Key minerals influencing apple quality in Chinese orchard identified by nutritional diagnosis of leaf and soil analysis

We investigated the correlation between leaf/soil minerals and fruit quality in apple trees grown in orchards, with the ultimate goal of improving the latter. Leaf mineral nutrients; soil nutrients in the 0–20, 20–40, and 40–60 cm layers; and fruit quality traits in 32 apple orchards in China were monitored for 2 years. Significant factors associated with fruit quality were identified via correlation analysis. An analysis of leaf data revealed that leaf nitrogen (N) and leaf magnesium (Mg) levels were extremely high in 75 and 89% of the orchards, respectively. In the Bohai Gulf region, 94% of the orchards showed significantly higher values than the standard. The soil pH values of the orchards in eastern China like eastern Shandong or Liaoning were lower than 7.0, while the pH values in the Loess Plateau of northwestern China like Shaanxi were much higher than 7. Soil alkali-hydrolyzable N levels in 47% of the orchards were lower than the optimal level of 70 mg kg–1. Generally, the soil alkali-hydrolyzable N levels of orchards in the Bohai Gulf region were significantly higher than those in the Loess Plateau region. The available P levels in the orchards of the Bohai Gulf region were up to three times higher than those of the Loess Plateau region. However, although the available potassium (K) in most orchards was sufficient (51.39–309.94 mg kg–1), leaf K content in 73% of the orchards was low, possibly due to fruit bagging or fruit overload. Approximately 63% of the orchards in Shandong and 29% of the orchards in Shannxi showed leaf Fe deficiencies. In the Loess Plateau, most orchards showed high leaf Ca levels, a strong correlation was observed between leaf and soil phosphorus/potassium (P/K) content and fruit organic acid content. The amounts of fruit soluble sugar or fructose were positively correlated with soil calcium/potassium (Ca/K) levels and leaf calcium/boron (Ca/B) levels in most orchards. The excessive leaf N levels caused by the extensive application of N fertilizers had a negative effect on fruit quality in most apple orchards in China. P, K, Ca, and B were key minerals associated with fruit quality.