Nutritional Diagnosis of the Mineral Elements in Tainong Mango Leaves during Flowering in Karst Areas

The balance of the mineral nutrition in mango leaves during the flowering period affects the flowering of mango trees and fruit production. Because the soil in karst areas has a slow and unbalanced supply rate of nutrients, mango orchards in a karst area generally have a low yield. There are few studies on the fertilization of mango orchards in karst areas, especially on the diagnosis of leaf mineral nutrition. In this study, mango orchards in the typical karst areas of Guangxi province, one of the main mango-producing areas in China, were selected from the low-yielding and medium-yielding mango orchards. Surface soil samples and leaf samples from mango orchards in full bloom were collected to test for macronutrients and micronutrients. The Diagnosis and Recommendation Integrated System (DRIS) graphical method, the DRIS method, the Modified DRIS (M-DRIS), and the Deviation from Optimum Percentage (DOP) index diagnostic methods were applied to the leaves. The results showed that the DRIS graphical analysis yielded appropriate ratios of N, P, K, Mg, S, Fe, Mn, Cu with the corresponding three elements, Ca, Zn, and B, which can be used as reference diagnostic criteria. Based on the values of the DRIS diagnostic criteria for high-yielding orchards, the critical ranges of the suitable values of the mineral nutrients in the Tainong mango leaves during flowering were determined as N (14.87–17.27 g/kg), P (0.69–0.89 g/kg), K (4.45–6.90 g/kg), Ca (9.51–16.55 g/kg), Mg (1.44–2.20 g/kg), S (0.75–1.06 g/kg), Fe (0.10–0.13 g/kg), Mn (0.61–1.02 g/kg), Cu (5.41–8.89 mg/kg), Zn (7.91–18.95 mg/kg), and B (8.38–16.23 mg/kg). The results of the DRIS, M-DRIS, and DOP index methods were analyzed to determine the order of the fertilizer requirements for the low-yielding orchards: Mg > Fe > S > Zn > B > Cu > K > N > P > Mn > Ca, and for the medium-yielding orchards: Mg > Fe > B > Zn > S > Cu > N > Mn > K > P > Ca. The soil and leaf correlation analysis showed that the soil exchangeable Ca and effective Fe were significantly negatively correlated. Leaf Ca and Fe elements had a mutually antagonistic effect, leaf Mn-rich contents inhibited the uptake of the Mg and Fe elements, and low-yielding orchards had an excess of Mn and a deficiency of Mg. We found that there is lack of the Mg and Fe, a low content of the S and B, and an excess of the Ca and Mn in the mango orchards of the Baise karst area. We suggested that the DRIS graphical method is suitable for the diagnosis of three nutrient elements, and either the DRIS or M-DRIS index method can be chosen. The present research can be used for the precise fertilization of mango orchards in karst areas to improve the yield and quality of local mango orchards.