METHOD AND DEVICE FOR USING A NUTRITIVE INORGANIC IRON COMPOSITION

Abstract

Plant fertirrigation method comprising the formation of a series of mother solutions, the feeding of each of the mother solutions into a dilution system in order to form a fertirrigation solution, the supply to a fertirrigation device by means of a transfer device, supplying said fertirrigation device with said fertirrigation solution, and an addition of iron and of at least one polyphosphate.

Claims

1. A method for the fertirrigation of plants, comprising making available a series of mother solutions, feeding each of the mother solutions into a dilution system, in order to form a fertirrigation solution, and supplying a fertirrigation device by means of a transfer device, and supplying said fertirrigation device with said fertirrigation solution, wherein the method further comprises an addition of iron and at least one polyphosphate to an aqueous phase with formation of at least one iron solution and at least one polyphosphate at a pH of between 4.5 and 6.2.

2. The plant fertirrigation method according to claim 1, wherein one of said at least one iron solution and at least one polyphosphate at a pH of between 4.5 and 6.2 is a mother solution in said series of mother solutions.

3. The plant fertirrigation method according to claim 1, wherein one of said at least one solution of iron and at least one polyphosphate at a pH of between 4.5 and 6.2 preferably a pH greater than or equal to 5.5, has a pH of between 4.5 and 6.2, advantageously is the fertirrigation solution.

4. The plant fertirrigation method according to claim 1, wherein said addition of iron and at least one polyphosphate is implemented by supplying, separately or together, a source of iron and a source of at least one polyphosphate, each source being, independently of each other, in the form of a solid, solution or suspension.

5. The plant fertirrigation method according to claim 1 wherein said at least one addition of iron and at least one polyphosphate to an aqueous phase with formation of at least one solution of iron and at least one polyphosphate at a pH of between 4.5 and 6.2 is an addition of a mother presolution of iron and at least one polyphosphate to a mother solution, which forms the solution of iron and at least one polyphosphate at a pH of between 4.5 and 6.2.

6. The plant fertirrigation method according to claim 1, wherein said at least one solution of iron and at least one polyphosphate at a pH of between 4.5 and 6.2 is a mother presolution of iron and at least one polyphosphate at a pH of between 4.5 and 6.2 added to at least one mother solution of said series of mother solutions.

7. The plant fertirrigation method according to claim 1, wherein said addition of iron and at least one polyphosphate is implemented by supplying a composition containing iron and at least one polyphosphate, in solid, solution or suspension form.

8. The plant fertirrigation method according to claim 1, wherein said at least one mother solution of said series of mother solutions containing iron and at least one polyphosphate also contains ions selected from the group consisting of sulphates, phosphates, nitrates, chlorides, potassium, sodium, ammonium and mixtures thereof.

9. The plant fertirrigation method according to claim 1, wherein said at least one mother solution of said series of mother solutions containing iron and at least one polyphosphate or said mother presolution containing iron and at least one polyphosphate contains micronutriments selected from the group consisting of boron, manganese, zinc, copper, molybdenum, cobalt and mixtures thereof.

10. The plant fertirrigation method according to claim 1, wherein the pH of said mother presolution or of one of said mother solutions or of said fertirrigation solution is adjusted by adding a mineral acid.

11. The plant fertirrigation method according to claim 1, further comprising oxidation of said mother solution containing iron and at least one polyphosphate and/or of said mother presolution containing iron and at least one polyphosphate.

12. The plant fertirrigation method according to claim 1, wherein the iron and said at least one polyphosphate are added in a P.sub.poly/Fe ratio of between 5 and 50, preferably between 8 and 32, where Fe represents the total number of moles of iron and P.sub.poly represents the number of moles of phosphorus in polyphosphate form.

13. The plant fertirrigation method according to claim 1, wherein said at least one polyphosphate comprises potassium tripolyphosphate, and/or tetrapotassium pyrophosphate, and/or potassium tripolyphosphate, and/or sodium tripolyphosphate, and/or sodium acid pyrophosphate, and/or tetrasodium pyrophosphate and/or ammonium pyrophosphate, and/or ammonium polyphosphate or mixtures thereof.

14. The plant fertirrigation method according to claim 1, wherein the pH of the fertirrigation solution is below 6.2.

15. The plant fertirrigation method according to claim 1, wherein said fertirrigation solution is at a pH below 6.

16. The plant fertirrigation method according to claim 1, wherein the iron and said at least one polyphosphate are added in said fertirrigation solution at a supply point having mixing conditions that are sufficiently intense with a view to avoiding any precipitation.

