Greenhouse Hardware
The physical components of both the germination area and the NFT area are of fundamental importance to hydroponic spinach production. It is necessary to have an idea of the physical components associated with each area, as well as a good understanding of their purposes.
Germination Area
The first 8 days of spinach production take place in the germination
area.
At any given time, a facility capable of producing e.g., 1000 plants per day, seven days per week, will have approximately 11,200 seedlings in the germination area. This allows for culling 20% of the seedling before transplanting. This requires a Controlled Environment germination area of about 140 square feet (4 m2), not including aisles.
Seedlings develop best with closely controlled temperature, relative humidity, carbon dioxide, irrigation and lighting conditions. These conditions can best be met in an area with the following equipment:
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Ebb and Flow Benches
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Solution Tank and Plumbing
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Supplemental Lighting
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Aspirated Box
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Sensors
Ebb and Flow Benches
An empty ebb and flow bench. Conventionally, the Ebb and Flow bench is periodically filled from a storage tank and drained back into the storage tank so the seedlings can be irrigated. We found, after many tests, spinach roots did not have adequate aeration when the cubes were immersed then drained. We changed our procedure to wet the root cube with a capillary mat which is continuously immersed in nutrient solution. The height of the capillary mat above the solution controls the moisture and aeration of the root cube. The nutrient solution, in turn, is continuously circulated through an Ebb and Flow bench.
Rockwool slabs, containing the seeds, are placed on landscape fabric, which covers capillary mat fabric. The rockwool slabs are elevated about 4 inches (10 cm) above the floor of the ebb and flow bench. The edges of the capillary mat fabric are draped into the nutrient solution. The lower photo illustrates the arrangement of the rockwool slab on the fabric and the capillary mat draped into the nutrient solution. The ebb and flow benches were specifically designed to supply water and nutrients by sub-irrigation. Through a pump and piping, the fertilizer solution is pumped into the Ebb and Flow bench.
Solution Tank and Plumbing
In our small test facility, a fiberglass tank (A) holds the nutrient solution used for sub-irrigating the seedlings. Approximately 250 L of nutrient solution is sufficient to prime the system, fill the bench, and provide nutrient solution for the first 8 days of growth. A small (Teel Model 1P808A 1/50 h.p.) pump (B) is used to pump the solution to the bench. The piping (C) should be flexible to adjust to individual germination area needs. A throttling or gate valve (D) is included to control the flow of the nutrient solution to the Ebb and Flow bench. The bottom of the sub-irrigation bench (E) is visible in this photo.
In a commercial facility, a 2000 L tank could be used for storage of the nutrient solution; the pump would have to be scaled up. Two pumps in parallel are suggested to provide redundancy. Some means for checking the pumps to assure both are working should also be included.
Supplemental Lighting
Type
Cool White Fluorescent (CWF) lamps (A) are recommended in the germination area, but High Pressure Sodium
(HPS) can also be used. Heat generated by the lamps must be dissipated from the germination area in order to maintain the temperature set points.
Use of incandescent lamps (B) is discouraged, because the red light emitted from these lamps causes the seedlings to 'stretch'.
Fluorescent lamps are rich in blue light, which results in short, squat seedlings.
Configuration
The lamps should be configured for uniform distribution of light over the entire
growing area. During the first 24 hours the seeds are in the germination area
no light is provided. For the remaining 7 days, light intensity is maintained
at 250 micromol m-2s-1 of PAR
(Photosynthetically Active Radiation). The photoperiod (or day length) at
this intensity is 12 hours, followed by 12 hours of darkness. Light output of
CWF lamps decays over time and it is important to realize output of CWF lamps
varies as a function of air temperature. Maintain a constant temperature in the
lamp area and regularly measure light output of the lamps. When the light intensity
drops below an acceptable level (e.g., 200 micromol m-2s-1),
new lamps should be installed. A quantum
sensor should be used to measure light intensity.
A lamp maintenance program can be employed to minimize fluctuations in light intensity. In this program, every ten weeks 25% of the lamps are replaced. CW fluorescent lamp life is about 7500 hours, and by 5000 hours about 50% of the light output is dissipated. If the lamps were replaced all at one time, there would be an enormous change in total light output between installation and replacement. Replacing only a portion of the total lamps means some lamps are working at their maximum light output and some are declining therefore, fluctuation in total light output is minimized. This exchange system improves uniform lighting, which is critical for uniform plant production.
Aspirated Box
An example of an aspirated box, which houses and protects the computer sensors from light or localized temperature changes. The position of the box should be close to the plant canopy to measure the environmental parameters at plant level. This may not be possible in all germination areas. The box is equipped with a small fan (A) which draws air past the sensors (B). Sensors are located upstream from the fan.
Sensors
See "Sensors" under Computer
Technology for full details.
