Process for hydroponic cultivation of plants

In a process for hydroponic cultivation of plants an irrigation solution containing water and at least one nutrient is circulated through a main loop at a first point where a disinfectant is added to the irrigation solution. At a second point at least one nutrient is added to the irrigation solution. At a third point the plants are supplied with the irrigation solution, and at a fourth point the irrigation solution drained of the plants is collected. The main loop is provided with one or more sub-loops through which the irrigation solution is recirculated.

FIELD OF THE INVENTION
 This invention concerns a process for hydroponic cultivation of plants
 using a particular system for the irrigation of plants. More specifically
 it concerns the provision of a system of plant irrigation which enables
 controlling unwanted microbial growth within this system.
 BACKGROUND OF THE INVENTION
 In order both to speed up the growth process of plants and to provide a
 year round supply for the consumer, many vegetables are grown either
 completely or partially isolated from the outside environment, for example
 in greenhouses. The conditions under which the plants are grown can
 thereby be closely regulated and parameters such as temperature, water,
 nutrient make-up, light etc. carefully controlled. The water and nutrient
 level can especially be controlled where a substrate other than soil is
 used for the plants to grow in, for example rockwool, perlite, silica gel,
 and plastic foams. These do not actually supply the plant with minerals
 but serve simply as a substrate for the root system. In such so-called
 hydroponic systems, the nutrients such as potassium, sodium, molybdenum,
 phosphate, nitrate, etc. are usually applied dissolved in the water.
 Although this can be carefully controlled, some excess water supply will
 inevitably occur. The excess can then either be discharged to the
 environment or recycled, possibly with further treatment before re-use.
 The former option is now less favoured for the following reasons. First,
 plant nutrients dissolved in the water can cause eutrophication. Secondly,
 the water may in addition contain pesticides which could also have a
 negative impact on the environment if discharged in large quantities.
 Thirdly, it is becoming increasingly undesirable to discharge water to
 drain, not just on cost grounds but also to conserve a restricted supply.
 Re-use of the excess water does however bring with it its own problems,
 especially where the water is utilised for the same plants. The chief
 reason for this is that plant pathogens can be carried through in the
 water and the irrigation system then often provides suitable breeding
 conditions for the plant pathogen, which makes re-use of this water highly
 unsuitable. It is often desirable therefore for a suitable disinfectant to
 be added at some point in the water recycle to remove or reduce the
 pathogen population. Well known disinfectants in this area include
 hypochlorite, ozone and UV irradiation as well as biological control
 mechanisms. A further group of compounds suitable for disinfecting the
 substrate and the water supplied to the plant is disclosed in British
 Patent Application 2,224,441, where organic percarboxylic acids,
 particularly peracetic acid are used.
 Simply killing any plant pathogens in the water via disinfection does
 however have an unwanted side-effect due to the biological equilibrium of
 the water being thus disturbed. According to the Derwent.RTM. abstract of
 Danish Patent Application DK 9300538 the population of one particular
 group of fungal species Trichoderma spp. is normally kept down due to
 competition from other micro-organisms present in the water. When these
 competitors are removed however, for example by disinfection, then the
 population of the Trichoderma can increase. It can eventually form a
 fibrous mat which can block up pipework and nozzles used to irrigate the
 plants. Removal of this material is a time consuming process, involving
 temporary dismantling of the pipework of the irrigation system, which
 would, and especially in warmer weather, need to be carried out
 frequently.
 In the above-mentioned disinfectants which can, potentially, be used, the
 following disadvantages may be mentioned. Hypochlorite suffers from two
 main problems. First, the solutions containing the hypochlorite will also
 comprise a large quantity of sodium chloride which may not be suitable for
 the particular plant grown. Secondly, hypochlorite is increasingly coming
 under pressure from environmental considerations due to their capacity to
 form unwanted chlorinated organic compounds via side reactions. For ozone
 and UV treatment, expenditure and equipment can be expensive.
 Biological control is also difficult to achieve as care must be taken,
 first to select a control which can adequately maintain the targeted
 micro-organism(s) at an acceptable level, and secondly to select a control
 which will not cause problems itself--for example by reaching an
 unacceptable population size.