17. A plant fertirrigation installation comprising: a dilution system comprising an opening for feeding mother solution and a fertirrigation solution outlet, a series of vessels for mother solutions and/or mother presolutions each having an outlet manifold equipped with a transfer means and connected respectively to said feed opening of said dilution system or to said mother solution vessels, and a fertirrigation device provided with manifolds arranged to supply plants with nutriments, connected to said fertirrigation solution outlet of said dilution system by means of a manifold provided with a transfer means, wherein said installation further comprises a means for supplying iron and at least one polyphosphate as well as pH adjustment means connected to at least one vessel in said series of vessels for mother solutions and/or mother presolutions or to said dilution system.

18. The plant fertirrigation installation according to claim 17, wherein said series of mother solution vessels comprises a first mother solution vessel and a second mother solution vessel, said means for supplying iron and at least one polyphosphate being connected to said second vessel, which also comprises said pH adjustment means.

19. The plant fertirrigation installation according to claim 17, wherein said series of mother solution vessels comprises a first mother solution vessel and a second mother solution vessel, said means for supplying iron and at least one polyphosphate being connected to a presolution vessel connected to said second vessel by means of a transfer manifold, said presolution vessel comprising said pH adjustment means.

20. The plant fertirrigation installation according to claim 17, wherein said second vessel also comprises oxidation means chosen from agitation, a pumping device, an aeration device, and an addition of oxidants.

21. The plant fertirrigation installation according to claim 17, wherein said series of mother solution vessels comprises a first mother solution vessel and a second mother solution vessel, said means for supplying iron and at least one polyphosphate being connected to a presolution vessel connected to said second vessel by means of a transfer manifold, said second vessel comprising said pH adjustment means.

22. The plant fertirrigation installation according to claim 17, wherein said series of mother solution vessels comprises a first mother solution vessel, a second mother solution vessel and a third mother solution vessel, said means for supplying iron and at least one polyphosphate being connected to said third vessel, which also comprises said pH adjustment means.

23. The plant fertirrigation installation according to claim 17, wherein said presolution vessel or said third mother solution vessel further comprises oxidation means chosen from agitation, a pumping device, an aeration device, an addition of oxidants such as oxygen, air, hydrogen peroxide, Javel water, etc.

24. The plant fertirrigation installation according to claim 17, wherein said series of mother solution vessels comprises a first mother solution vessel, a second mother solution vessel and a third mother solution vessel, said means for supplying iron and at least one polyphosphate being connected to said third vessel, said pH adjustment means being connected to said dilution system or to one of said first or second mother solution vessels.

Description

[0082] In the drawings, FIG. 1 depicts schematically a form of implementation of the first embodiment of the installation and method according to the present invention.

[0083] FIG. 2 depicts schematically a form of implementation of the second embodiment of the device and method according to the present invention.

[0084] FIG. 3 depicts schematically a form of implementation of the third embodiment of the device and method according to the present invention.

[0085] FIG. 4 depicts schematically a form of implementation of the fourth embodiment of the device and method according to the present invention.

[0086] FIG. 5 depicts schematically a form of implementation of the fifth embodiment of the device and method according to the present invention.

[0087] In the figures, the identical or similar elements bear the same references.

[0088] The plant fertirrigation method according to the present invention comprises

[0089] making available a series of mother solutions A, B, . . . in a series of mother solution vessels 1, 2, . . . ,

[0090] feeding each of the mother solutions (A, B, . . . ) into a dilution system 6, in order to form a fertirrigation solution F,

[0091] supplying a fertirrigation device 7 by means of at least one transfer device 8, supplying said fertirrigation device 7 with said fertirrigation solution F,

[0092] adding iron and at least one polyphosphate to an aqueous phase with formation of at least one solution D of iron and at least one polyphosphate at a pH of between 4.5 and 6.2, advantageously greater than or equal to 5.0 and less than or equal to 6.0, preferably a pH greater than or equal to 5.5.

[0093] In a preferred embodiment of the invention, the method makes it possible to achieve an iron yield at the discharge from the manifolds 9 of the fertirrigation system 7 of more than 85%, preferably more than 90%. The iron yield being defined as the concentration of soluble iron measured at the discharge from the manifolds 9 of the fertirrigation system 7 (or just before the fertirrigation solution is put in contact with the substrate or substrates and the plant or plants) with respect to the theoretical iron concentration at this point. The theoretical iron concentration is calculated on the basis of the initial iron concentration (mother solution or mother presolution) and the dilution factor of the step of preparing the fertirrigation solution F.

