The term hydroponics describes soilless cropping systems that rely on water to transport the necessary nutrients to plant roots. The main difference between hydroponics and soil-based growing systems is the lack of a buffering capacity for nutrients.Typically for hydroponics, a small amount of inert rooting medium is used to anchor the first roots immediately after germination and early growth stages of the plant. Hydroponic systems also allow for better control of nutrient uptake and improved temperature and oxygen management of the root zone. The more control a grower has, the more optimized conditions can be provided to the plants.
As the complexity of control systems increases, however, the opportunity for error also increases. For example, a malfunction of the water and nutrient supply system can easily result in crop damage if not corrected before the roots dry out. But because hydroponic systems generally have improved control, crop yields are often higher compared to soil-based systems. Many greenhouse hydroponic systems are also designed for year-round production and use supplemental lighting to sustain crop production during the darker winter months.
Classical Substrate-Based Hydroponics Systems
A traditional hydroponic system requires water-tight growing containers, storage tanks for water and nutrients, pumps and a growing substrate. The
irrigation system must distribute water and nutrients to containers with plants at a frequency and rate corresponding to the volume of and drainage from the container and the substrate’s water holding capacity.
In hydroponic growing systems, plant roots can be placed in air and sprayed periodically (aeroponics), in a thin film of constantly flowing water (nutrient film technique) or submerged in a layer of water (floating hydroponics). Other hydroponic systems use conventional irrigation systems, such as ebb and flow, when growing plants in soilless media.
In aeroponic systems, the plant roots are suspended in the air, often in the dark to prevent algal growth, and periodically sprayed with a nutrient solution. The plants are placed in a support structure that does not have to be positioned horizontally. The spray nozzles should not clog and are capable of high-duty cycles. The mist should reach all parts of the roots evenly.
Nutrient Film Technique
Here the plant roots are suspended in a thin layer of continuously flowing nutrient solution. Plants are placed in shallow troughs and kept upright by the trough covers. The nutrient solution is then supplied to the high end of the troughs and moves by gravity to the low end before being collected and recirculated. The distance between troughs can be adjusted during the plant growth cycle to maximize plant yield and space efficiency. The root mass of mature plants can be substantial, making proper nutrient solution flow through the root matrix a challenge.
Floating hydroponic systems feature plant roots partially or totally submerged in several inches of nutrient solution. The plants are positioned in floating rafts, allowing for easy movement during the production system. The nutrient solution is pumped around to ensure proper mixing while oxygen has to be introduced to maintain an adequate dissolved oxygen concentration (at least 4 mg/L is recommended for most crops). The nutrient solution can act as a temperature and nutrient buffer in case of problems, buying the grower time when trying to solve them.
Ebb and Flow Systems
Plants in ebb and flow systems sit in a reservoir flooded with a nutrient solution for a specific period of time. The solution is then drained, allowing fresh oxygen to reach the roots to ensure maximum growth. This system is low cost, easy to operate and build, and provides high nutrient availability for the plants. Provisions must be included to prevent malfunctioning of the flood/drainage system, which could quickly result in crop damage. Attention must be given to the fact that continuous flooding and draining may lead to increased salt levels in the root zone, resulting in toxicity and/or deficiency of certain nutrients.
Other Hydroponic Systems
These systems use conventional irrigation systems, and while plants are typically started in small volumes of soilless media, the volume of rooting media is often significantly enlarged when plants are positioned at final density. Examples include the European systems for growing tomato, pepper and cucumber.
A good substrate must provide proper support and good pore sizes for aeration and prevent clogging the irrigation system, which can negatively affect the nutrient solution. Some of the growing substrates used in hydroponics includes:
Soilless Potting Mixes: These mixes usually consist of peat moss that can be supplemented with perlite, vermiculite and plant nutrients. Other substances, such as bark, peanut shells or rice hulls, may be used. Because the soilless potting mixes have a lower fertilizer holding capacity than a field soil, a fertilizer solution is typically applied several times during crop production along with the irrigation water. In this sense, growing in soilless substrates is akin to hydroponic production.
Perlite and Vermiculite: Perlite is a generic term used for naturally-occurring siliceous rock. The crude rock pops in a manner similar to popcorn when the rock is quickly heated to above 1600°F. The water inside vaporizes and creates small, light-weight bubbles. Perlite is chemically inert, physically stable and provides high porosity and aeration.
Vermiculite is a mineral that has been superheated until it is expanded into layered flakes. It has a better water-holding capacity than perlite and a natural wicking property capable of soaking up water three to four times its volume. Perlite and vermiculite mixes are common hydroponic growing media that provide good drainage and aeration, and good water and nutrient retention, respectively.
Rockwool: Rockwool has been widely used in commercial greenhouse production. It is made from basalt rocks and chalk. The rock is heated and the resulting liquid is spun into fibers that can be pressed into different shapes. This substrate has high water-holding capacity and provides for good aeration. Rockwool needs to be pre-soaked with water to lower the pH before it is used for plant production.
Coconut Coir (Fiber): Coco Peat, also known as coir or coco, consists of leftover material after the fibrous layer from coconut shells is removed. Coco coir provides for better aeration than rockwool and has a higher water-holding capacity. This is often considered an advantage for hydroponic systems using intermittent watering cycles. Coco coir can hold and release nutrients over extended time periods. There are various grades of coco coir used for substrate, and proper selection is important to eliminate concerns with high salt levels in growing media.
Expanded Clay: Expanded clay, also known as grow rocks, has good aeration mostly due to the round shapes of the rocks, which allow the formation of air pockets throughout the root zone. The hollow clay pellets are inert, pH neutral, have good capillary action and do not contain plant nutrients. Thus, there is a need for precise control of all nutrients in solution. The pebbles do not hold a lot of water, so frequent watering is also necessary to prevent roots from drying out. This substrate can
effectively be used in ebb and flow systems.
Preventing Water-borne Contaminants and Diseases
Most hydroponic systems recirculate the nutrient solution and only replenish the amount of water lost through evaporation and/or plant uptake. Once contaminated with undesirable chemicals or disease organisms, they can spread quickly throughout the system. Therefore, treatment systems using various strategies like UV light, ozone, heat treatment or filtration have been developed to keep the nutrient solution as clean as possible. Known sources of potential contamination include particulate settlements and surface films that are sometimes difficult to clean in hard-to-reach part of plumbing systems.
Environmental Controls Help Predict Plant Responses
Greenhouse hydroponic cropping systems allow for almost complete control of the range of environmental factors that influence plant growth and development. These factors include temperature, light intensity and spectrum, humidity, air composition (i.e. carbon dioxide concentration) and movement, root zone conditions, as well as pest and disease pressures. The interactions among these factors become a critical issue, but they are challenging to investigate and understand. For example, the interactions among daily light and temperature sums, carbon dioxide concentration and evapotranspiration will be responsible for many of the plant responses observed. But definitively predicting the result of a change in one of these factors proves challenging.
Accurate sensors that provide reliable data about the environmental conditions experienced by plants, as well as careful measurement of plant growth and development, are necessary to better understand the entire production system. The adoption of computer control allows for virtually continuous measurement and recordkeeping of environmental conditions, and this has become a great tool for hydroponic growers. The ultimate goal is to provide optimum growing conditions while minimizing labor, energy and other resource inputs, environmental impact and cost.