Mary Ann Rose and Hao Wang
Commercial micronutrient fertilizers and a biosolids compost were used in the pro-duction of container Rhododendron 'Girard Scarlet.' In some cases, using a micro-nutrient fertilizer or a compost-amended medium resulted in higher micronutrient concentrations in the potting medium or in plant foliage. However, there were no visible differences in plant size, color, and quality among controls, which received no micronutrient source, and the other treatments. Analysis of the bark-based medium suggested that the bark may have supplied sufficient micronutrients for a full year.
In the 1960s, soilless growing media were first developed to replace the use of soil mixes in container production. These new lightweight media were an improvement over heavy, soil-containing media because they did not require steam sterilization and had greatly improved physical characteristics, in particular, superior air capacity. However, soilless-medium components had a much lower capacity to store and supply nutrients than soil-containing mixes; because of this, slow-release fertilizers were developed to maintain a constant supply of N, P, and K. Micronutrient fertilizers also were developed to be used with soilless media to supply trace elements formerly supplied by soil. Many growers use these micronutrient fertilizers today.
Research with biosolids compost (Ticknor et al., 1985) and pine bark (Niemiera, 1992; Wright and Hinesley, 1991) has suggested that micronutrient fertilization may be unnecessary when those materials are used as growing-medium components. Even phytotoxic levels of micronutrients may be found in some types of medium components; for example, hardwood bark may contribute toxic levels of Mn (Bunt, 1988; Svenson and Witte, 1992), and compost may have high levels of B (Lumis and Johnson, 1982; Rosen et al., 1993). However, some researchers have found that the micronutrient supply from medium components is insufficient (pine bark: Handrek, 1995; Whitcomb et al., 1975) and that micronutrient fertilizer addition may improve growth (Wright et al., 1997).
Although previous research does not consistently support the need for micronutrient fertilizer supplements in soilless media, manufacturers are nonetheless producing an increasing number of slow-release fertilizers with minors packages (slow-release N-P-K + minors products). Micronutrients in these products may be bulk-blended with N-P-K prills, or may occur within, or as part of, the prill coating (Brian Birrenkott, The Scotts Co., personal communication). In a preliminary experiment (Rose, unpublished), micronutrient availability from several of these products and a stand-alone micronutrient fertilizer (Micromax, The Scotts Co., Marysville, Ohio) was examined. One of the slow-release + minors fertilizers and the Micromax fertilizer increased dry weights relative to controls that received no supplemental micronutrient fertilizer. However, significant growth effects were observed on only one of four sampling dates and for one species (Rhododendron). Foliar micronutrient concentrations were not different among treatments at any date. Overall, the preliminary research did not strongly support the use of micronutrient fertilizers in soilless media.
The primary goal of this experiment was to determine whether typical sources of micronutrients used in container nursery production supplied micronutrients over a full year, and whether they improved growth of container Rhododendron. Biosolids compost was used as a micronutrient source in one treatment; all other sources were commercial fertilizers.
Four-inch-square potted liners of Rhododendron 'Girard Scarlet' were potted June 7, 1996, in trade two-gallon (6.1 liter) pots containing a 3:1:1:0.2 ratio by volume of pine bark, hardwood bark, peat, and sand. The medium was amended with gypsum, dolomitic lime, and granular sulfur at 4, 2, and 1 lb. per cubic yard, respectively. The micronutrient treatments consisted of:
All plants were top-dressed with 3.8 grams nitrogen (2 lbs. N per cubic yard) from either the 18-6-12 (no micronutrient package) or the slow-release + minors fertilizers. All slow-release fertilizers were eight- to nine-month release formulas.
Table 1 presents how much manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), and boron (B) was supplied to each pot in each treatment. The amounts vary widely because it is impossible to equalize amounts applied from sources that contain micronutrients in varying proportions. Our decision was to instead hold constant the amount of nitrogen applied from each slow-release product. The stand-alone product, Step Hi-Mag, was used at the labeled rate, and it is clear that this product supplies a much higher level of micronutrients than the slow-release-plus minors products applied at typical nitrogen fertilization rates (Table 1). There is no guaranteed micronutrient analysis for Technagro compost, but the authors tested the material and report the test results in Table 1. The compost supplied about twice as much Fe and Zn as the Step Hi-Mag.
