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Biological Significance
Temperature
Temperature is one of the factors controlling plant growth. Generally,
as temperatures increase, chemical processes proceed at faster rates. The
temperature of the plant environment should be kept at optimum levels for
fast and successful maturation.
Relative Humidity
The relative humidity (RH) of the greenhouse air influences the transpiration
rate of plants. High RH of the greenhouse air causes less water to transpire,
which causes less transport of nutrients from roots to leaves and less
cooling of the leaf surfaces. High humidities can also cause disease problems
in some cases. For example, high relative humidity encourages the growth
of botrytis and mildew.
CO2
CO2 concentration of the greenhouse
air directly influences the amount of photosynthesis (growth) of plants.
Normal outdoor CO2 concentration is around
350 parts per million (ppm). Plants in a closed greenhouse, during a bright
day, can deplete the CO2 concentration
to 100 ppm, which severely reduces the rate of photosynthesis. In greenhouses,
increasing CO2 concentrations to 900-1500
ppm speeds growth. CO2 is commonly supplied
to the greenhouse by vaporizing liquid CO2.
Lights
Light measurements should be taken with a quantum sensor, which measures
Photosynthetically Active Radiation (PAR, 400-700 mm wavelength) in the
units micromol m-2s-1.
PAR is used by plants for photosynthesis. Measurements of PAR give an indication
of the possible amount of photosynthesis and growth being performed by
the plant. Foot-candle sensors and lux meters are inappropriate, because
they do not directly measure light used for photosynthesis.
Dissolved Oxygen
Dissolved oxygen (DO) measurements indicate the amount of oxygen available
in the nutrient solution for the roots to use in respiration.
Pak choi grows satisfactorily at a DO level between 4 and 16 ppm.
pH
The pH of a solution is a measure of the number of hydrogen
ions. The pH of a solution can range between 0 and 14. A neutral
solution has a pH of 7. That is, there are an equal number of hydrogen
ions (H+) and hydroxide ions (OH-). Solutions ranging from pH 0-6.9 are considered
acidic and have a greater concentration of H+.
Solutions with pH 7.1-14 are basic or alkaline and have a greater
concentration of OH-.
The pH of a solution is important because it controls the availability
of the fertilizer salts. A pH of 5.8 was found best for pak choi,
however a range of 5.6-6.0 is acceptable. Nutrient deficiencies may occur
at ranges above or below the acceptable range. Electrical Conductivity
Electrical conductivity (EC) is a measure of the dissolved salts in
a solution. As nutrients are taken up by a plant, the EC level is lowered
since there are fewer salts in the solution. Alternately, the EC of the
solution is increased when water is removed from the solution through the
processes of evaporation. If the EC of the solution increases, it can be
lowered by adding RO water. If the EC decreases, it can be increased by
adding a small quantity of a stock solution.
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