TABLE OF CONTENTS

 

TABLE OF CONTENTS.................................................................................................... i

1.0 .. INTRODUCTION AND BACKGROUND............................................................. 1

2.0... MATERIALS AND METHODS  ........................................................................... 1

3.0... RESULTS ................................................................................................................. 3

4.0... DISCUSSION ........................................................................................................... 5

5.0... REFERENCES ......................................................................................................... 6

6.0... ACKNOWLEDGMENTS  ....................................................................................... 7

APPENDIX A ................................................................................................................ A-1

 

LIST OF TABLES

 

TABLE 1 ............................................................................................................................. 2

TABLE 2  ............................................................................................................................ 3

TABLE 3  ............................................................................................................................ 4

TABLE 4  ............................................................................................................................ 4

TABLE 5  ............................................................................................................................ 5

 

LIST OF FIGURES (ONLLY 4 Figures incl. in this electronic format)

 

FIGURE 1- Full System View, Week 1...................................................................................

FIGURE 2 - Full System View With Reflective Paper Hung On Fourth Side.............................

FIGURE 3 - Individual Seedling, Week 1................................................................................

FIGURE 4 - Full System View, Week 2..................................................................................

FIGURE 5 - Individual Seedling, Week 2................................................................................

FIGURE 6 - Full System View, Week 3..................................................................................

FIGURE 7 - Full System View, Week 4..................................................................................

FIGURE 8 - Individual Seedling, Week 4................................................................................

FIGURE 9 - Full System View, Week 5..................................................................................

FIGURE 10 - Individual Seedling, Week 5..............................................................................

FIGURE 11 - Full System View, Week 6................................................................................

FIGURE 12 - Individual Seedling, Week 6..............................................................................

FIGURE 13 - Full System View, Week 7................................................................................

FIGURE 14 - Full System View, Week 8................................................................................

FIGURE 15 - Individual Seedling, Week 8 .............................................................................

FIGURE 16 - Full System View, Week 9................................................................................

FIGURE 17 - Full System View, Week 10..............................................................................

FIGURE 18 - Hanging To Dry.................................................................................................

 

 

 

 

1.0    INTRODUCTION AND BACKGROUND

 

This experiment tested the use of recycled glass as a hydroponic rooting medium.  The hypothesis of this study was that glass is a viable rooting medium and will produce no greater or lesser growth yields than an expanded clay aggregate. Results supported the hypothesis -- there was no statistical difference between the glass-grown basil and the control-grown basil. This report describes the experiment conducted, presents results, and discusses the significance of the findings.  

 

2.0 MATERIALS AND METHODS

 

THE EXPERIMENTAL SYSTEM

To test the use of recycled mixed colored glass as a hydroponic rooting medium, 20 basil plants were grown in glass and 20 basil plants were grown in an expanded clay aggregate over a period of ten weeks and two days (May 7 - July 16). An expanded clay aggregate was selected as a control because it is a commonly used hydroponic medium. It is popular among hydroponic hobbyists because it is light, clean, and sterile. The following sections provide a description of the experimental system (Figures 1 arid 2).  

 

Lighting

A reflector and 1000 watt halide bulb hung over the four foot by eight foot growing area. An electrically powered chain mover slowly shifted the light from left to right over 20 minute intervals throughout the day. The lighting system was operated by a timer that turned the lights on at 6:30 a.m. and off at 8:30 p.m., providing 14 hours of light each day. The growing area was surrounded by three walls, each lined with reflective paper. A fourth piece of reflective paper hung across the open side to reflect as much light as possible onto the plants (Figure 2).  

 

As a follow-on to this report, the CWC sponsored another test of the use of glass/sand mixtures to make topsoil.

