The United Nations Population Fund (
UNPF, 2007) estimated that the world population will increase by 72% in 2050 and reach 9.2 billion people. This growth in population will lead to increased urbanization in the least developed countries (LDCs), where the population is projected to grow from 2.7 to 5.1 billion people. This growth will create increasing demand for food to humans to levels that remain anonymous globally. For instance, in 2011, the United Nations Food and Agriculture Organization
(UNFAO, 2011) noted that the demand for food, especially ensuring sufficient, timely and cost-effective food supply that can meet the need of the increasing population is a critical challenge that the world currently faces. As a way forward, different crop productions have been developed and are practiced across the world. For example,
Yeh and Chung (2009) indicated that people in urban areas have embarked on indoor crop production in order to meet the rising food needs. One of the fastest growing indoor farming systems that has been reported to attract many urban dwellers is use of soil-less culture commonly known as; hydroponic farming.
The science of hydroponic farming
Hydroponics also known as: “
Hydro culture”, “
Nutri-culture”, “
soil-less culture”, “
soil-less agriculture”, “water culture” “tank farming” or
“chemical culture”
(Lakkireddy et al., 2012) is an agricultural science, which involves the cultivation of crops in a water-based solution rather than soil and this agri-system was substantiated by Jean Boussingault in 1860. The term “Hydro” generally refers to “water” while Ponics refers to “working (
Rajkumar et al., 2018). The water-based solution is composed of artificial chemical nutrients, which support crop growth
(Steinberg et al., 2000) and the crops can be cultivated with or without a medium which is used to provide support to the plant (
Hanger, 1993;
Jensen, 1999). The medium used include organic substances (Fig 1) such as; rock wool and inorganic materials such as; vermiculite, perlite, volcanic porous rock, expanded clay granules as well as synthetic materials (
Gruda and Schnitzler, 2006;
Lakkireddy et al., 2012). In addition to the varieties of medium used, there are also factors and characteristics that should be considered while selecting the medium for crop production (Table 1).
@figure1
Due to its nature of being practiced in controlled environments, hydroponic farming can be practiced in countries with intense arid areas including Algeria, Eritrea, Tanzania, Ethiopia, Kenya, North and South Sudan and South Africa (
Seawater Greenhouse Limited., 2010). Hydroponics has been used to grow a number of plants (Table 2).
The history of hydroponics and hydroponic nutrients
Hydroponic farming dates back to 1699 when scientist John Woodward started adopting water culture without using any concrete substance (
Hewitt, 1966). Hershey recognized Woodward as the first individual-English Physician to practice water culture in 1699 after Woodward researched on Helmont’s theory which stated that plant components are exclusively formed out of water (
Hershey, 1991). After Woodward’s discovery, other scientists namely Boussingault, De Saussure and Wilhem Knop carried out research and identified the nutrients that supported plant growth and Knop’s hydroponic nutrient composition was the most famous (Table 3) which had been used for a number of years worldwide under soil-less culture (
Benton, 1982).
Later on, a modified hydroponic nutrients composition (Table 4) necessary for plant growth was discovered during the mid-1900s’ (
Russell, 1953).
Plant pathologist Fredrick. W. Gericke from the University of California finally popularized the hydroponic system in the 1930s’ (
Gericke, 1937). He initially named this system aquiculture but later re-named it hydroponics because aquiculture mainly involved growing of aquatic plants. According to
Anon (1940), hydroponics was first successfully practiced on the Wake Island in the 1930s with the growth of fresh vegetables since it was the only solution for vegetable production on the Island. Later on in the 1960s and 70s, commercial hydroponic farms were established in different countries, that is: Italy, Denmark, Russia, Holland, German, Iran, United Arab Emirates, Japan, United States of America, Belgium etc after which many automated farms were established worldwide in the 1980s followed popularization of home-made systems in 1990s (
Mamta and Shraddha, 2013).
Much as hydroponics was initially developed mainly to cater for the production of fresh produce in the non-arable areas of the world
(Murali et al., 2011), some congested cities such as; New York in the United States of America (USA) and Montreal in Canada, have advanced the technology to the extent that it can be easily performed on apartment rooftops (
Fahey, 2012). This is a form of hydroponics called “vertical farming”, that is; growth of crops on vertically inclined planes or on skyscrapers (
Despommier, 2011). Developed countries such as; The Netherlands have already benefited from hydroponics, in terms of increased crop quality and yield
(Stoner and Clawson, 1995).
