Effect of slow release iron fertilizers on chlorosis in grape

Abstract

Iron‐EDTA (Fe‐EDTA), ferrous sulfate (FeSO₄), and a combination of both (50:50) were coated with a natural resin material and tested in a concentration of 75 and 100 mg Fe/kg soil, respectively, for three years in pot experiments with the susceptible grape (Vitis vinifera L.) vine rootstock Couderc 3309. In 1990, two locations in the field were fertilized with coated Fe‐EDTA. The application of coated Fe‐EDTA increased the iron (Fe) content in the youngest leaves about 10% and the chlorophyll content about 71% compared to the three year average values of the untreated plants. The most obvious effect was a better growth, which resulted in a higher dry weight and a greater Fe uptake in the leaves and shoots. Organic acids decreased about 11% and chlorosis symptoms, as in untreated plants, did not appear. By itself, FeSO₄ was only applied in the first year, after which chlorosis symptoms occured. The combination of Fe‐EDTA and FeSO₄ resulted in an increase of iron content, dry weight, Fe uptake and chlorophyll content and a slight decrease of organic acids. All data except the Fe content were higher by singly applying coated Fe‐EDTA. At one location of chlorotic plants under field conditions coated Fe‐EDTA (8% Fe), applied in the amount of 15 g and 30 g per plant increased chlorophyll content as well. As no chlorosis occured at the second location chlorophyll and Fe content remained unchanged. It was concluded that coated Fe‐EDTA may serve as an Fe fertilizer if the water supply is sufficient.