Using the Pour Through Nutrient Extraction Procedure Production Regimes: Optimum Fertilizer Rates and Associated Leachate Electrical Conductivity Levels of Twelve Foliage Plants

University of Florida, IFAS
Central Florida Research and Education Center - Apopka
CFREC-Apopka Research Report RH-92-24

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

Since many foliage plants can be grown satisfactorily with a wide range of fertilizer rates, the best fertilization rate, from an environmental and economic standpoint, cannot always be determined by visual inspection. Even though excess fertilizer may not harm some plants, excess nitrogen (N) and phosphorous (P) ions can be lost through leaching and eventually contaminate groundwater supplies. Growing regimes designed to have minimum detrimental impact on the natural environment should include only enough fertilizer for good plant growth, combined with irrigation practices that limit or eliminate leachate formation.

Environmentally sound fertilization rates can be determined by monitoring growing medium electrical conductivity (EC) levels. Medium EC levels, except in cases where salty irrigation water is used, are generally associated with fertilization rates (the higher the fertilizer rate the higher will be the associated EC level). Electrical conductivity levels will also rise when leaching is limited and plants are over-fertilized because plants are not utilizing all the available ions.

This paper presents the results of several years of research with 12 foliage plants to find the lowest fertilization rates (and associated leachate electrical conductivity levels as determined by the pour-through method) associated with production of high quality plants. The pour-through method was used to determine leachate electrical conductivity because it has been shown to be as satisfactory as other more expensive and time consuming methods of measuring EC. Unlike sampling methods that require removal of medium from containers, the pour-through method does not disturb plant roots, a major advantage when periodic monitoring is done. As nurseries implement more environmentally and economically sound growing regimes, the pour-through method should become more widely used by foliage growers for self-determination of EC levels periodically during the crop production cycle.

Materials and Methods

GREENHOUSE PRODUCTION: The nine genera grown under greenhouse conditions were 'Silver Queen' aglaonema [Aglaonema Schott. 'Silver Queen'], 'Apollo' zebra plant [Aphelandra squarrosa Nees 'Apollo'], parlor palm [Chamaedorea elegans Mart.], 'Banana' croton [Codiaeum variegatum (L.) Blume 'Banana'], dumb cane [Dieffenbachia maculata (Lodd) G. Don 'Camille'], corn plant [Dracaena fragrans (L.) Ker-Gawl. 'Massangeana'], compact Boston fern [Nephrolepis exaltata (L.) Schott. 'Bostoniensis Compacta'], heart-leaf philodendron [Philodendron scandens oxycardium (Schott) Bunt.] and 'Petite' peace lily; [Spathiphyllum Schott. 'Petite']. Liners obtained from local growers were planted into 6-inch containers using Vergro Container Mix A (Canadian peat moss:coarse grade vermiculite:perlite, 2-1-1 by volume, plus starter nutrient charge, (Verlite Company, Tampa, FL 33610). Plants were grown in glass greenhouses where they received 1500 or 2000 ft-c maximum light intensity with air temperatures ranging from 68 to 95°F, depending on season. Plants were watered overhead with tap water (pH-7.4, EC-0.3 millimhos/cm) as needed to promote healthy growth. Containers were top-dressed with Osmocote 19-6-12 (Grace/Sierra Co. Milpitas, CA 95035) fertilizer, at 3-month intervals, at the ten rates shown in figures 1 through 9.

SHADEHOUSE PRODUCTION: Three genera grown under shadehouse conditions were Norfolk Island pine [Araucaria heterophylla (Salisb.) Franco], areca palm [Chrysalidocarpus lutescens H. Wendl] and 'Warneckii' dracaena [Dracaena deremensis Engl. 'Warneckii']. Seedlings of Norfolk Island pine and Areca palm and rooted cuttings of 'Warneckii' dracaena were potted into 8-inch pots, three per container. Growing medium used for shadehouse production was composed of Florida sedge peat:pine bark:builders' sand in a ratio of 6:3:1 v/v, amended with 7 lbs/yd³ dolomite and I lb/yd³ Micromax (micronutrient blend manufactured by Grace/Sierra Co.). Plants were grown in a shadehouse covered with 73% polypropylene shadecloth, so that maximum ft-c at plant level was 2000 in winter and 3500 in summer. Air temperatures ranged from 45 to 95°F, depending on the time of year, and plants were watered overhead with tap water (pH-7.4, EC-0.3 millimhos/cm) as needed to promote healthy growth. Containers were also top-dressed with Osmocote 19-6-12, at 3-month intervals, at the rates shown in figures 10 through 12.

Electrical conductivity and pH of medium leachate from containers of both greenhouse and shadehouse grown plants were determined monthly, using the pour-through method. Leachate was collected in the following manner: 1) leachate was collected at times when growing medium was judged to be about 50% saturated with water, determined by sight and touch; 2) a 2000- or 4000-ml beaker, depending on pot size, was placed under every pot sampled; 3) deionized water was poured onto medium surfaces until about 50-ml of leachate was collected in the beaker. Electrical conductivity (millimhos/cm) 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).

