Effects of Fertilization Rate on Draceana deremensis 'Warneckii' Growth and Electrical Conductivity of the Medium Leachate

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University of Florida, IFAS,
Central Florida Research and Education Center - Apopka
CFREC-Apopka Research Report RH-91-10

C.A. Conover and R.T. Poole*

Growing medium soluble salts (SS) levels are determined by various factors including soil type, fertilizer composition, fertilizer application rate, salinity of irrigation water and the amount of water applied at each watering. When SS levels are adequate plant roots can easily absorb water from the medium/water solution. As salts levels increase, roots are less able to extract water from the solution and root systems will be smaller than root systems of plants grown in media with lower SS levels, even though no differences in foliage may be detected. When plants develop less roots compared to shoots their ability to survive in interior environments is compromised. Even higher concentrations of SS in media cause noticeable injury to foliage because plant roots become desiccated and can no longer extract needed nutrients and water.

Soluble salts of the growing medium solution, as measured by electrical conductivity, are utilized by many foliage plant producers as a method of determining soil fertility levels. Although data suggesting optimum SS ranges are available for many species of foliage plants, little information has been published regarding optimum SS levels as determined by the pour through nutrient extraction method compared to the other three methods of determining SS levels.

The pour through method of SS measurement has several advantages over other methods of SS determination when the same medium must be sampled several times. The growing medium is not removed, coated fertilizer beads are not ruptured, root damage in minimal and no specialized equipment is needed to extract the solution from the medium. Also, less time is required to perform each extraction compared to other methods of SS determination. The purpose of this research was to compare SS levels of medium receiving various amounts of fertilizer, determined by measuring the electrical conductivity of medium leachate, with growth of Draceana deremensis 'Warneckii'.

The experiment, initiated 20 August 1990, compared the effects of 10 fertilization rates on D. 'Warneckii' growth using 9 replications per treatment. Rooted D. 'Warneckii' cuttings were potted in 8 inch plastic containers, 3 per pot, using a growing medium of Florida peat:pine bark:sand in a ratio of 6:3:1 v/v, amended with 7 lbs dolomite and 1 lb Micromax /yd³ (Micronutrient manufactured by Grace/Sierra Co., Milpitas, CA 95035). After potting, plants were placed in a shadehouse covered with 73% propylene shadecloth, so that they received a maximum of 2000 ft-c in winter and 3500 ft-c in summer. Production temperatures ranged from 45 to 95°F, depending on time of year and plants were watered 4 times per week using overhead irrigation. Osmocote 19-6-12 (Grace-Sierra Co., Milpitas, CA 95035) was applied to medium surface of each pot on 15 October 1990, 15 March and 21 May 1991 at rates of 4.3, 8.6, 12.9, 17.2, 21.5, 25.8, 30.1, 34.4, 38.7 or 43.0 g/8 inch pot. These rates are equivalent to 20, 40, 60, 80, 100, 120, 140, 160, 180 or 200 lbs N/1000 ft²/yr.

After fertilizer applications started, electrical conductivity (ymhos/cm) and pH of the medium leachate from pots growing D. 'Warneckii' were determined monthly, using the pour-through method. Deionized water was applied to medium surface until about 50 ml was collected in a beaker placed under the pot. The amount of SS dissolved in the collected water (leachate) was measured with a conductance meter (YSI model 35, Yellow Springs Instrument Co. Inc., Yellow Springs, OH 45387). Leachate pH was measured using a selective ion analyzer (Fisher Accumet model 750, Fisher Scientific Co., Pittsburgh, PA 95219). Plant height was measured initially and at three month intervals. Plants were graded when research was completed, based on a scale of 1 = poor quality, unsalable, 3 = fair quality, salable and 5 = excellent quality plants. Research was concluded on 21 June 1991.

There was a dramatic increase in growth and quality when D. 'Warneckii' plants were fertilized at the rate of 8.6 g/8 inch pot compared to 4.3 g 19-6-12/8 inch pot, the lowest rate tested (Table 1). Height increased slowly as fertilization level increased from 12.9 to 38.7 g/19-6-12/8 inch pot. However, the higher fertilizer levels tested, 21.5 to 43.0 g 19-612/8 inch pot, did not produce better quality plants than the 17.2 g 19-6-12/8 inch pot rate.

The electrical conductivity (µmhos/cm) of the medium leachate from pots containing test plants generally increased as fertilization rate increased (Table 2). However, the leachate electrical conductivity readings obtained in this experiment were well within the general range recommended for good foliage plant production. The pH of the leachate generally decreased as fertilization levels increased.

The recommended 19-6-12 Osmocote application rate for production of acclimatized Draceana deremensis cultivars grown in 8 inch containers is 7.2 g/pot/3 months. This rate was determined based on research evaluating plants adaptability to indoor conditions. Optimum fertilizer rates for acclimatized potted foliage production are those that produce good quality plants with root systems large enough to support top growth when plants are placed indoors. Over fertilized plants may not show signs of SS injury during production; however, these plants will have smaller root systems, making them less able to adjust to indoor conditions.

The plants in this experiment fertilized with 8.6 g/8 inch pot, close to the rate recommended for the production acclimatized Draceana deremensis cultivars, had leachate conductivity readings ranging from 144 to 189 ymhos/cm. Plants grown at several of the fertilization levels tested received the same high plant grade. The lowest fertilization rate producing the highest graded plants was 12.9 g/8 inch pot. The electrical conductivity of leachate from pots fertilized at the 12.9 g/8 inch pot rate ranged from 202 to 247 ymhos/cm.

* Professor and Center Director (retired 7/96), and Professor of Physiology, respectively, University of Florida, IFAS, CFREC-Apopka, 2807 Binion Road, Apopka, FL 32703-8504.

Additional Reading

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.
2. Conover, C.A., R.T. Poole and R.W. Henley. 1975. Growing acclimatized foliage plants. Florida Foliage Grower 12(9):1-7.
3. Poole, R.T. 1981. Soluble salts interpretation. Foliage Digest 4(6):11-13.
4. Poole, R.T., C.A. Conover and A.R. Chase. 1985. Soluble salts interpretation for ornamental crop production. Proc. Trop. Reg. Amer. Soc. Hort. Sci. 27:33.
5. Poole, R.T. and A.R. Chase. 1987. Response of foliage plants to fertilizer application rates and associated leachate conductivity.
6. Wright, R.D. 1986. The pour-through nutrient extraction procedure. HortScience 21:227-229.

^ZPlants were graded on a scale of 1 = poor quality, unsalable, 3 = fair quality, salable and 5 = excellent quality plants.
^Y19-6-12 Osmocote 3 month release rate fertilizer (Grace-Sierra Co., Milpitas, CA 30905) was applied to medium surface on 15 October 1990, 15 March and 21 May 1991. Application rates were equal to 20, 40, 60, 80, 100, 120, 140, 160, 180 or 200 N/1000 ft²/yr).
^Xns, **; Nonsignificant and significant at P = 0.01 respectively.

^Z19-6-12 Osmocote 3 month release rate fertilizer (Grace-Sierra Co., Milpitas, CA 30905) was applied to medium surface on 15 October 1990, 15 March and 21 May 1991. Application rates were equal to 20, 40, 60, 80, 100, 120, 140, 160, 180 or 200 N/1000 ft²/yr).
^Yns, *, **; Nonsignificant, significant at P = 0.05 and P = 0.01 respectively.