A METHOD AND A SYSTEM FOR GROWING PLANTS

Abstract: Plants, for example vegetables, are grown in inactive growth media, including aquaculture, where the plants are supplied with an aqueous liquid containing a growth or health adjusting agent, such as, for example, nitrogen. The growing is effected by dividing a growth area into a reference area (10) and a production area (11) with substantially the same growth conditions, and supplying to the plants in the reference area (10) a first aqueous liquid in which said agent is present in an empirically determined concentration. The agent is supplied continuously or intermittently to the first liquid in such amounts that the concentration of the agent in said first liquid is substantially maintained at the empirically determined level. The plants in the production area (11) are supplied with a second aqueous liquid with a second fixed, preferably lower, concentration of said agent, and the agent is supplied continuously or intermittently to said second liquid in amounts controlled in dependency of, for example proportionally to, the amounts of the agent added to the first liquid. Thus, it becomes possible to control the plants' content of the growth or health adjusting agent, such as, for example, to reduce the content of nitrate, without involving any substantial yield reduction or quality deterioration.

Description: A METHOD AND A SYSTEM FOR GROWING PLANTS The present invention relates to a method for growing plants, whereby an aqueous liquid containing a growth or health adjusting agent is supplied to plants which are arranged in a growth area with substan- tially the same growth conditions. The nutrients or growth adjusting substances which are absorbed by plants, such as vegetables, are normally not poisonous in the amounts in which they are absorbed by people who eat the vegetables in qu¬ estion in a normal amount. However, under certain growth conditions, the plants may absorb the substances in question to a particularly high degree, which might involve a toxic effect or an effect inju¬ rious to the health of those persons who eat the plants. For example, nitrate may be absorbed by vegetables or other plants to a large or small extent depending on the nitrate availability and the other growth conditions during the growth of the plant. Nitrate as such is not poisonous in the amounts in which it is normally eaten by people, but during passage of the mouth and the gastrointestinal canal the nitrate may to some extent be reduced to nitrite. Nitrite is far more poisonous and by reaction with secondary or tertiary amines or amides may form nitrosamines which are thought to be carcinogenic. The accumulation of nitrate in plants is a result of an interaction between the amount of nitrate in the growth medium, the nitrate absorption of the plants, the nitrate reduction of the plants and the speed of incorporation of nitrogen in new tissue. So far it has been attempted to limit the nitrate accumulation in plants by reducing the nitrate concentration in the growth medium, for example by using slow-acting nitrogen fertilizer, nitrification inhibitors, low concentrations of nitrate-N in the growth medium, etc. Slow-acting nitrogen fertilizers comprise various types of nitrogen fertilizers in which the nitrogen is slowly released from the fertilizer particles. Thus, the possibility of the plants of absorbing nitrogen as nitrate is limited. The nitrate content in the plants therefore becomes lower than with the use of water-soluble nitrate fertilizers. The release of nitrogen from the fertilizer particles is, however, dependent on the temperature, water content and pH of the growth medium. Results of growth tests with, among other things, lettuce which was supplied with sulphur-coated urea show that the content of nitrate could be reduced to one fifth com- pared with the cases in which non-coated urea was used. However, the use of sulphur-coated urea involved a yield reduction of 222, which was probably caused by lack of nitrogen. Nitrification inhibitors are chemical substances which prevent the microbial oxidation of ammonium into nitrate in the growth medium. The most used ones are N-serve with the active substance 2-chloro-6- (trichloromethyl)-pyridine and didrine, which consists of dicyanodi- amide. Test results show a reduction in the nitrate content of plants with the use of nitrification inhibitors, but at the same time a tendency towards a yield reduction. This may be due to a moderate ammonium poisoning of the culture. Growth of lettuce in water culture with low concentrations of nitrate may mean less nitrate in plants, but often involves a yield reduc¬ tion. Further, the nitrate reduction in plants to be obtained by reduction of the nitrate concentration in the water culture solution is strongly dependent on the time of year, and the quality of the lettuce may reach an unacceptable level. Substitution of nitrate with other anions in growth in water culture has been attempted under laboratory conditions. An exchange of nitra¬ te in the nutrient solution with chloride in the period before har- vest of the plants involved a reduction in the nitrate content of about 65% and a yield reduction of 50%, while substitution of nitrate with sulfate involved a reduction in the nitrate content of 70% and a yield reduction of 30%. The availability of nutrients or growth adjusting substances for plants also contributes to the control of the flower and fruit for¬ mation of the plants. There is therefore a need to control the supply of nutrients to flower and fruit plants at certain times of the growth period so as to obtain an improved flower formation and/or a larger fruit yield. The invention attempts to provide a method of the type mentioned above in which it becomes possible to reduce and control the supply of nutrients to plants, for example in a manner so that the nitrate content of the plants is reduced without involving a yield reduction or quality deterioration of any significance, or so that the flower and/or fruit formation is improved. The method according to the invention is characterized in that the growth area is divided into a reference area and a production area, that the plants in the reference area are supplied with a first aqueous liquid in which the growth or health adjusting agent is present in an empirically determined concentration, that the agent is continuously or intermittently added to the first liquid in such amounts that the concentration of the agent in said first liquid is substantially maintained at the