| Table 1. Micronutrients Applied to Each Two-Gallon Pot (Mg/Pot) Calculated from Manufacturers' Guaranteed Analyses, or from Compost Analysis. All Slow-Release Products Applied at 2# N/ Cu. Yd. | ||||||
|---|---|---|---|---|---|---|
| Treatment Name and Micronutrient Source | Slow-Release Fertilizer | Milligrams per Pot | ||||
| Mn | Fe | Cu | Zn | B | ||
| Control (no source) | Osmocote 18-6-12 | - | - | - | - | - |
| Biosolids compost | Osmocote 18-6-12 | 228 | 1172 | 25 | 135 | 5 |
| STEP HiMag | Osmocote 18-6-12 | 210 | 560 | 35 | 70 | - |
| Customblend | Customblend 19-5-8 | 50 | 180 | 10 | 20 | - |
| Sierra | Sierra 17-6-10 | 22 | 90 | 11 | 11 | 4 |
| Osmocote Plus | Osmocote Plus 16-8-12 | 17 | 119 | 12 | - | 5 |
| High N Plus | High N Plus 22-4-8 | 17 | 173 | 9 | 9 | 3 |
| Nutricote Plus | Nutricote Plus 18-6-8 | 13 | 42 | 11 | 3 | 4 |
There were 18 single-plant replications of each treatment, divided between three blocks. Recently matured leaves were sampled for foliar analysis a year after potting on June 1, 1997. The growing medium was sampled for nutritional analysis twice -- Sept. 12, 1996, and May 27, 1997. Nutritional analysis was performed at The Ohio State University's Research-Extension Analytical Laboratory, Wooster, Ohio. One year after potting ( June 1, 1997), shoots from all 144 plants were harvested to measure dry weights as an estimate of growth. All data were analyzed as a randomized complete block with three replications. A protected LSD test was used to compare all means (p < 0.05).
None of the treatments improved growth (dry weight data not shown) one year after potting. Control plants, which received no supplemental micronutrient fertilizer, were excellent in quality and color.
By the end of the growing season (Table 2, Sept. 12), there were few differences in medium micronutrient concentrations between the control and the treatments receiving the commercial micronutrient fertilizers -- both Step Hi-Mag and Customblend had higher Mn in the medium; Customblend also had significantly higher copper levels. The compost-amended medium produced higher levels of Mn, Fe, and Zn than controls for both sampling dates. The compost medium also had higher levels of phosphorus on the first date.
| Table 2. The Growing Medium pH, Phosphorous, and Micronutrient Concentrations (ppm) of Container-Grown Rhododendron 'Girard Scarlet' on Sept. 12, 1996, and May 27, 1997 (0.005 M DPTA Extraction). Underlined values indicate that they are significantly greater than controls. | |||||||
|---|---|---|---|---|---|---|---|
| September 12, 1996 Treatment | pH | P | Mn | Fe | Cu | Zn | B |
| Osmocote 18-6-12 | 4.5 | 14.8 | 2.1 | 30.2 | 0.3 | 4.2 | 2.3 |
| Osm. + compost | 5.6 | 25.6 | 14.1 | 58.5 | 1.6 | 28.4 | 1.4 |
| Osm. + STEP | 4.8 | 18.7 | 7.6 | 30.1 | 1.5 | 11.1 | 2.0 |
| Customblend 19-5-8 | 4.8 | 12.5 | 7.6 | 30.3 | 2.4 | 11.5 | 0.9 |
| Sierra 17-6-10 | 5.0 | 10.1 | 4.3 | 19.0 | 0.7 | 12.3 | 1.1 |
| Osm. Plus 16-8-12 | 5.3 | 13.4 | 2.0 | 20.0 | 1.4 | 4.2 | 1.9 |
| High N Plus 22-4-8 | 4.7 | 5.9 | 2.9 | 28.4 | 0.4 | 5.2 | 0.9 |
| Nutricote Plus 18-6-8 | 5.2 | 3.6 | 2.3 | 17.8 | 1.1 | 4.4 | 0.9 |
| May 27, 1997 Treatment | pH | P* | Mn | Fe | Cu | Zn | B |
| Osmocote 18-6-12 | 4.7 | - | 5.1 | 23.5 | 1.6 | 11.2 | 0.1 |
| Osm. + compost | 5.1 | - | 16.9 | 77.2 | 1.9 | 26.8 | 0.3 |
| Osm. + STEP | 4.5 | - | 4.3 | 26.3 | 2.3 | 10.8 | 0.1 |
| Customblend 19-5-8 | 4.6 | - | 5.4 | 32.4 | 6.6 | 14.9 | 0.1 |
| Sierra 17-6-10 | 4.7 | - | 2.8 | 13.6 | 0.9 | 7.7 | 0.1 |
| Osm. Plus 16-8-12 | 4.8 | - | 2.0 | 19.5 | 2.1 | 5.3 | 0.3 |
| High N Plus 22-4-8 | 4.8 | - | 1.5 | 33.0 | 0.8 | 7.1 | 0.1 |
| Nutricote Plus 18-6-8 | 4.8 | - | 1.4 | 12.2 | 1.9 | 5.2 | 0.2 |
| Adequate ranges for micronutrients (DPTA Extr.) | 5-30 | 15-40 | 0.5-1.5 | 5-30 | 0.7-2.5 | ||
| * Did not analyze for P in 1997. | |||||||
Plants in the micronutrient treatments did have significantly greater foliar Mn and/or Cu than controls (Table 3); however, control plants had excellent color, quality, and size and were not inferior to any treatment.