In Africa, hydroponic farming has been reported in South Africa for production of high-quality vegetables
(Baumgartner and Belevi, 2001;
Gruda, 2009). Recently, in the 2000s, farmers in East Africa, Kenya, Tanzania and Uganda have adopted the technology mainly for fodder (barley) and vegetable (spinach, lettuce, etc.) production on a small scale (
Kibiti and Gitonga, 2017;
Naluyima, 2015).
Types of hydroponic farming
Wick system
This system also known as passive technique uses a wick to draw the required nutrients from the tank into the growing medium. Much as it is the simplest and inexpensive system, it is more suitable for small plants which do not require a lot of nutrients since the wicks do not offer quick supply of the nutrients (
Keith, 2003).
Ebb and Flow (Flood and Drain)
This is a system which works by flooding the plant tray with the nutrient solution using a pump connected to the solution pool at given time intervals with the use of a timer. The solution is then drained back to the nutrient storage tank or vessel.
Nutrient film technique (NFT)
With this technique developed by Allen Cooper in the 1960s’, channels/tubes where the plant roots are immersed are used to constantly supply them with water rich in dissolved nutrients required for plant growth without a timer (
Wilcox, 1982). It has an advantage of the exposing the plant roots to sufficient supplies of nutrients, oxygen and water and doesn’t require a timer (
Omics, 2017).
Aeroponic Hydroponic System
This method doesn’t require any growing medium for crop production (
Runia, 1995). The nutrients and moisture are supplied to the plant roots suspended in air in form of mist with the use of a pump that is timed. The timer ensures that after every few minutes, mist is released. The disadvantage with this system is that any interference with the pump can lead to drying of the plant roots
(Murali et al., 2011).
Deep water culture (Direct Water Culture)
Under this system, the plants are put in baskets/net cups and roots are suspended directly in a highly oxygenated nutrient solution. The plants are able to survive because of addition of dissolved energy (
Sandlers, 2016). It is easy to construct and operate (
Railey, 2018).
Source of lighting under hydroponic farming
LED (Light Emitting Diodes) lighting
LED lights were first identified as a source of light for indoor agriculture (
Robert, 2008) that is practiced under controlled environments (
Yano and Fujiwara, 2012) in 1980s’. They were invented by Engineer Henry Joseph Round in 1907 (
González, 2012). A LED is a semi-conductor source of light which has capability of converting electricity to light when an electric current (electrons) is applied
(Shaw et al., 2004).
Studies have revealed that LED lights offer a high source of visible radiation
(Bula et al., 1991) for cultivating agronomic and horticulture crops indoors especially with the use of white, blue or red-blue LED lights
(Brown et al., 1995; Duong et al., 2002; Kurilcik et al., 2008; Yanagi and Okamoto, 1997). Red and blue light play a key role during plant development, photosynthesis and physiology (
Kopsell and Sams, 2013;
Olle and Virsile, 2013).
For example, the quality of blue light can be used to control plant shape, height and influence photosynthesis
(Cope and Bugbee, 2013). LED systems have been reported to have minimal red radiation, which affects flowering time for short day crop species (
Craig and Runkle, 2013). Results from a study by
Sabzalian et al., (2014) indicated that plants grown under LED lighting exhibited better flowering and productivity than those grown in a greenhouse.
They further highlighted that blue and red wavelengths play a role in controlling the closure and opening of the stomata, which affects the height and size of the plant as also indicated by
Folta et al., (2007). An experimental study carried out by
Tehrani et al., (2016) further revealed that 2 hours of red light resulted into maximum germination (83%) as compared to 8 hours of blue light (59%). None the less, blue light further plays a role in stimulating; Vitamin C; polyphenol and carotenoid components
(JohnKhan et al., 2010; Lefsrud et al., 2008). On the contrary, green LED light has the potential to drive photosynthesis (Folta
et al., 2007;
Kang et al., 2013).
