Foliar fertilizer

Abstract

Nanocrystalline compounds containing essential nutrients have been synthesized to have effective physical and chemical characteristics, including a high contact surface area/total surface area ratio that provides maximal leaf surface contact, limited mobility and improved solubility, a net positive charge, soluble salt-forming groups, and reactive surface edges for cation exchange to release nutrient cationic ions into the water film on leaf surfaces.

Claims

1. A foliar fertilizer composition comprising water-soluble fertilizing nanoparticles having a ratio of contact surface area to total surface area of greater than 1:4, a platelet morphology, and an overall positive surface charge in water at neutral pH, wherein the fertilizing nanoparticles consist essentially of: a cationic nutrient element selected from the group consisting of zinc, copper, iron, manganese, boron, molybdenum, potassium, and magnesium; and one or more groups capable of forming a water-soluble salt with the cationic nutrient element.

2. The foliar fertilizer composition of claim 1, wherein the ratio of the contact surface area to the total surface area of the water-soluble fertilizing nanoparticles is greater than 1:3.

3. The foliar fertilizer composition of claim 1, wherein the ratio of the contact surface area to the total surface area ratio of the water-soluble fertilizing nanoparticles is between 1:3 to 1:2.

4. The foliar fertilizer composition of claim 1, wherein the water-soluble fertilizing nanoparticles have a thickness less than about 500 nm.

5. The foliar fertilizer composition of claim 1, wherein the water-soluble fertilizing nanoparticles have a thickness less than about 250 nm.

6. The foliar fertilizer composition of claim 1, wherein the water-soluble fertilizing nanoparticles have a thickness less than about 150 nm.

7. The foliar fertilizer composition of claim 1, wherein the water-soluble fertilizing nanoparticles have a thickness less than about 100 nm.

8. The foliar fertilizer composition of claim 1, wherein the ratio of the contact surface area to the total surface area of the water-soluble fertilizing nanoparticles is within a range of from 1:4 to 1:2.

9. The foliar fertilizer composition of claim 1, wherein a ratio of the contact surface area of the fertilizing nanoparticles to a volume of the water-soluble fertilizing nanoparticles is at least 10.

10. The foliar fertilizer composition of claim 1, wherein a ratio of the contact surface area of the fertilizing nanoparticles to a volume of the water-soluble fertilizing nanoparticles is at least 50.

11. The foliar fertilizer composition of claim 1, wherein a ratio of the contact surface area of the of the fertilizing nanoparticles to a volume of the water-soluble fertilizing nanoparticles is at least 100.

12. The foliar fertilizer composition of claim 1, wherein the one or more groups capable of forming a water-soluble salt with a cationic nutrient element are selected from the group consisting of nitrate, chloride, sulphate, phosphate and acetate.

13. The foliar fertilizer composition of claim 12, wherein the cationic nutrient element is zinc, and wherein the one or more groups comprise at least one nitrate group.

14. The foliar fertilizer composition of claim 13, wherein the water-soluble fertilizing nanoparticles comprise a zinc hydroxide nitrate.

15. The foliar fertilizer composition of claim 14, wherein the water-soluble fertilizing nanoparticles comprise Zn.sub.5(OH).sub.8(NO3).sub.2.2H.sub.2O.

16. The foliar fertilizer composition of claim 15, further comprising an aqueous liquid carrier.

17. The foliar fertilizer composition of claim 16, wherein the solubility of the water-soluble fertilizing nanoparticles in the aqueous liquid carrier is between 0.1-100 mg/L for micronutrient elements and 100-1000 mg/L for macronutrient elements.

18. The foliar fertilizer composition of claim 16, wherein the water-soluble fertilizing nanoparticles are dispersed in the aqueous liquid carrier.

19. A method of delivering a nutrient to a plant, comprising: providing a water-soluble foliar fertilizer composition comprising a fertilizing nanoparticles dispersed in a liquid carrier, the fertilizing nanoparticles having a ratio of contact surface area to total surface area of greater than 1:4, a platelet morphology, and an overall positive surface charge in water at neutral pH, wherein the fertilizing nanoparticles consist essentially of: a cationic nutrient element selected from the group consisting of zinc, copper, iron, manganese, boron, molybdenum, potassium, and magnesium; and one or more groups capable of forming a water-soluble salt with the cationic nutrient element; and applying the foliar fertilizer composition to the plant.

