Smart city garden: Or how to make greens even greener

Since childhood, we have heard that vegetables and greens are good for health. And research shows that plant-based food production is also preferable from a climate perspective. However, not all industrial agriculture is equally good for the environment. It requires a lot of water, fertilizers leaking into groundwater can cause flowering of water bodies, and pesticides and herbicides can harm wild animals and even humans. Together with SFU, we are looking into whether it is possible to grow vegetables using less water, and how digital technologies can help optimize the process.

Divide and rule

Agriculture is primarily associated with the soil: “From the soil, everything will be born,” “Whose soil is, his bread.” The idea of growing gardens and vegetable gardens without soil at first glance seems somewhat irreverent, but most importantly — impossible. How did people not only develop this idea but also make “landless” crop farming more efficient than traditional farming?

Soil performs three main functions. Firstly, it mechanically holds and supports the plant, preventing it, for example, from falling from the wind. Secondly, it serves as a kind of depot for nutrients needed by plants, which are introduced by humans or formed naturally, for example from humus. Thirdly, the soil also serves as a depot for water: during rain or watering, it absorbs water, allowing the roots to gradually absorb it. However, even the use of modern irrigation systems that supply water directly to the roots of the plant does not allow it to be used with one hundred percent efficiency. Some of the liquid still evaporates under the sun’s rays or goes into deeper layers of the soil. If plants have been treated with fertilizers or pesticides, then the water carries them along with them — into groundwater, and then into reservoirs. As a result, instead of being beneficial, such substances can cause serious harm. Fertilizer leakage causes blooms in water bodies, and pesticide and herbicide leakage can be dangerous to wildlife and humans.

When plants are grown hydroponically, that is, without soil, these functions are performed separately from each other.

^{Hydroponic installation. Jeremy Thompson / Flickr / CC BY 2.0 DEED}

For mechanical support, artificial frames are used: the structural supports are made of plastic and metal, and inert porous materials are used for the roots — for example, gravel, expanded clay, perlite, sawdust, and mineral wool.

Plant roots receive water and all the necessary microelements from a nutrient solution. In this case, the unabsorbed solution can be collected and reused, so that water use efficiency approaches 100 percent.

Fewer fertilizers are also needed, and other agricultural “chemicals” — pesticides and herbicides — can often be completely abandoned. Weeds and harmful insects simply have nowhere to live in such a garden.

Finally, hydroponics reduces transportation costs. Modern logistics make it possible to taste products from all over the world, but you have to pay for their transportation, not only in money but also in emissions. The ecological footprint of an avocado that comes to the table from South America can be equal to the ecological footprint of beef. But vegetables grown in the neighboring block do not bear any additional transport load.

On a small raft

The simplest hydroponics option is a mesh raft. The plant in this case is fixed so that its stem and branches remain above the raft, and the roots are immersed in the water below it. The Aztecs used such vegetable gardens on rafts back in the 11th–12th centuries AD. To secure the plants, they coated the rafts with volcanic clay. It also, apparently, served as a source of necessary salts. There is evidence that Chinese peasants were also able to make similar rafts.

^{Some authors believe that hydroponic technologies were also used in the construction of one of the Seven Wonders of the World — the Hanging Gardens of Babylon. However, there is no exact description of the gardens left, and even scientists do not have a consensus on their location. Hanging Gardens of Babylon / Wikimedia Commons / Public Domain}

The experiments of the Flemish chemist Jan Baptista van Helmont can be considered the development of the ideas of hydroponics. Van Helmont wanted to find out how plants gain mass. In 1624, he planted a 2 kilogram willow seedling in a pot with 90 kilograms of soil and for the next five years added nothing to the pot except distilled water and rainwater. At the end of the experiment, the tree weighed 77 kilograms, and the mass of the soil decreased by only 2 grams. Van Helmont concluded that “75 kilograms of wood, bark, and roots arose from water alone.” At the end of the 17th century, English naturalist John Woodward grew mint without soil, but with different types of water: distilled, rain, and polluted.

The term “hydroponics” (from the ancient Greek ὕδωρ — “water” and πόνος — “work”), however, appeared only in the 1920s. Its author is the father of modern hydroponics, an agronomist from the University of California William Gericke. Colleagues initially treated Guericke’s ideas with great skepticism. The school administration even forbade him from experimenting with hydroponics in the university greenhouses, so Guerika had to plant the plants in his backyard. However, a 7.6-meter-tall tomato vine grown without soil caused a sensation in scientific circles.

The first practical success came to hydroponics already in the 1930s, during the development of intercontinental flights. American Airlines used the Pacific Wake Atoll to refuel their planes. It housed a fuel station and housed employees. The atoll’s rocky and salty soil was unsuitable for farming, and the delivery of fresh food from the mainland was difficult. This is where hydroponics comes to the rescue. Hydroponic plantations on Wake Atoll produced enough vegetables to feed the entire station staff with fresh salad. During World War II, the United States actively used hydroponics to supply Pacific bases. In 1945, hydroponic plantations, at the initiative of the British Air Ministry, appeared in Bahrain and at the Habbaniya airbase in Iraq.

