January/February – 1992
Story Title: Sand Culture
Author: Leo Wright
“Sand culture is one of the most efficient and cost-effective methods of soilless culture, and is widely used in the dry arid regions of the Middle East. Although it is not used in Australia on a commercial scale, it has proved popular among some growers for crop trials. Its simplicity and low capital cost makes it an attractive alternative to existing growing methods.”
Sand culture is one of the most efficient and cost-effective methods of soilless culture, and is widely used in the dry arid regions of the Middle East. Although it is not used in Australia on a commercial scale, it has proved popular among some growers for crop trials. Its simplicity and low capital cost makes it an attractive alternative to existing growing methods.” by Leo Wright History records that Aztec Indians were among the first to use sand culture techniques. During the 12th century the Aztecs developed extraordinary irrigation systems in the Mexican basin, with swamp reclamation their most significant achievement, even including the colonisation of surrounding lakes. The Aztecs grew beans and squash on primitive rafts covered with sand removed from shallow lake beds. The plant roots grew through the sand down into the nutrient-rich water of the lake. A few of these so-called ‘floating gardens’ can still be seen on the lakes surrounding Mexico City today.
In modern times, sand culture re-emerged during the 1960s, ironically in Puerto Penasco, Mexico, where successful trials led to commercial operations in Mexico, the U.S. Southwest and in the Middle East. Using coarse beach sand, leached free of excess salts, vegetables were either sown directly in the sand or planted as seedlings.
The Advantages The sand culture system has many advantages over traditional hydroponic techniques. The fact that it is an ‘open’ (‘run-to-waste’) system, whereby the nutrient solution is not recycled, greatly reduces the likelihood of diseases such as Fusarium and Verticillium spreading in the medium. It also means there is no nutrient imbalance since plants are fed with fresh nutrient solution at each irrigation cycle.
Another advantage is the excellent capillary action of sand, which results in lateral movement of nutrients so that there is an even distribution of nutrients throughout the root zone. Additionally, water retention is high owing to the smallness of the sand particles, allowing fewer irrigation cycles during the course of the day. Unlike other systems, particularly NFT (Nutrient Film Technique, in which there is no medium), in the event of a fractured pipe, power or mechanical failure, there is more time available to repair the system before plants consume existing water in the medium and begin to experience stress due to dehydration.
Practical advantages of sand culture include lower construction costs, simplicity of operation, and easy maintenance and service.
The disadvantages of sand culture are few, and in some cases are in common with other hydroponic techniques. A major disadvantage is the need to use chemical or steam sterilisation between crops in order to destroy media-borne pathogens. Such methods are thorough, although somewhat time- consuming.
Like all drip irrigation systems, feed lines can become blocked with fine nutrient particles, grit or sand. This can be overcome by using in-line mesh filters which can be easily cleaned. Perhaps the overriding factor which makes sand culture unattractive to end-users is the seemingly high consumption rate of nutrients because of the need to run to waste.
However, with careful management, the waste should account for no more than 8% to 10% of the total nutrient solution added. Salt build-up is another common problem, but this can be corrected by flushing the medium periodically with fresh water. Again, regular and careful monitoring of the drainage water for evidence of salt accumulation is important to prevent excess salt problems.
Sand Culture Systems There are basically two proven methods of utilising sand as a growing medium. The first is to use plastic-lined beds in above ground troughs; the other is to spread sand over the entire floor of the growing area. Both methods use a sand depth of between 300 and 400 mm. Sand beds can be easily constructed with wooden sides, or with concrete reinforcement wire, cut and bent to form an above ground trough and lined with thick, black plastic film.
The bed must be watertight as leaks mean wasted nutrients. Polyethylene film (6-20micron), or swimming pool liner are ideal. If using black polyethylene film, it should be doubled over for maximum strength – a single layer of polyethylene will stretch and tend to mould around sharp objects once sand is shovelled into the bed, which may cause it to rupture.
The bottom of the trough should have a 1:400 incline (150mm drop per 60 metres), so that it can be drained or leached when required. A drain pipe should be set along the entire length of each sand bed, which in turn should be connected to a main pipe at one end to collect waste water from all beds and to conduct it away from the greenhouse or growing area.
For sand beds deeper than 400mm, 50mm agricultural pipe, covered with blue metal, can be used to channel excess nutrients away. For a standard sand bed, 70mm pressure pipe can be used and this should be seated on a shallow layer of blue metal. The pipe should have drainage holes, cut across the pipe with a saw every 450mm, with the holes positioned on the undersurface of the pipe to discourage plant roots entering the pipe. For greenhouse operations, the floor should have a gradient of 150mm per 30 metres, (1:200 incline), and should be covered with 6 micron black plastic film. Generally, two layers of black film are used to overlap and cover the entire floor.
