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University of Florida, IFAS
Central Florida Research and Education Center - Apopka
CFREC-Apopka Research Report RH-92-1
R.T. Poole and C.A. Conover*
The pH of growing medium controls availability of nutrients used for plant growth. Most foliage plants grow best in highly organic soilless mixes when pH is within a range of 5.3 to 6.5, although good quality plants have been obtained using mixes with lower and higher pH levels (2, 3, 4). Medium pH is usually adjusted to the appropriate level by additions of lime at the time components are mixed. Over time medium pH may drop due to acidifying irrigation water and/or fertilizer, some of which can also acidify the medium. In order to raise pH levels of medium in containers with established foliage plants, the most widely recommended method has been addition of hydrated lime. Calcium hydroxide (hydrated lime) is surface applied in a water suspension of no more than one pound/100 gal or less to 100 ft2 of surface area (1). Plants should be retreated no more frequently than at 4 week intervals, if necessary, because it is thought that rapid pH change or the corrosive action of the hydroxide ion may harm plants.
In recent years, several foliage plant growers in Central Florida have followed these suggestions for pH adjustment and have not obtained desired results. Although these recommendations are found in popular literature and we have used them ourselves, we could find no evidence in scientific journals of experiments testing the ability of surface applied liming agents to raise medium pH during plant production.
Materials and Methods
This 3 x 5 x 2 factorial experiment, with 5 replications per treatment, was initiated on 15 March 1991. Dieffenbachia maculata 'Camille' ('Camille' dieffenbachia) growing in 3 inch pots were transplanted into 6 inch containers filled with a medium composed of 3 Florida sedge peat: 1 builder's sand (3:1 by volume). The medium was amended with 1.5 lbs/yd3 Micromax (Grace/Sierra Co., Milpitas, CA 95035) and 0, 2.5 or 5 lbs/yd3 dolomite. 19-6-12 Osmocote (Grace/Sierra Co., Milpitas, CA 95035), a three month release rate fertilizer, was surface applied, 5.7 g/6 inch pot, when transplanting was completed. Plants were then placed in a greenhouse where maximum light intensity was 1500 ft-c and air temperature ranged from 65 to 90°F. Irrigation water with a pH measuring 7.2 was applied 3 times per week using overhead irrigation.
On 10 April 1991, each 6 inch pot received 0.0, 0.9, 1.8, 2.7, or 3.6 g Ca(OH)2 suspended in 100 ml of water, poured onto the medium surface. Four weeks later, on 8 May 1991, these treatments were repeated for only half of the pots, so that an equal number of pots tested received either one or two applications of Ca(OH)2.
Electrical conductivity (µmhos/cm) and pH of the medium leachate were recorded on 2 April, 18 April, 3 May, 16 May, 6 June, 3 July and 2 August 1991. Initial height was measured on 19 March 1991 with final plant height measured on 2 August 1991. Plants were graded (based on a scale of 1 = dead, 2 = poor quality, unsalable, 3 = fair quality, salable, 4 = good quality, salable and 5 = excellent quality plants) when experiment was terminated on 13 August 1991.
Results and Discussion
'Camille' dieffenbachia plant height and grade were slightly affected by incorporated dolomite levels (Table 1). Plants growing in medium containing no dolomite were only 0.8 inches shorter and received slightly lower grades than plants in medium with 5.0 lbs/YD3 dolomite. Increasing Ca(OH)2 from 0 to 1.8 g/6 inch pot increased plant height, but further increases in Ca(OH)2, up to 3.6 g/6 inch pot, apparently caused some stunting. The number of Ca(OH)2 applications (1 or 2) alone did not significantly affect plant quality or height.
Interaction between Ca(OH)2 rate and number of applications, however, produced lower grade 'Camille' dieffenbachia plants at the highest rate (3.6 g/6 inch pot) with two applications (Table 2). Ca(OH)2 rate and number of applications also interacted to influence pH of medium leachate, which rose slowly over time when rate and number of applications increased from 1 to 2 (Table 3).
The pH of leachate increased as dolomite incorporation rate increased and pH of all dolomite treatments remained proportional over time (Table 4). Ca(OH)2 rate and number of applications did not significantly increase pH until 3 July, nearly 3 months after planting, and this delayed effect was also observed when the second Ca(OH)2 application occurred.
