Agriculture Reference
In-Depth Information
Introduction
The increasing demand by world markets for high quality yield has lead more and
more horticultural crop production systems into protected environments. Cover-
ing the crop does not only protect it from external natural hazards, but also allows
for artificial manipulation of the crop micro environment to facilitate optimal
plant performance, extend production duration, induce earliness of flowering, and
improved production, and/or better quality product. According to Kacira (
2011
) the
top 5 worldwide countries by protected cultivation area are China with more than
2,760,000 ha, Korea with 57,444 ha, Spain with 52,170 ha, Japan with 49,049 ha
and Turkey with 33,515 ha.
A spectrum of covered structures is used by growers, depending on the crop,
the climatic region and the anticipated benefit. These structures can be generally
classified into two categories: screen constructions and greenhouses. The former
are covered by permeable porous screens while the latter by impermeable transpar-
ent plastic films or glass. The two groups can also be classified according to the
nature of the internal climate control, passive for the screen constructions and ac-
tive for the greenhouses, although sometimes a combination of both structures and/
or climate control approaches is used. Passive climate control means that once the
house is constructed, no actions are undertaken by the grower to artificially modify
the microclimate. There is strong interaction between inside and outside conditions
and exchange processes between the crop and the outside atmosphere are governed
by system attributes. On the other hand, active climate control means that besides
the structure and cover, systems are installed that enable manipulating of the inside
microclimate. In greenhouse structures the inside is more isolated from the outside,
than in screen-constructions.
The simplest type of the first category is a porous screen cover made of plastic
threads, horizontally deployed above the crop which protect crops from the sun
and physical damage by reducing the incoming radiation and wind speed. A more
advanced type of cover is the screenhouse (also called net-house) which, in addition
to the horizontally deployed screen, includes screened sidewalls. Such structures,
if made of sufficiently dense screens are insect-proof, thus avoiding insect invasion
into the crop and allow for a significant reduction of pesticide application. In the
analogy with screens, perforated transparent foils are used to cover the plants, in
order to improve their earliness of maturity.
A more advanced structure type is the naturally ventilated tunnel or greenhouse.
This structure is covered by an impermeable transparent plastic film which may in-
clude roof and/or side vents that allow the natural ventilation of the interior by wind
or buoyancy forces. Opening and closing these vents can be operated either manu-
ally or automatically by a control system. These structures provide better climate
control than screen constructions. In northern European countries such structures
are covered by glass for higher radiation transmittance.
The most sophisticated structure is the so called Hi-Tech greenhouse. Generally,
these structures can be equipped with any climate control system, thus allowing a
