Chelation and Mineral Nutrition
CHELATION is a natural process.  In order to prevent absorbed nutrients from precipitation resulting from the interaction of nutrients, such as iron forming precipitation with phosphorus, upon entering plant cells cationic nutrients will immediately form chelates with ORGANIC ACIDS such as citric acids, malonic cid, and some amino acids.  This chelation process will then enable the nutrients to move freely inside the lants.
 
CHELATION in soil increases nutrient availability to plants.  Organic substances in the soil either applied or produced by plants or microorganisms are the natural chelating agents. The most important substances having this nature are Hydroxamate Siderophores, Organic Acids and Amino Acids.
Hydroxamate Siderophores are naturally produced by soil microorganisms and are  essential in natural ecosystems to solubilize and transport nutrients, especially iron to plant roots. Under Iron deficient I conditions, microorganisms will produce siderophores to overcome the iron starvation.  Neilands and co-workers at the University of California found that Rhizobium meloti was able to correct the iron starvation using this mechanism. Neilands, Cline and co-workers of Colorado State University reported the abilities and mechanisms by which sunflower and sorghum acquire iron supplied as a ferrated hydroxamate siderophore.  Research on oats by Read and co-workers of Colorado State and the University of Texas found that the absorption of iron from ferrichrome was nearly two orders of magnitude greater than that from the EDDHA treatment when there was excess supply of the ligand.  Their results indicated that iron uptake by monocots may be more efficient from naturally occurring chelates than from synthetic chelates.
Organic acids and amino acids such as citric acid and glycine  are also naturally occurring chelating agents.  Glycine is the simplest amino acid with a molecular weight of 75.  Chelates of glycine with cations such as iron, zinc, and copper have been fully studied.  The chelates usually contain 2 moles of ligand (glycine) and one mole of metal as demonstrated in the following figure.
Research conducted in USSR by Tronov and co-workers indicated that glycinates greatly stimulate the growth of plants.  Their results showed that zinc glycinate (zinc glycine chelate) increased the total, stem, root, and foliage weights by 194, 215, 254 and 147%, respectively.  Respective effects of manganese glycinate (manganese glycine chelate) were 79, 108, 110, and 15%.
Citric acid is one of the organic acids commonly used as chelating agents. Other naturally occurring organic acids such as malonic acid and gluconic acid also play an important role in plant mineral nutrition.
 

Chelating agents as well as minerals contribute to the stability of chelates. A strong chelating agent may bond the mineral too strongly and make itunavailable to plants. On the other hand, a weak chelating agent may not be able to protect the chelated minerals from chemical reactions with other compounds and thereby reduce their availability to plants. Combination of chelating agents can improve product stability and broaden product effectiveness.

 
WHAT IS A CHELATE?
The word chelate derives from the Greek word “chel”, meaning a crab’s claw, and refers to the pincer-like manner in which the metal is bound.  Chemically, a chelate is a compound from complexing of cations with organic compounds resulting in a ring structure.  Typical structure of chelates with known organic acids are shown below for citric acid, tartaric acid, gluconic acid and glycine.
   
 
   
   
 
THE SIGNIFICANCE OF CHELATION PROCESS IN SOIL ARE:
 
1.  Increase the availability of nutrients.  
Chelating agents will bind the relatively insoluble iron in high pH soil and make it available to plants.
 
2.  Prevent mineral nutrients from forming insoluble precipitates.
The chelating agents of the metal ions will protect the chelated ions from unfavorable chemical reactions and hence increase the availability of these ions to plants.  One example is iron in high pH soil.  In high pH soil, iron will react with hydroxyl group (OH-) to form insoluble ferric hydroxide (Fe(OH)3) which is not available to plants.
      
Fe+3 + 3 OH- --------> Fe (OH)3
Soluble   Insoluble
Chelation will prevent this reaction from happening and hence render iron available to plants.
 
3.  Reduce toxicity of some metal ions to plants.
Chelation in the soil may reduce the concentration of some metal ions to a non-toxic level.  This process is usually accomplished by humic acid and high-molecular-weight components of organic matter.
 
4.  Prevent nutrients from leaching.
Metal ions forming chelates are more stable than the free ions.  Chelation process reduces the loss of nutrients through leaching.
 
5.  Increase the mobility of plant nutrients.
Chelation increases the mobility of nutrients in soil.  This increased mobility enhances the uptake of these nutrients by plants.
 
6.  Suppress the growth of plant pathogens.
Some chelating agents may suppress the growth of plant pathogens by depriving iron and hence favor plant growth.