 The above problem with Trichoderma can be exacerbated when percarboxylic
 acids are used. Percarboxylic acids are usually supplied as an equilibrium
 solution containing the parent carboxylic acid, and furthermore once the
 percarboxylic acid has carried out its disinfection then the carboxylic
 acid remains as a by-product. As some carboxylic acids can be readily used
 as a food source by Trichoderma the result is that although its population
 may initially drop, the end result of application of the percarboxylic
 acid can be an increase in Trichoderma's population.
 Potential problems with percarboxylic acids have been further exemplified
 in the prior art when they have been applied to such recycle systems, for
 example in the Derwent.RTM. abstract of the Dutch Patent Application
 9201631. In this patent percarboxylic acids were used to control bacteria
 and other micro-organisms. A further treatment was however also required
 using ultra-violet radiation to kill off viruses and moulds, along with
 occasional treatment of the water with acidified percarboxylic acids to
 remove various deposits which built up during prolonged use of the
 irrigation system.
 Conventional recycle systems are particularly prone to fouling by unwanted
 microbial species. Especially during periods of low water uptake by the
 plants, large volumes of water remain stagnant, and hence provide an
 excellent breeding ground for species such as Trichoderma. Although much
 research, as outlined above) has been carried out into methods of
 disinfection of the irrigation water, the aforementioned problems have not
 been solved.
 It is therefore an object of the present invention to solve these problems
 by providing a process for hydroponic cultivation of plants which makes it
 possible to control plant pathogens whilst at the same time inhibit the
 selective growth of other non pathogenic microbial species such as
 Trichoderma spp.
 SUMMARY OF THE INVENTION
 According to the current invention, there is provided a process for
 hydroponic cultivation of plants wherein an irrigation solution containing
 water and at least one nutrient is circulated through a main loop at a
 first point of which a disinfectant is added to the irrigation solution,
 at a second point of which at least one nutrient is added to the
 irrigation solution, at a third point of which the plants are supplied
 with the irrigation solution, at a fourth point of which the irrigation
 solution drained of the plants is collected and recirculated to the first
 point, the main loop being provided with one or more sub-loops through
 which the irrigation solution is recirculated.
 One of the essential characteristics of the invention is the combination of
 a suitable configuration of the irrigation system in conjunction with a
 percarboxylic acid. This combination can indeed not only bring about the
 desired removal of the plant pathogens but also control other non
 pathogenic microbial species such as fungi, for example Trichoderma.
 The process of the invention uses a circulation system provided with one or
 more sub-loops. The starting point of the sub-loop is advantageously
 situated between the second point at which at least one nutrient is added
 to the irrigation solution and the third point at which the plants are
 supplied with the irrigation solution. It can also be advantageous that
 the sub-loop, which starts at any point of the main loop, joins the main
 loop again at the second point where at least one nutrient is added to the
 irrigation solution. His particularly preferred that the sub-loop leaves
 the main loop at a point situated between the second and the third points,
 and joins the main loop again at the second point. According to a first
 alternative the sub-loop comprises a circulation pump which pumps the
 irrigation solution from the starting point back to the second point.
 According to a second alternative the sub-loop comprises a pipework which
 feeds the irrigation solution from the starting point to a feed machine
 wherein the concentration of nutrients in the irrigation solution is
 controlled and optionally adapted, the irrigation solution leaving the
 feed machine being circulated back to the second point. According to a
 third alternative the sub-loop comprises an irrigation solenoid which
 diverts the irrigation solution at a point situated between the second and
 the third points back to the second point. Two or three of these
 alternatives can be combined.
 It is to be understood that the main loop of the process of the invention
 can contain other elements than the four points mentioned before. These
 other elements can be tanks, collectors, pumps, filters, etc. For
 instance, the main loop can be provided with a means of flushing it with
 rinse water.
