METHOD AND APPARATUS FOR CULTIVATING PLANTS DESCRIPTION OF THE INVENTION The invention relates to methods for growing plants in which the flow of water for irrigation is controlled through the environment of the roots of the plant. In particular, this relates to methods in which the plants are grown in a growth substrate, in particular mineral wool growth substrate. This also relates to an apparatus for carrying out the method. It is well known to cultivate plants in a natural or artificial growth substrate, in particular a growth substrate of mineral wool, such as rock wool or glass wool. The water and, if necessary, the fertilizer and other additives are supplied to the growth substrate, generally causing the water, which optionally contains the fertilizer and other additives, to flow through the substrate. It is important that the plants receive a supply of adequate water, oxygen and other materials such as fertilizer which are transported by the water. Water is one of the means by which oxygen is transported within the growth substrate (although oxygen does not penetrate the growth substrate by other means, such as directly from the air). In particular, if the water is supplied from a sprinkler placed on a mineral wool growth substrate, the droplets falling on the substrate are highly oxygen rich. This oxygen is transported within the substrate and lodged in the roots of the plant. Similar considerations apply to other additives dissolved in water, such as fertilizer. A higher flow rate of water within the substrate increases the supply rate of additives carried by the water. It is advantageous to have a suitable water flow for other reasons. The increased water flow leads to increased turbulence around the roots, which increases the rate of transfer of beneficial components such as water and fertilizer into the roots. The water flow also undesirably removes released byproducts within the growth substrate of the plants. However, simply increasing the flow rate of water supply to the aqueous growth substrate can cause problems. In particular, the maximum flow rate is usually determined by the maximum flow rate of water through the growth substrate under gravity. If the water supply flow exceeds this direct flow then the excess water simply spills. It is possible to modify the growing substrate so that a higher maximum direct flow rate is obtained. However, this generally requires the reduction in the density of the growth substrate, in particular in the case of mineral wool. This by itself leads to a lower aqueous distribution through the substrate. The water level at the top of the growth substrate is much lower than at the bottom of the growth substrate. The upper part may become excessively dry and the bottom may become over-saturated. It would be desirable to actively control the flow of water through the substrate. Prior publications EP-A-300,536 and EP-A-409,348 describe active water flow systems. EP-A-300, 536 describes a system in which the flow of water through the growth substrate is controlled by a capillary system. The water conduits extend into the growing substrate and are connected to a water pump. This is adjusted to a predetermined speed to pump water out of the substrate. The piping system is substantially filled with water and the flow rate is determined essentially by the speed setting for the water pump. This publication discusses the "suction pressure", but this is in the context of the required force exerted by the plant to remove water from the substrate. The "suction pressure" elevated in this sense correlates with the aqueous content of the low substrate and the purpose of this publication is to maintain an appropriate substrate aqueous content and a suitably appropriate suction pressure. The EP-A-409, 438 is related to the same water pumping system. In addition, coupling members are provided between the channeling system and the growth substrate. The intention of these is to avoid the growth of plant roots within the canalization system. It is stated that one advantage of the coupling members is that they remain more humid than the surrounding growth substrate and prevent air from entering the piping system from the side of the panel. WO95 / 31094 describes a drainage system for the active and passive liquid drainage of growth substrates. A series of growth substrates are provided each having a "suction plug" coupled to a siphon hose which drains into a riser tube. There is no indication of the material from which the "suction plug" is made. Although all these systems are effective and useful, there is capacity for improvement in certain areas. In particular, the previously described systems require that the surface on which the plants are grown, for example, the floor of a greenhouse, be almost exactly horizontal. Otherwise, the pressure in the system and the water flow rate vary according to the height at which a block of the growth substrate is placed (eg, mineral wool). An additional potential problem lies in the fact that the pipeline system is substantially filled with water. There is thus an intact water path from one plant to any other plant in the system. This has the potential to allow the transfer of plant viruses and other infections through the entire culture. O94 / 03046 describes another system for growing plants in mineral wool. In this system, the aqueous content of the mineral wool is kept constant by supplying water to the mineral wool growth substrate through irrigation pipes and removing it through drainage pipes. A common pipe system is used for water supply and drainage. In