Importance of Trace Elements in Soil
Trace elements are essential nutrients and their relative deficiency or excess can potentially influence optimum development to a considerable extent.
- There are always trace elements in the soil but the supplies may be inadequate for agriculture.
- There may be an imbalance of nutrients.
- The essential elements may not be available for a number of reasons.
Whatever the cause, disturbances in trace element nutrition results in great economic loss in many parts of Australia, by preventing proper stock and crop development.
Hidden Trace Element Deficiencies
The widespread trace element deficiencies are not generally recognized, even by many investigators, because too much stress has been placed on visual, clinical symptoms, such as chlorosis of leaves or abnormal growth.
These symptoms are of the greatest value in indicating the presence of nutritional deficiencies, but it must be appreciated that not all deficient plants show them. Deficiencies not acute enough to show visual signs may be severe enough to halve the potential yield.
Difficulties in correcting trace element deficiencies
(Disadvantage of using inorganic salts)
The addition of water soluble sulphate salts of copper, zinc, manganese, cobalt and iron, to correct trace element deficiencies in the soil is very inefficient and sometimes dangerous, as certain soil factors tend to have the same general effect on the availability of all of them these are:
- High rate of leaching of the sulphate salts from the soil (especially sandy soils).
- All tend to become less available to plants as the soil pH is raised (liming).
- High pH-value favours the oxidation or iron, manganese and copper and they become less soluble.
- The application of large quantities of phosphate fertilizers adversely effects the supply of trace elements. The uptake of both iron and zinc is reduced in the presence of excess phosphate.
- On soils high in organic matter, copper appears to be bound tightly by the organic matter.
- Cobalt, zinc, manganese and iron, in water soluble form, are fixed by certain silicate clays, especially of the 2 to 1 type.
Chelated (“Protected”) forms of trace elements
A chelate is an organic compound which combines with and protects metallic trace elements, including iron, copper and zinc.
A specific example of the protecting action of chelating compounds may be cited, when inorganic salts of iron, added to a calcareous soil are precipitated and rendered unavailable to plants.
If oxidized iron sulphate is added the reaction would occur as follows:
Iron (Soluble) Alkaline Soil Solution Iron-Hydroxide (Insoluble)
If, instead of the sulphate salt of iron chelate was used, the oxidized (ferric) iron would have been protected from precipitation.
Synthetic Chelating Agents
The most widely used synthetic chelating agent is EDTA and is effective for iron chelation but inferior for other trace elements. In calcareous soils, it is detrimental to plant growth, in some conditions. Also iron EDTA is fixed in insoluble form on soil clays.
The stability of the synthetic metal chelates or the replacing power of the elements in decreasing order is: Iron (3) > Copper > Zinc > Iron (2) > Manganese > Calcium > Magnesium >.
For example, if zinc containing synthetic chelate is added to a soil which has a significant quantity of available iron, the following reaction would occur:
Zn (chelate) Fe++ Fe (chelate) Zn++
The released zinc from its chelated form is not protected from becoming unavailable to plants.
The release of nutrient elements form the protecting chelate does not occur when the highly stable natural chelates are employed.
Synthetic chelating agents:
- EDTA – Ethylene diamine tetracetic acid.
- DTPA – Diethylene triamine pentaecetic acid.
- HEEDTA – Hydroxyethyl ethylene diamine triacetic acid.
- CDTA – Cyclohexane trans 1, 2 diamino tetracetic acid.
- EDDHA – Ethylene bis Alpha-imino-2hydroxyphenyl-acetic acid.
Natural Organic Chelates
These are extracted from natural materials and modified for a uniform chelating power for all the micro-nutrients. The first Australian manufactured product of this kind is produced by “Agmin Chelates Pty. Ltd.”.
As a Foliar Spray:
- Easily absorbed by plants.
- Translocated readily within the plants.
- Easily decomposed within the plant so that the trace element becomes available.
- Not detrimental to plants at concentrations necessary to control deficiencies.
As a Soil Application:
- Having high stability not easily replaced by other nutrient elements in the soil.
- Each of the metals chelated are very stable against hydrolysis.
- Trace elements chelated are resistant to micro-biological decomposition.
- The chelated micro-nutrients are soluble in water.
- Not easily precipitated by ions or colloids in soils.
- Metal chelates are available to plants either at root surfaces or within the plant.
- Chelated micro-nutrients are not detrimental to plants at concentrations required to prevent deficiencies.
(The extracted plant material is sulphonated propylene phenolic acid with a large number of methoxyl group producing metal chelates with extremely high stability constants.)
|Micro-Nutrients||General Soil Type and Conditions||Crops Most Likely Susceptible|
|Copper||Sandy soils, peats & mucks, over-limed acid soils. (Excess applied gypsum).||Cereal grains, vegetable and tree fruits.|
|Zinc||Calcareous soils after leaching and erosion, acid-leached soils, after heavy phosphorous, coarse sands, subsoil exposed by land levelling.||Beans, soybeans, citrus, corn, sorghum, onions, potatoes, tree fruits, flax, sugar beets, rice.|
|Manganese||Sands, mucks and peats, alkaline particularly calcareous soils.||Legumes, cereal grains, tree fruits, cotton, leafy vegetables.|
|Molybdenum||Highly weathered acidic leached soils, acid soils.||Cauliflower, citrus all legumes.|
|Iron||Alkaline soils, particularly when cold and wet.||Beans, soybeans, corn, sorghum, tree fruits and ornamentals.|
|Boron||Acid leached soils, coarse-textured sandy soils, peats and mucks, drought conditions, over-limed acid soils.||Lucerne, lupins, peas, apples, beets, clovers, citrus, cotton, cauliflower, celery, corn, sweet potatoes, tomatoes, tree crops, sugar beets.|
|Cobalt||Is essential for the symbiotic fixation of nitrogen. This element is a component of vitamin B12 which is essential for the hemoglobin in nitrogen fixing nodule tissue.||Ruminants have essential need for cobalt and the deficiency in pasture soils increases the incidence of disease in sheep and cattle.|