LED lights have benefits of; having a long-life span; providing ideal light spectrum for growth of crops/plants
(Murali et al., 2011); producing limited heating compared to high- intensity light sources
(Bula et al., 1991); and producing quality yield among vegetables
(Demers et al., 1998; Hao and Papadopoulos, 1999). Nevertheless, they have a drawback of being costly compared to other lightening systems such as; High-Pressure Sodium (HPS) (
Nelson and Bugbee, 2014).
However, Scientist
Robert (2008) highlighted LED lights as having the potential of being cost-effective in the long-run due to their long-life span as compared to other horticultural lamps. Likewise, according to Haitz law, LED light costs have dropped by a factor of 10 each decade as their performance keeps doubling (Steigerwald
et al., 2002).
Hydroponic farming under solar lighting
This hydroponic farming system can take place either indoors or under a green house. The green house can be set up outside or on top of mixed-use buildings to maximize interactions between building environments and agriculture-like energy (
Caplow, 2009). Indoor farms are further divided into; store front glasshouses (double skin building) and leveled indoor farms (Specht
et al., 2013) which majorly favor shade-tolerant plants (
Brock, 2008). These indoor growth systems use the natural energy from the sun instead of LED lights for food production and are thus more eco-friendly and energy-efficient. This necessitates access to a window in order to access solar energy (
Brooke, 2016).
High-pressure vapor sodium (HPS) lamps are also used as a lighting source in greenhouse production because of their high efficiency besides offering a suitable light spectrum required for photosynthesis (
Christina, 2011). Greenhouse hydroponics is categorized under urban agriculture because it assimilates environmental and urban economics especially in the growth of horticultural crops (
Mougeot, 2008;
Pearson.et_al2010). Farmers can also cultivate high-quality vegetables
(Gruda, 2009) and flowers with solar powered hydroponics.
Soil-less culture vs. Soil culture
Hydroponics, when compared to soil culture systems has been considered superior in terms of plant nutritional balance in its composition and other attributes (Table 5).
Advantages of hydroponic farming
Hydroponic farmers are not affected by climate change conditions since they have control over climatic conditions such as; humidity, temperature, light among others (
James et al., 2000). This enables them to have all year round food production thus increasing their profit margin (
Max, 2017). Since it is a soil-less farming system, there are no soil-borne diseases and pests under this technology (
Mamta and Shraddha, 2013). It’s a farming system that reduces labor demands since it doesn’t require weeding, fumigating pests (
Graff, 2009) or cultivating the land in preparation for planting (
Max, 2017). Studies have shown that hydroponic farming system has the potential of removing atmospheric carbon dioxide
(Park et al., 2010). This air which is produced through human respiration and Volatile organic compounds (VOCs) heavily contaminates indoor surroundings
(Aydogan and Montoya, 2011;
Kim et al., 2008;
Oh et al., 2011). Carbon dioxide is a narcotic
(Milton et al., 2000) which has been associated with decline in student academic performance and work performance when increased in circulation (
Seppänen et al., 2006;
Shaughnessy et al., 2006).
As earlier noted, the technology was first developed for non-arable areas thus it is a favorable technology for regions without soil or areas without fertile land (
Sonneveld, 2000). In Uganda and Kenya, hydroponic gardening has been reported to be consistent with fodder production mechanism (
Naluyima, 2015).
However, it is important to note that hydroponics has a limitation of necessitating careful observation of the system as it does not offer opportunity for making rapid changes (
Singh and Singh, 2012). The system also requires heavy financial capital and adequate knowledge to effectively implement (
Sonneveld, 2000).
The prospect of hydroponic faming in Africa
There is already a plea from the scientific community for the potential of application of hydroponic farming technology in Low developed countries which are faced with water scarcity challenges (
Butler and Oebker, 2006). The current food insecurity challenges presented by climate change coupled with inadequate access to healthy foods in urban centers (
Alkon and Norgaard, 2009) call for strategic research and studies that could help fast track the adaptation of hydroponic farming systems in African countries. Hydroponics has the capacity to feed huge numbers of people in Africa where there is a scarcity of water and crops (
Kibiti and Gitonga, 2017). The urgent need for hydroponic-based urban farming is further motivated by the assertion that 70% of the world’s population will live in urban areas by 2050 (
Walsh, 2009). However, the initial costs of setting up the technology still remain a barrier to its implementation (
Mamta and Shraddha, 2013).
