QBARS - v26n3 The Role of Transition Metal Chelates in Rhododendron Nutrition

The Role of Transition Metal Chelates in Rhododendron Nutrition
Fred R. Davis, Ph.D., Bent, Ohio

A considerable amount of information concerning the role of micro-nutrients in the nutrition of various plants has been amassed over the last two decades. Some of the early work was done with ornamentals using various chelated iron compounds (Wallace, et al, 1953). Minor element deficiencies manifest themselves in the leaf structure by turning pale green or yellow as a chlorosis. In some cases, there is die - back of the twigs and a retarding growth effect. Necrosis and malformation of leaves is sometimes observed under severe deficiency conditions.
The soil may have sufficient amount of the metal present, but certain conditions, such as high pH, excess water or excess alkaline materials, may form insoluable salts which prevent the plants from getting a sufficient amount of the metal ion for proper chlorophyll synthesis.
The need for chelated metallic compounds four correcting these deficiencies over simple metallic salts is that they are effective over a wide range of soil conditions. They have been synthesized for the extremely acid soil, as well as for the highly alkaline and calcareous soils. Most of the commercially available metal chelates contain about 10 to 15% as the metallic element. The chelating agents generally used are ethylenediamine tetraacetic acid (EDTA) or the sodium salt of EDTA and diethylenetriamine pentaacetic acid (DTPA). Although several other polyamine carboxylic acids have been looked at, these seem to be the most effective. Methods for the chemical determination of various chelating agents and metal chelates in plant tissues has been described by Wallace (1953, 1955) and Darbey (1952). This paper reports the results of nutritional and physiological studies made on rhododendrons using transition metal chelates.

Materials and Methods
The following rhododendrons were used in these experiments: 'Nova Zembla' 'Lee's Dark Purple,' 'King Tut,' 'America' and a Dwarf Red Catawbiense Seedling. These plants were all at least two and three years old, although some field grown specimens of the above were used. All the plants, except the field grown specimens, were grown in a German peat moss and perlite mixture. The pH of this mixture was around 5.5 to 6.0.
Five different metal chelates were used: NaFe-EDTA, NaFe-DTPA, Na 2 Zn-EDTA, Na 2 Mn-EDTA and "ChelIron." "Chel-Iron" is the trade name of a mixed chelated iron compound containing a wide spectrum fungicide along with some other micronutrient ions.
The metal chelate solutions were made up as a concentrate, then diluted to the necessary concentration. All solutions had the wetting agent Triton B-1956 added to them so as to insure even distribution of the solution on the leaf surfaces. This is very important since foliar application was used in all these experiments.
I might point out here that it is rather easy to prepare your own metal chelates. I give the following procedure to prepare an iron-EDTA complex. Dissolve 5.5 g FeS0 4 .7H 2 0 in 200 ml of distilled water, then dissolve 7.5 g Na 2 -EDTA in another 200 ml of water; while heating and continuously stirring, add the FeS0 4 solution. After cooling, adjust the volume with water to 1000 ml of solution. It is better here to use the disodium salt of EDTA because it is much more soluble than the free acid. The zinc and manganese chelates of EDTA can be made via manner similar to the above procedure for the iron-EDTA complex.
The concentrations (mg/ml) used in these experiments were: 0.25, 0.50, 0.75, 1.00, 1.25, 1.50, 2.00 and 5.00.

