The Essential Micronutrients Required by Plants

Introduction

In my previous article (see Maximum Yield, September/October 2006), the roles for the major elements — carbon (C), calcium (Ca), hydrogen (H), magnesium, (Mg), nitrogen (N), oxygen (O), 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 micronutrients boron (B), chlorine (Cl), 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 of their control and management than most of the major elements, 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.

Let’s look at each of the micronutrients, using tomato as the plant for basing sufficiency. In this issue of Maximum Yield, we will address boron, chlorine, copper, and molybdenum. In the next issue, iron, manganese, and zinc.

Boron (B)

Content in Plants. The sufficiency range for B in tomato leaf tissue is between 25 and 75 ppm of the dry weight, with the critical values being closer to either the lower or upper concentration of the sufficiency range. Boron accumulates in the leaf margins at concentrations 5 to 10 times that in the whole leaf blade. Therefore, the per cent of margin to leaf blade can significantly influence a B assay of a leaf tissue sample taken for analysis.

Function. Boron is important in carbohydrate synthesis and transport, pollen growth and development, and cellular activities (division, differentiation, maturation, respiration, etc.).

Deficiency Symptoms. Plants deficient in B exhibit various visual symptoms. The first is slowed and stunting of new growth followed by a general stunting of the whole plant, and when the deficiency is severe, the growing tip of the plant will die. The plant itself will be brittle (due to cell wall deterioration), and leaf petioles and stems will easily break from the main stem. Fruit development will be slow or non-existent, depending on the severity of the deficiency. Fruit quality will be impaired when B is inadequately supplied. When the deficiency is severe, the growing tip of both tops and roots will die.

Excess Symptoms. Because B accumulates in the leaf margins, an early symptom of excess B is discolouration and eventual death of the leaf margins. Normally, discolouration along the whole length of the leaf distinguishes B excess from Ca deficiency, where just the leaf tip and margin at the tip turn brown and die. Boron toxicity can easily result from excess B in the nutrient solution or from B found in natural waters. The B level in the tomato plant should be monitored by means of a plant analysis for evaluating both deficiency and excess.

Content in a Nutrient Solution. Hydroponic formulas usually call for a B concentration of about 0.3 ppm in the nutrient solution; the borate (BO33-) anion and molecular boric acid (H3BO3) are the forms found in solution and utilized by plants.

Nutrient Solution Reagents. Boric acid (H3BO3), Solubor (Na2B4O7.4H2O + Na2B10O16.10H2O) and borax (Na2B4O7.10H2O) are the primary reagent sources. Since B is not an uncommon constituent of some water supplies, its B concentration should be determined and an assay made on the final prepared nutrient solution to determine its B content.

Chlorine (Cl)

Content in Plants. Leaf content of Cl in tomato leaves will range from low parts per million levels (20 ppm) in the dry matter to per cent (0.15 per cent) concentrations. Leaf levels in excess of 1.00 per cent would be excessive for tomato. A sufficiency range for Cl in tomato leaves has not been firmly established.

Function. Relatively little is known about Cl function in the tomato plant. For some plants, possibly tomato, when Cl-deficient it will wilt and is easily susceptible to various fungus diseases.

Deficiency/Excess. Since the Cl- anion is ubiquitous in the environment, deficiencies are not likely to occur, except under unusual circumstances. There is far greater danger in Cl excesses from exposure of plants to salt-affected Cl-based environments. Symptoms of Cl toxicity include burning of the leaf tips or margins and premature yellowing and loss of leaves.

Content in a Nutrient Solution. Because Cl is a common contaminant in water and reagents used to prepare a nutrient solution, this element does not normally have to be added to a nutrient solution formulation. Care should be taken to avoid adding sizable quantities of Cl to the nutrient solution by using reagents such as potassium or calcium chloride (KCl or CaCl2, respectively). If present in high concentration in the nutrient solution, the Cl- anion will inhibit the uptake of other anions, particularly NO3-. Chlorine exists in the nutrient solution as the chloride (Cl-) anion.

Copper (Cu)

Content in Plants. The sufficiency range for Cu in tomato plant leaf tissue is between 5 and 16 ppm in the dry matter. The range between deficiency and toxicity is fairly narrow, toxicity occurring when the Cu leaf tissue concentration is in excess of 15 ppm.

Function. Copper plays a role in electron transport in photosynthesis processes, is a constituent of a chloroplast protein, and is also known to be an enzyme activator.

Deficiency Symptoms. When deficient, the tomato plant will be stunted and chlorosis develops on the older leaves. Plants moderately Cu-deficient may be dark green in colour and slow growing. Copper deficiency affects the developing fruit; they will be small and imperfectly formed. Death of the growing tip of the plant may also occur with Cu deficiency.

Excess Symptoms. In hydroponic systems Cu toxicity can result in significant root damage if the Cu content of the nutrient solution is greater than 0.1 ppm.

Content in a Nutrient Solution. The normal concentration range for Cu in nutrient solutions is from 0.001 to 0.01 ppm. Copper exists in the nutrient solution as the cupric (Cu2+) cation. It has been suggested by some that if the Cu concentration is raised to 4 ppm in nutrient flow systems, some degree of root fungus control can be obtained. Additional research is needed to determine if such Cu levels will indeed control common root diseases and not damage or kill plant roots. Such Cu concentrations should not be used for other types of hydroponic growing systems. There is evidence that if a chelated form of Fe is present (as either FeEDTA or FeDTPA) in the nutrient solution, Cu uptake and translocation in the plant can be impaired. However, there are not sufficient data to recommend an increase in the concentration of Cu in the nutrient solution to compensate for this effect.

Nutrient Solution Reagents—Copper sulfate (CuSO4.5H2O) is the primary reagent source for Cu in nutrient solution formulas. However, there may be sufficient Cu contamination from contact with Cu-containing equipment (pipes, etc.) to supply all that is required.

Molybdenum (Mo)

Content in Plants. Plant Mo requirement is very low, the critical level in tomato plant tissue being less than 0.5 ppm of the dry matter. The Mo concentration found in normally growing tomato plants is usually between 0.5 to 1.0 ppm, but it may be considerably greater with no apparent toxic effect on the plant itself.

Function. Molybdenum is an essential component of two major enzymes involved in N metabolism. Nitrogen (N2) fixation by symbiotic N-fixing bacteria requires Mo, and the reduction of the NO3- anion by the enzyme nitrate reductase requires Mo. Therefore, the tomato plant receiving all of its N by root absorption of the NH4+ cation either does not require Mo or has a reduced Mo requirement.

Deficiency Symptoms. Molybdenum deficiency symptoms are unique in some ways, sometimes giving the appearance of N deficiency when insufficient. Plant growth and flower development are restricted.

Concentration in a Nutrient Solution. Hydroponic formulas call for 0.05 ppm Mo in the nutrient solution. Molybdenum exists in the nutrient solution as the molybdate (MoO42-) anion.

Nutrient Solution Reagents. Ammonium molybdate [(NH4)6Mo7O24.4H2O] is the primary reagent source. There may be sufficient Mo added as a contaminate in the major element source reagents used to make the nutrient solution, or Mo may exist in the rooting media itself sufficient to meet the crop requirement.

In the March/April 2007 issue of Maximum Yield, we will be discussing the micronutrients iron, manganese and zinc.