20. The method of claim 19, wherein the water-soluble fertilizing nanoparticles are fertilizing nanocrystals.

21. A method of formulating a foliar fertilizer composition, comprising: providing water-soluble fertilizing nanoparticles having a ratio of contact surface area to total surface area of greater than 1:4, a platelet morphology, and an overall positive surface charge in water at neutral pH, wherein the fertilizing nanoparticles consist essentially of: a cationic nutrient element selected from the group consisting of zinc, copper, iron, manganese, boron, molybdenum, potassium, and magnesium; and one or more groups capable of forming a water-soluble salt with the cationic nutrient element; and dispersing the fertilizing nanoparticles in a liquid carrier.

22. The foliar fertilization composition of claim 1, wherein the water-soluble fertilizing nanocrystals have a thickness of about 50 nm to about 100 nm and a lateral dimension of about 0.2 m to about 1 m.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) In order that the invention may be readily understood and put into practical effect, preferred embodiments will now be described by way of example with reference to the accompanying figures, wherein:

(2) FIG. 1A shows a series of XRD patterns for three zinc-containing fertilizing compounds;

(3) FIG. 1B shows a series of FTIR spectra for three zinc-containing fertilizing compounds;

(4) FIG. 2 shows two scanning electron micrograph images of sample A (zinc hydroxide nitrate) as a fertilizing compound of the present invention;

(5) FIG. 3 is a scanning electron micrograph image of sample B (zinc oxide);

(6) FIG. 4 is a scanning electron micrograph image of sample C (zinc oxide); and

(7) FIG. 5 is a series of diagrammatic representations of the contact area of different morphologies of fertilizing compounds.

DETAILED DESCRIPTION

(8) The present inventors have provided fertilizing nanocrystals demonstrating reliable and controlled dissolution of nutrients into the water film on a leaf surface. Nanocrystalline compounds containing essential nutrients have been synthesized to have effective physical and chemical characteristics, including a high contact surface area/total surface area ratio for maximal surface contact, suitable chemical composition and charge balance to achieve a net positive charge, and reactive surface edges for cation exchange to release nutrient cationic ions in the water film on leaf surfaces. The nanocrystals are the source of nutrients and slowly dissolve to release nutrient cations to maintain a concentration of between about 1-100 mg/L nutrient ion in the water film on leaf surfaces for penetration into leaf cells.

(9) The present invention is predicated, at least in part, on the development of a foliar fertilizing compound that takes the form of nanocrystal platelets or sheets and has an overall positive surface charge or potential in water. The morphology of the nanocrystal platelets, combined with the overall positive surface charge or potential, has been observed to provide surprisingly large gains in terms of the efficiency of delivery of a nutrient element to a plant through its leaf surface. Although not wishing to be bound by any particular theory, it is postulated that the platelet shape and nanosized dimensions of the nanocrystal provide for a high overall surface area to volume ratio, meaning the compound is somewhat better when placed to dissolve and become bioavailable to the plant and, particularly, a high contact surface area to total surface area ratio leads to reduced mobility of the compound on the leaf and a much improved solubility/release profile, while the overall positive surface charge or potential results in good dispersion over, and strong adherence onto, the leaf surface, thereby reducing post-application loss. The fertilizing compound, due to its chemical composition, has a suitable solubility range in water such that it can be delivered to the plant leaves in sufficient quantities to form a slow release system without demonstrating phytotoxicity.

(10) Although the invention will be demonstrated herein with particular reference to a zinc hydroxide nitrate fertilizing compound, it is believed that the principles discussed are equally applicable to a range of nutritional element-containing compounds capable of providing suitable nanoparticulate morphology and an overall positive surface charge or potential.

(11) The term foliar fertilizer, as used herein, refers to a composition suitable for application onto the leaves of a plant, which, upon dissolution, is capable of delivering a desired nutrient to the plant. The foliar fertilizers described comprise a partially soluble fertilizing compound suspended or otherwise dispersed or contained within an aqueous solution.

(12) The term contact surface area, as used herein, relates to the surface area of the fertilizer particle, which is in either direct contact with, or is immediately adjacent to, the leaf surface. For a variety of shapes, this is likely to be the surface with the greatest individual surface area as this will be a more stable landing position for the particle to take when it locates on the leaf surface. For example, for the platelets or sheet-like nanoparticles described herein, the contact surface area is one of the two large surfaces as opposed to a side or edge of the platelet or sheet.

(13) The terms dispersed or dispersion, as used herein, refers to the presence of a fertilizing compound within an aqueous solution forming a foliar fertilizer composition. The fertilizing compound will have limited solubility in the aqueous solution such that solid particles thereof will be suspended or able to be suspended therein.

