Essential Micronutrients Required by Plants; Part 1

May 8, 2008

toxicity of a geranium IN MY PREVIOUS ARTICLE (see Maximum Yield, November / December 2006), the roles for the major elements – carbon (C), calcium (Ca), hydrogen (H), magnesium, (Mg), nitrogen (N), oxygen (0), phosphorus (P), potassium (K), and sulfur (S) – on the nutrition of plants were discussed. The major elements are found in the plant dry weight in per cent concentrations. For the micro-nutrients boron (B), chlorine (CI), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn), plants require considerably smaller concentrations
(less than 0.01 per cent of the dry weight) to sustain sufficiency.

The commonly used unit of concentration for the micronutrients is parts per million (ppm), avoiding the confusion that would come with decimals if the concentration in the plant’s dry weight were in per cent. In metric units, micronutrient concentration is expressed in milligrams per kilogram (mg/kg). One ppm is equal to 1 mg/kg. Another equivalence for solution concentration used in this article is milligram per liter (mg/L) equal to parts per million (ppm).
Interestingly, all but one of the micronutrients, Fe (iron), have been established as “essential” between the years 1922 and 1954. Essentiality for Mn was established in 1922, B and Zn in 1926, Cu in 1931, Mo in 1939, and Cl, the last to be identified as essential, in 1954. For all the major elements and Fe, their essentiality has been known since the 1800s. So, what we know today about the micronutrients is of more recent history.

Another change that has taken place recently has been the use of the word “micronutrient” as the proper term, rather than the words “trace element” or “minor element,” terms that are found in the older literature and, unfortunately, are still occasionally used to identify the micronutrients. Today, “trace element” is used to designate those non-essential elements found in plants in low concentrations, at the parts per million (ppm) level, in the dry weight of plants.

In the case of several of the micronutrients, the range of plant content sufficiency is quite narrow. Departure from this narrow range results in either a deficiency or toxicity when below or above, respectively. In addition, deficiency or toxicity symptoms can be difficult to evaluate visually and, therefore , require an analysis of a specified plant part for confirmation by means of a plant analysis.

The micronutrients, as a group, are far more critical in terms their control and management than most of the major element particularly in soilless culture systems. Most micronutrient deficiencies can usually be corrected easily and quickly, but when dealing with excesses or toxicities, correction can be difficult, if not impossible. If toxicity Occurs, the grower may well have to start over. Therefore, great care Must be taken to ensure that an excess concentration of a micronutrient be not introduced into the rooting media, either initially or during the growing season.
The availability of some of the micronutrients, particularly Fe Mn, and Zn, are significantly affected by the pH of the rooting medium, particularly in organic soilless mixes. A pH greater than 5.5 can result in a micronutrient deficiency, though a recommended quantity of that micronutrient had been added to the mix. The level of a major element, particularly high P, in a rooting medium will affect the uptake of Cu, Fe, Mn, and Zn. Therefore, proper control of the pH and concentration of major elements in a rooting medium are important to ensure “available” sufficiency.

Since the requirements for some of the micronutrients are relatively low, there may be sufficient concentration in the natural environment (i.e., in the water used to make a nutrient solution, in the inorganic or organic rooting media substance, as a contaminate in a major element supplying reagent or fertilizer or from contact with piping, storage tanks, etc.) to preclude the necessity to add micronutrient. Therefore, it is best to analyze a prepared nutrient solution after constituting it and after contact with the environment to determine its micronutrient content. In addition, careful monitoring of the rooting media and plants will ensure that the plant’s micronutrient requirement is being satisfied but not exceeded.

Article source: Maximum yield magazine February 2007

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