D.J. Eakes, J.L. Sibley, C.H. Gilliam, B.H. Murphree, and J.W. Olive

Dolomitic limestone at rates of 5 to 10 pounds per cubic yard of medium is commonly added to pine bark or peat moss based organic growing media used in container production of woody ornamental plants. Dolomitic limestone is used as an amendment to adjust medium pH and serve as a source of calcium and magnesium. Plant growth responses to the addition of dolomitic limestone vary according to the ornamental species being grown and rate of liming material applied.

Dolomitic limestone is usually preplant incorporated as a finely ground powder. Uniform distribution of the liming material throughout the growing medium during mixing is questionable. Pelletized dolomitic limestone has increased in popularity over the past few years for soil application of lime due to its ease of handling. No information is available on how pelletized lime compares to ground dolomitic limestone as a liming material in a container growing medium. Therefore the objective of this study was to compare pelletized dolomitic limestone with finely ground dolomitic limestone as a woody ornamental container medium amendment.

METHODS
In April, 1996, uniform liners of ‘Soft Touch’ Japanese holly and ‘Fashion’ azalea were potted in a pine bark:peat moss (3:1 by volume) substrate amended with 14 pounds of Osmocote 17-7-12 and 1.5 pounds of Micromax per cubic yard. Treatments consisted of dolomitic limestone pre-plant incorporated at the rate of 5 or 10 pounds per cubic yard of medium as finely ground or pelletized dolomitic limestone. A control with no dolomitic limestone added to the growing medium was also evaluated for a total of five treatments. All plants were produced under overhead irrigation in trade gallon containers placed in full sun. Canopy growth index (GI) [(height + width1 + width2 )/3 where width1 is at the widest point, and width2 is perpendicular to width1] and foliar color ratings (FCR) were determined monthly. Foliar color ratings were on a scale of 1 - 5 with 1 being bleached foliage and 5 being dark green. Medium leachate solutions were collected to determine pH on 7, 42, 60, 120, 180, 180, 240, 270, and 330 days after potting (DAP).

RESULTS
Medium solution pH increased with increasing rate of dolomitic limestone regardless of lime form (Table 1). Finely ground dolomitic limestone increased medium solution pH to a greater extent than pelletized lime when applied at a similar rate, and differences in pH increased as lime rate increased. Addition of ground dolomitic limestone at 10 pounds per cubic yard of medium reduced FCR by 60 DAP and GI 330 DAP for ‘Fashion’ azalea compared to all other treatments (Table 2). The 10 pounds of finely ground dolomitic limestone treatment was the only treatment that had a medium solution pH that rose above 6.5 and remained above 6.0 for the duration of the experiment. All other treatments had medium solution pHs below 6.0 by 60 DAP. Amending with dolomitic limestone had no affect on FCR of ‘Soft Touch’ Japanese holly, regardless of the product form or rate. In contrast, the addition of dolomitic limestone at 10 pounds per cubic yard of medium in the pelletized form or at 5 pounds in the ground form per cubic yard of medium increased the GI for ‘Soft Touch’ Japanese holly when compared to the unlimed control plants. Holly plant size in all other treatments was similar.

In summary, finely ground dolomitic limestone was more effective in increasing medium solution pH when compared to the pelletized material. Finely ground dolomitic limestone preplant incorporated at the rate of 10 pounds per cubic yard of medium reduced the size and quality of ‘Fashion’ azalea. Lime form and rate had no affect on the foliar color of ‘Soft Touch’ Japanese holly plants but the addition of pelletized dolomitic limestone at 10 pounds per cubic yard or ground dolomitic limestone at 5 pounds per cubic yard of medium increased plant size.