 At the first point of the main loop a disinfectant is added to the
 irrigation solution. The addition of disinfectant may be operated for
 whole of daily irrigation period. Alternatively, the disinfectant can be
 added for part of daily irrigation period interspersed with periods of
 circulation of irrigation liquid without disinfectant. Such a period
 without disinfectant may be of particular benefit within the last hours as
 final irrigation to remove any residual carboxylic acids in the irrigation
 system thereby reducing the possibility of excess growth of Trichoderma or
 other organisms.
 The addition of disinfectant may be carried out directly into the
 irrigation solution circulating through the main loop. It may be
 advantageous to add the disinfectant in one or more treatment tanks
 wherein the irrigation solution is allowed to stand while being treated
 with the disinfectant to improve reduction of the population of pathogenic
 or problematic organisms. It can be advantageous to use several treatment
 tanks, for instance 2, to reduce stagnation time and build up of
 carboxylic acids which may encourage growth of problematic organisms such
 as Trichoderma.
 Although the system is designed to utilise a wide range of known
 disinfectants, it is especially suitable for use in conjunction with
 percarboxylic acids. The percarboxylic acid can be any percarboxylic acid
 of sufficient. solubility. Examples of such percarboxylic acids include
 low molecular weight aliphatic peroxyacids, containing up to 6 carbon
 atoms. Examples include performic acid, peracetic acid, perpropionic acid,
 perbutyric acid, dipersuccinic acid, diperglutaric acid, and diperadipic
 acid. The alkyl part of the chain may be optionally substituted with one
 or more substituents selected from halo-, nitro-, amido-, hydroxy-,
 carboxy-, sulpho-, or phosphono- groups. Contemplated from this group are
 monochloroperacetic acid, dichloroperacetic acid, trichloroperacetic acid,
 and trifluoroperacetic acid. Further examples include the
 monopercarboxylic acids of dibasic carboxylic acids such as
 monopersuccinic acid, monoperglutaric acid, monoperadipic acid, and also
 percitric acid and pertartaric acid. Additionally the substituent may be
 further derivatised to give groups such as esters or ethers. Examples of
 these are monoester percarboxylic acids of formula:
 ##STR1##
 where R represents an alkyl group having from 1 to 4 carbons and x is from
 1 to 4.
 A mixture of percarboxylic acids, particularly a mixture of mono- and di-,
 persuccinic, perglutaric and peradipic acids, may be employed if desired.
 Especially suitable are the monoester percarboxylic acids given above, and
 more especially, mixtures of these comprising x=2, 3, and 4. The
 compositions may alternatively or additionally include aromatic and
 substituted aromatic peroxyacids, such as monoperphthalic acid or salts
 thereof, sulphoperbenzoic acid or salts thereof chloroperbenzoic acids,
 and tolueneperbenzoic acids. Especially preferred are peracetic acid and
 perpropionic acid. Particularly preferred is peracetic acid. Performic
 acid is also convenient.
 The percarboxylic acid can also be added at other points than the first
 point in the main loop of the circulation system, care being taken however
 that the concentration of percarboxylic acid which reaches the plant is
 preferably below a maximum value. This maximum value depends on a variety
 of factors such as the percarboxylic acid employed, the crop being
 cultivated and treated, and the age of the crop when treated. In most
 cases, the concentration of percarboxylic acid in the irrigation solution
 which reaches the plants should be at most 200 mg/l. For younger plants or
 seedlings, the maximum value is 150 mg/l because higher values may have
 deleterious effects. Concentrations of at most 140 mg/l, especially at
 most 120 mg/l, are convenient.
 The percarboxylic acid is generally added in an amount sufficient to reduce
 the population of the pathogen to the desired level. It is preferred that
 the percarboxylic acid is added in sufficient quantities to give a
 concentration in the irrigation solution which reaches the plants of at
 least 20 mg/l, especially at least 80 mg/l.