[0094] In a first embodiment of the method according to the invention illustrated in FIG. 1, said series of mother solutions comprises a first mother solution A in a first mother solution vessel 1 and a second mother solution B in a second mother solution vessel 2, said first mother solution A comprising calcic nutriments, preferably in nitrate or chloride form, and magnesian nutriments, preferably in nitrate form, said second mother solution B comprising sulphate and phosphate anions, and mixtures thereof, as well as iron and at least one polyphosphate fed for example by a hopper P in solid form. The second mother solution B is said solution D of iron and at least one polyphosphate having a pH of between 4.5 and 6.2, advantageously greater than or equal to 5.0 and less than or equal to 6.0, preferably greater than 5.5, by adjustment thereof by the pH adjustment means 10 connected to said second vessel 2 using acid.

[0095] Naturally, depending on the fertirrigation installations present at the horticulturists or farmers and the possibility of adapting same, the method according to the invention exists according to various embodiments. The first mentioned above in fact aims to adapt the conventional method by introducing the calcic and magnesian nutriments in the first vessel, the other nutriments then being introduced in the second vessel, including iron and at least one polyphosphate.

[0096] In this embodiment, the only ions with which the inorganic nutritive composition comprising iron and at least one polyphosphate is not compatible are calcium and magnesium. The latter are isolated in the first mother solution A in the first mother solution vessel 1 while the inorganic nutritive composition is used in the second mother solution B in the second mother solution vessel 2, for example with monopotassic phosphate (MKP), potassium nitrate (KMO.sub.3) or potassium sulphate (K.sub.2SO.sub.4). However, magnesium, which is often found in sulphate form in the second mother solution B in conventional methods, is now added in the form of nitrate in the first mother solution A.

[0097] The pH of the second mother solution B must be greater than 4.5 and preferably greater than 5.0 and advantageously greater than 5.5 in order to guarantee the stability of the iron during its storage time. Moreover, if the pH of this second mother solution does not exceed 6.2, any risk of precipitation in the dilution step is removed, as explained above.

[0098] The other oligoelements or micronutriments (B, Mn, Zn, Mo, Co, Cu and mixtures thereof) may be introduced either in the first mother solution A or in the second mother solution B, however, in the light of the pH conditions imposed in the second mother solution B, it will be preferred to introduce them in the first mother solution A.

[0099] In a second embodiment according to the present invention illustrated in FIG. 2, said series of mother solutions comprises a first mother solution A contained in a first mother solution vessel 1 and a second mother solution B contained in a second mother solution vessel 2. The first mother solution A comprises calcic nutrients, preferably in the form of nitrate or chloride, and magnesian nutriments, preferably in the form of a nitrate. The second mother solution B comprises sulphate and phosphate anions and mixtures thereof as well as iron and at least one polyphosphate. The iron and said at least one polyphosphate are supplied using a mother presolution C contained in a mother presolution vessel 3 to said second mother solution B in the second mother solution vessel 2 by means of a transfer manifold 11 connected firstly to the presolution vessel 3 and secondly to the second mother solution vessel 2. Means 10 for adjusting the pH are connected directly to the mother presolution vessel 3. The mother presolution C has sufficient acidity to provide a pH of between 4.5 and 6.2, advantageously greater than or equal to 5.0 and less than or equal to 6.0, preferably greater than 5.5 in said second mother solution B.

[0100] In this particular embodiment of the method according to the present invention, a mother presolution C comprising iron and at least one polyphosphate is supplied in order to be transferred subsequently in the second mother solution B in solution form. In this case, the pH of the mother presolution C will be adjusted directly by means of the addition of acid so that the pH of said second mother solution B has a pH of between 4.5 and 6.2, more particularly between 5.0 and 6.0, and preferably greater than 5.5. This can be achieved in various ways, such as for example by adjusting the pH of the mother presolution C to a predetermined pH by the means 10 for adjusting the pH so that the pH in the second mother solution B is in the required range of 4.5 and 6.2, more particularly between 5.0 and 6.0, preferably greater than 5.5. In this case, the second mother solution B is the solution of iron and at least one polyphosphate at the pH in the aforementioned required range (B=D). Another way lies in adjusting the pH also in the second mother solution B, in which case the second vessel 2 is also connected to pH adjustment means 10 (not illustrated), or by ensuring that the second mother solution B has a pH between 4.5 and 6.2, more particularly between 5.0 and 6.0, preferably greater than 5.5, as well as the mother presolution C. In the latter case, the mother presolution C is the solution D (C=D) containing iron and at least one polyphosphate at a pH between 4.5 and 6.2, more particularly between 5.0 and 6.0, preferably greater than 5.5, but also the second mother solution B (B=D).

