Hydroponics: Aggregate culture
There are many variations of this system of cultivation which consists in principle of growing plants in containers filled with an aggregate such as sand and gravel or some similar inert material and watered with nutrients in solution.
Choice of aggregate
Certain types of aggregate, certain sizes of the particles making them up and certain mixtures of aggregate have given better results than others. Mixtures of 20% by volume of coarse gravel, 35% fine gravel, 35% coarse sand and 10%, fine sand give good results, as do mixtures of peat and coarse sand, containing 25-40 % by volume of peat.
Sands or gravels which contain large amounts of chalk or limestone should be avoided, because they are strongly alkaline and will have an adverse effect on the pH of the mix or solution. The tiny particles of fine sand will stick drainage together when water is added; cover the basic layer of fine sand with a 1 cm (1/3”) layer of sand or gravel to prevent this.
Vermiculite is a lightweight granular material, rather like mica, which has been heated to expand the granules. This material is clean and sterile, but unless it is mixed with an equal amount of sand, it can be too retentive of moisture, especially in winter.
Polyurethane foam plastic chips have performed particularly well in tests. They are very light in weight—an obvious advantage for roof-top gardens and window boxes. However, some may contain chemicals that will harm plants.
Peat is easier to manage when it is mixed with coarse sand than on its own, when patches of poor growth can occur.
Sawdust has also given satisfactory results.
Lignite is also known as ‘brown coal’, and is highly absorbent. Although organic in origin, it is resistant to breakdown, unlike peat. Perlite, an expanded volcanic residue, is soft and light in weight and has given good results either alone or mixed. This is the material used in the kit based on the flotation technique.
Hydroleca or leca, is an expanded clay aggregate used for ornamentals.
Aggregate culture systems
The type of installation for use with aggregate culture varies from a simple waterproofed box with plug holes, to highly sophisticated, sub-irrigated tanks fitted with pumps and other gadgets.
Two container measurements are crucial—the depth and the width. A depth of 20-23 cm (8-9”) will be suitable for most crops, though forand root crops a 30 cm (12”) depth is preferable. Seed trays can of course be shallower; 5-7.5 cm (2-3”) will suffice. The tray or bed width should not exceed 75 cm (2-1/2′) unless there is access from both sides, otherwise it will be difficult to reach the centre.
Plastic lined tanks are good, but metal tanks should be coated with asphalt to prevent traces of the metal from harming the plants. Tanks can also be made of concrete, wood or earthenware. It is a good idea to paint the inside of the tank with bitumen paint, to prevent release of substances toxic to the plants and, in the case of wood, to prevent it rotting. The paint must be petroleum-based.
The container should slope slightly towards one end to ensure adequate drainage, and should be provided with plug holes in the sides to permit them to be washed or flushed out as necessary.