 The concentration of the percarboxylic acid in the solution added, can
 contain up to 40% w/w percarboxylic acid. Preferably it will contain from
 between about 1% w/w to about 15% w/w percarboxylic acid, and particularly
 preferably from about 5% w/w to about 12% w/w. It is especially preferred
 to use a solution containing about 12% w/w percarboxylic acid. Although it
 may be supplied as either an equilibrium solution or a non-equilibrium
 solution, it is more usual on logistical and stability grounds for it to
 be supplied as an equilibrium solution. Such equilibrium solutions will
 contain, in addition to the percarboxylic acid, also acetic acid and
 hydrogen peroxide. The acetic acid can be present in an amount up to about
 40% w/w, but is usually present up to about 20% w/w. The hydrogen peroxide
 will be present in an amount up to about 30% w/w, but is preferably
 present in an amount up to 20% w/w. In addition the solutions may contain
 compounds commonly recognised in the art as percarboxylic acid or hydrogen
 peroxide stabilisers, such as for example dipicolinic acid,
 alkylphosphonic acids, and alkali metal stannates. They may also contain
 corrosion inhibitors such as for example phosphates and polyphosphates, or
 acids such as sulphuric acid or nitric acid.
 At the second point of the main loop at least one nutrient is added to the
 irrigation solution. Any classical nutrient for hydroponic cultures can be
 used.
 At the third point of the main loop the plants are irrigated with the
 irrigation solution. This can be done in various ways. For instance, the
 irrigation solution can be sprayed on the plants. Alternatively, the
 irrigation solution can be added to the substrate in which the plants
 grow. Both alternatives can be combined.
 At the fourth point of the main loop, part of the irrigation solution which
 has not been taken up by the plants is drained of the plants and collected
 in order to be recirculated to the first point.
 The process of the invention can be employed to control the growth of a
 wide range of microbial species. Common examples of these are tomato
 pathogens such as Phytophthora cryptogea, Pythium aphanidermatum,
 Thielaviopsis basicola and Colletotrichum coccodes, as well as other
 fungal pathogens such as Fusarium oxysporum, and Penicillium cyclopium.
 The process of the invention may particularly find use in the cultivation
 of a wide range of crops such as tomato, strawberry, cucumber and other
 soft fruits.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 illustrates one aspect of the process of the present invention.
 Irrigation solution drained of the plants is collected, via pipework, 22,
 in the collection tank 1. The irrigation solution is then disinfected
 before being passed, via pipework 3, to a feed tan, 2, in which nutrients
 and fresh make-up water can be added to the irrigation solution. It may be
 advantageous if 3 comprises retention tanks in which disinfection can take
 place. A circulation pump, 4, then pumps the irrigation solution further
 in one or more of three alternative directions. The first alternative is
 that the irrigation solution is passed via a non-return valve, 11, to a
 main feed delivery line, 14 which sends the irrigation water in the
 direction of the plants. The second alternative, which constitutes a
 sub-loop in accordance with the invention, is that the irrigation solution
 is circulated via pipework, 6, back to the feed tank 2. The third
 alternative, which also constitutes a sub-loop in accordance with the
 present invention, is that the irrigation solution is fed into a feed
 machine (i.e. nutrient supply), 5. The feed machine 5 is equipped with
 sensors which determine the concentration in the irrigation solution of
 the nutrients necessary for the plants. If the concentrations are outside
 a predetermined level then the feed machine can adjust them accordingly.
 The irrigation solution then leaves the feed machine 5 and re-enters the
 feed tank 2 via pipework, 7. A further sub-loop is provided in the form of
 irrigation solenoid, 15, and pipework (return line), 8. In the event of
 low irrigation solution up-take by the plants, solenoid 15 allows
 irrigation solution to be diverted, via pipework 8 back to the feed tank
 2. The force required for circulation can be provided by motorised valve,
 9. When irrigation solution is required by the plants it is allowed
 through irrigation solenoid, 15, and then proceeds via sub-feed lines, 16,
 to the nozzles, 17, which supply the individual plants 18. Any excess
 irrigation solution 19 is either recycled back to tank 1 via pipework 22,
 or discharged to drain, 21, upon opening outflow valve, 20. The system can
 further be provided with a means of flushing the pipework with rinse water
 should this become necessary, namely that rinse water, 12, is allowed to
 enter the system upon opening rinse valve, 13.
 Having described the invention in general terms, specific embodiments
 thereof are described in greater detail by way of example only.
