Why you should use iron chelates in limestone soils

Agriculture has gone hand in hand in recent years with technology. And that is why the current trend in the use of products is to use few doses to achieve good results. Hence the current importance in the XXI century of the use of iron chelates and other essential elements for plants.

Within this «technological line», protecting the iron molecule with chelates (as can also be done in other types of micronutrients), has meant a leap in the world of deficiency correctors, chlorosis and other problems of modern agriculture. 


In a calcareous soil, with high pH and high concentrations of bicarbonates, the mobility of calcium is very low.

This does not mean that there is no presence of iron in the soil, since it is one of the most abundant minerals (3.8%), but rather because it suffers great loss of mobility, insolubilization and low utilization rate.


When we speak of positional isomers in iron chelates , it refers to the structural position or arrangement of the chelating agents protecting the metal ion that we are not interested in losing. For example, iron.

It is only the position it has when it comes to protecting the molecule and, later, it will limit the way the metal ion is released.

A comment that is often heard among people related to agriculture is that ortho-ortho (o, or EDDHA) is better than ortho-para (o, p EDDHA), but it is neither better nor worse, it is different .

The correct use of the iron chelate will depend on the situation of the crop, soil and how the deficiency is presented. We talk about it.

The o, or EDDHA chelate is a type of chelate whose positional isomer is arranged in a more stable manner against o, p EDDHA. This means that the iron molecule will be more protected. Not for this reason, the fact of being more stable is more beneficial for our crops, since it will take longer to degrade and we will not correct a chlorosis as quickly as with an o, p EDDHA.

That is to say, the o, p EDDH configuration is less stable and, therefore, it is more susceptible to being absorbed by the roots quickly (or getting lost in the soil…).

The ideal is a homogeneous mixture between the configuration o, o (ortho-ortho) and or, p (ortho-para), so that we can prevent or correct a chlorosis quickly (o, p) but also future ones (o, o).

With this image we make sure to verify the vital importance of iron for a crop.


A complex is an organic substance that also forms protective structures for metal ions. However, these structures are not as chemically controlled as chelates and are therefore less stable.

The big difference between chelates and complexes is that the latter have a recognized acceptance when they are applied in a foliar way.

Likewise, it no longer makes sense to use an iron chelate (an element that protects the ion against negative agents present in the soil) in a foliar way, since we add complexity to the molecule that we want to introduce into the plant and there are not so many negative variables when we make foliar applications.

Using a complex in a foliar form has its advantages. The fact of being made up of organic components allows easier assimilation through the epidermal route than other non-organic chelates.

In fact, a clear example of the ease of absorption of organic elements is urea (CH4N2O), which is even usually applied with herbicides and other products to increase its systemic levels and at the same time nourish the plant with nitrogen.

This can also be done without the need for a prior degradation of the urea to the ammonia phase and this, in turn, to the nitrate form.

In short, chelates for irrigation application and complexes for foliar application.

Different complexes used in modern agriculture.

  1. Iron citrate (Fe)
  2. Cysteine ​​for protection of zinc (Zn)
  3. Gluconato de Zinc (Zn)


In the market we can find different types of iron chelates, the most used being EDDHA.

However, we can find other sources of chelation such as EDTA, IDHA, HEDTA, DTPA, HBED, etc.

The latest chelates that are coming to the market, a source of different studies and research, can have a pH stability range even higher than EDDHA (above pH 12), so they are very suitable for situations of limestone and soil. pH greater than 8 or 9.

other chelates, for example the 2nd best known on the market, EDTA, have a limited pH range, which makes it more effective for chelating other ions such as zinc and manganese,  as it offers greater stability or pH range than iron.

In the case of EDTA in iron, the pH range is from 4 to 6.5 on average, so it would not be efficient to use it in limestone soil with a pH higher than 7. It would be more focused on foliar applications, hydroponic crops or soils. more acidic.

Another iron chelate , DTPA, has a pH range similar to EDTA.


Although we do not apply iron chelates to the soil, over thousands of years, plants have evolved enough to have the potential to modify the environment from which they cannot move.

Therefore, in situations of iron demand, but since it is in insoluble forms, plants are capable of secreting enzymes and molecules that rescue these insoluble forms of the metal.

They are known as phytosiderophores and act like current iron chelates , modifying the chemical characteristics of trapped iron to solubilize it.

However, this represents an energy expenditure for the plant, as occurs with rhizodeposition .

Therefore, it is not effective to leave the iron obtaining by the plant to free will, especially if it is practically insolubilized and blocked, as occurs in limestone or high pH soils.


Normally, in the case of EDDHA iron chelate, it usually comes in the form of a soluble powder, given its low solubility in water.

Others, such as EDTA chelate, has more solubility and we can see it in liquid concentrations of up to 7.5%. Also as a chelate of other metallic elements such as zinc (EDTA zinc chelate), manganese (EDTA manganese chelate) or even copper.


In this case, as it is powder, the dosage is defined in grams / foot or kg / ha.

Grass:  0.5-1 kg / ha every 4-5 weeks in times of greatest need.

Stone and pip fruit trees: 

  • From plantation to start of production: 5-25 gr / foot and campaign
  • In production, normal size: 20-80 gr / foot and campaign
  • In production, great development: 50-150 gr / foot and campaign


  • Citrus seedlings: 5-15 gr / seedling and season.
  • Start of production: 10-30 gr / foot and campaign.
  • In production: 50-100 gr / foot and campaign.
  • Very developed trees: 50-150 gr / foot and campaign.

Horticultural crops

General dose: 1-5 gr / m2 of surface.

However, for more specialized crops, other calculations are usually made based on the iron contribution in mg / L or ppm.

The calculation is made from Steiner’s micronutrient recommendations, which are as follows:

Microelementos (Steiner):

ppm (mg/L)1,5-20,80,060,150,40,05

Therefore, for iron and iron chelates, which are the protagonists of this article, we would be talking about 1.5-2 ppm of Fe in continuous solution. 

This is very interesting because we ensure that there will always be iron available for the plant, from start to finish. Much better than applying the gr / ft that we talked about earlier at the critical moment of the campaign.

Selecting the value of 1.5 ppm of Fe, more than enough for any high-yield crop, we make the following calculations:

Grams Fe = [1.5 (ppm) / 0.06 (% of Fe richness)] * m3 irrigation water.

Let’s imagine that we have a tank for micronutrients and we want it to last for 100 m3 of irrigation water. The calculations would be like this:

Grams Fe = [1.5 (ppm) / 0.06] * 100 = 2500 grams = 2.5 kg of 6% iron chelate.

From a soil analysis ( learn to decipher a soil analysis ) and knowing its pH, we can select the best chelate, which we know will have a high yield and will not be lost.

For example, for a soil with pH 8.5 in high value horticultural crops (pepper, tomato, cucumber, etc.), we would select EDDHA iron chelate, with sufficient stability to guarantee a continuous supply of iron.

We leave you the option of buying EDDHA iron chelate , in this case with a fairly balanced proportion, 7.5% Fe, of which Chelated Iron [o, o] EDDHA 3.2% and Chelated Iron [op] EDDHA 1.6 %.


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