 

Glass and Clay Aggregates

Table 1
Glass and Clay Sieve Results
Nominal Opening	Sieve	Glass Percent Passing	Clay Percent Passing
9.5mm 4.74mm 2.36mm	3/8 inch	100%	70.4%
4.74mm	No. 4	82.6%	5.9%
2.36mm	No. 8	3.5%	2.2%
1.18mm	No. 16	1.36%	2.1%
0.6mm	No. 30	1.23%	2.1%
0.3mm	No. 50	.6%	2.0%
0.15mm	No. 100	.21%	1.8%
0.075mm	No. 200	.07%	-

 

Table 1 provides details of the glass and clay analyses.  The glass used ranged in size between Sieve Number 200 and Sieve Number 4 with most of the particles falling in a range of between Sieve Number 8 and Sieve Number 4.  The clay used ranged in size between Sieve No. 100 with most of the particles falling in the range of between Sieve Number 4 and 3/8 inch.

To ensure that the glass was sterile and contained no organic matter, it was heated in a kiln at 1,000 degrees Fahrenheit for four hours. The clay was also sterilized by being manufactured in a kiln where temperatures exceeded 1000 degrees Fahrenheight.  

 

The Plants

Three day old basil seedlings started in one-inch rockwool cubes were set into four inch pots containing growing medium, 20 with only glass, 20 with only clay. (The young plants were started in rockwool because the small seeds would have fallen through the spaces between the glass and clay media particles and because the glass and clay would not have held enough water for the seedlings.) The young plants were set on two tables, each measuring four feet by four feet. One table housed the glass and the other, the clay. Each table held four trays and each tray contained five potted basil plants. Under each of the tables were nutrient solution tanks. The set up prevented the clay-system nutrient solution from mixing with glass-system nutrient solution.  

 

The Nutrient Solution Feeding System

The plants received nutrients through an ebb and flow system. Both nutrient solution tanks contained a small pump that flooded the trays for a period of four minutes, four times a day (at 6:30 am, 10:00 am, 1:30 pm, and 5:00 pm).  

 

Each reservoir contained 20 gallons of water. On an as needed basis, nutrients and water were added. On a weekly basis, pH, conductivity and temperature were monitored. Appendix A, Log Notes, provides a day by day and weekly summary of observations and system care.

 

3.0 RESULTS

Three basil plants were damaged during the course of the experiment and were therefore excluded from results. At harvest, plants were cut from their roots and dried to eliminate transient differences in moisture content. Dry weights were obtained by weighing the plants on a Sartoris balance and are presented in Table 2.  

Table 2
Growth Yields of Clay and Basin-Grown Plants
	Plant Number	Glass in Grams	Clay in Grams
	1	25.07	17.86
	2	10.66	15.84
	3	26.00	18.85
	4	26.96	19.07
	5	28.62	20.06
	6	27.77	20.80
	7	18.02	9.58
	8	18.35	23.47
	9	24.16	29.84
	10	15.50	21.53
	11	18.20	24.70
	12	21.61	19.65
	13	25.64	16.30
	14	17.12	26.35
	15	19.32	27.27
	16	28.04	41.64
	17	21.70	20.80
	18	14.97	19.76
	19	19.95	-

Table 3
Descriptive Statistics for Glass-Grown and Clay-Grown Basil Plants
	Sample Size	Mean	Median
Glass	19	21.5	21.6
Clay	18	21.9	20.4

All statistics were calculated using the Minitab Release 8.2 statistical computing program. Mean dry weight of the glass-grown plants was 21.5 grams, with a standard deviation of 5.1 grams. Mean dry weight of clay-grown plants was 21.9 grams, with a standard deviation 6.8 grams. To test the significance of the difference between the two sample means, the Fisher statistic was calculated'. No significant (p= 0.05) difference in dry weight was found between plants grown in glass and plants grown in expanded clay aggregate.  

Conductivity and pH

Table 4
Weekly pH Levels
Date	Week Number	Glass Solution pH	Clay Solution pH
5-8	1	6.2	6.2
5-17	2	6.3->6.2	6.3->6.2
5-21	3	6.2	6.0
5-28	4	6.2	6.2
6-4	5	6.2	6.2
6-2	6	6.2	6.2
6-9	7	6->6.2	6->6.2
6-26	8	6->6.2	6->6.2
7-2	9	5.6->6.0	5.7->5.9
7-9	10	5.8->6.1	6->6.2

During the course of the experiment conductivity and pH were measured each week. Conductivity provides a measurement of the total nutrients within the growing liquid solution. It was measured in order to determine whether glass-grown basil used nutrients at the same rate as clay-grown basil. pH was measured to monitor acidity and compare nutrient uptake - differences in acidity indicate differences in the way nutrients are absorbed by plants. Table 4 provides weekly pH readings and Table 5 provides weekly conductivity readings.  