(14) Zinc is an essential micronutrient that is often applied as a component of a foliar fertilizer composition in the form of ground zinc oxide. Although generally effective, it has been found that it can be difficult to achieve an even distribution of this compound on the leaf surface and, coupled with its rather low solubility and problems with its being easily dislodged from the leaf surface by wind and rain, can mean that inadequate amounts of zinc are entering the plant.

(15) The present inventors postulated that optimization of the morphology and charge characteristics of a zinc-containing fertilizing compound could result in improved delivery, retention on the leaf surface and availability of the zinc to a plant leaf surface.

(16) Three samples of a zinc-containing fertilizing compound were synthesized and characterized as set out in the examples section. Sample A was shown to be zinc hydroxide nitrate (Zn.sub.5(OH).sub.8(NO3).sub.2), which typically exists in the dihydrate form as Zn.sub.5(OH).sub.8(NO3).sub.2.2H.sub.2O. Samples B and C were both zinc oxide but the different synthetic conditions employed in their production resulted in nanoparticles with different morphology characteristics.

(17) Zinc hydroxide nitrate, Sample A, was synthesized by a variation on a known synthetic method, as described in the examples section. Samples B and C were synthesized in a relatively similar manner but with key variations as set out in the example section. The particular process conditions used produced zinc-containing fertilizing compounds with corresponding morphologies as discussed below.

(18) FIG. 2 shows two scanning electron micrograph (SEM) images of sample A in which the platelet or sheet-like morphology of the material can be clearly seen. The thickness of the platelets are between about 50-100 nm, while the lateral dimension was generally in the range of 0.2-1 m. The zinc hydroxide nitrate synthesized can thus accurately be described as having formed a nanomaterial or being nanoparticulate. Particularly, the images shown in FIG. 2 can be said to show nanocrystals.

(19) The platelet shape of the zinc hydroxide nitrate nanocrystals means that they have a very high leaf contact surface area to total surface area ratio. This has been found to provide surprisingly large gains in efficacy over larger amorphous particles and even morphologies such as nanocubes, nanorods and the like as, first, a greater proportion of the zinc hydroxide nitrate is exposed to the environment, which will solubilize the material and allow it to enter the plant leaf and, second, more of the material is in physical contact with the leaf surface. This second point results in the zinc being made available to the plant in a more efficient manner and also means the zinc hydroxide nitrate nanoparticles are less likely to be mobile on the leaf surface and, therefore, inadvertently displaced, as can happen with shapes having a lower contact surface area to total surface area ratio and greater resulting mobility, such as spherical particles.

(20) In general terms, the smaller the size of a crystal with a particular shape, the larger the specific surface area (or surface area to volume ratio) and, thus, the greater the likelihood of a larger relative contact area between crystal and leaf. In relation to the nanocrystals provided by the present invention, this can be further considered by the ratio of the contact surface area (i.e., the area of crystal in contact with or immediately adjacent the leaf surface) to the total surface area of the crystal. By way of example, for a sphere, the theoretical contact area approaches zero, as it is a point contact, and so the ratio is close to zero. For a cube the ratio is 1/6, for a very long square prism, the ratio is close to 1/4, and for a very thin sheet, the ratio is close to 1/2. Thus, for a nanocrystal of sheet-like or platelet morphology, as seen from sample A, more surface area is effectively available as the leaf contact area. This is shown in FIG. 5.

(21) FIG. 3 shows that Sample B produced a typical zinc oxide crystal shape, nanorods, with a hexagonal cross-section. The side length of the hexagonal cross-section was about 100 nm, while the length of the rods was in the range of 200-400 nm.

(22) FIG. 4 is an SEM of the particles of Sample C and it can be seen that the crystal size was approximately 50-100 nm, on average, without noticeable morphological features. These crystals are aggregated into particulates of a hundred to a few hundred nanometers in size.

(23) The uptake of each of Samples A, B and C, along with a commercial zinc-containing foliar fertilizer (Activist 30% Zn in which the zinc is present as zinc oxide) was tested on capsicum plant leaves, as set out in the examples section. The results of these tests are summarized in Table 1, wherein the parameter LSD 0.05 refers to Fisher's least significant difference analysis with 5% limitation.

(24) TABLE-US-00001 TABLE 1 Foliar zinc uptake from various samples Applied Zn Zn uptake Fertilizers (g) (g/leaf) % applied dose Sample A 288 26.85 9.32 Sample B 300 16.49 5.50 Sample C 300 15.67 5.22 Activist 30% Zn 268 9.84 3.67 LSD.sub.0.05 6.95 2.38

(25) The results show that the zinc hydroxide nitrate (Sample A) is significantly more effective at delivering zinc into the plant leaf than either of Sample B or C or the commercially available treatment. In terms of the percentage of the applied zinc dosage to reach the interior of the leaf, Sample A was more efficacious than the commercial treatment in making bioavailable almost three times as much zinc for a similar total applied amount.

