‘Newhall’ navel orange.