Plants were grown for a time sufficient for production of a good quality crop. Some plants, like zebra plant and compact Boston fern, were "finished" or ready-for-sale after only 3 months, while others, such as areca palm and Norfolk Island pine, were grown for l year before considered to be ready-for-sale. Plant foliage was graded (plant grade) when crops were "finished" (based on a scale of l = dead, 2 = poor quality, unsalable, 3 = fair quality, salable, 4 = good quality and 5 = excellent quality plants). Greenhouse grown crops also received a root grade when ready-for-sale (based on a scale of 1 = 0 - 20% root ball covered with healthy roots, 2 = 21 - 40% root ball covered with healthy roots, 3 = 41 60% root ball coverage, 4 = 61 - 80% root ball coverage and 5 = 81 - 100% root ball covered with healthy roots).

Results

In the results and conclusion sections, the term "excessive fertilization rate", when applied to individual plants, refers to all application rates greater than the lowest rate producing best quality plants.

GREENHOUSE PRODUCTION

Aglaonema 'Silver Queen'. Salable plants were produced at the lowest rate tested, but results indicate a rate near 7.2 g/6-inch pot/3-months would yield highest graded plants with excellent roots (Fig. 1). Plants fertilized at higher rates received lower plant and root grades. Root and plant grades of aglaonemas were highly correlated. EC readings above 1.5 millimhos/cm are indicative of excess fertilizer application.

Aphelandra squarrosa 'Apollo'. 'Apollo' zebra plants with good root and plant grades were produced over a wide range of fertilization levels, a reflection of their tolerance of high EC levels (Fig. 2). The 4.8 g/6-inch pot/3-month rate would be the level of choice to reduce potential for N and P in leachate. EC readings above 3.0 millimhos/cm are indicative of excess fertilization.

Chamaedorea elegans. Parlor palm root and plant grade were highly correlated (Fig. 3). Best quality plants were produced within a narrow fertilization range. Plants getting 2.4 to 7.2 g/6-inch pot/3-months had high plant and root grades, although those receiving 2.4 g were somewhat lighter green in color. Plant and root grade decreased rapidly as fertilizer rate increased beyond 7.2 g/6-inch pot/3-months. EC levels above 3.0 millimhos/cm are probably indicative of excess fertilization which will reduce plant quality.

Codiaeum variegatum 'Banana'. 'Banana' crotons were graded after two months spent in an interior environment (Fig. 4). Best plant grades were received by plants top dressed with 7.2 g/6-inch pot/3-months. Leachate from these pots had maximum EC level of 3.5 millimhos/cm.

Dieffenbachia maculata 'Camille'. Fertilization at 4.8 g/6-inch pot/3-months produced 'Camille' dieffenbachia with highest quality foliage and roots, although comparable quality 'Camille' dieffenbachia were grown with rates ranging from 2.4 to 12.0 g/6-inch pot/3-months (Fig. 5). This shows that 'Camille' quality is not enhanced by excessive fertilizer, which elevates EC levels. Fertilization rates producing EC levels above 3.0 millimhos/cm are excessive and could cause N and P pollution.

Dracaena fragrans 'Massangeana'. Plant grades of corn plants jumped from about 3.0 to almost 5.0 when fertilizer rate increased from 2.4 to 4.8 g/6-inch pot/ 3-months (Fig. 6). Plant grades remained in the 4.5 to 4.8 range when plants received the 7.2 to 24 g/6 inch pot/3 months rates tested in this experiment. 'Massangeana', like 'Camille', is a good example of a plant that requires a moderate amount of fertilizer for good growth, but can tolerate much higher fertilization levels. These results clearly demonstrate that excess fertilizer rates did not improve 'Massangeana' plant quality, but created higher levels of EC in leachate. A fertilizer level of 7.2 g/6-inch pot/3-months and the maximum EC level associated with this rate (2.0 millimhos/cm) is recommended to reduce risk of N and P pollution and still achieve high quality plants.

Nephrolepis exaltata 'Compacta'. Compact Boston fern plants receiving from 4.8 to 24 g/6-inch pot/3-months had comparable root and shoot grades (Fig. 7). Plants fertilized with the lowest rate tested, 2.4 g/6-inch pot/3-months, were unsalable (plant grade below 3.0). However, suggested fertilizer level is 7.2 g/6 inch pot/3 months, based on an EC level maximum of about 1.5 millimhos/cm. As demonstrated with 'Camille' and 'Massangeana', even though this fern is tolerant of high fertilizer conditions, it just makes no sense to apply unneeded amounts.

Philodendron scandens oxycardium. Heart-leaf philodendrons received high plant and root grades when fertilized over a wide range, but both root and plant grades gradually decreased when rates were increased over 4.8 g 6-inch pot/3-months, the fertilizer rate associated with highest plant grades (Fig. 8). An EC level of about 2.0 millimhos/cm would mean plants were receiving an adequate amount fertilizer without creating excessive N and P ions in leachate.

