By Eric W. Stein, Ph.D.
Associate Professor of Management Science and Information Systems, Penn State
President, NE Decision Sciences Institute
This article identifies the potential environmental effects large-scale indoor farming may have on air, water, and soil. We begin with an overview of what indoor farming is with a focus on greenhouses and indoor vertical farms (eg, plant factories). Next, the differences between these 2 primary methods of indoor farming are presented based on their structural requirements, methods of growing, media, nutrient sources, lighting requirements, facility capacity, and methods of climate control. We also highlight the benefits and challenges facing indoor farming. In the next section, an overview of research and the knowledge domain of indoor and vertical farming is provided. Various authors and topics for research are highlighted. In the next section, the transformative environmental effects that indoor farming may have on air, soil, and water are discussed. This article closes with suggestions for additional research on indoor farming and its influence on the environment.
Indoor farming, vertical farming, vfarm, zfarm, plant factory, water, air, soil, sustainability, carbon cycles, drought, information technology, greenhouse gases, climate change, environment, agtech
Stein EW. The Transformative Environmental Effects Large-Scale Indoor Farming May Have On Air, Water, and Soil. Air, Soil and Water Research. January 2021. doi:10.1177/1178622121995819
Open field farming has been practiced the same way for centuries as the primary means of growing food. Its origins can be traced back to wheat production 11 000 years ago in the Middle East, which later spread to the Mediterranean, north-Africa, and elsewhere.1 Given limitations on the amount of arable land, water scarcity, increased awareness of sustainable development, and the well-documented environmental effects of open field agriculture, other farming methods have been developed in the past few decades. The primary alternative to open field farming is referred to as indoor farming, which has received relatively little attention in terms of environmental impacts. The goal of this article is to introduce indoor farming in its many forms to environmental scientists, outline key areas of research, and highlight the effects large-scale indoor farming could have on the environment. Research needs to be done to better understand the cumulative and transformative environmental effects indoor farming methods may have on water, air, and soil as it realizes its potential to supply a significant portion of the population with fresh food.
What Is Indoor Farming?
Indoor farming is a relatively new method of growing vegetables and other plants under controlled environmental conditions. These farm systems are variously referred to as indoor farms, vertical farms, vfarms, zfarms, greenhouses, controlled environment agriculture (CEA), and plant factories.2,3 Indoor farms are sometimes confused with urban farms, which typically represent small outdoor farms or gardens to grow vegetables that are located in urban areas. It also should be noted that mushrooms have been grown indoors in compost under controlled conditions without light for more than one hundred years.4 For the purposes of this article, we will focus on characteristics of controlled environment indoor vertical farms and greenhouses, which are the primary architectures used for the large-scale production of leafy greens and other vegetables that require natural or artificial light.
The many faces of indoor farming
Greenhouses have been the workhorse for indoor growers for decades, especially in the production of flowers and ornamental plants. The modern high-tech greenhouse designs were pioneered in the Netherlands and have since been embraced all over the world. Several examples of these farms are evident throughout the United States and the largest span hundreds of acres. For example, according to Greenhouse Grower,5 Altman Plants (CA) has almost 600 acres under glass followed by Costa Farms (FL) with 345 acres. These are mainly used in the production of ornamental plants.