this system, as in the systems of EP-A-300,536 and EP-A-409, 346, discussed above, there is a continuous connection between the water in the growing substrate and the water in the drainage system. Another known system for growing plants is known as the nutrient film technique (NFT) system. In this system plants are grown in blocks of small propagation or even in no substrate at all, the plants and blocks if used, are contained in a plastic container, such as a plastic film container. If water is used it is sprayed into the container and into the propagation block, and drained from the plastic container through holes. Such systems suffer from the problem that the drainage process is significantly affected by the uniformity of the surface on which the plants are grown. An uneven surface results in uneven drainage and different plants are subjected to different degrees of saturation. O03 / 005808 describes a system which addresses all these problems effectively. A system comprising a liquid extraction and air blocking device integrated within a growth system and which is part of a channeling system which uses a cavity partially filled with liquid and partially filled with air to induce the flow is described. controlled release of liquid from the substrate. Specifically, it describes a method for growing plants which comprises providing plants, supplying water so that the roots of the plant come into contact with a body of water and extract water through a suction device provided in contact with the body of water and within of a first conduit, extracting water through the first conduit and into a second conduit, and the second conduit is at least partially filled with air and the first and second conduits are connected so that the first conduit is released into the space air in the second conduit. In the preferred modalities, plants are provided in a growth substrate, water is supplied to the growth substrate and extracted from the growth substrate by the suction device, which is provided in the growth substrate. This system has numerous advantages over previous systems such as EP-A-300,536, EP-A-409,348 and O95 / 31094 and O94 / 03046. It is described that the suction device is capable of draining water from the growth substrate by capillary force. The suction device is made of a porous material, including stone (especially volcanic stone), ceramic, mineral wool or porous glass. Organic polymeric foam and organic polymeric fibers are also described as potential materials for the suction device. In practice, stone, especially volcanic stone, is adequate. It has been found that the preferred suction device materials in this publication have certain disadvantages. In particular, after a period of use, the nutrients in the water irrigating the system tend to precipitate on the surface of a stone or ceramic suction device. Due to the small pore size of the suction device, this can result in the obstruction of the suction device. The present invention seeks to address this problem and does so by providing specific types of material for the suction device. According to the invention there is provided a method for growing plants which comprises providing plants, supplying water so that the roots of the plant make contact with a body of water and extract water by means of a suction device provided in contact with the body of water and within a first conduit, extracting the water through the first conduit and into a second conduit, wherein the second conduit is at least partially filled with air and the first and second conduits are connected so that the first conduit is released within the air space in the second conduit, characterized in that the suction device is formed from a foam formed of a polymer selected from phenol urea formaldehyde polymer; urea polymer melamine formaldehyde; polyurethane; furanic polymers; and ethylene, propylene and butylene homopolymers, copolymers and terpolymers. In this way, the polymeric foam can be formed from, for example, polyethylene, polypropylene or polybutylene and ethylene-propylene-butylene terpolymers can also be used, as well as ethylene-propylene, ethylene-butylene and propylene-butylene copolymers. Within the term "foam" materials are included which are, in a micro scale, a three-dimensional mesh. In the preferred embodiments, the pressure in the ducts is controlled by an air pump. It is found that the use of the types of organic polymeric foam mentioned above alleviates problems with precipitation of nutrients in the pores of the suction device and allows trouble-free and prolonged operation of the process. Although O03 / 005808 mentions generally organic polymeric foams, the above specific materials are not mentioned. In particular, there is no mention of the fact that these specific materials can address the problem of nutrient precipitation within the pores of the suction device. The invention comprises a device for extracting liquid and blocking air which is integrated into a growth system and which is part of a duct system which uses a cavity partially filled with liquid and partially filled with air to induce controlled release of liquid from the substrate. The liquid withdrawing and air blocking device is generally in the form of a suction device such as a suction pad inserted into the growth substrate. The suction device is formed from one of the defined materials and is capable of forming an air bubble when the pressure in the conduit system tends to draw air through it. Since the pressure to draw water into the system increases, the water flow increases, generally up to