Hydroponic commercial agriculture, raising mostly tomatoes, cucumbers, peppers, and lettuce, is a rapidly growing industry with an annual rate of growth ranging from 15-25%. The center of growth is in the desert southwest of the United States, primarily in Texas and Arizona, and extends into Mexico. This region is favored because of ample winter sunlight and low winter heating costs.

However, in the near future, increased environmental constraints are expected to limit hydroponic crop production by limiting mining, raw material for most substrates as well as make the post-use disposal of hydroponic substrates such as rockwool more difficult. The ability to respond to such constraints will become increasingly important. One effective approach is to promote the use of by-products as alternative hydroponic substrates.

Growstones hydroponic substrates are a sustainable alternative. Manufactured from recycled glass bottles, these substrates reduce environmental impact at both ends of the production chain. They offer an alternative to strip-mined materials like pumice, perlite, and basalt (Rockwool). Being a non-toxic, chemical-free manufactured material, after a crop season Growstones can be simply disposed by using it as a soil amendment to improve the texture of heavy soils or the water holding capacity and aeration of sandy soils.

Furthermore, Growstones are engineered to provide an effective balance of aeration and moisture, a set of characteristics that favor root growth. Growstones can be manufactured with larger pore size to enhance aeration and drainage, or smaller pore size to enhance water retention. As an example, Growstones with small pore size have an air filled porosity of approximately 50% and a water holding capacity of 35% at saturation. In both cases, large air spaces in between aggregates enhance oxygen diffusion throughout the entire slab even after saturation. Growstones are also extremely light when dry.

A research trial was conducted at the Controlled Environment Agriculture Center at The University of Arizona in 2006-2007. Tomato plants (TOV, cv. Clermont) were grown in three different substrates in an A-frame greenhouse. The purpose was to compare the performance of the new hydroponic substrate manufactured from recycled glass, Growstones and Rockwool for hydroponic greenhouse tomato production in an arid climate and to establish whether or not Growstones are a suitable alternative to Rockwool, for decades the industry standard hydroponic substrate for vegetable crop production.

The experiment consisted of a randomized block design in a side-by-side comparison of yields, fruit quality, and plant balance between three different growing media: Growstones with large pore size (GS2), Growstones with small pore size (GS3), and Rockwool (control).

Prior to transplant, all grow slabs were rinsed to remove residual salts. This rinse was done using the regular industry practice of pH adjusted nutrient solution (5.5 – 6.0). For that purpose all six drippers were placed in their final position in the slab and the slabs were rinsed with a diluted solution of nitric acid to adjust pH to 5.5 – 6.0. A slit was cut at approximately 0.5 cm from the bottom of each slab to allow for proper drainage for the remainder of the crop season. No further pH adjustments were necessary for any of the three substrates.

Crop management followed standard practices for commercial greenhouse tomato production. Plant density was 2.4 plants per m2 during the entire season with 6 plants per slab in all substrates. Fruit pruning was performed weekly to keep 5 - 6 fruits per truss.

Trusses with 80% ripe fruit were considered ready for harvest. In all substrates, harvest started 15 weeks after transplant and fruits were harvested weekly for the following 17 weeks and twice weekly for the last 5 weeks.

Water flow/retention and oxygenation properties of Growstones require slight adjustments in the crop irrigation schedule (frequency and duration) compared to Rockwool. In general, Growstones were irrigated with twice the frequency and half the duration of Rockwool. At the end of the day a similar volume of water was provided to all treatments. For example, during the winter young plants in Growstones were irrigated every 19 minutes for 1 minute. Later fully grown plants were irrigated every 13 minutes for 2 minutes. This was necessary to maintain 30 to 40% drainage during the fall-winter months and 40 to 60% during the warmer spring-summer months. It was also necessary to keep the drainage electrical conductivity (EC) below 3.0 mS per m, and drainage pH between 6.5 and 7.0. EC at the drip was kept at 2.5 mS per m, and pH between 5.5 and 6.5. All plants were provided the same modified Hoagland nutrient solution.

Growing a successful tomato crop depends on the ability to maintain a balanced crop or steer the crop between vegetative and reproductive growth. Balance is important since excessive vegetative or reproductive growth can lead to reduced yields in the short or long term. Growstones is a highly steerable substrate. Due to its drier nature is possible to better control root zone moisture content at any given time compared to Rockwool. This, in turn has a significant impact on the way plant grows.

Cumulative yields were very similar in all substrates with a slight advantage for Rockwool. At the end of the crop production period (5.5 months) Rockwool, GS3 and GS2 plants produced 40, 38 and 37 kg per m2, respectively (Figure 1). No CO2 enrichment was used.

 

Figure 1. Cumulative yields (kg per m2) during production cycle (January to June 2007) from tomato plants growing in Growstones with large pore size GS2 (-●-), Growstones with small pore size GS3 (-○-), and Rockwool (-▼-). Averages calculated from 18 plants per substrate (N=18).·

Cumulative yields at the end of the crop season were above industry levels in all substrates. In fact, when summers are not excessively hot, inter-planting techniques allow for continuous greenhouse production year round. If one could assume a linear increase in cumulative yields with time, these results would correspond to total production of 81 kg per m2 in GS2; 83 kg per m2 in GS3; and 87 kg per m2 in Rockwool for year-round production with inter-planting. These results are particularly important considering no CO2 enrichment was used during the trial. For a medium level technology greenhouse, tomato yields around 70 - 75 kg per m2 are considered common in the industry with CO2 enrichment.

Tomato fruit quality as evaluated by its sugar content (Brix, %) was higher in GS2 (4.9%) and GS3 (4.8%) than in Rockwool (4.6%). Growstones substrates resulted in a significant increase in sugar content of tomato fruits (Brix, %) compared to Rockwool. This was likely due to the drier nature of Growstones, as drier substrates tend to limit water influx into fruits compared to wetter substrates. Thus, for the same amount of sugars produced through photosynthesis and with abundant water and nutrient supply, fruit sugar content is higher in drier substrates.

Dr. Paula Costa received her Ph.D in Agriculture and Biosystems Engineering from The University of Arizona, Tucson in 2007. Her research interests include the study of plant responses to manipulation of greenhouse environment, and irrigation strategies for root growth optimization. Dr. Costa is the director of R&D and Technical Support for Growstone, LLC a company that developed and manufactures hydroponic growing media made from recycled glass bottles. Her work has been published in ISHS as well as in multiple Trade Press Magazines in the US and Mexico. Dr. Costa can be reached at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it . More information on Growstones can be found at http://www.growstone.com/.