| Table 3. Foliar Micronutrient Concentrations (ppm) of Rhododendron 'Girard Scarlet' the Year After Potting. Underlined values indicate that they are significantly greater than controls. | |||||
|---|---|---|---|---|---|
| Treatment | Mn | Fe | Cu | Zn | B |
| Osmocote 18-6-12 (control) | 46.6 | 60.4 | 2.4 | 44.0 | 31.8 |
| Osmocote + compost | 83.9 | 60.6 | 3.0 | 51.8 | 28.3 |
| Osmocote + STEP | 134.4 | 64.7 | 2.5 | 58.0 | 30.1 |
| Customblend 19-5-8 | 134.6 | 58.3 | 3.6 | 53.9 | 45.7 |
| Sierra 17-6-10 | 85.3 | 58.2 | 3.4 | 52.5 | 38.2 |
| Osmocote Plus 16-8-12 | 52.8 | 61.6 | 3.7 | 47.9 | 34.8 |
| High N Plus 22-4-8 | 97.4 | 64.4 | 4.0 | 54.4 | 40.8 |
| Nutricote Plus 18-6-8 | 62.0 | 56.2 | 5.1 | 46.1 | 47.0 |
| Adequate ranges for foliar micronutrients | 50-200 | 35-250 | 6-25 | 20-200 | 6-75 |
Even though plants in some micronutrient treatments had higher levels of micronutrients in the medium or in the foliage, the plants grown without any micronutrient source (controls) were just as big and had just as good color. Since all plants have micronutrient requirements, where did the controls obtain micronutrients? Most likely, the source was the bark in the potting media. Table 4 provides the extractable levels of micronutrients in fresh medium components and in irrigation water. Comparing the values in Table 4 to the micronutrient adequacy ranges in Table 2 suggests that at least when fresh, both types of bark supply adequate Fe and Mn. Compost may supply adequate amounts of all micronutrients tested, whereas the municipal water used in this experiment appeared to be a negligible source.
While this study demonstrated no growth or plant-quality improvement from any of the micronutrient sources used, the study does not prove that with different plants, different media, and different circumstances, micronutrient supplements may not be important. Furthermore, it is possible that controls may have developed micronutrient deficiencies had they been grown longer than one year. This study and the authors' previous work do suggest that in a one-year crop, the primary value of micronutrient supplements may be as "insurance."
| Table 4 . Micronutrient Concentrations (ppm) in the Irrigation Water and Fresh Potting Medium Components (DPTA Extraction). | |||||
|---|---|---|---|---|---|
| Mn | Fe | Cu | Zn | B | |
| Irrigation water | 0.03 | 0.03 | 0.06 | 0.30 | 0.06 |
| Technagro compost | 45.4 | 37.3 | 2.2 | 57.5 | 1.8 |
| Pine bark | 81.4 | 37.4 | 0.2 | 2.2 | 0.6 |
| Composted hardwood bark | 93.1 | 38.7 | 0.3 | 2.7 | 0.5 |
| Peat | 0.2 | 21.1 | < 0.1 | 0.7 | 0.1 |
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