an extraction force of at least 30 cm of water column. The pressure can be increased to an extraction force at which the suction device releases air into the first conduit instead of water because the force tending to draw water through the system is greater than the force that holds water in the system. suction device. In a particularly preferred embodiment of the invention, plants are provided in a growth substrate, water is supplied to the growth substrate and extracted from the growth substrate by the suction device, which is provided in the growth substrate. In this way, the liquid extraction and air blocking device is preferably integrated into the growth substrate. It is not necessary to provide a level surface and in this way the system can be easily and expressly applied in any greenhouse without requiring floor leveling in the first place. The first conduit is released into the air space in the second conduit. In a preferred embodiment, at least two and preferably a large number of conduits are provided, each connected with a suction device in contact with the body of water which comes in contact with the roots of the plants. When plants are grown on a growth substrate, it is common to provide a large number of panels each containing one or a small number of plants. In this case, each suction device is usually associated with a single panel, and in some cases, a suction device can be associated with each plant. Thus, although it is possible that viruses and other infectious agents from one plant can be extracted from the growth substrate within the first conduit and then released into the second conduit, there is no water path between the second conduit and other first conduits. associated with other plants. In this way, the risk of transfer of viruses or other infectious agents is greatly reduced. It is possible to control the flow of water through the surrounding environment of the roots of the plant, for example, a growth substrate, simply by means of modifying the pressure in the duct system by an air pump and obtaining the consequent advantages discussed. above, such as controlling the oxygen delivery rate, the delivery rate of other additives, control of aqueous content, pH, EC (electrical conductivity), nutrients such as nitrogen and micro elements, and the removal of undesirable byproducts. It is also possible to achieve this with a high density growth substrate which gives good aqueous distribution. It is possible to change the air pressure within the piping system quickly and easily and thus modify the flow rates and water content without difficulty. If a growth substrate is used and the suction device is placed at the bottom of the growing substrate, then the water is extracted from the bottom of the substrate and the tendency to aqueous saturation at the bottom of the substrate is reduced. The invention also provides an apparatus suitable for use in the cultivation of plants. This comprises a growth environment adapted to contain plants and water such that the roots of the plant are in contact with a body of water, the growth environment is provided with a suction device formed from a foam formed of a polymer selected from phenol urea formaldehyde polymer;
urea polymer melamine formaldehyde; polyurethane; furanic polymers; and ethylene, propylene and butylene homopolymers, copolymers and terpolymers and arranged to draw water from the growth environment and connected to a first conduit at one end of the first conduit. The first conduit is connected at its other end to a second conduit and the apparatus comprises means for draining water from the second conduit. The apparatus is classified so that the second conduit is at least partially filled with air in use. The apparatus also preferably comprises an air pump arranged to control the air pressure in the piping system. As in the method of the invention, the growth environment is preferably a growth substrate and the suction device is preferably provided in the growth substrate. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic view of an apparatus according to the invention. Figure 2 shows a part through the cross section of an apparatus according to the invention. Figure 3 shows a different part through the cross section of an apparatus according to the invention. Figure 4 shows a further schematic view of an apparatus according to the invention. In the invention, it is essential that the body of the water in contact with the roots of the plant is in contact with the defined suction device. The suction device is capable of extracting water from the growth substrate. That is, it is capable of incorporating water against pressure. Thus, although the invention may include a system for applying vacuum or pumping, the suction device is such that water can be incorporated therein. In particular, it is capable of initially extracting water from the growth substrate by capillary force. The polymeric foam is preferably formed from phenol urea formaldehyde polymer; urea polymer melamine formaldehyde; polyurethane or a furanic polymer. Other embodiments are formed from homopolymers, copolymers and terpolymers of ethylene, propylene and butylene, for example polyethylene, polypropylene and polybutylene. The suction device is preferably formed from either a phenol urea formaldehyde foam or a polyethylene foam. More preferably, this is a phenol urea formaldehyde foam. One type of such foam is marketed under the name Oasis ™, which has a three-dimensional mesh (or network) structure. Alternatively, the suction device can be formed from a polymer of urea