Effects of Dolomite Source, Dolomite Rate and Fertilizer Rate on Change in pH of Growing Medium Leachate

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C.A. Conover, Ph.D.(1), R.J. Steinkamp(2) and K. Steinkamp(3)

University of Florida
Institute of Food and Agricultural Sciences
Central Florida Research and Education Center
CFREC-Apopka Research Report RH-95-4

A basic ingredient of most potting mixes used in foliage plant production is peat, which tends to be acidic. Dolomite is added to growing medium to raise pH to the range of 5.5 to 6.5 and to supply plants with calcium and magnesium needed for healthy growth. Dolomite also slowly dissolves in the growing medium over time, helping to counteract the acidifying effects some fertilizers and/or irrigation water can have on growing medium pH. The need for new research on pH has been identified by industry as important due to observation of low pH in many soil and water samples.

Nursery supply companies offer several different dolomitic limestone products which vary in particle size (grade). In a previous test, finer grades of limestone were more effective than coarser grades in raising pH of container medium of boxwood (Buxus sempervirens), grown in 2-gal plastic containers over a two year production period (Leda and Wright, 1991).

The following two experiments were performed to explore the short-term effects of different dolomite products having different grades on pH of growing medium in 6-inch (15-cm) pots during production of Dieffenbachia maculata 'Camille'. Whether low pH influences foliage crop quality and whether different dolomite sources could influence pH were the main questions prompting this research. A secondary purpose was to demonstrate effects of using recommended versus excess fertilizer rates on crop quality as well as pH.

For both experiments Dieffenbachia maculata 'Camille' (Camille dieffenbachia, Agristarts II, Inc., Kelly Park Road, Apopka FL 32712) in 72-pack trays were transplanted into 6-inch (15cm) plastic tub pots on June 6, 1994. A customized form of Farard #4 growing medium (Fafard of Florida Inc., 3723 Hogshead Road, Apopka, FL 32703) containing no lime was used. The four different dolomite products incorporated into growing medium at transplanting time were Soil Doctor Dolomite, James River Dolomite, Asgrow Dolomite and James River Camadil Dolomite. The particle size distribution of each of the four products is given in Table 1. 'Camille' dieffenbachia were grown to salable size in a greenhouse where maximum light intensity was about 2000 ft-c (266 µmol•m^-2•s ^-1) and air temperatures ranged from 65°F to 90°F (18°C to 35°C). Plants were watered overhead one to three times per week as necessary.

Electrical conductivity (µmhos/cm) and pH of leachate collected from growing medium of Camille dieffenbachia were measured when experiments were initiated in June, five days before fertilizer treatments started, then monthly until research was terminated in September, 1994. Plant size index was determined by the formula; (height + width) ÷ 2. Growth was determined by the formula; final plant size index - initial plant size index. Plants were graded at termination of research based on a scale of 1 = dead, 2 = poor quality, unsalable, 3 = fair quality, salable, 4 = good quality and 5 = excellent quality.

In experiment 1, growing medium was amended at transplanting time with 1 lb/yd³ (0.59 kg/m³) Micromax (The Scotts Company, 6656 Grantway, Allentown, PA 18106) and 7.5 lb/yd³ (4.45 kg/m³) of one of the four dolomite products tested. This was a 4 fertilizer rate x 4 dolomite product factorial test with 5 replications per treatment. Starting on June 13, 1994, plants were fertilized once a week. Liquid fertilizer made from concentrated stock solutions was used, with NH₄ and NO₃ each providing 50% of the nitrogen source. When fertilized each pot received 100 ml fertilizer solution made up of NH₄NO₃, K₂SO₄ and H₃PO₄ so that fertilizer formulation was 6N-2P-5K. Fertilizer rates tested were 1222, 1833, 2444 or 3666 lb N/A/Yr, (1369, 2053, 2737 or 4106 kg/ha/yr) with 1833 lbs being the recommended level.

Electrical conductivity (µmhos/cm) and pH of the growing medium leachate were determined on June 9, July 7, August 9 and finally on September 7, 1994. Initial plant height and width (cm) were measured on June 8, 1994 and final height and width were determined on September 21, 1994. Plants were graded on September 26, 1994, when research was terminated.

