– HYDROPONICS –
Hydroponics is the technique of growing plants in water. It is generally thought that plants need soil for growing. In fact, what they need is the nutrients (vitamins and minerals) and moisture contained in the soil and these can be supplied through water, as well as through soil. Hydroponics is not a new process.
As long ago as the 1690’s, an English physician tried growing plants in water in a laboratory experiment. However, it was not until the 1800’s that German researchers used this method to develop many of the formulas for plant nutrient solutions still in use today. About a generation ago, hydroponics moved out of the research laboratory into practical use. In the past 40 years, hydroponic farming has progressed in a number of areas, especially in those where water is in short supply and temperatures are too extreme for ordinary agriculture.
This is because hydroponic farming is the only economical solution in such desert areas. Each year, more than 2.7 million kilograms of vegetables and fruit arc produced by hydroponic farming. These are mostly tomatoes but cucumbers, lettuce and melons are also grown. On hydroponic farms, each tomato plant produces an average of 6 kilograms of fruit twice a year – a total of 12 kilograms every year.
An ordinary soil-grown plant, on the other hand, produces only a total of 9 kilograms per year. In hydroponic farming, plants are grown in greenhouses. The greenhouses measure 8 by 39 metres and consist of steel frames covered with strong transparent plastic that is resistant to weather and lets in a maximum amount of light. The plants are fed by inorganic nutrients dissolved in water which is supplied by a plastic pipeline. The feeding and watering system is automated. Electric sensing devices (sensors) determine when the plants are hungry or thirsty. The sensors send messages which automatically start the water and nutrient delivery system.
When the sensors ‘know’ that the plant have had enough, the system turns off automatically. Nothing is left to chance within the greenhouses. Temperature, humidity and air circulation are carefully controlled. Air conditioning and heating equipment keep the temperature at 29°C by day and 18°C by night. In recent years, hydroponic farming has expanded to many parts of the world. An application of the method has occurred in Italy, for example, where the largest hydroponic installation – 50,000 square expansion of such a bar, due to heating, may be used to operate switches and valves.
In a gas refrigerator, there is a reservoir containing ammonia water. When the lower gas flame is burning, the ammonia water rises through the tube to the ‘generator’. The upper gas flame drives off the ammonia gas, which passes into the ‘condenser’. The cold air around the condenser rapidly brings down the temperature of the gas. Then the cooled gas, now condensed into a liquid, passes into the ‘evaporator’, which contains hydrogen.
In the evaporator, the ammonia expands rapidly, especially since its expansion in hydrogen is greater 45 than it would be in air. This rapid expansion greatly lowers its temperature. It is the cooling of the gas in the evaporator which lowers the temperature of the whole refrigerator and freezes the water in the ice-cube trays.
A. What do the following refer to?
1. ‘it’ (line 4):
2. ‘they’ (line 11): 3. ‘doing so’ (line 11):
4. ‘do* (line 13): 5. ‘one’ (line 18):
6. ‘Many’ (line 19):
7. MV (line 31):
8. ‘if (line 32): 9. ‘such a bar” (line 35):
B. Mark the best choice.
1. In selecting a good refrigerant, we must choose one that .
a) evaporates quickly b) is cheap c) is not explosive d) All of the above.
2. When a substance in gaseous form is allowed to pass through a small hole,
a) unlike the molecules of evaporating liquids, it absorbs heat energy from its surroundings
b) like the molecules of evaporating liquids, it absorbs heat energy from its surroundings
c) it might absorb heat energy from its surroundings
d) it absorbs heat energy from the pump