For vegetables, greenhouses were originally designed for tomatoes, but now are used in the production of kale, microgreens, lettuces, herbs, squash, and other types of fresh produce. These greenhouses, formerly located in rural areas, are now being positioned near urban and peri-urban areas to bring operations closer to population centers to save money and reduce the carbon footprint associated with transportation miles. For example, BrightFarms (brightfarms.com) has greenhouse operations located just outside of Philadelphia and Cincinnati to produce lettuces and other leafy greens. Gotham Greens (gothamgreens.com) situated its first greenhouse on top of a warehouse in Brooklyn, NY and has since expanded to other cities. AppHarvest (appharvest.com) is a venture located in Kentucky whose greenhouses cover more than 60 acres to produce tomatoes and other vegetables. What is common to greenhouse design is that all growing takes place on a single level, they are clothed in materials such as glass that transmit natural sunlight, and include climate control and irrigation equipment. They may also use a modest amount of supplemental artificial lighting during winter months.6
Growing leafy greens and other plants in buildings has emerged in the past 25 years whereby plants are grown vertically and hydroponically using artificial lights. Indoor vertical farms are typically located in warehouses or similar structures that have been retrofitted to provide superior heating, ventilation, and cooling (HVAC) for the benefit of plant production and racking systems to support the production systems.7-9 The PVC grow systems transport nutrient-rich water to the root zone of the plants, and the water is then returned to the main reservoir. Designed as closed re-circulating systems, indoor vertical farms only use a fraction of the amount of water as greenhouses or open-field methods (see also section “Water Use”). The advent of cost-effective LED lighting technologies has allowed farmers to provide the plants with just the right wavelengths of light, intensity, and photo-period to optimize growth.10 Other advances include automation, IoT, and artificial intelligence; ie, all of the information technologies that contribute to “smart farming.”11
Although modern LEDs are very efficient compared to HID, high-pressure sodium or florescent lamps, the capital and operating costs of these artificial lighting systems are significant,10 as are the climate control systems that are also required. Greenhouses, for example, require significant investment in heating and cooling equipment to maintain stable temperatures and humidity, which result in significant operating costs in buildings with low R-value membranes (eg, glass). The chief benefit of this design is that the light comes free, although growing is limited to a single level. Indoor vertical farms, however, can benefit from well-insulated structures that reduce heating and cooling costs and growing can take place on multiple levels. That said, these savings come at the expense of relatively high electricity usage for artificial lighting.10 These operating costs can be mitigated with the increasing efficiencies of LED’s, sensing systems that modulate light to the maximum required for the plants, pairing indoor farms with renewable energy sources such as solar and geo-thermal, and architectures that favor energy efficiency.9
Methods of indoor farms
Indoor farms are characterized by several parameters:
- Growing Method and Media
- Source of Nutrients
- Lighting Requirements
- Facility Capacity
- Climate Control
Most indoor farms use hydroponic methods of growing; i.e, plants are grown in water. Seeding takes place in an inert material such as stone-wool or peat, which is irrigated with nutrient–rich water. Water is administered using a variety of techniques ranging from fine mist sprayers (aeroponics), to shallow water (NFT) irrigation, to deep water culture (DWC) immersion to flood and drain methods.9 All are effective and have their pros and cons. Nutrients for larger scale hydroponic production systems typically come from dissolved salts that ionize in the water. In some smaller systems, the nutrients come from the nutrient-rich water of fish farms (ie, aquaponic systems) that are proximate to and coupled with the plant production system.9
In greenhouse production facilities, most lighting comes from the sun, which may be supplemented with artificial light, especially in the northern latitudes during winter. Plant factories and vertical farms, however, use only artificial lighting but are designed to maximize growing area using stacking methods. One common design is characterized by horizontal multi-tier growing systems starting at ground level that may include up to a dozen growing levels or tiers. Aerofarms (aerofarms.com) and Bowery Farms (boweryfarming.com) use this type of design for their production processes. An alternative is to use vertical drip irrigation grow systems. This design is characterized by vertical multi-site growing systems starting at ground level that extend upwards of 8 ft. In these systems, plants grow “sideways” toward artificial lights that are positioned at a right angle. Plenty, Inc. (plenty.ag) uses systems like these obtained in the acquisition of Bright Agrotech. Several examples of vertical farming ventures can also be found in Al-Kodmany.9
All indoor farming methods share the characteristic of offering CEA. Controlled environment agriculture offers the grower complete control over several environmental variables including, but not limited to: light intensity and wavelength, photo-period, wind velocity, temperature, and humidity. Water culture is further managed to obtain optimal results based on nutrient levels, PH, and dissolved oxygen.9,12 In most cases, pesticides and herbicides are eliminated. More advanced farms such as Fifth Season (fifthseasonfresh.com) benefit from extensive use of sensors, IoT, robotics, automation, and control systems designed to optimize yields and minimize labor. Another valuable aspect of CEA farms is their ability to produce plants with certain desired morphologies and nutritional profiles based on the control of lighting wavelength, temperature, and nutrient levels. SharathKumar et al13 go so far as to suggest that with CEA, we are moving from genetic to environmental modification of plants.
