SL353 Iron (Fe) Nutrition of Plants 1 George Hochmuth 2 1. This document is SL353, one of a series of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date, August 2011. Visit the EDIS website at http://edis.ifas.ufl.edu 2. George Hochmuth, professor, Department of Soil and Water Science; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611. The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A&M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean Introduction Iron is one of 16 essential elements for plant growth and reproduction (some scientists also consider nickel to be essential, making 17 in total). Iron (Fe) is one of the most abundant elements on the planet. In 1844, Eusebe Gris showed that certain chlorosis in plants could be reversed by treating roots and leaves with iron solutions. Iron is a micronutrient and is required by plants in small amounts. Most annual plants have a requirement for Fe on the order of 1 to 1.5 lb Fe per acre, compared with nitrogen (N) at 80 to 200 lb per acre. is publication provides information on plant nutrition and soil fertility for agricultural and urban plant production and management practitioners. e information should provide a detailed basic understanding of soil science and plant physiology for diagnosing and correcting Fe problems in plants and soils. e audience for this publication includes county agents, crop consultants, environmental managers, and fertilizer and lawn care specialists. Forms of Fe in the Natural Environment e most abundant form of Fe in soils is ferric oxide (Fe 2 O 3 ) or hematite, which is extremely insoluble and imparts a red color to the soil. e oxide form is commonly hydrated. In aerobic soils, the oxide, hydroxide, and phosphate forms control the concentration of Fe in solu- tion and its availability to plants. In typical aerated plant production systems of acceptable reaction (pH) of +/- 6.0, the concentrations of ferric (Fe +++ ) and ferrous Fe ++ iron are on the order of 10 -15 molar (very low concentration). As pH increases by one unit, activity of Fe +++ decreases by 1000-fold due to the formation of insoluble Fe +++ hydroxide. Under reducing conditions—addition of H + or other reductants—Fe solubility increases. Under such situations, Fe can be adsorbed on soil as an exchangeable ion. In certain soil situations, carbonate or sulfide compounds may form with Fe. Commonly in waterlogged situations, ferric iron is reduced to the ferrous state. If sulfates also are abundant in the soil, these become oxygen sources for bacteria and black-colored ferrous sulfide is formed Where organic matter is present in soils, Fe may be present in its reduced state as Fe ++ in the soil solution or adsorbed onto soil particle surfaces. Organic matter in soils plays a major role in the availability of Fe to plants. Biochemical compounds or organic acids (aliphatic acids or amino acids) and complex polymers (humic and fulvic acids) can form soluble complexes with Fe, or act as chelating agents and thereby increase Fe availability to plants (chelating agents are organic compounds that complex with Fe and help hold Fe in more soluble forms). Function of Fe in the Plant Although required by plants in small amounts, Fe is involved in many important compounds and physiological processes in plants. Iron is involved in the manufacturing process of chlorophyll, and it is required for certain enzyme functions. Fe’s involvement in chlorophyll synthesis is the reason for the chlorosis (yellowing) associated with Fe Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office.