empirically determined level, and that the plants in the production area are supplied with a second aqueous liquid having a second fixed, preferably lower, concentration of said agent, and that the agent is continuously or intermittently added to said second liquid in amounts controlled in dependency of the amounts of the agent added to the first liquid. This first liquid is normally a plant nutrient solution, and said growth or health adjusting agent may, for example, be a plant nutri¬ ent, a trace element, a hormone agent, an insecticide and/or a fungi¬ cide. The agent is added to the first liquid in varying amounts so that the concentration of the agent in the first liquid is substanti- ally maintained at the empirically determined level, and the supply will thus correspond to the varying absorption of the agent by the plant depending on the changing growth conditions. As the amounts of the growth or health adjusting agent added to the second liquid which is supplied to the plants in the production area are controlled in dependency of the amounts of the agent added to the first liquid supplied to the plants in the reference area, it is possible to obtain a desired ratio between availability of the agent for the plants in the production area and the tendency or need of the plants for absorption of the agent under the changing growth conditions. The supply of the growth or health adjusting agent to the plants in the production area may be controlled according to any desired pat¬ tern or program on the basis of the supply of the agent to the first aqueous liquid. For example, the amounts of the agent added to the second aqueous liquid may be reduced relatively more at night than in the daytime and/or reduced especially much in the period immediately before harvesting of the plants or in the period in which the flower and/or fruit formation may be particularly influenced. However, in the preferred embodiment the agent is added to the second liquid in amounts which are proportional to the amounts of the agent added to the first liquid. The plants in the reference area and the production area may be separately supplied with the growth substances or nutrients they need beyond said growth or health adjusting agent, and said first and second aqueous liquids then only need to contain this agent. However, these aqueous liquids may further contain one or more plant nutrients and/or growth or health adjusting agents. Thus, an aqueous liquid may be used which contains one or more plant nutrients, the ratio between these nutrients substantially corresponding to the ratio in which the plants in question are known by experience to absorb these nutrients. The plants grown by the method according to the invention may be of any type. However, the method according to the invention may be used particularly advantageously when growing vegetables, such as for example lettuce, head lettuce, spinach, leeks, Chinese cabbage, celery, etc., and when the agent used is nitrogen or nitrate, as the nitrate absorption of these vegetables may then be controlled. The amounts of nitrogen per plant unit in the production area which are added to the second aqueous liquid may advantageously constitute between 50 and 100%, preferably 60-70%, of the amount per plant unit in the reference area which is added to the first aqueous liquid. It has been found by experience that the nitrate accumulation in the plants may then be kept at a desired low level without involving any substantial yield reduction. In the production area the absorption of the plants of the growth or health adjusting agent, for example nitrate, is partly dependent on the amount (availability) of the agent in the second liquid, and partly on the external environmental factors to which the plants are exposed, such as light, temperature, humidity, the carbon dioxide concentration, etc. The environmental factors may thus be said to control the tendency of the plants to absorb the agent. These en¬ vironmental factors are the same in the production area as in the reference area, but in the reference area the amount (availability) of said agent is maintained at a fixed level for which reason the amounts of the agent added here correspond to the plants' absorption of the agent, in response to the given environmental factors. Therefore, in accordance with the method of the invention, when the addition of the agent to the second liquid is controlled in dependen¬ cy of the addition of the agent to the first liquid, it is possible to obtain a desired ratio between the external environmental factors and the availability of the agent in the second liquid. Thus, in practice, control is obtained of the relative availability of the agent - i.e. relative to the external environmental factors - in the second liquid, and thus control of the plants' absorption of the agent which is independent of these environmental factors. The invention further relates to a system for growing plants in inactive growth media, including aquaculture, of the type comprising a first water culture circuit, a supplementary liquid container and first supply means adapted to supply a supplementary liquid to the first circuit continuously or intermittently from the supplementary liquid container so that the concentration of a health or growth adjusting agent contained in a liquid circulating in this circuit is substantially maintained at an empirically determined level, and the system according to the invention is characterized in that it compri¬ ses a second separate water culture circuit and second supply means adapted to continuously or intermittently supply said agent to the second circuit in amounts which are substantially proportional to the amounts of the agents supplied to the first circuit. The invention will now be described in detail with reference to the drawing, in which: Fig. 1 schematically shows a system according to the invention, Fig. 2 is a graph indicating the fresh weight of plants grown by the method according to the invention, and Fig. 3 is a graphic indication of the nitrate content in lettuce plants grown partly by a conventional growth method and partly by the method according to the invention. Fig. 1 shows a growth area which is divided into a reference area 10 and a production area 11, respectively, with substantially the same growth conditions, and in which plants are grown in an inactive growth medium, for example lettuce plants in water culture. The reference area 10 is associated with a first liquid circuit 12 comprising a container 13 for a water culture