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Iron Fertilizers (part 2)

Share | 2016-04-18 14:39:19 From:topfert Comment:0 Look:1968 [ L M S ]

Iron can be applied as ferrous sulfate or in a chelated form.

Ferrous sulfate (FeSO4) contains about 20% iron. This fertilizer is inexpensive and is mainly used for foliar spraying. Applied to soil, it is often ineffective, especially in pH above 7.0, because its iron quickly transforms to Fe3+ and precipitates as one of the iron oxides.

Iron chelates. Chelates are compounds that stabilize metal ions (in this case - iron) and protect them from oxidation and precipitation. 

Iron chelates consist of three components:   

Fe3+ ions

A complex, such as EDTA, DTPA, EDDHA, amino acids, humic-fulvic acids, citrate.

* Sodium (Na+) or ammonium (NH4+) ions

Different chelates hold iron ions in different strengths at different pH levels. They also defer in their susceptibility to iron replacement by competitive ions. For example, at high concentrations, calcium or magnesium ions may replace the chelated metal ion.


Fe-EDTA - This iron chelate is stable at pH below 6.0. Above pH of 6.5, nearly 50% of the iron is unavailable. Therefore this chelate is ineffective in alkaline soils. This chelate also has high affinity to calcium, so it is advised not to use it in calcium-rich soils or water. 

Note that EDTA is a very stable chelate of micro-elements, other than iron, even in high pH levels.

Fe-DTPA - this iron chelate is stable in pH levels of up to 7.0, and is not as susceptible to iron replacement by calcium.


Fe-EDDHA - this chelate is stable at pH levels as high as 11.0, but it is also the most expensive iron chelate available.


In soilless media and hydroponics, pH monitoring of water and media is relatively easier than in soils. When regular testing is performed, and pH control is adequate, it is possible to prefer the inexpensive, less stable iron chelates. 


On the other hand, in alkaline soils, where it is difficult to effectively decrease pH levels, it is advised to use more stable iron chelates, such as EDDHA.



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