(26) Samples B and C produced relatively similar results to each other and both were improved over the commercial treatment, although by an amount just under the determined limit of statistical significance. The better delivery of zinc into the leaf, as observed for Samples B and C by comparison to the commercial treatment, is believed to be due purely to the smaller, nanoscale size of their particles. The Activist 30% Zn contains zinc oxide, just as Samples B and C do, but the smaller particle sizes of B and C result in a higher overall bulk solubility and so more of the zinc is available to the leaf.

(27) The success of the zinc hydroxide nitrate as a fertilizing compound can be attributed to a number of features resulting from its particle morphology and/or physicochemical characteristics. These features include, but are not limited to, the platelet/sheet-like shape of the nanocrystals providing for a high surface area to volume ratio, high contact area to total surface area ratio and low mobility on the leaf surface; the nanoscale dimensions of the platelet improving solubility of the material; the surface charge profile or zeta potential of the zinc hydroxide nitrate; and the chemical composition of the zinc hydroxide nitrate itself assisting in providing an optimal solubility profile.

(28) In one general embodiment of the invention, the fertilizing compound is present in a foliar fertilizer composition in the form of particles having at least one dimension less than about 1000 nm, preferably less than about 500 nm, more preferably less than about 250 nm, even more preferably less than about 150 nm, most preferably less than about 100 nm. These nanoscale dimensions allow the fertilizing compound within a foliar fertilizer composition to be dispersed evenly, in appropriate amounts, across the leaf surface.

(29) Although the platelet morphology described herein is optimal, it will be appreciated that other nanoparticulate shapes may be suitable so long as they provide a sufficiently large contact surface area to total surface area ratio, attain a reasonable rate of solubilization, and, therefore, release of the bound zinc. Preferably, the contact surface area to total surface area ratio of the nanoparticulate shapes will be greater than 1:6, more preferably greater than 1:4, even more preferably greater than 1:3, and still more preferably approaching 1:2.

(30) As discussed, it is preferred that the fertilizing compound exists in a form that has a high contact surface area to total surface area ratio to ensure good contact over a maximal area of the leaf surface and to increase the amount of compound exposed to solubilizing conditions. As an alternative to the ratio above, this may be described as being a contact surface area to volume ratio of the fertilizing compound particles of at least 1/m, preferably at least 10/m, more preferably at least 20/m, even more preferably at least 50/m, and most preferably at least 100/m. This ratio can be calculated as shown below for certain crystal shapes relating to those shown in FIG. 5, Panels a-c.

(31) FIG. 5(a) Cube: Contact surface area (Sc)=a.sup.2 Volume (V)=a.sup.3 therefore, Contact surface area to volume ratio: R (Sc/V)=1/a If a=0.01 m (10 nm), then R (Sc/V)=100/m If a=0.1 m (100 nm), then R (Sc/V)=10/m If a=1 m (1000 nm), then R (ScN)=1/m If a=10 m, then R (Sc/V)=0.1/m

(32) FIG. 5(b) Square prism (standing): Contact surface area (Sc)=a.sup.2 Volume (V)=a.sup.2b therefore, Contact surface area to volume ratio: R (Sc/V)=1/b, depending on b (height or thickness) Suppose a=1 m (1000 nm), If b=0.01 m (10 nm), then R (Sc/V)=100/m (sheet) If b=0.1 m (100 nm), then R (Sc/V)=10/m (plate) If b=1 m (1000 nm), then R (Sc/V)=1/m If b=10 m, then R (Sc/V)=0.1/m (rod)

(33) Note that a cylinder would give approximately the same result.

(34) FIG. 5(c) Square prism (laying down): Contact surface area (Sc)=ab Volume (V)=a.sup.2b therefore, Contact surface area to volume ratio: R (Sc/V)=1/a (depending on a scale) Suppose b=1 m (1000 nm), If a=0.01 m (10 nm), then R (Sc/V)=100/m (rod) If a=0.1 m (100 nm), then R (Sc/V)=10/m (rod) If a=1 m (1000 nm), then R (Sc V)=1/m If a=10 m, then R (Sc V)=0.1/pm (plate standing)

(35) The contact area to volume and/or total surface area ratio of a nanoparticle with platelet or sheet-like morphology is thus much higher than other common morphologies, providing distinct advantages when used as foliar fertilizers that have not previously been considered.

