One way to decrease the need for soil and water resources is with greenhouse production (Figure 6.2), although not all crops may be suitable for this. Greenhouse crops such as tomatoes, cucumbers, lettuce, peppers, herbs, and strawberries are grown with hydroponics. Technically, the term hydroponic describes plants grown in water that has been amended with soluble nutrients, but now it is also often used to describe plants grown in some type of sterilized material to give the roots a place to anchor. These materials can be gravel, sand, or rock wool, which is spun fibers from melted rocks that have been compressed into cubes or slabs. The benefits of hydroponic versus field-grown crops are lower water use, less disease, fewer fertilizers required, less labor, reduced need for pesticides, less damage to the environment, and higher yield. Water is kept in a closed system and recycled. The nutrient levels and pH of the water are closely monitored and adjusted as needed. The water can be reused for many months.
Another similar method is the ebb and flow system used with potted plants. The pots are filled with a sterilized media composed of various combinations of peat moss, pine bark, vermiculite, or sand. Beneficial microbes that help prevent pathogens from causing disease are often inoculated into the potting media. Organic growers may use well-ripened compost that has been sieved through a small mesh screen. The pots are kept in large, waterproof plastic trays, which are periodically flooded with water. The plants absorb the water from the bottom of the pot by capillary action in the roots and the excess water is drained into a reservoir container and recycled for many months.
Hydroponic and ebb and flow methods can be modified to meet many of the requirements for organic or sustainable certification and they are less degrading to the environment than conventional agriculture in the field. However, not all greenhouses are pesticide free. Insect infestations in the greenhouse may be more difficult to deal with than outdoors in the field because predatory insects and other creatures that normally consume these pest insects cannot get in to feed.
Plants infected with viruses or other microbial infections are susceptible to insect infestations, and fungal infections are common. Therefore, many commercial greenhouses regularly spray insecticides. Many of these chemicals persist on greenhouse benches, on plants, and on the floor. Commercial greenhouses have a dedicated quarantine area that is used to hold new plants for a few weeks prior to their introduction into the greenhouse. This is to allow time to observe and remove any pests that may have hitched a ride, before they get into the greenhouse.
Sticky cards colored blue or yellow are often used to monitor and trap harmful insects, especially thrips and whiteflies, in the greenhouse. The insects are attracted to the colorful cards and get stuck to the surface. Additionally, pheromone traps may be helpful. Insects release pheromones to lure members of the opposite sex; these can be used to entice pests into the traps.
Insufficient sunlight may cause problems with plant growth and development. Sunlight in greenhouse structures made of glass is at 89% of unobstructed light. Polyethylene double-pane plastic transmits 84%. Greenhouses must be kept clean to transmit this amount of light. Plastic also transmits less light when it is scratched. Rain cleans the outside of a greenhouse, but the inside must be regularly maintained and kept free of plant debris and standing water. Disease is spread through standing water on the floors and from fallen leaves and withered flowers that may harbor Botrytis mold spores.
Plants need protection from high-intensity light during the summer, so shade cloth or whitewash is used on the windows to block some of the light. The greenhouse must also have an adequate ventilation system since good air circulation is required to prevent diseases. Carbon dioxide, which is required for photosynthesis, may become depleted if the greenhouse is closed during inclement weather. Injection of carbon dioxide into the closed greenhouse can increase photosynthesis and speed up the time to harvest as long as there is sufficient light. The amount of carbon dioxide required to increase plant growth is not high enough to cause any problems to humans who breathe the air.
The temperature in the greenhouse is crucial and must be within the range required for the crops. Heating costs are higher in cooler climates. Locations near the equator have longer growing seasons and more hours of sunlight, and thus have no need to heat the greenhouses during the winter. As a result, they can offer greenhouse crops at a lower cost. International competition has been particularly tough on the American cut-flower industry because these crops can be efficiently transported over long distances with minimal damage.
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