Ten elements constitute about 99% of the weight of the surface of the earth. While Fe accounts for 5% of the total weight, in plants it is needed in a very small amount of the total plant weight and is classified therefore as "micro-nutrient"
Other nutrient elements are found in small concentration in t rocks and soils and are measured in units of g/t of soil weight..
Iron deficiency observed in plants in most cases does not mean a deficiency of this element in the soil but rather in ability of the plant to take it up. It is hard to rely on soil testing to establish the deficiency in soil of a micro nutrient. Practically, in order to identify in the field which of the micro nutrient is deficient in a certain soil, it is easier to test the response of plants to a complete nutrient solution that contain all the micro nutrients and than to establish next to it a series of plots each containing the complete micro nutrient solution minus one.
In that way, for example, manganese deficiency was identified in soils recovered from a bottom of the Hula lake in northern part of Israel after it was drained and turned into an agricultural operations..
Some of the micro nutrients may become toxic, when their concentration in the plant tissue increases even slightly above normal levels. There are situations in which testing the plants will not be effective, as the analytical error is greater than the difference between the lack and the toxic levels.
Selenium is not vital for plants. It may accumulate in them without damage to the plant. Hot blood animals, eating those plant parts containing excess concentrations of selenium are likely to suffer serious poisoning . We had a case of Se poisoning in which cows that died from selenium poisoning. The soils did not contain Se but it source in the cows diet came from lubrication grease used in harvesters which cut the grass to feed the cows.
In testing the influence of the concentration of heavy metals in the nutrient solution root elongation was used as a measure of their toxicity to plants.Manganese, in concentrations up to almost 10 000 g/m3 was harmless. In contrast, copper is toxic to plants even at a concentration in solution of only about 40 g/m3. The sensitivity of the plants and their response to shortage or excess of any micro nutrient differs from plant to plant and even among cultivars of the same species.
The essential role of micro nutrient , like their potential toxicity, is particularly emphasized in soil less or solution cultures, where the buffer factor of the soil does not exist.
The concentration of the metallic ion in the soil solution is difficult to establish: the metal ion reaches the soil solution from the primary minerals (magmatic rocks, and granite). These ions during the long soil forming processes are facing many types of reaction sites.
The main tow chemical environments determining the level of soluble micro nutrient in the soil solution are: Oxidation-reduction reactions and the pH.
This equation show the complete dependence of the metal concentration in the solution on the pH. If these metal ions would have stayed in the plant in their simple ionic forms their concentration in the plant would have suffered from internal changes in pH. These elements are usually found in the plants not in ionic form but are forming chelate complexes..
The iron is taken up by the plants only in the form of divalent iron. In soil analysis there is usually no distinction made between divalent iron and trivalent iron,. This also may leads to error in availability estimation.
When the Fe metal is bound to a complex (chelate) such as citric acid or haem compounds in the cell - its free ion characteristicsare hidden and it does not respond to the inorganic chemical low s of naked Fe ion in solution.
In spraying micro nutrient solutions on the foliage, there might be a danger in using a chelate (such as EDTA) as the chelate may bind to the calcium from the plant tissue (the middle lamella of the cells and the cytoplast membranes contain Ca. The protein in the plasma membrane is connected to the phospholipids skeleton of the membrane through a Ca bridge. Free EDTA might extract the Ca from it sight in the leaf or root membranes and may inflict far more damage than the supply of metal which is added is intended to be beneficial. In foliage application simple metallic salts might be safer to use.
A good summary on chelate structure and principles of their mode of operation can be found in : Lehman (1963) Principles of Chelation Chemistry, Soil Science Society of America Proceedings. 27, 167-170.
The stability constant Kma is a measure of the affinity bond of the metal to the complex with a specific organic molecule. As seen in the following table
The complex bond strength of the chelate increases with the increase in the ionic polarization. The stability constant increases from Ca >Cd>Cu>Zn.
The stability constants of certain metals chelate complexes dependent on the structure and composition of the active group in the chelate. Carboxylate are weaker than amino groups (NH) which are weaker (and cheaper) than HS groups.
The complex stability increases with the increase in the numbers of ring in which the metal is part of. The metal is bound to the chelate by forming a 5 or 6 member rings.
The most influential factor on the bond strength and on the stability constant is the solution pH.
For example EDTA holds iron well up to pH 6, but between pH 6 and pH 8 it progressively loses iron and replace it with calcium. Therefore "Koratin" , a commercial solution of micro-nutrient solution based on EDTA is good and stable source up to a pH of 6.5 or a little higher than that but not at a basic pH.
The synthetic chelate, which might be effective in preserving the solubility of the metals in the soil solution, do not penetrate the root. The metal leaves the chelate on the root surface before it is taken up by the root
The citric acid inside the plant is the natural organic chelate in the transport of iron and other metals in the plant. At severe low levels of P in the plant, citric acid is exuded out of the root , helping to solubilize external non soluble P and iron.
Arnon tested (as early as 1939) the composition of tomato plants grown in nutrient solutions with and without aeration, with and without manganese additions in the presence of ammonium or nitrate in the nutrient solutions..
When the leaves were analyzed there was a difference primarily among those fed with manganese and those which did not received manganese in the solution. The differences due to the forms of nitrogen and the level of aeration was only secondary. In the root analysis it was not possible to distinguish between the manganese absorbed and that which precipitated on the root surface. When the plant roots were fed with nitrate, and particularly if manganese was added to the solution - much more manganese was found in the roots than in those which were fertilized with ammonium. With ammoniacal nutrition, more than with nitrate nutrition, aeration influenced the amount of manganese in the root.