melamine formaldehyde. Suitable polymers are marketed under the brand name Fytocell (TM) by the company Fytogreen. It is produced from an aminoplast resin and has an open cell structure. Similar products which can be used are marketed under the Fytofoam (TM) and Hydrocell (TM) brands by the same company. A network in which the mesh is formed substantially of square or rectangular mesh in which the distance between the points of intersection is from about 20 to about 100 microns, especially about 40 to about 60 microns, is preferred. The strands forming the mesh are preferably in the range of 2 to 20 micrometers, but the particularly preferred strands have a thickness at the high end of this range, for example, 4 to 20 micrometers. The thickness is preferably from 1/10 to 1/5 of the distance between the points of intersection of the mesh, preferably from 1/8 to 1/5. The material used for the suction device must be sufficiently hydrophilic to give the desired capillary action. Certain particular foams are formed from polymers which are inherently hydrophilic enough to allow this, but otherwise the foam also preferably includes a wetting agent. Suction devices that have a density of at least 60 kg / m3 are preferred, especially when the suction device is formed from phenol urea formaldehyde foam. The density of the suction device can be high, for example up to 900 kg / m3. In particular, for polyethylene suction devices the density can be from 600 to 820 kg / m3. The urea melamine formaldehyde materials can have a density / dry matter content from 14 to 20 kg / m3. The polymeric foam generally has an open foam structure. The suction device must hold water more tightly than the air. Preferably, water is maintained against a force of at least one water column of 10 hours, preferably at least one water column of 13 cm, more preferably at least one water column of 20 cm, more preferably at least one column of water of 30 cm. Some can hold water against a force of up to 200 cm of water column. The ability of the suction device to hold water may be greater or less according to the nature of the growth substrate (when used). For example, when the growth substrate is a stone wool suction device capable of holding water against a force of at least one column of water of 5 cm gives acceptable results. However, where the growth substrate is earth, better results are achieved when the suction device holds water against a force of at least one 50 cm water column. Where the pressure in the second conduit is below the (preferred) atmosphere, generally the suction device holds water more tightly than the air in a water column of value determined by: the elevation of the second conduit on the suction device subtracted from the difference in pressure in the second duct below the atmospheric one (often referred to as low pressure). In practice, the suction device must maintain water against a force substantially equal to the low pressure in the second conduit. When a growing substrate is used, preferably the material of the suction device has an average pore size smaller than the average pore size of the growth substrate. The suction device can be described as substantially air blocking. That is, it does not allow substantial passage of air through the body of water in contact with the roots (ie, through the growth substrate if used) and into the first and second conduits. The air pressure in the first and second conduits is generally predetermined and is preferably below atmospheric pressure. The entry of air into the second conduit through the suction device will affect and modify this pressure to some degree. This also has the effect of subjecting different suction devices in a simple system to different air pressures, in which the claimed system seems to be avoided. However, in systems in which the pressure is significantly below atmospheric for example about a 20 cm column then a low degree of air passage within the side conduit through the suction device is not problematic. In this way, the suction device is the blocking of air to the extent that it prevents the entry of substantial amounts of air into the second conduit which has a substantial effect on the air pressure in the second conduit. In systems that include an air leak from the air pump inside the system you can take care of the air pump. The suction device generally has a total volume from about 2 to 100 cm3. Usually, suction devices are provided as separate entities within individual panels of growth substrate (each panel containing one or a small number of plants) or separately within a large panel (containing many plants), each suction device being associated with a small panel or a small number of plants within a large panel. Suction devices of this nature can be described as "suction plugs". The devices can take any shape or size. Usually, the suction device is generally cylindrical or oblong. However, it does not need to be a simple element. For example, this may be in the form of two or more elements in the form of a separate clasp. The size of the suction device is generally chosen to be appropriate to the environment of the roots of the plant, whether it is a growing substrate panel or a body of water. It is also possible that the suction device is not a suction plug, but is provided by a layer of material next to the base of a panel. For example, a growth substrate panel can be provided from mineral wool in which a top layer is formed from mineral wool and a base layer is formed from the defined foam, such as phenol formaldehyde urea foam or polyethylene foam. Such a