b'IronThe trace element iron is part of various enzymes and ensures the formation of chlorophyl, the respiratory processes and photosynthesis. Iron is immobile in the plant. This means that iron must be supplied continuously. The plant absorbs iron in the form of Fe2 +. The trivalent iron Fe3 + (iron rust) cannot be absorbed directly by the plant, but is converted at the plant root into absorbable Fe2 +.The reason that iron is used in various chelate forms is because iron easily reacts with other (in) organic substances and is therefore no longer soluble and absorbable by the plant. The pH of the dripping water, the composition of the water and the pH of the substrate determine which type of chelate to choose. The most common chelates at a pH between 3 and 6.5 are EDTA and DTPA. As the pH increases, sulphate will bind with iron and precipitate. For crops with recirculation it is important to know how the iron chelate is produced. The 6% DTPA is made with ammonium. 3% DTPA is made with sodium, which is also in the solution. The content of sodium in 3% DTPA can be more than 10%. This sodium accumulates in the system, increasing the need to flush more drain water. All chelates are sensitive to degradation by UV light. This means that entry of UV light into the A-bin must be avoided. Deficiency symptomsA lack of iron can be seen by the yellow discolouration of the youngest leaves that have yet to develop. The leaves can even turn almost white. The markings are sharp with the veins that still remain green. The root volume is greatly reduced.The cause may lie in a low soil temperature, resulting in less active uptake, a too high pH, a too high level of phosphate resulting in iron phosphate or in a fast-growing crop.118'