What is the iron source in soilless cultivation?

In the early stage of the development of soilless cultivation, the iron source was the use of inorganic salts such as ferrous sulfate and ferric chloride. Ferric chloride has a molecular weight of 270.30 and contains 20.66% iron. It is yellowish brown or orange with a slight smell of hydrochloric acid. It is very easy to dissolve in air and soluble in water. Ferrous sulfate, commonly known as green alum, can provide iron and sulfur, contains 20.09% iron, is a blue-green crystal, is unstable, easy to lose water and oxidizes to brown ferric sulfate, and is even more unstable under high temperature and strong light or in the presence of alkaline substances. As ferrous sulfate is a by-product of some industries, with a wide range of sources and low price, it is not only an important iron source for soilless crops, but also a raw material for making other iron fertilizers such as ammonium ferrous sulfate and iron chelate. Because inorganic iron salts such as ferric chloride and ferrous sulfate are easy to become ferric phosphate or ferric hydroxide precipitation when the alkalinity is high, and even in neutral conditions, ferrous sulfate can be oxidized into basic salts, while low-valent ferrous sulfate is easy to be oxidized to high-valent iron by oxygen in the air, which often results in iron deficiency in plants, and later iron salts in cultivation are changed to organic iron acids, such as iron citrate and iron salts. Although these compounds are more effective than ferric chloride and ferrous sulfate, they are very unstable, so their effect is not ideal. with the development of modern chemical science, it is clear that the organic compounds which can form chelating ring interact with iron or chelate iron. This kind of chelated iron is used as iron source in soilless culture nutrient solution, so now it is used instead of inorganic iron salt and organic acid iron. Iron chelates are light brown or dark brown powdery substances, which can form stable compounds in soilless culture nutrient solution and maintain their effectiveness for a long time. The chelates of iron mainly include disodium iron ethylenediamine tetraacetate, disodium iron diethylenetriamine pentaacetic acid and so on. Because iron disodium ethylenediamine tetraacetate is cheap and stable, disodium iron ethylenediamine tetraacetate is the most commonly used at present. In preparation, ferrous sulfate and disodium iron ethylenediamine tetraacetate are dissolved respectively before they are mixed and chelated.