[0101] In a third particular embodiment of the method according to the present invention illustrated in FIG. 3, said series of mother solutions comprises a first mother solution A in a first mother solution vessel 1 and a second mother solution B in a second mother solution vessel 2, said first mother solution A of the first mother solution vessel 1 comprising calcic nutriments, preferably in the form of nitrate and chloride, and magnesian nutriments, preferably in the form of nitrate, said second mother solution B of the second mother solution vessel comprising sulphate and phosphate anions and mixtures thereof as well as iron and at least one polyphosphate. The iron and said at least one polyphosphate are supplied from a mother presolution C contained in a presolution vessel 3 to said second mother solution B by means of a transfer manifold 11 connected firstly to the presolution vessel 3 and secondly to the second mother solution vessel 2. The pH adjustment means 10 are connected to the second mother solution vessel 2. The second mother solution B is then said solution D (B=D) of iron and at least one polyphosphate having the pH between 4.5 and 6.2, advantageously greater than or equal to 5.0 and less than or equal to 6.0, preferably greater than 5.5, by adjusting it by means of acid.

[0102] In this particular embodiment of the method according to the present invention, a mother presolution C comprising iron and at least one polyphosphate is provided in order to be transferred subsequently into the second mother solution B in solution form. In this case, the pH of the second mother solution B, in which the presolution is added, will be adjusted directly to a value between 4.5 and 6.2, advantageously greater than or equal to 5.0 and less than or equal to 6.0, more particularly greater than 5.5, by means of an addition of acid.

[0103] Advantageously, in the second and third embodiments, the mother presolution C comprises iron and at least one polyphosphate and has a P.sub.poly/Fe ratio of between 5 and 50 and preferably between 8 and 32, where Fe represents the total number of iron moles and P.sub.poly represents the number of phosphorus moles in polyphosphate form.

[0104] Preferentially, in the second and third embodiments, the mother presolution C is formed from an inorganic nutritive composition based on iron and at least one solid or liquid polyphosphate.

[0105] Preferentially, in the second and third embodiments, the mother presolution C is formed from a source of iron and at least one solid or liquid polyphosphate added independently or not.

[0106] In a fourth preferred embodiment of the method according to the present invention illustrated in FIG. 4, said series of mother solutions comprises a first mother solution A in a first mother solution vessel 1, a second mother solution B in a second mother solution vessel 2 and a third mother solution E in a third mother solution vessel 5. The first mother solution A comprises calcic nutriments, preferably in the form of nitrate or chloride, the second mother solution B comprises sulphate and phosphate anions and mixtures thereof and said third mother solution E comprises iron and at least one polyphosphate; the magnesian nutriments being able to be supplied in said mother solution A or in said second mother solution B; said third mother solution E being said solution D (E=D) of iron and at least one polyphosphate having a pH of between 4.5 and 6.2, advantageously greater than or equal to 5.0 and less than or equal to 6.0, preferably greater than 5.5, by adjustment thereof by means of acid.

[0107] In this embodiment, a third mother solution E comprising iron and at least one polyphosphate is formed separately (such as for example in a third mother solution vessel 5 or from mother presolutions (not illustrated) supplied in the third mother solution vessel 5). It is then supplied not in the second mother solution B but directly in the fertirrigation solution F, in the dilution tank 6 or in the irrigation line 12, by means for example of its own transfer device 13. However, in this embodiment, the pH will be adjusted to a value between 4.5 and 6.2, advantageously greater than or equal to 5.0 and less than or equal to 6.0, preferably greater than 5.5, by the pH adjustment means 10 connected to the third vessel 5, in particular by adding acid in the third mother solution E. Although this embodiment requires two additional appliances in terms of equipment, a vessel 5 for preparing a third mother solution E and an additional system 13 or 14 for metering this solution in the dilution vessel 6 or in the irrigation line 8, it is nevertheless less constraining from a chemical point of view. In particular the magnesian nutriments can be added in the form of sulphates or nitrates in said second mother solution B.

