Effect of Foliarly Applied Acids and Ferrous Sulfate on Leaf Ferrous Iron Content and Leaf Greenness of Lychee Trees

Iron deficiency is a major nutritional problem of lychee trees ( Litchi chinensis Sonn.) grown in calcareous soils. Applica­tions of chelated iron (Fe) to calcareous soil are efficacious but very expensive. The effects of foliarly applied organic acids, organic acids plus ferrous sulphate (FS), or a chelated Fe (EDDHA) soil drench on Fe nutrition of 18-month-old ‘Mauritius’ lychee trees in containers and 8-year-old ‘Kaimana’ lychee trees in calcareous soil in an orchard in southern Florida were investigated during 2006. All foliar treatments included the organosilicone adjuvant Freeway ® . Trees in containers were sprayed at 2-week intervals six times with one of the following foliar treatments: ascorbic acid alone (AA), citric acid (CA), sulphuric acid (SA), ascorbic acid plus FS (AA+Fe), citric acid plus FS (CA+Fe), or sulphuric acid plus FS (SA+Fe). Additional treatments were application of chelated Fe (EDDHA) as a soil drench applied once at the beginning of the treatment period or trees receiving no Fe (Control). Trees in the orchard were sprayed four times at 14- to 18-day intervals with one of the following treatments: AA, AA+Fe, CA+Fe or SA+Fe or soil drenched two times with chelated Fe (EDDHA) or the control. Leaf chlorophyll indices were determined with a SPAD meter. For trees in containers, SPAD, and leaf total and ferrous Fe content were significantly higher in trees receiving all foliar acid+Fe and EDDHA treatments compared to trees in the acid alone and control treatments. Similarly, SPAD values and leaf total and ferrous Fe content of leaves of orchard trees were significantly higher for all foliar acid+Fe treated leaves compared to EDDHA, AA, or control treatments. Economic analysis indicated that foliar applications of acid+Fe were 51% to 75% less costly than soil applications of EDDHA.