The following points highlight the two types of elements present in soil. The types are: 1. Useful Elements 2. Toxic Elements.
Type # 1. Useful Elements:
The elements included in this category are sodium, cobalt, silicon, selenium, etc.
Sodium:
It promotes growth of several plant species especially salt-loving species and these species accumulate large amount of sodium. Some of the species are nonresponsive and do not absorb sodium. Some of the halophytes need this element for successful glycolysis. Excessively absorbed sodium may even extrude out through salt glands. Several of the mangroves fail to absorb this element.
Cobalt:
This is essentially required by microorganisms and is a part of vitamin B12. The essentiality of Co is very difficult to show. It is essential for the nitrogen fixing systems and assumingly affects nitrogen fixation.
Silicon:
Diatoms have silicon in their outer covering. Silicon is also impregnated in the leaves, glumes and awns of several cereals and this element may constitute up to 25% of the dry weight of these plants.
It decreases transpiration, provides resistance to several pathogens but possibly does not participate in plant metabolism. Silicon also reduces Fe and Mn toxicity possibly by precipitating these elements.
Selenium:
Some species especially Astragalus accumulate large levels of this element whereas some species are tolerant to high levels of selenium. Some amino acids contain selenium as well. It counteracts the phosphorus toxicity. The blue colour of the flowers of Hydrangea macrophylla is related to the content of aluminium in the floral tissues.
Some species of Aspergillus have shown to require gallium element. The general consensus is that several of the above mentioned elements act in a purely indirect manner to benefit plant growth chiefly by increasing the availability of essential elements and by decreasing the concentrations of toxic substances or by establishing a more favourable balance of ions in the growth medium.
Some of the elements have been shown to replace other elements successfully or the replacement caused reduced function. Thus calcium could be partially replaced by barium and strontium. Potassium deficiency could be replaced by rubidium and caesium. Magnesium could be partly replaced in some plant species by beryllium.
Type # 2. Toxic Elements:
Heavy metals like silver, lead, mercury show strong toxic effects due to their high accumulation. In some plants resistant varieties may be evolved through mutations, etc. In nitrogen fixing microorganisms tungsten causes toxicity. Geranium causes inhibition of silicon metabolism. Likewise aluminium has been shown to affects phosphorus-metabolism and a heavy level of inorganic phosphate accumulates in the roots.
Chelating Agents:
As inorganic salts, micronutrients like Fe, Zn, and Cu are relatively insoluble in nutrient solutions and the insolubility is well marked especially when pH is above 5. Thus metals are used as metal chelates. The chelate or complex iron is stable. The ethylenediaminetetra acetic acid (EDTA) is combined with the metals like iron-EDTA, zinc-EDTA, etc.
EDTA is a ligand and is generally used to overcome deficiency of specific metals. A good chelating agent must have specific characteristics like it should be microbe resistant and also form a stable chelate with the given micronutrient. In the soil natural chelates of micronutrients also are found. Recently amino acids and proteins are used as chelating agents.
When amino acids are combined with the elements the uptake is very high. It is regarded that chelating agents usually function as effective micronutrients sources by keeping the elements in solution. Plants possess ligands which check precipitation inside the xylem or in the vacuoles and cytoplasm of cell.
Amino acids, proteins and some organic acids are the best chelating agents in nature. Iron is present as chelate of malic acid in the xylem of soybeans. Except sodium and potassium most of the elements are present as chelated minerals inside the plant body. It may be stated that iron and Mg are tenaciously chelated in the cytochrome pigments and chlorophyll.