layer can be provided in individual panels or in a simple large panel arranged to transport a large number of plants. The plants are generally commercial crops of the type grown in greenhouses. The crop can be for example tomato, cucumber, sweet pepper, eggplant, rose or fungus. According to a preferred aspect of the invention, the plants are grown on a growth substrate. Any natural or artificial growth substrate, for example, soil, peat, coconut, perlite or artificial vitreous fibers (MMVF), and mixtures of any of these may be used. Other suitable growth substrates include blends of polyurethane and granular mineral fibers, as described in O02 / 00009. If the suction device is formed from phenol urea formaldehyde foam such as that marketed under the name Oasis ™, then the growth substrate is not made of this material. Preferably, the growth substrate is formed from mineral wool such as glass wool, or preferably, rock wool. A mineral wool growth substrate can be made in a conventional manner by providing a molten mineral and forming fibers from the melt. During the production of the fibers or, less preferably, after the production of the fibers, the binder can be applied to the fibers. When the binder is used, it is preferably a hydrophilic binder. The growth substrate preferably contains a wetting agent. This can be used in addition to the binder. Alternatively, a simple material can be used which acts as the binder and the wetting agent. The growth substrate may contain other additives known in the art to modify and improve properties, such as clay and lignite. In one embodiment, the growth substrate is in the form of a series of small propagation blocks, each containing a plant, and the propagation blocks are contained in a plastic container such as plastic sheet. This is a modality of the NFT system discussed above. Another modality of the NFT system does not use the growth substrate at all. Instead, the plants are grown with their roots in contact with a body of water contained within a plastic container such as a plastic sheet. In the method, water is supplied to the plants, for example, to the growth substrate where it is used. This can be by any conventional means, for example, spray feeding. This method is particularly preferred because the water is rich in oxygen when it reaches the environment of the plants, for example, the growth substrate. Irrigation can be continuous or periodic. The water may contain fertilizers, biologically active additives such as fungicides where it is appropriate for the growing culture, and other additives. The suction device is connected to one end of a first conduit, which generally has a narrow diameter. The internal diameter is preferably from 1 to 10 mm, more preferably from 2 to 6 mm, in particular approximately 4 mm. The other end of the first conduit is connected to a second conduit. The second conduit is filled at least partially with donaire. This allows the pressure in the system to be controlled by an air pump. The first conduit is discharged into the air space in the second conduit so that in the preferred system where several first conduits feed a second single conduit there is no continuous water path between the plants. Generally, the first conduit is connected to the upper part of the second conduit. Generally, also the first conduit is substantially full of water during the flow of water in use. The relative volumes of air and water in the pipeline system will vary according to the flow of water required and the dimensions of the pipes. However, preferably not more than 80%, more preferably not more than 60%, in particular not more than 40%, of the internal volume of the channeling system is absorbed by water. More preferably, less than 20%, in particular less than 10% of the internal duct volume is absorbed by water. The pressure in the duct system is generally from 3000 Pa below 3000 Pa on the atmospheric pressure, preferably from 2000 Pa below 2000 Pa on the atmospheric pressure. This is preferably below atmospheric pressure, for example from 100 to 2000 Pa below atmospheric pressure. It is possible to provide a system in which the air pressure inside the ducts is above atmospheric, as long as the point of discharge from the first duct within the second duct is at a lower elevation than the suction plug. This means that the gravitational force causes the water to move from the suction plug to the second conduit. Pressure on atmospheric pressure will reduce this tendency, but as long as the total force causes the water to tend to move to the second conduit then any combination of elevation and air pressure can be used. It is preferred that the point of discharge from the first conduit into the second conduit be at a greater elevation than the suction device. Preferably, the total of the second conduit is at a higher elevation than the suction device and more preferably at a greater elevation than the total growth substrate. In this case, the pressure in the duct system is below atmospheric pressure. This has an advantage that if an air bubble should appear in the first conduit then it will automatically move to the second conduit, without any change in the pressure in the system that is required to be induced. For optimum operation of the preferred system comprising two or more suction devices each associated with a first conduit, the two or more first conduits discharging into a second single conduit, the difference in elevation between the suction device and the point at which The first conduit discharged into the second conduit must be the same for each suction