A 30mm-50mm diameter drainage pipe, cut in the manner described earlier, is placed on top of the plastic at a uniform spacing of 1-2 metres between pipes, depending upon the grade of sand in use. The finer the sand, the closer the pipes should be spaced.
Drain pipes must run parallel with the slope of the floor. At the low end of the greenhouse a connecting pipe is installed to conduct excess nutrients away from the greenhouse where it can be used for outside irrigation. Once the pipes are in place, the entire greenhouse floor is covered with sand to a depth of 300mm.
A simple home system can be designed along the same lines as commercial units, but on a much smaller scale. It should consist of a bed or growing tray, nutrient reservoir and a trickle feeding system operated by a pump that is controlled by a timer. The growing tray can have small holes in the bottom of the plastic liner or a perforated plastic pipe can be used for drainage.
One of the oldest and still popular hydroponic methods is to use a wick system. This consists of a double pot, one containing the sand and plant, and the other the nutrient solution. A fibrous wick is set into the growing pot about one-third of the way with the other end suspended in the nutrient solution. As the sand dries out, capillary action draws more solution through the wick to the plant root zone. Ideal Sand Aggregate Research shows that beach sand is usually too fine and causes puddling, indicated by water coming to the surface upon vibration of the sand.
Examples of puddling can often be seen in footsteps while walking along wet beach sand. It is caused by the high percentage of silt and fine sand. The ideal sand aggregate is river sand, washed free of fine silt and clay.
The sand particle size should be between 0.6mm and 2mm in diameter, which allows the aggregate to drain freely and not puddle after an application of water. Cuming (1986) recommends a mixture of grades (30-40% of 0.5mm, 40-60% of 0.2mm to 0.5mm, and 5-15% of 0.2mm).
Drip irrigation System A drip irrigation system must be used with sand culture. In a greenhouse situation piping should be capable of delivering 6-10 litres per minute for each 100 square metres, or 30-45 litres per minute for each 500 square metres of growing area. However, the rate and length of irrigation cycle will depend upon the crop, its maturity, weather conditions and time of day.
The volume of water can be regulated using a flow control valve for each sand bed. The valve is adjusted for each bed according to plant requirements. In this way several plant types can be grown. The flow valve should be positioned upstream from the solenoid valve which automatically controls the irrigation cycle.
Where several beds are cultivated, there is no need to water them simultaneously. The water arrangement can be set so that individual beds are watered separately, thus ensuring mains pressure is not reduced.
For small sand beds, a single 13mm black polyethylene pipe can be run down the length of the bed with spaghetti lines inserted every 300mm. Larger beds will require 13mm poly pipe run along the inside of each plant row.
Emitters can be used to deliver nutrients to plants, adjusted to deliver 4-6 litres per hour. Alternatively, spaghetti tubing can be inserted into the main 13mm feed line. A short length (50mm) of 13mm poly tube can be attached to the end of each spaghetti line to channel nutrients out to each side of the irrigation point.
Emitters, above ground pipes and fittings should be black to prevent algae growth inside the piping system.
If a timer is used, it should be programmed to deliver nutrients two to five times daily, depending on the maturity of plants, weather and seasonal factors. As discussed earlier, enough water is added to each cycle to allow 8%-10% drain off. Twice a week a sample of the drain off should be taken and tested for total dissolved salts. If the dissolved salts reach 2000 parts per million (ppm), then the entire sand bed should be leached free of salts using fresh water.
The Nutrient Tank
The nutrient tank should be large enough to meet feeding requirements for at least one week. And if several crops are grown which have different nutrient requirements, then two tanks should be used, each with its own specific nutrient formulation. In this event, independent irrigation systems must be connected to each nutrient tank.
Since sand culture is an open system, there is no need to change the nutrient solution regularly. However, the tank should be drained and cleaned periodically of any sludge or sediment which may accumulate owing to inert carriers in the fertilizer salts.
Sand culture beds can be constructed on any surface, including hard bedrock or stony ground with a minimum of fuss. That, combined with low installation costs, simplicity of operation and retention of moisture, even in hot weather, makes it an ideal system for Australian conditions. It is particularly well-suited to rain shadow areas. Perhaps the only significant disadvantage is that it is likely to become waterlogged during heavy rain periods in those systems exposed to the elements.
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