In experiment 2, the four dolomite products were incorporated into the same basic growing medium as used in the previous experiment, each product at three different rates. For each product tested, Camille dieffenbachia were also grown using the basic potting mix without dolomite. There were 5 replications per treatment.

Fafard #4 growing medium, containing no lime, was amended with 1 lb/yd³ Micromax. Treatments consisted of each of the four dolomite products incorporated at 0.0, 2.5, 5.0, or 7.5 lb/yd³ (0.0, 1.48, 2.96 or 4.45 kg/m³ ). Plants were fertilized once a week using the same liquid feed as used for experiment 1, applied at the recommended rate of 1833 lb N/A/yr.

Electrical conductivity (µmhos/cm) and pH of the growing medium leachate was determined initially on June 9, 1994. Electrical conductivity and pH were then determined once a month throughout production, on July 6, August 8 and September 6, 1994.

Results

In experiment 1, interaction of dolomite source and fertilizer rate affected pH of growing medium leachate on July 7, August 9 and September 7, 1994 (Figures 1, 2 and 3). From July 7 until termination, growing medium leachate with higher pH, fertilized at any rate, were mainly those with incorporated James River Camadil or Asgrow dolomite, followed by James River and Soil Doctor, in order of decreasing pH. The first two products are made up of more finer particles compared to particle size makeup of Soil Doctor Dolomite and James River Dolomite (Table 1). On June 9, 1994, five days before fertilizer treatments were initiated, pH of leachate collected from growing medium of Camille dieffenbachia averaged 5.4. Over time, the increase in fertilizer rate from 2444 to 3666 lb/A/yr had a much greater acidifying effect on growing medium leachate compared to when rate was increased from 1222 to 1833 or from 1833 to 2444 lb/A/yr.

Although all plant grades were above 4.0 (good quality), plants grown in medium containing Asgrow or James River dolomite were slightly better than plants grown in medium containing James River Camadil dolomite or Soil Doctor dolomite (Table 2). particle size of these two products were more similar, when compared to particle size of the two other products tested (Table 1). Soil Doctor Dolomite had more larger size particles and James River Camadil had more smaller particles.

The recommended fertilizer rate of 1833, the second lowest rate, produced the largest best quality plants (Table 2). Plants grew less and received lower plant grades as fertilizer rate was increased above the 1833 lb/A/yr rate.

When pH was measured soon after incorporation, on June 9, growing medium leachate from products made up of finer dolomite particles had higher electrical conductivity compared to leachate collected from growing medium getting one of the two products having coarser particles (Table 3). On later sampling dates, however, electrical conductivity of the growing medium leachate was not significantly different due to dolomite source. Electrical conductivity of leachate from samples collected from July through September increased as fertilizer rate increased.

In experiment 2, when dolomite sources were compared, no significant differences due to dolomite source were found in plant grade, plant growth and electrical conductivity and pH of the growing medium leachate (data not shown). However, for each dolomite source tested, incorporation rate did affect pH and electrical conductivity of leachate and most plant growth measurements.

Electrical conductivity of leachate was variable in its response to increasing dolomite rate, although it generally increased (Tables 4 through 7). However, over time conductivity decreased, and this probably related to use of nutrients by the plants as well as leaching of calcium and magnesium from dolomite.

When the amounts of Asgrow dolomite in growing medium increased plant growth increased but plant quality was not similarly affected (Table 8). Increasing dolomite incorporation rate from 0 to 5.0 lb/yd³ produced a small but significant increase in plant growth and quality for Camille dieffenbachia getting any of the other three dolomite products (Tables 9 through 11). When the amount of any of these three products in the growing medium was increased from 5.0 to 7.5 lb/yd³ plants grew less compared to plants getting the 5.0 lb/yd³ rate and quality did not further improve.

Quality of all Camille dieffenbachia was better than good when the experiment was terminated, but increasing lime rate did slightly increase plant quality for two of the four products tested, Soil Doctor Dolomite and James River Camadil Dolomite. These two products were on opposite ends of the particle size scale (Table 1). (In experiment 1 use of either of these two products incorporated at 7.5 lb/yd³ the highest rate tested in experiment 2, produced slightly lower quality plants).

Conclusions

Results from experiment 1 reinforce the importance of following recommended fertilizer guidelines. Higher than needed fertilizer rates, regardless of dolomite product used, adversely affected plant quality, had a more acidifying effect on growing medium pH and produced an unnecessary increase leachate electrical conductivity.