device / first conduit combination. It is not necessary that all the suction devices are at the same elevation with each other or that all the first conduits are at the same elevation with each other. However, the relative elevation of the end of the first conduit with respect to the suction device must essentially be the same for all pairs. It will be noted that the experienced person will be able to choose the relative elevations of the suction device and the point of discharge from the first conduit within the second conduit and the air pressure in the conduit system to obtain the desired force to extract water from the suction device to the second conduit. It is preferred that the height of the discharge point from the first conduit within the second conduit be no lower than any other point in the first conduit. That is to say, preferably no part of the first conduit is at a higher elevation than the discharge point within the second conduit. Preferably, the system comprises a number of growth substrate panels such as a mineral wool, each provided with a suction device and a first conduit, all of the first conduits conducting within a second single conduit. More preferably, a series of such systems is provided so that at least two, generally several second conduits are all fed into a third single conduit. The water then flows into the third conduit, in which a siphon is placed which removes water from the system. The siphon is preferably placed at the lowest point of the third conduit. The second conduit can be placed at any angle as long as it allows water to flow out of the system or, when preferable, into a third conduit. Generally, it is placed at an angle from 0 to 45 ° with the horizontal. Water drawn by siphon from the system is recycled in general, usually after disinfection. The system can be started by any suitable means to induce the initial flow of water through the suction device, for example, the use of an air pump or other means of suction or even gravity alone. In well sealed systems, no additional means to reduce or increase air pressure is necessary, but in practice, it is often convenient to include such means to control the pressure in the system over a long period of time. An air pump is preferably used to control the pressure in the system and can be connected at any point in the piping system, usually to the second or third conduit. It is often convenient to connect it to the third conduit. The air pump is regulated to control the air pressure within the desired range within the system. The water is extracted from the growth substrate within the conduit system by means of adjusting the forces so that the water tends to travel from the suction device to the second conduit. The system of the invention can be used in any culture method. This is particularly useful for controlling the flow rate of water in the oxygen handling system discussed in WO03 / 005807. A system of invention will now be illustrated for reference to the drawings. Figure 1 shows a series of panels 1 of the mineral wool growth substrate. In each panel 1 a plant 2 is placed for growth (see Figure 2). A suction plug 3 formed from the Oasis ™ material (phenol formaldehyde urea foam) connected to a first conduit 4 is provided in each panel. The first conduits 4 are joined to a second single conduit 5, described as a side conduit. In a preferred system there is a series of lateral conduits 5 within each of which a series of first conduits feed water. Two side ducts 5 are shown in Figure 1. The side ducts 5 are fed into a third duct 6. The third duct is described as a main duct. Connected to this main duct 6 is an air pump 7. At the lowest point of the main duct 6 is a siphon 8 used to remove water. It will be noted that the discharge point of each first conduit 4 within the lateral conduit 5 is at a greater elevation than the relevant suction plug 3. The first ducts 4 generally have an internal diameter of 1 to 10 mm, preferably approximately 6 mm. The second lateral conduits 7 generally have an internal diameter from 20 to 80 mm, preferably from 40 to 80 mm. The system is established as follows. Siphon 8 is filled with water. Panels 1 is filled with water. This allows the suction plugs 3 to be filled with water from the panels 1 for capillary action. The air pump 7 is then started so that the air pressure in the piping system is decreased. The air pressure is decreased, for example, approximately 10 Pa below the atmospheric pressure. Consequently, the water from the suction plugs 3 is withdrawn into the first conduits 4 as a result of the lower pressure in the conduit system and is sprayed into the lateral conduit 5 in the upper part of the lateral conduit 5. Figure 2 has a cross section through the side duct 5 showing the airspace and the water flowing along the bottom of the duct. In this way, the water removed from each panel is isolated from all other panels. Water flows along the base of the side conduit 5 and into the main conduit 6. The water is removed from the system by means of the siphon 8, which allows the water to leave independently of the air pressure and without affecting the air pressure.
In the illustrated system, the point at which the first conduits 4 are discharged into the lateral conduits 5 is at a higher elevation than the suction plugs 3. To drain water in this way through the first conduit 4, it is necessary that the air pressure be below atmospheric pressure to a sufficient degree to raise the water through the required lift. The relative elevation is the same for all pairs of suction plug / first ducts.