Increases in dolomite incorporation rate increased electrical conductivity of medium initially, but over time this trend was reversed, and electrical conductivity of medium with the higher dolomite levels was lowest after 2 months (Table 5). Leachate pH increased as dolomite incorporation rate increased and also increased over time for all dolomite levels tested. Nutrients become more soluble as the growing medium becomes more alkaline, which could explain why the leachate with higher pH had more electrical conductivity initially. When nutrients are easily soluble in the water medium solution, they become readily available for plant uptake and are also easily leached from containers, which could explain the drop in electrical conductivity of the more alkaline leachate pH over time. Ca(OH)2 rates and number of applications tested had very little effect on electrical conductivity.
Increasing Ca(OH)2 rate and number of applications raised the pH several months after the time of treatment, but plants grew less and received lower plant grades as rate and number of applications increased. Also, the amount of time needed to raise pH was too long considering the average crop turnover time for many foliage plant species. Therefore, these preliminary data indicate that present recommendations are inadequate for rapid pH adjustments. Calcium hydroxide is considered slightly soluble in water and calcium carbonate, another popular liming agent, is practically insoluble in water (5). Other forms of calcium such as calcium nitrate and calcium chloride which are both very soluble in water (5), but not commonly used to adjust pH, should be tested for ability to raise medium pH without harming foliage plants growing in the medium.
Table 1. Height and plant grade of Dieffenbachia
maculata 'Camille' grown in a medium treated with various
lime applications.
| Dolomite lbs/yd3 | Plant gradeZ 13 Aug 1991 |
Height (in) 2 Aug 1991 |
|---|---|---|
| 0.0 | 4.8 | 15.6 |
| 2.5 | 5.0 | 16.1 |
| 5.0 | 5.0 | 16.4 |
| Significancey | ||
| linear | * | ** |
| quadratic | ns | ns |
| Ca(OH)2 g/6 inch pot | ||
| 0.0 | 5.0 | 16.2 |
| 0.9 | 5.0 | 16.3 |
| 1.8 | 5.0 | 16.3 |
| 2.7 | 5.0 | 15.9 |
| 3.6 | 4.7 | 15.6 |
| Significancey | ||
| linear | ** | * |
| quadratic | ** | ** |
| No. of applications | ||
| 1 | 4.9 | 16.1 |
| 2 | 4.9 | 15.9 |
| Significancey | ||
| linear | ns | ns |
zPlants were graded based on a scale of 1 = dead, 2
= poor quality, unsalable, 3 = fair quality, salable, 4 = good
quality, salable and 5 = excellent quality plant material.
yns, *, **; Results nonsignificant, significant at P =
0.05 and P = 0.01, respectively.
Table 2. Interaction effects of Ca(OH)2 application rates and number of Ca(OH)2 treatments on Dieffenbachia maculata 'Camille' plant grade, 13 August l991.z
| Ca(OH)2 g/6" pot/ application |
Ca(OH)2 applied 10 April 1991 |
Ca(OH)2 applied 10 April and 8 May 1991 |
|---|---|---|
| 0.0 | 5.0 | 5.0 |
| 0.9 | 4.9 | 5.0 |
| 1.8 | 5.0 | 4.9 |
| 2.7 | 5.0 | 4.9 |
| 3.6 | 4.9 | 4.6 |
zResults significant at P = 0.024.
Table 3. Interaction effects of Ca(OH)2 application rate and number of Ca(OH)2 treatments on pH of leachate collected from pots containing Dieffenbachia maculata 'Camille', 2 August l991.z
| Ca(OH)2 g/6" pot/ application |
Ca(OH)2 applied 10 April 1991 |
Ca(OH)2 applied 10 April and 8 May 1991 |
|---|---|---|
| 0.0 | 4.4 | 4.1 |
| 0.9 | 4.6 | 4.7 |
| 1.8 | 4.9 | 5.0 |
| 2.7 | 4.8 | 5.5 |
| 3.6 | 5.1 | 5.6 |
zResults significant at P = 0.003.
Table 4. Leachate pH from pots
containing Dieffenbachia maculata 'Camille'. Plants potted
15 March 1991. Ca(OH)2 applied 10 April to all plants.
Ca(OH)2 - applied 8 May to the total number of plants.