Benefits and challenges of indoor vertical farms
Several benefits are associated with vertical farming,9 although the industry is not without its challenges (see Table 1). The principal sustainable benefits of indoor vertical farming are a large reduction in the use of water (see also section “Water Use”), the reduction or elimination of pesticides, and mitigation of the effects of excess fertilizer run-off. From an economic perspective, the ability to control the environment results in a stable supply chain, price stability, long-term contracts with distributors and retail markets, and high yields per square foot. The elimination of pesticides puts produce grown this way on par with organics, which command premium pricing. Indoor farms, if designed correctly, can reduce labor costs and may be located closer to urban centers. Some see a role for indoor farms to ameliorate food deserts, unemployment, and as a means to re-purpose abandoned buildings and lots.3,9,14-16 Finally, vertical farms provide resilience to climate change, flooding, droughts, etc.
However, the vertical farming industry is facing some key challenges. For instance, currently only a very small portion of fresh vegetables are produced indoors. The one exception is the mushroom industry, which represents a US$1.15 billion industry.17 Second, the USDA does not clearly identify vegetable production by method; eg, greenhouse, open field, vertical farm, etc, so data are not readily available. Third, profits have been elusive, especially for vertical farms.18 According to the 2019 Global CEA Census Report only 15% of shipping container farms and 37% of indoor vertical vertical farms were profitable vs. 45% for greenhouse operations.19 Another limitation of indoor farming is that a relatively small number of cultivars can be grown using indoor farming methods. The primary ones are leafy greens, herbs, microgreens, tomatoes, and peppers, although berries, root vegetables and other more exotic plants are being trialed.19 Another challenge for indoor farm start-ups are the high capital costs, which can range from US$50-150/ft2 for greenhouses to US$150-400/ft2 for vertical farms. For example, AppHarvest had to raise over US$150 million to fund its 60 acre greenhouse complex.20 Aerofarms raised US$42 million for a 150 000 ft2 vertical farm,21 which equates to over US$280/ft2. Cosgrove22 further reports that access to capital is impeding the growth of indoor farming, especially for smaller farms. One reason that indoor vertical farms are not easily profitable is that they have to compete against conventional farms, which still enjoy a cost advantage. As a result, indoor farms typically price product toward the high end and along the lines of pricing for organics,2 which limits market penetration. The 2 major factors contributing to the high costs of indoor and vertical farm operations are energy10,23,24 and labor, which account for nearly 3 quarters of the total.2,24 Despite these challenges, venture capital continues to pour money into indoor farming and agtech in the hopes of driving cost down and maintaining growth. Dehlinger25 reported that US$2.8 billion was invested by venture capitalists in Agtech companies in 2019.
Finally, the industry is struggling to share knowledge, establish standards, and create best practices, although progress is being made. For example, the Center of Excellence for Indoor Agriculture established a “Best in Class” award for growers and manufacturers (indoorgacenter.org). Indoor Ag-Con (indoor.ag) and the Indoor Agtech Innovation Summit (rethinkevents.com) hold online events and annual conferences to help promote knowledge sharing. Several specialized industry news outlets now exist including Vertical Farm Daily (verticalfarmdaily.com), Urban Ag News (urbanagnews.com), iGrow (igrow.news), Hortidaily (hortidaily.com), AgFunder Network (agfundernews.com), and others.