Since plants require calcium and magnesium for healthy growth, it makes sense that potting mixes containing dolomite would produce larger, better quality plants compared to growing medium without a source of those two elements.

When incorporated at the rates used here, dolomite products having more particles in the moderate size range shown in Table 1 may be slightly better for Camille dieffenbachia production compared to dolomite products made up of more large or fine size particles. Even though the two products with finer particles did a slightly better job of raising the pH level, pH of growing medium incorporated with either of the two products having moderate size particles was well within the acceptable limits for healthy foliage plant growth, and produced slightly better quality plants than the growing media containing either of the two dolomite products made up of finer particle sizes.

These results also demonstrate that pH in itself is not an especially good indicator of the potential for production of quality foliage plants as long as a reasonable level of dolomite has been incorporated into the growing medium.

Fig. 1. Interaction of dolomite source and fertilizer rate on pH of Ieachate collected from growing medlum on July 7. 1994.

Interaction significant at P = 0.0001.

Fig. 2. Interaction of dolomite souroe and fertilizer rate on pH of leachate collected from growing medium on August 9, 1994.

Interaction significant at P = 0.0001.

References

1. Conover, C.A. and R.T. Poole. 1990. Light and fertilizer recommendations for production of acclimatized Potted foliage plants. Nursery Digest 24(10):34-36. 58-59.
2. Leda, C.E. and R.D. Wright. Dolomitic lime particle size and container medium pH. 1991. J. Environ. Hort. 9(4):226-227.
3. Poole, R.T. 1985. Changing the pH of a potting medium. Foliage Digest 8(7):6-8.
4. Poole, R.T and C.A. Conover. 1983. Change in pH and soluble salts of container mixes. Proc. Fla. State Hort. Soc. 96:260-261.

1. Table 1. Particle size distribution of four dolomite sources utilized in the foliage industry. Dolomite sources were incorporated into growing medium used to produce Camille dieffenbachia. Products were passed though standard size sieves.

1. ^zSoil Doctor Dolomite manufactured by Soil Doctor Inc., PO Box 739, Polk City, FL 33868.
   ^yJames River Dolomite and James River Camadil Dolomite manufactured by James River Limestone Company, Inc., Buchanan VA.
   ^xAsgrow Dolomite manufactured by Asgrow Florida Company, Plant City, FL 33566.

Fig. 3. interaction of dolomite source and fertilizer rate on pH of leachate collected from growing medium on September 7, 1994.

Interaction significant at P = 0.0001.

1. Table 2. Effects of dolomite source and fertilizer rate on plant growth index and plant grade of Camille dieffenbachia. Experiment 1.
   

1. ^zGrowth index was defined as final plant size index - initial plant size index. Size index was defined as (height + width) ÷ 2. Initial size index was determined from height and width measured on June 8, 1994 and final size index was determined from plant height and width measured on September 21, 1994.
   ^yPlants were graded based on a scale of 1 = dead, 2 = poor quality, unsalable, 3 = fair quality, salable, 4 = good quality and 5 = excellent quality on September 26, 1994.
   ^xMean separation in columns by Duncan's multiple range test, 5% level. Means in same columns with same letters are not significantly different.
   ^wWeekly fertilizer treatments began on June 13, 1994, six days after initial height was measured.
   ^vns, *, **; Nonsignificant, significant at P = 0.05 or significant at P = 0.01, respectively.

1. Table 3. Effects of dolomite source and fertilizer rate on electrical conductivity (µmols/cm) of leachate from growing medium of Camille dieffenbachia during production^z. Experiment 1.
   

1. ^zCamille dieffenbachia were transplanted on June 6, 1994, from 72-cell pack liners into 6-inch tubs. Electrical conductivity of growing medium leachate was determined initially on June 9, 1994 and monthly thereafter until plants reached salable size and the experiment was terminated.
   ^yMean separation in columns by Duncan's multiple range test, 5% level. Means in the same columns with the same letters are not significantly different.
   ^xWeekly fertilizer treatments began on June 13, 1994, five days after initial leachate electrical conductivity levels were measured.
   ^wns,*, **; Nonsignificant, significant at P = 0.05 or significant at P = 0.01, respectively.

1. Table 4. Effects of incorporation rate of Asgrow dolomite on pH and electrical conductivity of the growing medium leachate from June 9, until September 6, 1994. Experiment 2.
   

