D.J. Eakes, C.H. Gilliam, G.J. Keever, and J.W. Olive

Pre-plant incorporation of dolomitic limestone and micronutrients as amendments to container media is a common nursery practice. Organic components, such as pine bark and peat moss are acidic and are inherently low in phosphorus, calcium, magnesium, iron, and various micronutrients required for optimum plant growth. Dolomitic limestone is commonly added as a source of calcium and magnesium as well as a means to increase medium pH for these organic substrates. However, raising medium pH can have a detrimental affect on micronutrient availability. Manganese, boron, copper, and zinc are all less available for plant uptake when medium pH is above 6.0.

Previous research has shown that plant response to these amendments can be beneficial, detrimental, or have no effect based on the rate applied and the plant species being produced. Little or no work has been done looking at the impact that one amendment has on the other when used in an organic container medium. Therefore, the objective of this work was to determine the effects of dolomitic limestone and micronutrient rates on container medium solution pH and the growth and quality of seven container-grown species.

METHODS
On May 31, uniform liners of ‘Formosa’ azalea, ‘Greenluster’ Japanese holly, ‘Burford’ Chinese holly, ‘Yellow Jacket’ chrysanthemum, ‘Wood’s Dwarf’ nandina, ‘Meta Peka’ hosta, and ‘October Glory’ red maple were potted in a pine bark:peat moss (3:1 by volume) medium amended with 14 pounds of Osmocote 17-7-12 (Scotts Co., Marysville, Ohio) per cubic yard. The six treatments were ground dolomitic limestone at 0, 5, or 10 pounds per cubic yard of medium combined with Micromax (micronutrient product from the Scotts Co., Marysville, Ohio) at the rate 0 or 1.5 pounds per cubic yard of medium pre-plant incorporated. All plants were produced at the Ornamental Horticulture Station in Mobile, Alabama, under overhead irrigation in trade gallon containers with the exception of the red maples which were grown in 10-gallon containers receiving drip irrigation.

Foliar color ratings (FCR), on a scale of 1 - 5 with 5 being dark green and 1 being bleached foliage, were made 30, 60, 120, and 360 days after potting (DAP) for all species except chrysanthemum. Plant growth indices (GI) [(height + width1 + width 2)/3, where width1 is at the widest point, and width2 is perpendicular to width1] for shrub species, and height and stem diameter for red maple were determined 360 DAP. Chrysanthemum FCR were made 30, 60, and 120 DAP, and GI was determined 150 DAP. Medium solutions were collected using the pour-through technique and the pH was determined on 7, 14, 45, 60, 90, 120, 200, 250, 270, 300, 330, and 360 DAP.

RESULTS
Foliar color ratings were similar among treatments within each species 30 DAP. However, by 60 DAP dolomitic limestone and micronutrient rates affected both red maple and dwarf nandina FCR. Best FCR for red maple occurred with trees receiving Micromax regardless of dolomitic limestone rate, and for those trees produced with no dolomitic limestone and no Micromax (see table). Dwarf nandina produced with 5 or 10 pounds of dolomitic limestone and 1.5 pounds of Micromax had the best FCR compared to plants in the remaining treatments. Within each of the other species, FCR were similar among treatments. At 120 DAP, red maple, dwarf nandina, and hosta FCR increased as dolomitic limestone rate decreased when no Micromax was supplied. When Micromax was added, plants in all dolomitic limestone rates had similar FCR and were similar to FCR of no dolomitic limestone, no Micromax plants. Although there was no interaction between dolomitic limestone and micronutrient rates for azalea, FCR increased as dolomitic limestone rates decreased (a rating of 4.1 for no dolomitic limestone to 3.5 for plants receiving 10 pounds of dolomitic limestone) or when Micromax was supplied (4.0 with Micromax and 3.5 without). No other plant species’ FCR were affected by dolomitic limestone or micronutrient rate 120 DAP, and FCR for plants in all treatments were similar within species 360 DAP.

Growth indices of dwarf nandina and hosta 360 DAP and chrysanthemum 150 DAP increased as dolomitic limestone rate increased regardless of micronutrient rate. Greatest GI for both holly species occurred with the 5 or 10 pound rate of dolomitic limestone regardless of micronutrient rate. Height and stem diameter of red maple and GI of azalea were not affected by dolomitic limestone or micronutrient rates 360 DAP.

As dolomitic limestone rate increased, medium solution pH increased on each observation date through the study. Regardless of dolomitic limestone rate, medium solution pH decreased over time. The pH for the 10 pound rate of dolomitic limestone decreased from 6.6 on 45 DAP to 4.9 on 360 DAP while the 0 pound rate decreased from 4.4 on 45 DAP to 3.6 on 360 DAP. Micronutrient rate had no effect on medium solution pH on any observation date during the study.

In summary, the addition of dolomitic limestone to the potting medium increased the size of dwarf nandina, hosta, chrysanthemum, and both holly species, while it had no affect on azalea or red maple. However, the quality of red maple, dwarf nandina, and hosta species declined with increasing amounts of dolomitic limestone when micronutrients were not supplemented in the potting medium. This work suggests that while dolomitic limestone is an inexpensive amendment, routine incorporation in substrates without micronutrients supplements may be unnecessary.