15 14 WE have emphasised in previous DownUnder editions that plants do not eat, they drink, hence all the mineral nutrition supplied to plants, whether in soil or soilless cropping, must be dissolved in water. Atoms and molecules dissolved in solution are electrically charged and called “ions”. Such ions are either positively charged (“cations”), eg, potassium calcium, ammonium, magnesium, iron, manganese, zinc and copper, or negatively charged (“anions”), eg, nitrate, phosphate, sulphate, molybdenum and boron. The charge of each ion is known as its valency. So, for example, calcium has valency +2, written as Ca2+, and nitrate has valency -1, written as NO3-. Because ions of opposite valency attract, bonds are constantly being formed between positive and negative ions, sometimes in the nutrient solutions and sometimes in the irrigation system and media. Once these bonds are formed, the combined molecules are no longer soluble and, hence, are no longer suitable for plant nutrition. Some of these combined molecules are very stable, will not supply nutrients to the target plants and can be difficult to remove from irrigation equipment. Chelates The minerals iron, manganese, zinc and copper can be supplied in hydroponics as sulphates, but, depending on pH, these minerals can precipitate with phosphates, carbonates or hydroxides in the nutrient solution or media and become unavailable to plants. Chelating these minerals protects them from precipitation, allowing them to be available to plants. The term “chelate” comes from the Greek word “chela”, meaning claw of an animal like a crab or scorpion. It is an apt description of the chelating agent, which is the organic molecule used to “envelop” or “bind” a target metal ion, protecting it from reaction with negatively charged ions in the nutrient solution and/or media. Chelates are usually organic molecules with long names like “diethylenetriaminepentaacetic acid” (DTPA) and so are normally known by their acronyms. Some chelates, such as EDDHA and HBED, exist as complex molecules with different configurations, or bonds, between the component atoms, giving them different spatial shapes. In the case of iron FeEDDHA, there are two configurations, known as “isomers” (Figure 1). Each isomer has different bonding strength with the iron it contains. The most effective chelating agent for alkaline soils is the ortho-ortho configuration. It is the most stable. For hydroponic use, where pH in the nutrient solution and media is managed, the ortho-para configuration works. For Fe-EDDHA chelate products, the higher the proportion of the ortho-ortho isomer in the product, the higher the stability of the metal-chelate. Once the metal-chelate bond is broken, the unprotected metal ion is liable to precipitate after reaction with anions in the nutrient solution or media. Chelating agents exist in nature. Certain plants, bacteria and fungi can produce natural chelating agents, called “siderophores”, in bacteria and fungi and “phytosiderophores” in plants. Organisms produce these compounds to absorb iron from their substrates. Soilless nutrition In soilless nutrient fertigation, the main cations, such as calcium, and anions, such as phosphate, are separated in twin tank hydroponic systems to minimise reactions that would potentially render the minerals in the nutrient solution unavailable. It is common practice to use chelated iron products in hydroponic nutrient solutions. The stability of iron in such solutions is largely dependent on pH, but in soiless cropping Chelates By Peter Anderson Northern Sales Agronomist, Haifa Australia also on the presence of other cations, notably unchelated calcium, magnesium, manganese, zinc and copper. While pH of the soilless nutrient solution is managed in hydroponics, it is true that many plants prefer pH of 6 to 6.5. At this pH, the stability of Fe-EDTA in the nutrient solution and the root zone is reduced (Figure 2). Non-iron EDTA chelated microelements are not a problem at a wide pH range (Figure 3). For Fe in hydroponic nutrient solutions, Fe-DTPA or Fe-EDDHA are a better choice than EDTA for Fe chelates, particularly if the pH of the nutrient solution and/or the root zone is above 6.5. EDTA is a suitable for Mn, Zn and Cu. Figure 4 shows that the Iron in the Fe-DTPA chelate is replaced by the unchelated copper, zinc and manganese in that order. This Fe is subsequently lost to precipitation. There is no Fe replacement in the Fe-EDDHA. If the non-Fe minerals were already chelated, they would not replace the Fe in the Fe-DTPA (or Fe-EDTA). If a proportion of the total-Fe is in the form of EDDHA, then this helps compensate for the loss of Fe from the Fe-DTPA (or Fe-EDTA), even if using unchelated Mn, Zn and Cu. Fig 2: pH Stability of iron (Fe3+) chelates in practical conditions Fig 3: Stabilities of non-iron EDTA chelates in practical conditions Fe-EDDHA and Fe-HBED have a practical lower stability limit of around pH 3.5. These are normally dissolved in the A-tank, so the pH of the A-tank needs to be above pH 3.5. Experiments in Europe with greenhouse tomato demonstrated that in a nutrient solution using Fe-DTPA, chelating the microelements Mn, Zn and Cu with EDHA delivered retention of more Fe in the solution and more again in the media (rockwool in this case) than using Mn, Zn and Cu sulphates. Plots combining Fe-DTPA with chelated Mn, Zn and Cu had leaves with significantly higher chlorophyl content when compared to plots with Fe-DTPA and unchelated Mn, Zn and Cu. Plots with greater retention of Fe had significantly higher yield, mainly through larger fruit. In conclusion, chelating Fe in hydroponic nutrient solutions ensures that iron is available for the crop. Not all Fe chelates are stable at slightly acid-neutral pH, and the presence of non-chelated Mn, Zn and Cu can replace Fe in some Fe-chelates, resulting in loss of this Fe. Chelates acidity akalinity Fe-HBED Fe-EDDHA Fe-DTPA Fe-HEDTA Fe-EDTA pH 1234567891011 Chelates acidity akalinity Mn-EDTA Zn-EDTA Cu-EDTA Ca-EDTA Mg-EDTA pH 1234567891011 Ortho-Ortho Soil and Hydroponic application Ortho-Para Hydroponic application Para-Para No application Source: Nouryon 4 5 6 7 8 100 80 60 40 20 0 Fe-DTPA Fe-EDDHA Cu-DTPA Zn-DTPA Mn-DTPA Relative to [metal] (%) pH Fig 4: Fe-DTPA (85% of total Fe), Fe-EDDHA (15% of total Fe) and unchelated Cu, Mn and Zn Source: Nouryon Source: Nouryon Fig 1: Isomers of EDDHA Click for further info Micronutrients
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