To compare the effects of various zinc (Zn) foliar fertilizers on correcting citrus Zn deficiency and to explore an effective correcting method, three common Zn fertilizers, Zn sulfate heptahydrate (ZnSO 4 .7H 2 O), Zn chloride (ZnCl 2 ), and Zn nitrate hexahydrate [Zn(NO 3 ) 2 .6H 2 O], were selected to spray the Zn-deficient citrus leaves, tested at different concentrations, with or without organosilicone surfactant. Zn content, chlorophyll levels, and photosynthesis characteristics of leaves were analyzed. Leaf Zn content was significantly increased with increase of the sprayed Zn concentration of the three Zn fertilizers. However, when the sprayed Zn concentration of ZnSO 4 .7H 2 O exceeded 200 mg·L −1 , and Zn concentration of ZnCl 2 or Zn(NO 3 ) 2 .6H 2 O exceeded 100 mg·L −1 , obvious necrotic spots formed on leaves. This necrosis disappeared when 0.025% organosilicone was added to the three Zn fertilizer solutions, even at a Zn concentration of 250 mg·L −1 . Meanwhile, the Zn contents of leaves increased one to four times for these treatments. Furthermore, foliar application of the three Zn fertilizers significantly improved chlorophyll levels and photosynthetic capacity of Zn-deficient leaves. The data of chlorophyll and photosynthesis characteristics indicate that the correcting effect of ZnCl 2 and Zn(NO 3 ) 2 .6H 2 O is better than that of ZnSO 4 .7H 2 O, and could be further improved via supplement of organosilicone. In conclusion, ZnCl 2 or Zn(NO 3 ) 2 .6H 2 O containing 250 mg·L −1 of Zn and supplemented with 0.025% organosilicone is a safe and effective formulation of Zn foliar fertilizer for correcting citrus Zn deficiency.
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Fu et al. (2016) studied this question.