Most commercial vegetable production greenhouses in North America use some form of “hydroponic culture”. The term hydroponics essentially translates as “water culture.” It is an advanced form of crop culture that allows for specific control of the delivery of nutrients to the plants. The term hydroponics can bring to mind a number of variations on the same theme. Hydroponic growing systems can include:
- Substrate culture where the roots are allowed to grow in an inert or semi-inert media
- Solution culture where the roots are immersed in ponds of nutrient solution
- NFT culture (nutrient film technique) where the roots are contained such that a thin film of nutrient solution constantly runs by the roots
- Aeroponics where the root systems are suspended within an enclosed area and are misted with nutrient solution
A general working definition of hydroponic culture that would include all the above systems would be a plant culture where the plants receive fertilizer nutrients every time they receive water.
Using this working definition of hydroponics also leaves room for the inclusion of soil as a growing medium. However, soil culture is not widely practiced in commercial vegetable greenhouses in Alberta. The main reason for moving out of soil into soilless culture is to escape problems due to soil borne diseases that can build up in the soil that is used year after year.
Soilless media such as rockwool and sawdust offer an initially disease-free growing medium. There are other advantages to moving the root system out of the soil and into confined spaces such as sawdust bags or rockwool slabs. The main advantages are realized in the improved management of watering and nutrition, topics discussed in more detail in following sections.
Growing media for the production greenhouse
The majority of North America commercial greenhouse vegetable production is based on substrate culture where the plants are grown in sawdust or rockwool. These substrates contain practically nothing in the way of plant nutrients and serve as a medium for the root system to anchor the plant. The growing media plays a significant role in defining the environment of the root system and allows for the transfer of water and nutrients to the plant.
Typically for sawdust culture, two or three plants are grown in 20 to 25 litre white plastic bags (white reflects more light) filled with spruce and/or pine sawdust. Rockwool culture uses approximately 16 litres of rockwool substrate for every two to three plants. The sawdust bags or rockwool slabs are placed directly on the white plastic floor of the greenhouse.
Sawdust is less expensive than rockwool in initial cost; however, standard density rockwool slabs can be pasteurized and reused for up to three years. Sawdust is a waste product of the lumber milling process, which is usually burned, so the use of sawdust as a growing media is an environmentally sound practice.
For sawdust culture, it is important to use a moderately fine sawdust. Lumber mills in Alberta understand the sawdust requirements for plant production and will supply “horticultural grade” sawdust if they are made aware the sawdust is to be used for plant culture. Sawdust that is too fine will break down over the production season, with a resulting loss of airspace around the roots that can lead to root death.
A Grodan Rockwool Grow Slab
There is always some sawdust decomposition during the growing season, which makes the product useful for further composting or adding to mineral soils to improve soil quality. Through the continued action of soil microbes, the sawdust residue is returned to the environment in an ecologically sound manner at the end of the cropping season. The waste from sawdust culture is confined to the plastic bags themselves, which are recovered when the sawdust bags are dumped. These bags can be recycled where facilities exist.
More information about hydroponic greenhouse design click here.
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