Spathiphyllum 'Petite'. Only 4.8 g of fertilizer was needed to produce good quality (4.0 plant grade) 'Petite' peace lilies (Fig. 9). Plants in containers top-dressed with 7.2 g/6inch pot/3-months received the highest plant grades. Plant grades and root grades decreased, and leachate EC levels increased, as fertilizer rates increased from 12 to 24 g/6-inch pot/3-months. Since high fertilizer rates increase risk of tipburn and reduce overall quality of 'Petite' peace lily, EC levels should not exceed 1.5 millimhos/cm.

SHADEHOUSE PRODUCTION

Araucaria heterophylla. Norfolk Island pines grew best with 12.2 g/8-inch pot/3months (Fig. 10) and increasing fertilizer rate beyond that level produced lower quality plants. An EC level near 1.0 millimhos/cm was associated with fertilization at the optimum level and is suggested.

Chrysalidocarpus lutescens. Plant grades of areca palms increased as fertilization rate increased, up to 24.5 g/8-inch pot/3-months (Fig. 11). Further rate increases did not improve plant grades. Based on our present data, a range between 20 to 24 g/8-inch pot/3months is probably the optimum fertilization level. An EC of 1.0 millimhos/cm would indicate appropriate fertilization level.

Dracaena deremensis 'Warneckii'. A minimum of 8.6 g/8-inch pot/3-months was needed to produce good quality 'Warneckii' dracaena (Fig. 12). Further increases in fertilizer rate did not improve quality of 'Warneckii' grown in this experiment. Data from this test show very low EC levels correspond to optimum fertilizer rate.

Conclusion

Medium leachate EC levels followed a pattern of increasing as fertilizer rates increased. When plant nutritional requirements were being met, plant grade curves flattened for soluble salt tolerant plants as fertilizer rates increased. When plants were fertilized at higher than optimum rates, the unused fertilizer accumulated in the growing medium, which elevated EC levels. Some of the foliage plants that grew well only when receiving a narrow range of fertilization rates were those that are known to be intolerant of high EC levels.

These results confirm for some growers what they already know. They are fertilizing crops at an unnecessarily high rate and relying on frequent medium leaching to prevent soluble salts buildup. Extra fertilizer is still viewed by many as an "insurance policy" guaranteeing a fast growing, good looking crop. Since fertilizer expenditures amount to about 3 % of total production costs, these growing regimes can remain profitable only as long the costs of environmental pollution are unaccounted for. Growers need to realize that the "premiums" on these "insurance policies" in the future may be readjusted to include fines and penalties for being in noncompliance with laws regulating N and P in site runoff, making them just too expensive to be profitable.

One reason growers are reluctant to implement "conservative" production practices is the fear that, without regular leaching, soluble salts might accumulate to harmful levels in the growing medium. Usually this will not be a problem when fertilizing at the environmentally correct level. We suggest adopting "conservative" production practices, along with a bimonthly program monitoring growing medium electrical conductivity levels.

Information provided in this paper can help growers determine whether they are over-fertilizing and, thereby, increasing the potential for N and P contamination of ground and surface water. The data in the text and tables can be used in two ways: 1) Check the suggested fertilizer rate in the text against the rate you are applying and also, see reference No. 1 for rates producing high quality acclimatized foliage plants; 2) Check growing medium EC levels of crops in your nursery against levels shown here to determine if you are applying too much fertilizer.

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

Further Reading

1. Conover, C.A. and R.T. Poole. 1990. Light and fertilizer recommendations for production of acclimatized potted foliage plants. Nrsy. Dig. 24(10):34-36, 58-59.

2. Hipp, B.W., D.L. Morgan, and D.Hooks. 1979. A comparison of techniques for monitoring pH of growing medium. Commun. Soil Sci. Plant Anal. 10:1233-1238.

3. Poole, R.T. and A.R. Chase. 1986. Growth of six ornamental plants and soluble salts of the growing media. Proc. Fla. State Hort. Soc. 99:278-280.

4. Poole, R.T. and A.R. Chase. 1987. Response of foliage plants to fertilizer application rates and associated leachate conductivity. HortScience 22(2) :317-318.

5. Poole, R.T. and C.A. Conover. 1990. Leachate electrical conductivity and pH for ten foliage plants. J. Environ. Hort. 8(4): 166-172.

6. Wright, R.D. 1986. The pour-through nutrient extraction procedure. HortScience 21(2):227-229.

7. Yeager, T.H., R.D. Wright and S.J. Donohue. 1983. Comparison of pour-through and saturated pine bark extract N,P,K, and pH levels. J. Amer. Hort. Sci. 108(1): 112-114.

Fig 1. Aglaonema 'Silver Queen' Plant & root quality vs. soluble salts

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