Investigation of soil nutrient status and the correlation between leaf nutrient and yield of navel orange in Fengjie county

Abstract: 【Objective】The Three Gorges Reservoir area is the suitable region for citrus cultivation, which is the core area of the citrus belt in the upper and middle stream of the Yangtze River planned by the Ministry of Agriculture and the quarantine disease free area of citrus in China. The nutrient levels in the soil and in the leaves of navel orange trees in Fengjie orchards were studied to explore the impacts of soil and leaf nutrients on fruit yield. The results will provide reference for proper fertilization in navel orange production in Fengjie.【Methods】Samples of soils and citrus leaves of 148 orchards were collected from 10 main producing townships in Fengjie during the fruit ripening stage. Most of the collected samples were from‘Fengjie 72-1'trees (Citrus sinensis Osbeck) grafted on red tangerine (Citrus reticulata Blanco) . Soil samples were collected from sampling points beneath 15-20 trees arranged in an S pattern in each orchard with 100 g of soil sample at depths of 0-40 cm per point located at the two opposite sides of each selected tree and 10 cm in-ward from the drip line. 120 leaves were taken from positions evenly distributed in the canopy of each tree. The leaves were sampled at the 2 nd or 3 rd node from the top of the spring vegetative shoots of current season. At the same time, and the fruit yield data in the year were collected. The pH, organic matter and the available nutrient contents in the soil and the nutrients in the leaves and their correlations with fruit yield were analyzed.【Results】The results showed that the pH value of the soils in‘Fengjie navel orange'orchards varied greatly, and the average value was 7.12. The soils were relatively rich in organic matter. The percentage of soil samples with organic matter exceeding 15 g · kg-1 was 85.81%. With an average content of 78.71 mg · kg-1, deficiency of alkali-hydrolyzable nitrogen in the soils the orchards was very serious. However, available P and K were relatively abundant, with an average value of 37.02 mg · kg-1 and 191.67 mg · kg-1, respectively. The spatial distribution of the two nutrients was more uneven than that of alkaline-hydrolyzable nitrogen, and the coefficient of the variation was 73.24% and 93.78% for available P and K, respectively. Available Ca was extremely rich, and non of the orchards had available Ca below the appropriate value, while available Mg was extremely deficient, and all the orchards were in the shortage of available Mg.Available Fe, Mn, Zn and Cu were relatively abundant, and their average contents were 40.72 mg · kg-1, 24.04 mg · kg-1, 2.69 mg · kg-1 and 2.17 mg · kg-1, respectively. The available B was relatively deficient, with an average value of 0.67 mg·kg-1. N, P, K, Ca, Fe, Mn and Cu in the leaves were abundant, with average values of 29.45 g · kg-1, 1.50 g · kg-1, 13.32 g · kg-1, 41.23 g · kg-1, 133.17 mg · kg-1, 42.97 mg · kg-1 and9.51 mg · kg-1, respectively. However, Mg and B in leaves were relatively deficient, with an average value of 3.90 g · kg-1 and 49.76 mg · kg-1, respectively. Zn, with an average of 16.65 mg · kg-1, was extremely deficient. There were great differences in nutrient elements in samples from different towns. Leaf P, K and Zn levels were significantly correlated with fruit yield, with the correlation coefficients of-0.332, -0.205 and-0.213, respectively.【Conclusion】Most of the soils in‘Fengjie navel orange'orchards were calcareous with an alkaline pH, which was much higher than the optimum range of 5.5-6.5 for citrus growth. It is necessary to improve soil physicochemical properties by applying more acidic fertilizers and organic fertilizer, which help to increase root activity and promote nutrient absorption and thus increase productivity. Besides, there was no positive correlation between the element contents in the soil and in the leaves. Because the level of leaf nutrient can directly reflect the nutrient status of trees, fertilization should be based on the levels of leaf nutrients, and soil nutrient status can be used as a secondary index. The soil management should focus on adjusting the pH, increasing soil organic matter, rationally applying nitrogen, phosphorus and potassium fertilizers, and increasing magnesium, zinc and boron fertilizers in the navel orange orchards in Fengjie.