[0108] The various mother solutions or mother presolutions of the aforementioned embodiments are transferred from one vessel to another by means of any conventional solution transfer system (11, 14, 8, 13) such as pumps, venturi, gravity, etc.

[0109] In an advantageous embodiment of the invention, each step of supplying each of the mother solutions (A, B, E) of the aforementioned embodiments, in a dilution vessel 6 or in an irrigation line 12, is a step of transfer by venturi entrainment of said mother solution by means of a flow of fertirrigation solution diverted at the discharge of said transfer device 8, which makes it possible to supply the fertirrigation device 7 (not illustrated).

[0110] All the steps of the method, whatever the embodiment envisaged, are performed at ambient temperature, that is to say without strict temperature control in the vessels of said mother solutions or of said mother presolutions, that is to say at temperatures fluctuating between 10 and 40 C. depending on whether the preparation installation is installed in the open air or in a greenhouse.

[0111] Advantageously, in the present invention, for example in the embodiments 1, 2, 3 or 4, oxidation of the mother solutions or mother presolutions can be envisaged by oxidation means 15, in particular in order to prevent the precipitation or coprecipitation of certain micronutriments such as copper. This is because, in the absence of oxidation, micronutriments such as copper may precipitate with iron or other ions such as pyrophosphates. Conventional oxidation techniques can be implemented with known oxidants such as oxygen, air, hydrogen peroxide, Javel water, etc.

[0112] By way of example, the oxidation treatments normally used are: bubbling with air by means of a disperser situated at the bottom of the vessel (the air being able to come from a compressor or a simple pump), agitation on the surface of the liquid under air, etc.

[0113] The duration of oxidation treatment varies between a few hours and several days depending on the size of the vessel and the aeration system used.

[0114] Advantageously, whatever the embodiment 1, 2, 3 or 4, a second pH adjustment means 10 may be present in the dilution tank 6 (not illustrated).

[0115] In a fifth embodiment of the method according to the present invention illustrated in FIG. 5, said series of mother solutions comprises a first mother solution A contained in a first mother solution vessel 1, a second mother solution B contained in a second mother solution vessel 2 and a third mother solution E contained in a third mother solution vessel 5. The first mother solution A comprises calcic nutriments, preferably in the form of nitrate or chloride, the second mother solution B comprises sulphate and phosphate anions and said third mother solution E comprises iron and at least one polyphosphate; magnesian nutriments being able to be supplied in said first A or said second mother solution B; the mixing conditions being sufficiently intensive at the point of supply of said third mother solution E in said fertirrigation solution F so as to prevent any precipitation of calcium phosphate; said fertirrigation solution F being at a pH below 6.2, preferably below 6.0 and preferentially below 5.8 after said supply of the third mother solution E.

[0116] In this embodiment, a mother solution E comprising iron and at least one polyphosphate is formed separately. This may have a basic pH and is supplied directly in the fertirrigation solution F, in the dilution system 6 or in the irrigation line 12 to which the pH adjustment means 10 are connected (which also comprise, in this particular embodiment, the means that make it possible to ensure sufficiently intensive mixing conditions at the point 16 of supply of said third mother solution E in said fertirrigation solution F). Advantageously, in the fifth embodiment, the mixing conditions may be provided by any conventional agitation means at the point of injection of said third mother solution E, such as agitators, recycling, gas bubbling, etc.

[0117] However, in this embodiment, the sufficient quantity of acid will be added before supply of said third solution E in the dilution device (dilution tank 6 or irrigation line 12).

[0118] In this particular embodiment, the fertirrigation solution F is the solution D of iron and at least one polyphosphate in the required pH range, that is to say 4.5 and 6.2, more particularly between 5.0 and 6.0, preferably greater than 5.5, but less than or equal to 5.8.

[0119] For example, in a variant, said series of mother solution vessels comprises a first mother solution vessel, a second mother solution vessel and a third mother solution vessel, said means for supplying iron and at least one polyphosphate being connected to said third vessel, said pH adjustment means 10 being connected to said dilution system or to one of said first or second mother solution vessels.

[0120] Naturally the present invention is in no way limited to the embodiments described above and many modifications can be made thereto without departing from the scope of the accompanying claims.

