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
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.