Small Hydroponics Experiment Comparing Two Systems

By Andrew Coggins, NCAT Sustainable Agriculture Project Coordinator

Hydroponics, aquaponics, and other forms of vegetable and fruit production based on soil-less systems have been around for many years. In November 2017, the NOSB (National Organic Standards Board) voted to allow these systems (operating within certain guidelines) to be classified as organic production. This continues to be a contentious issue with many organic producers, who believe that organic systems should be soil-based only. Nevertheless, these systems can be very efficient; independent of climate, season, and geography; very compact (space-efficient); and very water-efficient. The concern about 'nutrient density' remains, however – specifically, how nutrient-dense are the plants grown in water-based systems versus plants grown in soil based systems.

LED lights on newer system.
LED lights on newer system. Photo: NCAT
Original design with fluorescent tube.
Original design with fluorescent tube.
Photo: NCAT

On the SIFT farm this year, we’re conducting an experiment to start basil and lettuce seedlings in peat pods in a commercially available system and then transfer the plants using just compost tea as a natural method of plant fertilizer to keep them growing, comparing the commercially available system to a modified version (the “sustainable system”) that we built on-site with a few plumbing supplies. (We are still working on a more 'natural' alternative to the peat pods supplied with the commercial system).

Using the ‘Miracle-Gro’ Aero Garden
This unit is one of many hydroponics systems offered by Miracle-Gro in various shapes and sizes. We have two. Both are the same size, but the one at home (Aero Garden Sprout) has a single fluorescent white tube light, whereas the one in the office (Aero Garden Sprout LED) is newer and has multiple LED lights and the three-color spectrum of white, red, and blue – supposedly better for growing fruiting plants like tomatoes – though both systems work fine with leafy greens and herbs. This has slightly lower energy use and longer bulb life measured in years.

Growing time is usually four to six weeks, at which time we transfer the plants into bigger pots and the more ‘sustainable system,’ though the plants can stay in the Miracle-Gro system and be continually harvested, with the light adjusting up to 11 inches above the base as plants get bigger. The cycling is 16 hours on/8 hours off for the lights and air pump, with 4 ml of nutrient added with the seeds and thereafter every two weeks. As a final note, a slightly cheaper way to grow plants is to just buy a pack of 50 peat holders for $15, use your own seed, and use plastic plumbing joints to grow the plants in. A complete replacement pod pack (six pods) will cost around $16 from the manufacturers (but will include a 3-oz. bottle of plant feed). The plastic holders are not really reusable, however, as the roots get too entangled in them as the plants get bigger, and you have to cut the plastic away from the roots to transplant them.

Peat plugs and holders supplied with kit.
Peat plugs and holders supplied with kit.
Photo: NCAT
Re-usable plumbing connection holders.
Re-usable plumbing connection holders.
Photo: NCAT

A brief comparison of the two methods, based on six weeks from seed to harvestable plant, is shown below (using data supplied by Miracle-Gro,* which is readily available on its website).

Miracle-Gro system: Six-pod kit + 3 oz. (88 ml) nutrient bottle - $16 average (3 oz. nutrient bottle is $10 if sold separately, so assume remaining $6 for 6 plants = $1/plant). Prices approximate.
Three plants grown over six weeks = $3 + $1.36 cents (12-ml nutrient x $0.113/ml) + $1.81 (electric cost to run over 42 days) = $6.17 ($2.06/plant).

Modified system: Re-usable plumbing pipe connectors + 3 oz. (88.ml) nutrient bottle + bulk bought peat seed holders ($.30 each) + seed pack ($1.50), so assume about 32 cents/plant and peat holder.
Three plants grown over six weeks = $0.96 + $1.36 (12ml nutrient x $0.113/ml) + $1.81 (cost to run over 42 days) = $4.13 ($1.38/plant).

Buying a 1-liter (1,000-ml) plastic can of nutrient at $29.45 creates additional savings ($0.029/ml), of course, shaving another $1 approximately off every batch of three plants produced. It’s worth remembering, too, that if you don’t have a yard/garden, these systems will allow you to continually harvest herbs and salad greens for a period of time, giving additional added value.

Transferring to the Larger System
The lettuce was seeded on January 29 and transferred to the tub on March 7, a shade under six weeks, and this was plenty late enough. There is a fine line when transplanting - deciding when a plant has enough roots to support this or leaving it too long so that half the roots get destroyed in the process. Using plumbing connections helped, though. The tub lid is drilled with five holes to take five baskets and is takes just under four liters – two liters of compost tea and two liters of water/fill.

The ‘sustainable system’ showing the roots.
The ‘sustainable system’ showing the roots.
Photo: NCAT
Plant and plug in net basket with clay pellets.
Plant and plug in net basket with clay pellets.
Photo: NCAT

The compost tea is derived from hanging a bag of compost in a tub of water and letting the nutrients filter out (as per a tea bag, hence the name), and I so far have not needed to aerate this tea for 24 to 28 hours before adding it to the plants. The air pump and air stones were purchased from Walmart for just under $9 and $2, respectively, and run for eight to nine hours/day with the system just using the available heat and light in my office and nothing more. We have harvested the lettuce twice, and the container is just left by the window over the weekend. The plants don’t seem to suffer when the air pump is switched off for the weekend. We ran one of these tubs successfully for weeks in the stand-alone green house before Christmas, up to the point when temperatures inside the building dipped below freezing. We can test the ‘tea’ for pH, but this is a relatively new and cheap potential nutrient source across many areas of food production and since most is made locally in many different ways, there is no defined nutrient value to it.

* From Miracle-Gro website: “For example, in a cubicle in one of our offices at AeroGrow we are growing Gourmet Herbs (on 17 hours a day) in an AeroGarden 6 (which uses 42 watts, or .0042 kilowatts, of power). This is on for 31 days a month, at an average utility rate of approximately $0.11 per kilowatt hour for electricity. hours X kilowatts X days X utility rate = electrical cost per month” (Miracle Gro).

(17) X (.042) X 31 X .11 = $2.43 per month

To figure your own cost exactly, use the formula above but substitute the wattage of your model AeroGarden and the kilowatt/hour rate charged by your local utility company.”

We used a figure of $0.15 per kilowatt, based on our electricity bills.

Small air pump and air stones (Walmart).
Small air pump and air stones (Walmart).
Photo: NCAT
Liquid plant feed analysis.
Liquid plant feed analysis. Photo: NCAT


So, What Did we Learn?

Among things the experiment has already shown is that compost tea seems a perfectly adequate natural fertilizer for plant growth and does not need constant aeration or agitation to avoid the nutrients settling out of the solution. Secondly, the experiment has shown that when the plants were transferred to the plastic container in the office, the winter daylight hours were enough to sustain the plant. This has solved three problems we experienced last year using purchased organic fertilizer granules: nutrients separating out of the water, water pH levels varying, and plant roots becoming anaerobic, i.e, brown and slimy, due to lack of oxygen in the water.

Posted on: July 29th, 2019