 EXAMPLE 1
 (In Accordance with the Invention)
 A semi-closed irrigation system as described in FIG. 1 was employed to
 provide water to a tomato crop housed in a greenhouse. The tomatoes were
 grown on an inert rockwool substrate. Peracetic acid was added to area 3
 of FIG. 1 in amounts to maintain its level at around 100 mg/l peracetic
 acid. The peracetic acid employed contained approximately 12% w/w
 peracetic acid, 20% w/w hydrogen peroxide, and 16% w/w acetic acid. The
 dosage was controlled to within a peractetic acid range of 80-120 mg/l.
 This was allowed to stand for a minimum period of 60 minutes. Two small
 treatment tanks were employed to alternate the sequencing of treatment and
 discharge over a cycle time of approximately 1 hour. When a proportion of
 the treated solution was discharged to the nutrient feed tank, dilution
 occurred due to the increase in liquor volume and the addition of fresh
 make-up water. Water was circulated by means of a circulation pump 4, and
 nutrients provided by a feed machines. At the end of the day, a shut-down
 sequence, initiated by a timer, terminated the peracetic acid dosing
 operations 3-4 hours before the cessation of the normal irrigation
 process. Automated flushing sequences were incorporated to remove residual
 peroxygens from the treatment tanks (3) themselves and also to reduce the
 overall peroxygen residuals in the nutrient feed tank (2) and also via
 rinse valve (13) to feed lines (13, 15, 16 and 17) onto the plants (18).
 In addition the start-up sequence commenced approximately one hour before
 the first feed. Both treatment tanks were filled and dosed with peracetic
 acid to allow sufficient standing time for effective disinfection to
 occur. All materials for system and equipment were chosen for safety and
 compatibility with peracetic acid.
 The system was employed for a period of approximately 2 months in a
 standard commercial greenhouse and was evaluated alongside control systems
 for comparison. Replicated plots of tomato plants were used for treatment
 comparisons: (i) inoculated plants treated with peracetic acid introduced
 and circulated employing the system described, (ii) uninoculated control
 and (iii) inoculated control. Pathogenic fungi were introduced onto the
 tomato crops for treatments (ii) and (iii). Pathogen spread was monitored
 by seedling bioassay (introduction of seedlings at various points in the
 rows) leaving for 1-2 days the removed micro-organisms for microscopic
 inspection to observe root and foliar condition to determine pathogen
 spread and establishment in the crop and effects on plant growth, eg. leaf
 size, plant height, root size and colouration and including phytotoxic
 effects, and tomato yields were also monitored. The trial showed that the
 introduced pathogens (Phytophtora cryptogea, Thieleviopsis basicola,
 Colletotrichum coccodes, and Pythium aphanidermatum) were controlled,
 widespread dissemination of the pathogens was prevented and there was no
 significant build up of Trichoderma in the tanks, pipework or nozzles. In
 addition, this system of peracetic acid treatment did not show any
 phytotoxic effects, and yield improvements were noted.
 EXAMPLE 2
 (Given by Way of Comparison)
 A semi-closed irrigation system as described in FIG. 2, not according to
 the current invention, was employed to provide water to a tomato crop
 housed in a greenhouse. The numbering used in FIG. 2 corresponds to that
 used in FIG. 1 The tomatoes were grown on an inert rockwool substrate.
 Peracetic acid was added to area 3 of FIG. 2 in amounts to maintain its
 level at around 100 mg/l peracetic acid. The peracetic acid employed
 contained approximately 12% w/w peracetic acid, 20% w/w hydrogen peroxide,
 and 16% w/w acetic acid. The dosage wag controlled to within a peracetic
 acid range of 80-120 mg/l. Water was circulated by means of a circulation
 pump 4, and nutrients provided by a feed machine, 5. This system was
 evaluated in the same way and with the same treatments as outlined in
 example 1. Although the use of this system reduced the population of plant
 pathogens (same speces as example 1 were applied), was found to be not
 sufficient to suppress the growth of Trichoderma, with the result that
 after approximately 6-8 weeks the pipework and nozzles became blocked, and
 required cleaning.