 

While there were no significant difference in the pH between the clay and glass solution tanks, conductivity appeared slightly lower in the glass tank during weeks four through eight. This corresponds with differences in growth during those same weeks. As the clay-grown basil grew faster during weeks four through eight, it also used more nutrients, reducing the conductivity of its solution.

 

Table 5
Weekly pH Levels
Date	Week Number	Glass Solution Conductivity	Clay Solution Conductivity
5-8	1	1200	1200
5-17	2	1200	1200
5-21	3	1500	1500
5-28	4	1230	1170
6-4	5	1100	1080
6-2	6	1080->980	1080->980
6-9	7	880->1000	700->1020
6-26	8	930->960	640->1040
7-2	9	970->900	1030->900
7-9	10	790->960	700->940

 

Where two values are listed in Tables 4 and 5, the first is the pH or conductivity before changes were made to the system (i.e., adding water or nutrients) and the second is the pH or conductivity after changes were made.  

 

 

4.0 DISCUSSION  

The results of this experiment confirmed the hypothesis -- glass serves as a viable rooting medium and produces no greater or lesser growth yields than expanded clay aggregate. The results are somewhat surprising given the noticeable difference in plant growth apparent in weeks four through eight. The difference in growth may have been due to one of three factors.  

 

1.   The glass grown basil may not have been receiving enough oxygen due to the smaller air   spaces between the glass particles than between the clay particles. (Glass air spaces were smaller due to the smaller size of the glass particles. Also the relatively flat surfaces of the glass particles allowed for better packing than the rounded surfaces of the clay.)  

 

2.   Light may have permeated the glass and reached the roots. 

 

3.   The darker color of the clay may have absorbed more heat and may have provided the clay   grown roots with additional warmth. (The temperature of water was measured in the reservoir tank. not individual pots.)

 

These three factors would have been mitigated during week five when aerators were added to the tanks and reflective paper was placed at the bases of the plants. These changes would have allowed the glass-grown basil to "catch up."  

 

While our experiment indicates that glass can serve as a hydroponic rooting medium, the experience also brought to light some problems with the glass.  

 

1.   It was much heavier than the expanded clay aggregate.  Glass weighs fifteen ounces per cup while expanded clay aggregate weighs seven ounces per cup.

 

2.   The glass stuck to the fingers of the experimenters and they perceived this as inconvenient. This problem might be mitigated by wearing rubber gloves.

 

3.   It's edges caused skin irritation from handling.  In contrast, the expanded clay did not. This problem might also be mitigated by wearing rubber gloves.  

 

Based on the findings from this experiment, mixed-color recycled glass will not be the hot new hydroponic rooting medium of the '90s. It may, however, find itself a niche market among the environmentally minded hydroponic hobbyists.  

 

5.0 ACKNOWLEDGMENTS

 

ReTAP is a joint venture of the Clean Washington Center, Washington State's lead agency for market development of recycled materials, and the National Recycling Coalition, a 3,500 member nonprofit organization committed to maximizing the benefits of recycling. ReTAP is an affiliate of the national Manufacturing Extension Partnership (NIST MEP), a program of the U. S . Commerce Department's National Institute of Standards and Technology. The MEP is a growing nationwide network of extension services to help smaller U. S . Manufacturers improve their performance and become more competitive. ReTAP is also sponsored by the U. S. Environmental Protection Agency and the American Plastics Council.

 

6.0 REFERENCES

 

1) Reference: Applied General Statistics. 1967. F.E. Croxton, J.J. Cowden, and the S.K. Klein. Prentice-Hall, Englewood Cliffs, N.J. 754 pp.