The proven technology continued to evolve. For example, in the 1960s, the nutrient layer technique appeared, which is still very popular today. The development of fluorescent lighting has made it possible to develop hydroponic gardens indoors, and the advent of computers, probes, and sensors has helped automate the process.

First, second, and third for plants

What does the “nutrient solution” that provides plants with everything they need consist of? Scientists did not immediately arrive at its optimal composition.

Today it is well known that plants obtain organic substances through the process of photosynthesis, and absorb only inorganic substances with water. Seventeen elements are of greatest importance: oxygen, hydrogen, carbon, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, copper, manganese, molybdenum, zinc, chlorine, boron and nickel. However, there are only six main components of the nutrient solution: nitrogen in the form of ammonium (NH4+) and nitrate (NO3-), phosphorus in the form of hydrogen phosphate (HPO42-) and dihydrogen phosphate (H2PO4-), sulfur in the form of sulfate (SO42-), as well as potassium, calcium and magnesium in the form of cations of the listed salts. Their quantity can reach hundreds of milligrams per liter of solution. The remaining elements are added in much smaller quantities — milligrams per liter or less.    

^{Dependence of the availability of various elements on soil acidity. The wider the line, the faster the plant absorbs the nutrient. Turan M et al. / Recent Research and Advances in Soilless Culture, 2023}

The absorption of element ions by plants occurs at different rates and can depend on both temperature and the composition of the solution. For example, potassium and ammonium ions compete for absorption, potassium is usually less absorbed in the presence of sodium, and nitrogen absorption is hampered by an excess of chloride ions.

Today, there are several standardized nutrient solutions that even a novice gardener can use. Owners of industrial hydroponic farms, of course, most often prepare the solutions themselves, often keeping their exact composition a secret. However, it is known that each culture requires its optimal ratio of elements. Depending on the variety, the structure of the hydroponic installation, humidity, and temperature, this ratio can vary significantly.

For most plants, a slightly acidic solution environment is preferred (pH from 5.5 to 7.0). The most acidic environment (up to 5.0) is preferred by cucumbers and basil, and the most alkaline (up to 8.0) is asparagus. In this case, acidity can change already during the growing season. This usually occurs if cations and anions are absorbed at different rates. For example, if nitrate anions are absorbed faster than potassium cations, the plant may release OH- and HCO3- anions to balance the net electrical charge. In this case, the pH of the solution must be increased.

Now scientists are actively working on creating nutrient solutions based on wastewater, which is traditionally rich in phosphorus and nitrogen compounds. This could make hydroponics technology even cheaper and more environmentally friendly. Plants can absorb up to 70 percent of the nitrogen from wastewater and up to 80 percent of other important elements such as copper. At the same time, scientists did not note contamination of vegetables grown in this way with E. coli or other pathogens. However, the authors emphasize that the composition of wastewater can vary greatly depending on the region. There is still a long way to go before industrial protocols, especially before such vegetables enter the market.

Sun, air, and water

What else do plants need besides a nutrient solution?

Firstly, in oxygen. Moreover, not only leaves consume oxygen but also roots, albeit in smaller quantities. In nature, oxygen saturation occurs naturally (there is little oxygen in every raindrop), but in hydroponic installations, it is necessary to come up with special engineering solutions for this.

At first, scientists used the technique of periodic flooding. Pots with plants are flooded from time to time with a nutrient solution and then quickly emptied. When the solution flows out, a zone of low pressure is created, into which a fresh portion of air enters. However, this approach requires the constant presence of an operator and is not suitable for industrial cultivation.

The next step was the drip irrigation technique. Plants are placed in cubic trays, the solution enters each tray through thin capillaries, and its excess flows down the gutter. This technique is still used today.

^{Installation diagram using a thin nutrient layer.}

Another approach involves using a thin nutrient layer. The plants are placed in the holes of the inclined trough, and the solution is fed from the tank from the bottom up to its opposite end, to then flow back into the tank and return to the cycle. Thus, the solution forms a thin film in the gutter, and the area of contact with air increases. This allows the roots to be saturated with oxygen. There is a more advanced version of this technique: instead of pouring the solution through a chute, it is sprayed using several nozzles, which allows for a larger area of ​​contact with air. This approach is called aerohydroponics.

Secondly, plants will not survive without light. It is needed for photosynthesis, and in addition, it also has regulatory functions: changes in daylight hours determine many processes, including flowering.

The first hydroponic setups were located in open spaces and used natural sunlight. However, now such farms, as a rule, are located in completely enclosed premises, and the plants are located one below the other.

Instead of the sun, such smart gardens use fluorescent light sources; each level of plants has its row of lamps. This is the main problem with fluorescent hydroponics — the additional use of electricity. If in everything else — the use of water, fertilizers, and pesticides — hydroponics is much more environmentally friendly and more efficient than traditional farming, then this is its main Achilles heel.

However, scientists are looking for new phosphors and luminescent materials with a higher quantum yield, which will save energy. In some installations, plants are placed on the inside surface of the pipe. The lamp in this design is located inside the pipe, and as a result, its light is enough for a larger number of plants. If you power the lamps from solar panels placed, for example, on the roof of a building, then the electricity consumption will be completely reduced to a minimum.

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