Evaluation of soil fertility of major hilly orchards based on principal component analysis and cluster analysis

【Objective】Four fruit species including citrus, peach, waxberry and loquat are the main fruit trees cultivated in Liandu District. Previous surveys showed that most orchards were subjected to inadequate nutrient management practices. High application of chemical fertilizers, low application of organic fertilizers, and low application of medium and trace elements fertilizers led to soil nutrient imbalance, soil acidification and soil sloughing in orchards, which affected the growth of fruit trees, resulting in reduced yields and lower quality. The study analyzed the soils collected from the orchards of peach, citrus, waxberry, and loquat in order to comprehensively assess soil fertility and provide a scientific basis for soil management strategy.【Methods】A total of 58 orchards were surveyed, including 26 peach orchards, 17 citrus orchards, 9 waxberry orchards and 6 loquat orchards located within Liandu District. In total, 75 soil samples were collected from fruit-growing bases, among them, 33 samples were collect-ed from the peach orchards, 18 from the citrus orchards, 16 from the waxberry orchards, and 8 from the loquat orchards. The systematic sampling method was employed to collect 5 to 6 independent samples from the 0-30 cm plough layer near the drip line of the canopy. The sample point coordinates were recorded using GPS, and the orchard's terrain conditions, fertilizer management, and yield were investigated. The soil samples were naturally dried, and stones, plant debris, animal residues were removed before mixing and sieving. The key parameters selected for diagnosing soil nutrient abundance and deficiency included the soil bulk density (BD), soil pH, cation exchange capacity (CEC), total water-soluble salts (TS), soil organic matter (SOM), total nitrogen (TN), alkaline hydrolyzed nitrogen (AN), available phosphorus (AP), available potassium (AK), exchangeable calcium (Ca), exchangeable magnesium (Mg), available sulfur (S), available iron (Fe), available manganese (Mn), available copper (Cu) and available zinc (Zn). The differences in soil fertility among the orchards were conducted by principal component analysis and cluster analysis in this study.【Results】The soil bulk density of the orchards in our study ranged from 1.21 to 1.26 g · cm- 3 . The soil pH of the orchards was below 5.5, indicating a strongly acidic condition. In 66.67% of the citrus orchards and 68.75% of the prune orchards, the soil pH was below 4.5. In 84.85% of the peach orchards, the soil pH was below 5.5. Additionally, the soil pH of all loquat orchards was below 6.0. The cation exchange capacity of the orchards varied from 15.26 to 15.53 cmol·kg-1 . The average concentration of total water-soluble salts across the orchards was below 1 g · kg-1 . The soil salinization was observed in certain areas. The soil organic matter, total nitrogen, alkali- hydrolyzed nitrogen, available phosphorus and available potassium were generally categorized as between levels Ⅲ and Ⅰ. The soil organic matter levels were classified as level Ⅳ in 12.12% of the peach orchards, 11.11% of citrus orchards, and 6.25% of waxberry orchards. The total nitrogen levels in 3.03% of the peach orchards were classified as level Ⅴ. The alkali-hydrolyzed nitrogen was classified as level Ⅵ in 5.56% of the citrus orchards, and level Ⅳ in 6.06% of the peach orchards and 5.56% of the citrus orchards. The available phosphorus level was categorized as level Ⅳ in 6.06% of the peach orchards, level Ⅳ in 12.06% of the peach orchards. In 12.5% of the waxberry orchards, the available potassium was classified as level Ⅳ, whereas 12.12% of peach orchards ranged between level Ⅳ and Ⅴ. The levels of the exchangeable calcium, magnesium, available sulfur, iron, manganese, copper and zinc were sufficient for fruit tree growth in all orchards. However, some of the loquat orchards exhibited deficiencies in manganese and copper. The overall soil fertility ranking was as follows: loquat orchard ＞ citrus orchard ＞ peach orchard ＞ waxberry orchard, based on the principal component composite scores. The results from the cluster analysis indicated the combined soil fertility scores in the following order: loquat orchard and peach orchard ＞ waxberry orchard ＞ citrus orchard. The soil fertility quality of both loquat and peach orchards was classified as first class, whereas waxberry orchard was classified as second class and citrus orchard as third class.【Conclusion】Our study demonstrated that the levels of soil nutrients was typically high in the loquat orchards , and low in the citrus orchards . Overall, the orchards soil was characterized by high acidity, and salinization in some specific areas. The soil organic matter, total nitrogen, alkaline hydrolyzed nitrogen, available phosphorus and available potassium were abundant, however, the medium and trace elements were deficient in some orchards soil. It would be recommended to increase the application of organic fertilizers and medium and trace elements fertilizers, reduce the application of acidic chemical fertilizers, and apply quicklime or soil conditioners to neutralize soil acidity in the future.