Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office. HS1163 Foliar-Applied Micronutrients in Aquaponics: A Guide to Use and Sourcing1 Danielle Treadwell, Sarah Taber, Richard Tyson, and Eric Simonne2 Introduction maintaining sufficient crop micronutrient levels. It also summarizes the different sources of Aquaponics is an emerging type of agricultural micronutrients and their potential suitability for production system combining two production aquaponics systems. systems: aquaculture (fish farming) with hydroponic fruit, vegetable, or herb production. One unique This information should assist growers in challenge in aquaponics is maintaining water pH at evaluating products appropriate for their own values that are suitable for all the organisms in the systems, even in the face of a constantly changing system. The ideal water pH for hydroponic plants is array of product options. Both organic-compliant 5.5–6.5, while the ideal pH for fish and the micronutrient sources (USDA 2002) and accompanying biofilter system—discussed in more conventional micronutrient sources will be discussed. detail later—is between 7.0 and 9.0 (Tyson 2007). Individuals interested in organic production Plants may experience deficiency of some should be aware that the USDA National Organic micronutrients at pH greater than 6.5 because many Program final rule does not have a standard for micronutrients form compounds that do not dissolve greenhouse production, hydroponics, or aquaculture. in water at these pH levels. Biofilter bacteria are Therefore, aquaponic fish cannot be labeled and sold responsible for changing ammonia (NH ), which is 3 as "certified organic" (Diver 2006). However, quite toxic to fish, to relatively harmless nitrate hydroponic produce may be sold as certified organic (NO -). These bacteria can survive at the 5.5–6.5 pH 3 if it meets the organic requirements, and fish can be range, but do not convert ammonia to nitrate as marketed as "naturally grown" or in similar quickly, putting the fish at risk of suffering ammonia green-label categories. For more information on toxicity. certified organic production, visit "Introduction to This publication reviews the special aspects of Organic Crop Production" at water chemistry involved in aquaponics, especially http://edis.ifas.ufl.edu/CV118 and extension's pH, and how water chemistry contributes to "Organic System Plan Overview" at http://www.extension.org/article/20975. 1. This document is HS1163, one of a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date July 2010. Visit the EDIS Web Site at http://edis.ifas.ufl.edu. 2. Danielle Treadwell, assistant professor, Horticultural Sciences; Sarah Taber, graduate student, Plant Pathology; Richard Tyson, Extension agent IV, Seminole County; and Eric Simonne, associate professor and NE district Extension director 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 Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office. Foliar-Applied Micronutrients in Aquaponics: A Guide to Use and Sourcing 2 First Considerations with fish. The tilapia and produce aquaponics system at Micronutrients: pH the University of the Virgin Islands maintains pH by alternating between adding CaOH and KOH (Diver One of aquaponics' top management priorities is 2006). Other strong bases, such as sodium hydroxide the maintenance of correct water chemistry and (NaOH), would maintain correct pH but would be nutrient balance. Hydroponics typically operates at a poor choices for aquaponics because neither plants pH between 5.5 and 6.5 because several nutrients, nor fish take up sodium (Na) in any appreciable including iron (Fe), copper (Cu), zinc (Zn), boron amount, so it can easily accumulate in the water to (B), and manganese (Mn), become unavailable at a toxic levels. higher pH (Timmons and Ebeling 2002; Tyson Phosphoric acid, or H PO , is commonly used in 2007). However, both fish and biofilter bacteria 3 4 perform best at a pH of 7.0 or above. Biofilter aquaponics and aquaculture to adjust pH downward bacteria are required for fish culture because they (R. Tyson, pers. comm.). Other suitable materials for convert the ammonia (NH ) in fish waste, which is adjusting pH may be found by consulting books or 3 toxic to fish, into relatively harmless nitrate (NO -). periodicals on aquaponics, or by communicating with 3 Sizing of biofilters varies depending on stocking rate, other aquaponic growers. The National Sustainable type of substrate used for the biofilter, and amount of Agriculture Information Service's (ATTRA) NH + that is removed by the hydroponic plants. In publication Aquaculture Enterprises: Considerations 4 the case of tilapia, the NH concentration must be and Strategies (Gegner 2006) has a list of both 3 kept below 0.1 mg/L (0.1 ppm) by the combined NH written references and aquaponic growers' 3 removal of the biofilter and hydroponic plants associations in its resources section. (El-Shafai et al. 2004). Although not fatal, tilapia Organic producers must also consider whether growth is slowed at or above this level. acids, bases, and nutrients are allowable under the Tyson (2007) proposed a solution to this USDA National Organic Program final rule. dilemma by maintaining pH between 7.0 and 8.0 for Aquaponics constitutes a gray area under the organic intensively managed systems to increase nitrification, regulations because neither aquaculture nor and by applying foliar nutrients to compensate for the hydroponic produce is specifically addressed in the standards. For example, CaOH, KOH, and H PO are low availability of some nutrients in this pH range. 3 4 The amount of each nutrient to use in a foliar all allowable substances under the organic standards application on aquaponic plants has not been for various situations, but not to adjust pH. In this determined and would probably be different from case, it is best for growers to consult with their system to system. Therefore, the individual growers certifying agencies to find appropriate acids and would need to determine the appropriate amounts of bases for organic hydroponic produce. Fish cannot be micronutrients for foliar application. labeled as "certified organic" under any circumstances. Correct pH is essential to the healthy functioning of an aquaponic system. Therefore, growers should Different Aquaponics Systems Have check their system's pH daily, either with an Different Micronutrient Needs electronic pH meter or with a test strip (J. Rakocy and R. Tyson, pers. comm.). There are many hydroponic system types that can be modified to work with aquaponics; When the pH must be adjusted, growers are reciprocating and constant-flow systems are used limited to acids and bases that are nontoxic to both most often. A reciprocating hydroponics system is fish and plants and pose little or no risk of one in which the hydroponic media are alternately accumulation. For example, calcium hydroxide flooded and then drained on a regular basis, usually (CaOH) and potassium hydroxide (KOH) are good with the plant roots in some sort of granular or choices for raising pH, since Ca (calcium) and K fibrous media (such as sand, vermiculite, perlite, or (potassium) are nutrients required by both plants and coconut husk fiber). A constant-flow system is one in Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office. Foliar-Applied Micronutrients in Aquaponics: A Guide to Use and Sourcing 3 which the roots are always immersed in the nutrient systems and choose amendments that contain few or solution, which flows at a constant rate 24 hours a no ingredients capable of accumulating to toxic levels day. These systems can use some kind of solid for either fish or plants. soilless media, but the roots can also hang freely in the solution. Examples include the raft and nutrient Sources of Micronutrients film systems (where the plants are positioned in The most widely used sources of micronutrients floating foam or cork rafts and their roots are in agriculture are mined inorganic minerals and submerged in the flowing solution) and the trickling synthetic chelates. Chelate comes from the Greek bed system (where plants grow in a solid soilless term for "claw"; chelates are molecules that, when media with solution trickled through it) methods. dissolved in water, can "grab" onto positively Constant-flow systems have a distinct advantage charged ions, many of which are plant micronutrients 2+ 3+ 2+ in aquaponics, where micronutrient concentration is (such as iron [Fe and Fe ], copper [Cu ], 2+ reduced by a high pH (Tyson 2007, Tyson et al. manganese [Mn ], etc.). Chelated micronutrients 2008). In soil or reciprocating hydroponic systems, are still available to plants and prevented from the water and nutrients