EXAMPLES

[0121] The use of mineral nutritive compositions based on iron and at least one polyphosphate was tested in various fertirrigation methods, the examples most representative of the present invention and of the problem that they made it possible to solve being set out below.

[0122] Test Operating Method

[0123] The fertirrigation solution is formed continuously in a conventional 1000 litre dilution tank that the irrigation water passes through. The addition of nutritive elements in said dilution tank takes place via three different mother solutions, one of them containing the source of iron and polyphosphate, and other two all the remaining nutritive elements.

Comparative Example 1

[0124] The mother solution containing iron and at least one polyphosphate is that of example 1 of the patent application WO2014/056688. This solution, with an Fe content of 10 mmol/kg, is characterised by a basic pH of 9.4, in which no precipitate appears after 4 weeks of storage at ambient temperature or higher temperature (40 C.).

[0125] Each of the mother solutions is metered by means of a specific venturi system, a small fraction of the fertirrigation solution being taken off at the discharge from the irrigation pump in order to serve as a drive liquid for each venturi. The pH of the fertirrigation solution is moreover adjusted continuously between pH 5.4 and 5.7 by an automatic addition of nitric acid carried out directly in the dilution tank. A severe precipitation problem appears at the outlet of the venturi used for metering the mother solution containing iron and at least one phosphate after a few hours of operation.

[0126] The solid particles thus formed are deposited partly on the walls of the tube and end up by completely closing off the passage. Furthermore, the solid precipitate of calcium orthophosphate giving rise to this fouling entrains a large proportion of the iron by chemical coprecipitation. This may be the cause of a serious iron deficiency in the plants cultivated if the abnormality is not detected sufficiently quickly. According to the analysis of the fertirrigation solution taken off dropwise, the loss of iron resulting before the blocking of the venturi was already at least 20% compared with the calculation of theoretical concentration of iron that should have been present in the fertirrigation solution. This theoretical value was correctly reached by a dilute solution prepared in the same proportions in the laboratory in order to simulate the method.

Comparative Example 2

[0127] A variant of comparative example 1 was implemented by replacing the venturi system dedicated to the mother solution containing iron and at least one polyphosphate by a volumetric metering pump. In this case, said mother solution containing iron and at least one polyphosphate therefore ends up directly in the dilution tank without being partially diluted by the fertirrigation solution in the transfer system. During this second test, the yield of iron obtained dropwise was particularly non-reproducible, losses of 40% being observed at times.

Comparative Example 3

[0128] In a second variant of comparative example 1, the mother solution containing iron and at least one polyphosphate is raised to a pH of 7.0 by adding nitric acid before use thereof by the venturi metering system, the other operating conditions being identical to those of comparative example 1. As in comparative example 1, the stability of the mother solution is satisfactory: no precipitate being observed after 4 weeks of storage at ambient temperature or during storage at a temperature of 40 C. Such acidification of the mother solution does however not suffice to prevent the blocking of the venturi after a few hours.

Comparative Example 4

[0129] A variant of comparative example 3 was implemented by increasing the degree of acidification of the mother solution containing iron and at least one polyphosphate to pH 4.0. A precipitate then appears in said mother solution containing iron and at least one polyphosphate after a few days. Analyses of the mother solution containing iron and at least one polyphosphate when filtered made it possible to measure the proportion of iron remaining in solution. The results reveal significant losses of iron in the mother solution containing iron and at least one polyphosphate that reach respectively 20% and 55% after 1 and 4 weeks of storage at ambient temperature.

Example 1

[0130] During this test, the mother solution containing iron and at least one polyphosphate according to example 1 of the patent application WO2014/056688 was acidified by means of nitric acid in order to achieve a pH of 6.0 before use thereof via a transfer system of the venturi type in the dilution tank. The results thus obtained are satisfactory from all points of view: the problem of fouling at the outlet of the venturi disappeared and the soluble iron content of the fertirrigation solution taken off dropwise is continuously equal to the calculated value to within any analysis errors. Finally, the mother solution containing iron and at least one polyphosphate is stable: no loss of iron by chemical precipitation over 4 weeks of storage at ambient temperature or at a temperature of 40 C.

Example 2

[0131] Example 1 was reproduced replacing the transfer system of example 1 with a volumetric pump. The yield of iron dropwise always greatly exceeds 90%.

Example 3

[0132] Example 1 was reproduced by acidifying the mother solution containing iron and at least one polyphosphate to pH 5.0. The stability under storage of the mother solution is maintained up to 4 weeks at ambient temperature.