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Heat-expanded lightweight clay pebbles.Lightweight expanded clay aggregate (LECA) or expanded clay (exclay) is a lightweight aggregate made by heating clay to around 1,200 °C (2,190 °F) in a rotary kiln. The heating process causes gases trapped in the clay to expand, forming thousands of small bubbles and giving the material a porous structure. LECA has an approximately round or oblong shape due to circular movement in the kiln and is available in different sizes and densities. LECA is used to make lightweight concrete products and other uses.
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LECA was developed about in Kansas City, Missouri, to the production in a rotary kiln of a patented expanded aggregate known as Haydite which was used in the construction of SS Selma, an ocean-going ship launched in . Following in the USA was the development of a series of aggregates known as Gravelite, Perlite, Rocklite, etc. In Europe, LECA commenced in Denmark, Germany, Netherlands, and UK.
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Root ball of a hydroponically-grown cannabis plant with aggregate embedded in it
Cleaned root ball and main stem sitting atop the total amount of aggregate used to grow a single plant
LECA is usually produced in different sizes and densities from 0.1 millimetres (0.004 in) up to 25 millimetres (1.0 in), commonly 0&#;4 mm, 4&#;10 mm, 10&#;25 mm and densities of 250, 280, 330, and 510 kg/m3. LECA boulder is the biggest size of LECA with 100&#;500 mm size and 500 kg/m3 density.
Some characteristics of LECA are lightness, thermal insulation by low thermal conductivity coefficient (as low as 0.097 W/mK[1]), soundproofing by high acoustic insulation, moisture impermeability, being incompressible under permanent pressure and gravity loads, not decomposing in severe conditions, fire resistance, a pH of nearly 7, freezing and melting resistance, easy movement and transportation, lightweight backfill and finishing, reduction of construction dead load and earthquake lateral load, being perfect sweet soil for plants, and as a material for drainage and filtration.
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Common uses are in concrete blocks, concrete slabs, geotechnical fillings, lightweight concrete, water treatment, hydroponics, aquaponics and hydroculture.
LECA can be easily used for plant-growing substrate.LECA is a versatile material and is utilized in an increasing number of applications. In the construction industry, it is used extensively in the production of lightweight concrete, blocks and precast or incast structural elements (panels, partitions, bricks and light tiles). LECA used in structural backfill against foundations, retaining walls, bridge abutments etc., in addition, it can reduce earth pressure by 75% compared with conventional materials, and also increases ground stability while reducing settlement and land deformation. LECA can drain the surface water and groundwater to control groundwater pressure. LECA grout can be used for flooring (finishing) and roofing with thermal and sound insulation.
LECA is also used in water treatment facilities for the filtration and purification of municipal wastewater and drinking water as well as in other filtering processes, including those for dealing with industrial wastewater and fish farms.
LECA has many uses in horticulture, agriculture, and landscapes. Using LECA helps to alter soil mechanics. Using LECA provides many benefits across horticulture. It's commonly used as a growing medium in hydroponics systems since blended with other growing mediums such as soil and peat, it can improve compaction resistance, and drainage, retain water during periods of drought, insulate roots during frost, and provide roots with increased oxygen levels promoting very vigorous growth. LECA can be mixed with heavy soil to improve its aeration and drainage.
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In the horticultural practice of hydroponics, LECA is a favored medium for growing plants within; the round shape provides excellent aeration at the root level, while the LECA clay pieces themselves become saturated with water and plant food, thus giving the roots a consistent supply of both. So-called semi-hydroponics or passive hydroponics (also called "semi-hydro") is a popular, simplified method of hydroponics, most commonly utilized for houseplants and tropical species. A plant is potted in solely LECA (preferably in a container with multiple air holes on the sides and bottom, like an orchid pot) and placed into a second, sealed container, in which a "nutrient reservoir" of water and plant food is maintained. Only the very bottom of the pot needs to touch this reservoir; the LECA, being porous by nature, gradually wicks moisture and nutrients up and becomes saturated, allowing the plant to feed and drink at a consistent rate.
One of the main differences between semi-hydroponics and more advanced hydroponics is the nutrients and water--whether they are consistently being delivered or whether they are sitting beneath each plant, gradually needing to be replaced before evaporating or becoming stagnant. Other differences are elements such as the facilities, the equipment, the types of plants being grown and why, and the consistency of water and nutrient delivery. Hydroponics is often favored by farmers and growers of edible crops on a larger scale. LECA can be used successfully in these settings. Semi-hydroponics is much more popular with individual plant collectors, bearing in mind the need to flush out and replenish the plants' nutrient reservoirs periodically (approx. every 7-10 days); more advanced systems provide constantly flowing, filtered, nutrient-enhanced water over the plants' roots, which typically drains into another reservoir for recycling and reusing.
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Chapter 15
Ebb-and-flow benches (also called ebb-and-flood benches) combine an elevated benching system with a closed subirrigation system (See Figure 14.7). It provides for individual watering and fertigation of a number of different plants on different spacing without individual feed lines going to each plant and without overhead watering. Commercial ebb-and-flow systems are used chiefly for plants that are on the production tables for a relatively short period of time such as seedlings and the production of potted plants in the floriculture industry. As with other closed irrigation systems, the nutrient solution is recirculated and EC, pH and often temperature levels are adjusted at the main nutrient reservoir.
The ebb-and-flow system is one of the simplest hydroponic systems to set up and use, but it still takes some experience and effort to master. The ebb-and-flow system costs are usually very low in comparison to many other substrate systems, because it doesn't require any high tech, expensive components. The ebb-and-flow system is well-used because it can use many different types of growing media, expanded clay pellets, perlite, sphagnum moss, and rockwool. The disadvantage with an ebb-and-flow system is the high relative humidity that can build up in the canopy of plants.
Ebb-and-flow benches may be designed as stationary benches or as rolling benches, like Dutch trays. Either approach can work just fine, so the choice depends largely on personal preference and the desire to automate other production practices. The primary characteristic of an ebb-and-flow bench is the tray that makes up the bench surface. The tray may be made of hardened plastic or aluminum. The length and width of the tray varies depending upon the desired bench dimensions.
At one end of the tray there is an inlet and an outlet. The inlet allows fertilizer solution (or water) to be pumped into the tray. As fertilizer solution is pumped into the tray, it first floods the deepest of the channels. When the deepest channels are flooded, the solution floods the next level of channels. After both sets of channels are flooded, the solution continues to rise above the bottom surface of the bench. In a production situation, containers with plants would be placed on the tray surface. As the solution floods the tray and rises up around the bottom of the containers, the solution would come into contact with the root substrate inside of the containers and the fertilizer solution would move up and into the substrate in the container by capillary action.
Dutch trays are placed on steel tracks that serve to not only support the trays, but the trays (which have small wheel-like structures underneath) roll on the metal support structure (See Figure 14.8). This allows trays to be rolled together to maximize space usage efficiency, but the track system also serves as a transportation system around the greenhouse facility. When the plant material needs to be moved to another location in the facility, the trays may be rolled on the steel tracks to a major aisle or walkway.
Click on the following topics for more information on greenhouse production systems.
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