| Dolomite lbs/yd3 |
2 Apr | 18 Apr | 3 May | 16 May | 6 Jun | 3 Jul | 2 Aug |
| 0.0 | 3.8 | 3.7 | 3.7 | 3.8 | 4.0 | 4.2 | 4.2 |
| 2.5 | 4.6 | 4.5 | 4.6 | 4.8 | 5.3 | 5.2 | 4.9 |
| 5.0 | 5.4 | 5.6 | 5.6 | 5.9 | 6.7 | 6.2 | 5.5 |
| Significancez | |||||||
| linear | ** | ** | ** | ** | ** | ** | ** |
| quadratic | ns | ns | ns | ** | ** | * | * |
| Ca(OH)2 g/6" pot |
|||||||
| 0.0 | 4.7 | 4.7 | 4.6 | 4.8 | 5.0 | 4.7 | 4.2 |
| 0.9 | 4.7 | 4.6 | 4.7 | 4.8 | 5.3 | 5.1 | 4.7 |
| 1.8 | 4.6 | 4.6 | 4.7 | 4.8 | 5.3 | 5.2 | 5.0 |
| 2.7 | 4.6 | 4.6 | 4.7 | 4.8 | 5.5 | 5.5 | 5.2 |
| 3.6 | 4.5 | 4.6 | 4.7 | 4.9 | 5.5 | 5.3 | 5.3 |
| Significancez | |||||||
| linear | ns | ns | ns | ns | ns | * | ** |
| quadratic | ns | ns | ns | ns | ns | ns | ** |
| No. of applications |
|||||||
| 1 | 4.6 | 4.6 | 4.7 | 5.3 | 5.3 | 5.0 | 4.8 |
| 2 | 4.6 | 4.6 | 4.7 | 5.4 | 5.4 | 5.3 | 5.0 |
| Significancez | |||||||
| linear | nay | na | na | ns | ns | ** | ** |
zns, *, **; Results nonsignificant, significant at
P = 0.05 and significant at P = 0.01, respectively.
yna; Not applicable. Ca(OH)2 treatment 1
applied 10 April 1991, treatment 2 applied 8 May 1991.
Table 5. Electrical conductivity
(µmhos/cm) of leachate from pots containing Dieffenbachia
maculata 'Camille'. Plants potted 15 March 1991. Ca(OH)2
applied to all plants 10 April. Ca(OH)2 applied to
total number of plants on 8 May 1991.
| Dolomite lbs/yd3 |
2 Apr | 18 Apr | 3 May | 16 May | 6 Jun | 3 Jul | 2 Aug |
| 0.0 | 2493 | 2151 | 1425 | 1151 | 592 | 547 | 522 |
| 2.5 | 2756 | 2208 | 1325 | 980 | 351 | 401 | 376 |
| 5.0 | 2987 | 2533 | 1447 | 1007 | 336 | 392 | 398 |
| Significancez | |||||||
| linear | ** | ** | ns | * | ** | ** | * |
| quadratic | ns | ns | * | ns | * | * | ns |
| Ca(OH)2 g/6" pot |
|||||||
| 0.0 | 2723 | 2350 | 1518 | 1010 | 352 | 377 | 382 |
| 0.9 | 2794 | 2333 | 1383 | 991 | 385 | 427 | 435 |
| 1.8 | 2807 | 2280 | 1334 | 991 | 445 | 469 | 459 |
| 2.7 | 2778 | 2290 | 1380 | 1100 | 456 | 448 | 445 |
| 3.6 | 2616 | 2235 | 1384 | 1138 | 493 | 511 | 439 |
| Significancez | |||||||
| linear | ns | ns | ns | ns | * | * | ns |
| quadratic | ns | ns | ns | ns | ns | ns | ns |
| No. of applications |
|||||||
| 1 | 2737 | 2260 | 1356 | 1058 | 410 | 417 | 424 |
| 2 | 2754 | 2335 | 1445 | 1034 | 442 | 476 | 440 |
| Significancez | |||||||
| linear | nay | na | na | ns | ns | * | ns |
zns, *, **; results nonsignificant at P = 0.05 and
significant at 0.01, respectively.
yna; Not applicable. Ca(OH)2 treatment 1
applied 10 April 1991, treatment 2 applied 8 May 1991.
*Professor, Plant Physiology, and Professor and Center Director (retired 7/96), respectively. Central Florida Research and Education Center - Apopka, 2807 Binion Road, Apopka, FL 32703-8504.
Literature Cited
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. Joiner, J.N., C.A. Conover and R.T. Poole. 1981. Nutrition and fertilization. Chapter in: Foliage Plant Production, J.N. Joiner ed., Prentice-Hall Inc., Englewood Cliffs, NJ.
3. Peterson, J.C. 1981. Modify your pH perspective. Florist's Rev. 169(4386):34-35, 92-93.
4. Wright, R.D. 1983. Study indicates need for changes in nutrition programs for plants in containers. Amer. Nurseryman 157(1): 109-111.
5. Windholz, M., S. Budavari, L.Y. Stroumtsos and M.N. Fertig. 1976. The Merck Index. 9th edition, M. Windholz editor. Merck & Co., Inc., Rahway, NJ. pp. 210-213.