Travelling through Indiana last summer, I realized that many growers plant their crops in soil inside their high tunnels or greenhouses. Soilless production offers different benefits and challenges. This is the first article in a series focusing on soilless crop production in high tunnels and greenhouses. Today we are discussing Hydroponics.
What is Hydroponics? The word hydroponics technically means ‘working water’, derived from the Latin words “hydro” meaning water and “ponos” meaning labor. Hydroponics is a method to grow plants using a mineral nutrient solution, in water and without soil. Two types of hydroponics are commonly found: a) solution culture, and b) medium culture. Solution culture types include continuous flow solution culture (Nutrient Film Technique) and Aeroponics. Medium culture types include ebb and flow sub-irrigation, run to waste, deep water culture and passive sub-irrigation systems.
History. The first research published on the production of spearmint in water was conducted by John Woodward in 1699. Discoveries made in the late 19th century by German scientists Sachs and Knop resulted in the development of the technique of soilless cultivation. This work inspired Dr. W.F. Gericke (University of California) in the 1920’s to develop a solution culture technique. Following his work, Hoagland and Arnon (University of California) developed a complete hydroponic nutrient solution in 1938, the ‘Hoagland Solution’. The solution was modified several times by different researchers. However, the soilless cultivation technique was first used on a large scale during World War II to produce food for the American troops stationed on the in fertile Pacific islands.
Solution Culture:
- Nutrient Film Technique (NFT). This is the most common system used in solution culture. The water containing all the dissolved nutrients is recirculated on a continuous basis past the roots. A shallow stream (film) of water flows in a watertight, dark channel. The plant roots develop at the bottom of the channel, allowing for an abundant supply of oxygen and nutrients to the roots. The channel is installed at a recommended slope of 1:100, but 1:30 and 1: 40 is also used at times. As a general guide, the flow rate is 1 liter (0.26 gal.) per minute with an upper limit of 2 liter (0.53 gal.) per minute. Channel length should not exceed 10-15 meters (33-49 ft.). It is very important that the operator pay close attention to irrigation (24 hr. continuous), nutrient balances, water temperature and pathogens. This is a very productive system, but unforgiving when mistakes are made (Figure 1).
Figure 1. Lettuce is grown in channels using the Nutrient Film Technique
2. Aeroponics. In this system roots are continuously or discontinuously kept in a dark environment saturated with a mist or an aerosol of nutrient solution. This is a very popular system for the production of mini potato tubers (potato seed production). This system use less water than the NFT system, but is more capital intensive and requires a high level of technical knowledge. Production is very dependent on the functioning of the system, meaning the grower cannot afford an interruption in electricity or water supply or any mechanical failure.
Medium Culture: With medium culture a solid medium is used for root growth and anchorage, the plants are either sub- or top irrigated, and the plants are planted in a container. Today we are only concentrating on the Ebb and Flow and Run to Waste production systems.
- Ebb and Flow. In most cases growers make use of ebb and flow benches to subirrigate potted plants. However, large scale farms sometimes make use of big cement basins to subirrigate a large numbers of plants at one time. This technique is often used for seedling production and the production of potted plants in the floriculture industry. This technique does require large volumes of water. A root pathogen free environment is key to the success of this system.
Figure 2. Ebb and Flow benches used for seedling propagation
2. Run to Waste. This system consists of plants planted in a soilless substrate in individual containers and they are individually irrigated. Generally the excess irrigation water that runs out of the container is not collected for recycling purposes. However, this system is ideally suited for the collection of irrigation water if the high tunnel or greenhouse floor is covered with a plastic flooring film or when the container is situated in a trough that allows for the collection of water. It is important to note that disinfection of recycled water is absolutely imperative to avoid any root disease issues. More information in regards with irrigation water recycling will be provided in a follow up article.
Figure 3. Tomato plants grown in individual bags filled with substrate, using the Drain to Waste system
Different types of containers can be used in a run to waste system. It could be a plastic bag or a pot of different dimensions, a slab of substrate covered with plastic or it could be a continuous trough filled with substrate. It all depends on which system is best for the crop and the management style of the grower. Some very popular inorganic substrates include rockwool, perlite and vermiculite. Other popular organic substrates include pine sawdust, peat, coir (coconut fiber) and rice hulls. All of these substrates have different physical and chemical properties and will be discussed in more detail in the series of articles.
Figure 4. Cucumbers grown in a continuous trough filled with substrate, using the Drain to Waste system
However, a good growing media (substrate) should have physical and chemical properties that allow for nutrient retention, gas exchange/aeration, and water retention and drainage. The type of plant produced, and the cost and availability of the substrate will determine which substrate is best suited for maximum growth and yield. Each substrate have different characteristics and requires different cultivation techniques and management practices.
Soilless crop production allows the grower total control the production environment, especially the root zone. The root zone pH and electrical conductivity (EC) can be adjusted at any time. More frequent irrigations allow for better control over the pH, EC and nutrient availability, which in the end optimizes production. Recycling of the excess irrigation water can also bring great savings to the grower in terms of the amount of water and fertilizer used.
In the next Vegetable Crops Hotline issue I will focus on ‘Soil vs. Soilless High Tunnel and Greenhouse Production’ and we will also take a look at ‘Growth Substrates’.
