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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 from the plants, 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
The 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 1000-1500 ppm speeds growth.
CO2 is 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 pond nutrient solution for the roots to use in respiration.
Spinach grows satisfactorily at a DO level of between 4 ppm 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 spinach, 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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