Hydroponic Greenhouses

Hydroponic greenhouse cultivation provides an effective way to feed the plant and not the soil. In a greenhouse atmosphere, chemicals considered dangerous can be used within a controlled environment. Conventional gardening methods also use chemical fertilizer to feed plants, but this can harm both the plant and the soil. Soil can easily be depleted of its valuable nutrients when farming and growing practices fail to take into account the future of the land in use, which is why hydroponics is growing in popularity.

The primary vegetable crops grown in greenhouses include tomatoes, peppers, cucumbers, lettuce, herbs and strawberries.

Most plants grown hydroponically are raised in greenhouses under carefully controlled conditions. The production of high quality hydroponic vegetables requires an intensive time commitment. It is difficult and risky to leave the greenhouse for short or long periods of time due to the high demand for the production system and environmental controls to perform as needed. Growers who have been successfully producing crops at a profit have generally used dependable production systems and have invested their time and effort in developing a profitable market.

Hydroponic greenhouse production systems include soilless bag culture, rockwool, upright pots, beto buckets, nutrient film technique, floating systems, vertical stacking pots, troughs, gravel culture and others. Gravel is the most popular medium for root support, and a balanced mixture of all the necessary nutrients is periodically fed to the crops in a liquid form. This method is called “sub-irrigation culture”.

In large commercial greenhouses the method has been refined to such a degree that—once the seedlings have been planted—almost all the work is done by automation. Delicate sensors in the gravel can “decide” when the plants need more solution and turn on pumps, which then meter out the correct dosage.

Nutrient Management and Hydroponics

Plant fertilizers are composed of a mix of salts that are electrically conducting when dissolved into water. This characteristic leads to the use of electrical conductivity as a measure of fertilizer concentration. Computer programs have been developed that are suitable for balancing a nutrient mix to achieve close approximations to the desired molar ratios of elements. Two common hydroponics systems that use no root medium are:
• deep flow troughs (DFT).
• nutrient film technique (NFT)

Deep Flow Hydroponics

The classic hydroponic system in which plants are supported so that their roots hang into a nutrient solution, is generally called “deep flow hydroponics.” This system is appropriate for hobbyists and large scale production of leafy vegetable crops. The system consists of horizontal, rectangular-shaped tanks lined with plastic. The nutrient solution is monitored, replenished, recalculated and aerated. The rectangular pools act as frictionless conveyor belts where large, moveable floats of plants (lettuce) can be transported from transplant to harvest.

Nutrient Film Technique (NFT)

In a modification of the deep flow system called “nutrient film technique,” a thin film of nutrient solution flows through plastic lined channels containing the plant roots. The walls of the channels are flexible, which permits them to be drawn together around the base of each plant, excluding light and preventing evaporation. For lettuce production, the plants are planted through holes in a flexible plastic material that covers each trough. Nutrient solution is pumped to the higher end of each channel and flows by gravity past the plant roots to catchment pipes and a sump. The solution is monitored for replenishment of salts and water before it is recycled. Capillary material in the channel prevents young plants from drying out, and the roots soon grow into a tangled mat. This method is mainly used for tomatoes.
NFT was developed during the late 1960s by Dr. Allan Cooper at the Glasshouse Crops Research Institute in the U.K. With the NFT system, a thin film of nutrient solution flows through plastic channels, which contain the plant roots with no solid planting media. The root mat develops partly in the shallow stream of recirculating solution and partly above it. It is extremely important to maintain this basic principle of a nutrient film because it ensures the root system has access to adequate oxygen levels.
Greenhouse Construction

A greenhouse is defined as a structure covered with transparent or translucent materials such as glass, polyethylene (PE) or polyvinyl chloride (PVC). Greenhouse (or glass-houses) are relatively permanent structures (usually glass or plastic with aluminum or steel frames) equipped with several means of environmental modification. Major greenhouse hardware equipment includes:

• Hydroponic equipment for soilless agriculture, horticulture, floriculture and herbiculture.
• Heating and cooling systems.
• CO2 enrichment.
• Evaporative cooling pads.
• Exhaust fans.
• Roof vents.
• Thermal shade curtains.
• Computer control systems..
• Hybrid solar photovoltaic and wind energy systems for electric power generation.

Free-standing greenhouses are the most basic structural type. Cross-sectional shapes can be classified as arch, hoop, or gable (see illustration). Multispan greenhouses are typically connected by a series of roof gutters to create a single airspace. Large multispan greenhouses can cover several hectares (a unit of area equal to 10,000 square meters or 107,639 sq. ft.) under one roof, and they are the design of choice for larger commercial greenhouse operators. Floors are frequently made of concrete, although gravel floors with concrete walkways may be used to reduce cost.

Common commercial greenhouse shapes.
Light transmittance is important when selecting a covering material. Glass provides the most light to the plants and retains its light transmittance. However, various rigid and film plastic glazing materials are often used because of initial lower costs.

Mechanization and automation

Many greenhouse operations that were formerly done by hand are now mechanized or automated. Root media are mixed, fertilized and placed directly into flats and pots by machine. Seeding and transplanting can be done by machine. Plant watering and fertilizing (termed “fertigation” when combined) can now be automated. Automatic material movement at harvest, coordinated by a computer, is no longer unusual.

Greenhouse Management Software

“Virtual Grower” is a decision-support tool for greenhouse growers to monitor plant growth and control energy management in greenhouses. It was developed by the USDA–ARS (Application Technology Research) Unit at the University of Toledo. Users of the software can build a virtual greenhouse with a variety of materials for roofs and sidewalls, design the greenhouse style, schedule temperature set points throughout the year, and predict heating costs across more than 230 sites within the continental U.S. Different heating and scheduling scenarios can be predicted with the input of a few variables, with accurate data based upon historical records collected by USDA monitoring stations throughout the country.