solution with supply conduits 14 for pure, demineralized water with a control device (not shown) for maintaining a substantially constant amount of water culture solution in the liquid circuit, supply conduits 15 for sup- plementary amounts of nutrient solution(s) contained in one or more supplementary liquid containers 16, a supply conduit 17 through which the water culture solution from the container 13 is supplied to the plants of the reference area 10, and a return conduit 18. A pump 19 is arranged for circulating the water culture solution in the circuit 12. A measuring and control unit 20 is arranged at the return conduit 18 of the liquid circuit leading from the growth area of the referen¬ ce area. One or more pumps 21 (only one is shown in Fig. 1) are provided to meter a supplementary solution from the containers 16 to the container 13 through the supply conduit 15, and the function of these pumps 21 is controlled by control signals A from the measuring and control unit 20. The production area 11 is associated with a second liquid circuit 22 comprising a container 23 for a water culture solution with a supply conduit 24 for pure, demineralized water and comprising a control device (not shown) for maintaining a substantially constant amount of water culture solution in the liquid circuit, a supply conduit 25 for supplementary amounts of nutrient contained in one or more supplemen¬ tary liquid containers 26, an supply conduit 27 through which the water culture solution from the container 23 is supplied to the plants of the production area, and a return conduit 28. The water culture solution is circulated in the circuit 22 by means of a pump 29 inserted into the supply conduit 27. One or more pumps 30 (only one is shown in Fig. 1) , the function of which is controlled by control signals B from the measuring and control unit 20, serve to meter a supplementary solution from the container 26 to the water culture solution in the container 23. When the system according to the invention is in operation, a flow of water culture solution having an empirically determined concentration of one or more plant nutrients which it is desired to supply to the plants is supplied to the reference area 10. The plants, which, for example, are standing is open channels, then absorb nutrients in naturally controlled amounts (according to their "appetite"). At the return conduit 18 the remaining concentration of nutrient is measured by means of the measuring and control unit 20 which passes on control signals A, B to the pumps 21 and 30, respectively, on the basis of these measurements. The control signal A controls the operation of the pump 21 so that the amount of nutrient absorbed by the plants in the liquid circuit of the reference area 10 is replaced so that the concentration of the agent is substantially maintained at the empiri- cally determined level. The control signal B controls the pumps 30 so that an amount of nutrient dependent on the amount of nutrient absor¬ bed in the reference area is supplied to the water culture solution . of the production area 11. The supply is effected in such a manner that each plant in the production area has an amount of nutrient at its disposal which is a substantially fixed fraction of the amount at the disposal of each plant in the reference area. Measuring and control may be effected by means of a single unit 20, as shown in Fig. 1, or the tasks may be handled by separate apparatu¬ ses for measuring and control, respectively. In the preferred embodiment of the invention, the measurement is effected as a measurement of the electric conductivity and pH of the water culture solution, as the change in the conductivity reflects the amount of nutrient which has been absorbed during the circulation of the water culture solution in the reference area 10, and the pH reflects the acid/base ratio and/or the ammonium/nitrate ratio in the water culture solution. To ensure the acid/base balance in the water culture circuits the supply of supplementary liquid may be effect by a supply of nutrient from a basic (nitrate-containing) container when the pH is undesirab¬ ly low, and by a supply of nutrient from an acidic (ammonium-contai¬ ning) container when the pH is undesirably high. Measurement and control may take place substantially continuously, which means that a supply of the agent is effected when the content thereof in the liquid circuit falls to a predetermined, slightly lower level, or intermittently, i.e. measurements are made at certain intervals, for example every six or twelve hours, whereupon the amount of the agent absorbed by the plants during this time interval is supplied. Measurement and control may also take place as a combi- nation of the two methods explained above by means of electrical and/or electronic apparatuses. To monitor whether the process is under control, measurements of conductivity and pH may be made at the return conduit 28 of the production area. The control may be made for a single nutrient (nitrogen) or for a nutrient solution containing all or several of the nutrients neces¬ sary for the plants, and which are comprised in the solution in the ratio in which the plants are known by experience to absorb the various substances. The control may further be made for several nutrient solutions which separately contain certain of the nutrients necessary for the plants. EXAMPLE To verify the suitability of the method according to the invention for limitation of the nitrate content in vegetables, a number of experiments have been conducted. Such an experiment, and its results, will be described below with reference to Figs. 2 and 3. A greenhouse area was divided into reference and production areas where the plants (lettuce) in the production areas were supplied with 90%, 80%, and 65%, respectively, of the amount of nitrogen-containing nutrient absorbed by the plants in the reference area. In the ex- periment, a comparison was further made with plants grown according to the conventional method (in soil). During the experiment, 19, 24, 30 and 34 days after planting, respectively, samples were taken of the lettuce plants which were weighed and analyzed for nitrate con¬ tent. The fresh weight of the plants appears from Fig. 2 (only plants grown in aquaculture), and Fig. 3 shows the nitrate content of the plants measured in mg per kg fresh weight. It can be seen that the method according to the invention renders it possible to substantial¬ ly reduce the nitrate content in lettuce plants without any substan¬ tial reduction of the plant yield.

