Complexing Agents

Context:

Iron, manganese, zinc and copper ions are rapidly oxidized in oxygen-enriched water, thereby reducing their absorption capacity for plants. This is particularly important for iron, the deficiency of which can be a cause of chlorosis (yellowing of the leaves). Therefore, in order to keep such metal compounds, which would otherwise flake out due to oxygen oxidation or pH (as hydroxides), fixedly bound in solution, chelating agents or their compounds with, for example, iron, manganese, copper or zinc are added.

Examples of complexing agents:

• EDTA
• DTPA
• HEDTA/HEEDTA [7] (HydroxyEthylEthyleneDiamineTriaceticAcid)
• HBED (hexadentate phenolic aminocarboxylate = N,N-bis (2-hydroxybenzyl) ethylenediamineN, N-diacetic acid)
• Fulvic acids
• Phytic acid
• Citric acid or citrates
• Tartrates
• Humic acids
  and other chelate complexing agents

Some of the iron chelate complexes of the above-mentioned complexing agents are stable or unstable depending on the pH value, therefore the pH value of the finished nutrient solution or soil is essential for the iron absorption capacity of the plants. 

For plant uptake of the (trace) elements iron, copper, manganese, boron and zinc, a pH range between pH 5 and 6 is the best [7], EDTA is more commonly added to fertilizers for soil substrates, it has high affinity (will to bind) to calcium (and then keeps calcium in solution).

DTPA became standard for hydroponic fertilizers in Europe, and ecotoxicity is largely unexplored. Soluble humic substances occur naturally in humus soils, peat, and lignite. They reduce the toxicity of iron by keeping the iron bound as an iron(II) complex, but this also reduces the bioavailability of the iron
 

Inorganic fertilizers

Every aqueous hydroponic fertilizer is a complete fertilizer in which all of the nutrients mentioned are artificially added. Therefore, different formulas and approaches have been developed since the 1950s. 

„Most plants grow optimally with a certain ion concentration ratio!“

Typically, for multi-component fertilizers, the ratio of the core nutrients nitrogen (N), phosphorus (P) and potassium (K) in % of the commercially available reference base is given as „NPK value“.