Experimental Results & Discussion
All the cultivars responded to the foliar treatment with the iron chelate solutions whenever the concentration exceeded .25 mg/ml of solution. See Table I. 'Lee's Dark Purple' and the Dwarf Red Catawbiense Seedling were able to tolerate a higher concentration of the metal chelate than the 'Nova Zembla'. The addition of the manganese and zinc chelates to the iron chelate did promote an increase in the effect of iron at lower concentrations than without the manganese and zinc present. This is to be expected since their chemistry is very similar and their roles played in the synthesis of chlorophyll as metallic ion catalysts are essentially the same. It is also believed that they act as to lower the activation energy in the biosynthesis of the other pigmented substances.
It is important to realize that these transition metal chelates can be toxic to the plant if used indiscriminately. Experimentally, dosages ca. 5 mg/ml were toxic, producing withered and curled leaves with browning at edges. The toxicity is probably related to: 1) electrolyte disturbance or unbalance; 2) competition for adsorption on negative sites; 3) enzyme inhibition.
The foliar application of "Chel-Iron" with 1.5 mg/ml two times during the growing season to field grown specimens of 'America', 'Nova Zembla', 'Lee's Dark Purple' and Dwarf Red Catawbiense Seedling produced excellent results. Improved growth and greener foliage was observed along with increased bud formation.
Several quantitative experiments were carried out to determine the rate of transition metal ion assimilation into the plant and the location of the metal ion in the plant structures. These experiments were all carried out using extracts from the dried portions of the plant tissue. Atomic absorption spectroscopic techniques were employed because of the sensitivity and resolution of the method along with radioisotope tagging. It was found that the greatest concentration of Fe and Mn resided in the leaf, particularly in around the midrib region. This might be suspected since it has been postulated that the biosynthesis of the various plant pigments, e.g. chlorophylls, take place in the leaf and that the transition metallic ions lower the activation energy needed for this process to occur. Little or no iron was detected in the bud, albeit an appreciable concentration was found in the root tissue. The concentration gradient for the Fe remained rather constant for each variety of plant of a given age. Some further studies are being planned to elucidate the kinetics of these processes.

ANALYTICAL DATA CHART. Sample number 1 are leaves from the R. 'Nova Zembla' with good bud set and sample number 2 is from the plant with no buds.

No. Nitrogen Phosphorus Potassium Calcium Magnesium Sodium Chloride
1 1.94 0.188 0.90 1.91 0.198
2 1.97 0.188 1.02 1.78 0.268
No. Copper Zinc Manganese Iron Boron
1 16 74 490 220 56
2 16 76 450 195 51
Concentration Color and Growth Index
Cultivar (mg/ml) A B C
'Nova Zembla' 0.00 1 1 1
0.25 1 1 2
0.50 2 2 2
0.75 2 2 3
1.00 3 3 3
1.25 3 3 3
1.50 3 3 3
2.00 0 3 3
5.00 0 0 0
'Lee's Dark Purple' 0.00 1 1 1
0.25 1 1 1
0.50 1 2 2
0.75 2 2 2
1.00 2 2 3
1.25 3 3 3
1.50 3 3 3
2.00 3 3 3
5.00 0 0 3
Dwarf Red Catawbiense Seedling 0.00 1 1 1
0.25 1 1 1
0.50 1 2 2
0.75 1 2 2
1.00 2 2
1.25 2 3 3
1.50 3 3 3
2.00 3 3 3
5.00 0 0 3

A - Two year old plants treated with Fe EDTA for nine months by foliar application.
B - Three year old plants treated with a mixture of iron, manganese and zinc salts of EDTA and DPTA for nine months. This is a concentration of FE, with an equal concentration of Mn and Zn of .50 mg/ml.
C - Three year old plants treated with "Chel-Iron" for nine months.

Color and Growth Index Rating
0 - Plant foliage damaged
1 - No change in foliage or improvement in growth
2 - Noticeable improvement in growth and greener foliage


  • Darbey, A., Colorimetric Determination of the Sodium Salts of EDTA, Anal. Chem. 24:373-378, 1952. Lunt, O. R. and Wallace, A., Use of Iron Chelates, Calif. Agr. 9(6):4, 1955.
  • Wallace, A., et al, Chelating Agents as a Means of Supplying Micronutrients to Woody Plants in Alkaline and Calcareous Soils, Proc. Amer. Sec. Hort. Sci. 62:116-118, 1953.
  • Wallace A., et al, Chlorosis in Ornamentals, Calif. Agric. 7(IV): 13, 14, 1953.
  • Wallace, A., et al, Behavior of Chelating Agents in Plants, Proc. Amer. Soc. Hort. Sci., 65: 9-16, 1955. Wallace, A., et al, Comparisons of Five Chelating Agents in Soils, in Nutrient Solutions, and in Plant Responses, Soil Sci., 80: 101-108, 1955.