Claims: CLAIMS 1. A method of growing plants, whereby an aqueous liquid containing a growth or health adjusting agent is supplied to plants arranged in a growth area with substantially the same growth conditions, characterized in that the growth area is divided into a reference area (10) and a production area (11) , that the plants in the referen¬ ce area (10) are supplied with a first aqueous liquid in which the growth or health adjusting agent is present in an empirically deter¬ mined concentration, that the agent is continuously or intermittently added to the first liquid in such amounts that the concentration of the agent in said first liquid_is substantially maintained at the empirically determined level, and that the plants in the production area (11) are supplied with a second aqueous liquid having a second fixed, preferably lower, concentration of said agent, and that the agent is continuously or intermittently added to said second liquid in amounts controlled in dependency of the amounts of the agent added to the first liquid. 2. A method according to claim 1, characterized in that the agent is added to the second liquid in amounts which are proportional to the amounts of the agent added to the first liquid. 3. A method according to claim 1, wherein the said agent is a plant nutrien , characterized in that a first and a second aqueous liquid are used which contain one or more plant nutrients, the ratio between these nutrients substantially corresponding to the ratio in which the plants grown are known by experience to absorb these nutrients. 4. A method according to any of claims 1-3, characterized in that said agent contains nitrogen, and that vege- tables, preferably of the type having a tendency to accumulate nitra¬ te in the plant tissue, for example lettuce, head lettuce, spinach, leeks, Chinese cabbage, celery and the like, are grown. 5. A method according to any of claims 1-3, characterized in that flowers and/or fruit plants are grown, and that the flower and/or fruit formation of the plants is controlled by control of the supply of one or more nutrients to the plants in the production area. 6. A method according to claim 4, characterized in that the amount of nitrogen per plant unit in the production area (11) which is added to the second aqueous liquid, constitutes between 50 and 100%, and preferably 60-70%, of the amount of nitrogen per plant unit in the reference area (10) which is added to the first aqueous liquid. 7. A system for growing plants in inactive growth media, including water culture, and comprising a first liquid circuit (12), one or more supplementary liquid containers (16) and first supply means adapted to supply a supplementary liquid to the first circuit (12) continuously or intermittently from the supplementary liquid contai¬ ners (16) in such a manner that the concentration of a growth or health adjusting agent contained in a liquid circulating in this circuit is maintained at a substantially empirically determined level, characterized in that the system comprises a second, separate liquid circuit (22) and second supply means adapted to continuously or intermittently supply to the second circuit (22) said agent in amounts in relation to the amounts of the agent supplied to the first circuit (12). 8. A system according to claim 7, characterized in that the supply of said agent to the second circuit (22) is effected in amounts which are substantially proportional to the amounts of the agent supplied to the first circuit (12) . 9. A system according to claim 7 or 8, characterized in that the first supply means comprises electronic control and measuring equipment (20) for control of the supply of supplementary liquid to the liquid circuits. 10. A system according to claim 9, characterized in that an apparatus for measuring the amount of the agent absorbed in the reference area is preferably adapted to mea¬ suring the conductivity and the pH of the liquid in the first liquid circuit.

Inventors: ANDERSEN LILLIE, NIELSEN NIELS ERIK, ANDERSEN, LILLIE, NIELSEN, NIELS, ERIK
Applicants: ANDERSEN LILLIE, NIELSEN NIELS ERIK, ANDERSEN, LILLIE, NIELSEN, NIELS, ERIK
Application year: 1989
Main classification: A01G31
IPC codes: A01G31