1. ^zns, *, **; Nonsignificant, significant at P = 0.05 or significant at P = 0.01, respectively.

1. Table 5. Effects of incorporation rate of James River Camadil dolomite on pH and electrical conductivity of the growing medium leachate from June 9, until September 6. 1994. Experiment 2.
   

^zns, *, **; Nonsignificant, significant at P = 0.05 or significant at P = 0.01, respectively.

1. Table 6. Effects of incorporation rate of Soil Doctor dolomite on pH and electrical conductivity of the growing medium leachate from June 9, until September 6. 1994. Experiment 2.
   

^zns, *, **; Nonsignificant, significant at P = 0.05 or significant at P = 0.01, respectively.

1. Table 7. Effects of incorporation rate of James River dolomite on pH and electrical conductivity of the growing medium leachate from June 9, until September 6. 1994. Experiment 2.
   

1. ^zns, *, **; Nonsignificant, significant at P = 0.05 or significant at P = 0.01, respectively.

1. Table 8. Effects of incorporation rate of Asgrow dolomite on final plant size index, plant growth index and plant grade of Camille dieffenbachia. Experiment 2.
   

1. ^zFinal plant size index was deterrnined using the formula; (final height + final width) / 2. Final plant measurements were made on September 19, 1994.
   ^yGrowth index was determined using the formula; final size index - initial size index. Initial plant size index was determined when research began on June 8, 1994.
   ^xPlants were graded based on a scale of 1 = dead, 2 = poor quality, unsalable, 3 = fair quality, salable, 4 = good quality and 5 = excellent quality on September 26, 1994.
   ^wns, *, **; Nonsignificant, significant at P = 0.05 or significant at P = 0.01, respectively.

1. Table 9. Effects of incorporation rate of James River Camadil dolomite on final plant size index, growth index and plant grade of Camille dieffenbachia. Experiment 2.
   

1. ^zFinal plant size index was determined using the formula; (final height + final width) / 2. Final plant measurements were made on September 19, 1994.
   ^yGrowth index was determined using the Formular; final size index - initial size index. Initial plant size index was determined when research began on June 8, 1994.
   ^xPlants were graded based on a scale of 1 = dead, 2 = poor quality, unsalable, 3 = fair quality, salable, 4 = good quality and 5 = excellent quality on September 26, 1994.
   ^wns, * , **; Nonsignificant, significant at P = 0.05 or significant at P = 0.01, respectively.

1. Table 10. Effects of incorporation rate of Soil Doctor Dolomite on final plant size index, growth index and plant grade of Camille Dieffenbachia. Experiment 2.
   

1. ^zFinal plant size index was determined using the formula; (final height + final width) / 2. Final plant measurements were made on September 19, 1994.
   ^yGrowth index was determined using the formula; final size index - initial size index. Initial plant size index was determined when research began on June 8, 1994.
   ^xPlants were graded based on a scale of 1 = dead, 2 = poor quality, unsalable, 3 = fair quality, salable, 4 = good quality and 5 = excellent quality on September 26, 1994.
   ^wns, *, **; Nonsignificant, significant at P = 0.05 or significant at P = 0.01, respectively.

1. Table 11. Effects of incorporation rate of James River dolomite on final plant size index, growth index and plant grade of Camille dieffenbachia. Experiment 2.
   

1. ^zFinal plant size index was determined using the formula; (final height + final width) / 2. Final plant measurements were made on September 19, 1994.
   ^yGrowth index was determined using the formula; final size index - initial size index. Initial plant size index was determined when research began on June 8, 1994.
   ^xPlants were graded based on a scale of 1 = dead, 2 = poor quality, unsalable, 3 = fair quality, salable, 4 = good quality and 5 = excellent quality on September 26, 1994.
   ^wns, *; Nonsignificant or significant at P = 0.05, respectively.

1. (1)Professor of Environmental Horticulture and Center Director (retired 7/96), Central Florida Research and Education Center, 2807 Binion Road, Apopka FL 32703-8504.
   (2)Florida Technical Services Manager, Fafard Inc., 3223 Hogshead Road, Apopka, FL 32703-8504.
   (3)Technical Assistant, Central Florida Research and Education Center, 2807 Binion Road, Apopka, FL 32703-8504.