(36) The surface charge of a leaf is predominantly negative and this is a factor that is also not considered or addressed by prior art foliar suspension fertilizers. Most fertilizers employ metal oxides having a negative charge, at neutral pH, which does not provide for optimal dispersion onto and contact with the leaf surface. Zinc oxide nanoparticulate fertilizing compounds display a negative surface charge in water at neutral pH. They also use surfactants within the composition that can interfere with the surface charge matching between fertilizing compound and leaf surface. Preferably, nonionic or cationic surfactants are employed in the present formulations to maintain or enhance the positive charge of the suspension for improved adhesion with negatively charged leaf surfaces.

(37) The zinc hydroxide nitrate, synthesized as Sample A, has a positive surface charge or potential in water that can provide distinct advantages in terms of improving the dispersion of the compound evenly over the leaf surface as well as the contact between compound and leaf. The overall positive surface charge or potential means the nanocrystalline platelets of zinc hydroxide nitrate are attracted to the leaf surface and held in place so that they are less likely to be washed off or otherwise displaced after application. The positive surface charge is the charge presented on the platelet flat outer face and, while the edges of the platelets may display some negative charge, due to the size of this face, the overall surface charge is overwhelmingly positive.

(38) The solubility of the fertilizing compound in water is also a component of the present invention. As already discussed, this is influenced to some extent by the nanoscale size of the particles, as well as the high surface area (and contact area) to volume/total surface area ratios achieved. However, the chemical composition of the fertilizing compound is also key. Preferably, the fertilizing compound has one or more nitrate, chloride, sulphate, phosphate, acetate or like water-soluble salt-forming groups that aid in improving the solubility of the compound in comparison to a compound such as zinc oxide or zinc hydroxide.

(39) Preferably, the solubility of the fertilizing compound in water is between 0.1-100 mg/L for micronutrient elements and 100-1000 mg/L for macronutrient elements. For zinc and manganese a suitable range is 5-50 mg/L; for copper 1-5 mg/L, for molybdenum 0.1-1 mg/L and for calcium and magnesium 100-500 mg/L.

(40) The fertilizing compound will be delivered to the plant in the form of a foliar fertilizer comprising the fertilizing compound dispersed in a liquid carrier. Preferably, the liquid carrier is an aqueous carrier. The liquid carrier may be water based, but contain one or more suitable surfactants or additives for stability or like formulation purposes. A suitable stability additive is carboxymethyl cellulose (CMC) to form a particularly preferred foliar fertilizer composition.

(41) Although the discussion herein has focused on the synthesis of zinc-containing fertilizing compounds, it will be appreciated that the principles of forming a nanoscale compound with high contact surface area to total surface area ratio, suitable solubility and overall positive surface charge or potential can be applied to nano- or submicron particles of a range of other essential elements. In one embodiment, the fertilizing compound may contain a plant nutrient element selected from the group consisting of zinc, copper, iron, manganese, boron, molybdenum, chlorine, phosphorus, potassium, calcium, magnesium and sulphur.

EXAMPLES

(42) Sample Preparation

(43) Three zinc-containing samples were prepared as herein described. Sample A was synthesized by following a modified precipitation method. A 3.75 M solution of Zn(NO.sub.3).sub.2 (75 mmol in 20 ml deionized water) was poured with 0.75 M NaOH (37.5 mmol in 50 mL deionized water), i.e., giving a OH/Zn ratio of 0.5, with mechanical stirring at a rate of 500 rpm at room temperature. The stirring was continued for a period from 10 minutes to 24 hours. The precipitate was then collected by filtration, washed with deionized water and dried at 65 C.

(44) Sample B was synthesized using a similar process as for Sample A but the OH/Zn ratio was changed to 1.6 (8/5). In brief, a 1.88 M solution of Zn(NO.sub.3).sub.2 (18.8 mmol in 10 ml deionized water) was poured with 0.75 M NaOH (30.0 mmol in 40 mL deionized water), i.e., giving a OH/Zn ratio of 1.6, under mechanical stirring at a rate of 500 rpm at 50 C. The stirring was continued for a period of 1 to 24 hours. The precipitate was then collected by filtration, washed with deionized water and dried at 65 C.

(45) Sample C was synthesized via the same process as that of sample B but with the concentration of zinc nitrate reduced. A 0.47 M solution of Zn(NO.sub.3).sub.2 (23.5 mmol in 50 ml deionized water) was poured with 0.75 M NaOH (37.5 mmol in 50 ml deionized water), i.e., giving a OH/Zn ratio of 1.6, under mechanical stirring at a rate of 500 rpm at 50 C. The stirring was continued for a period from 1 to 24 hours. The precipitate was then collected by filtration, washed with deionized water and dried at 65 C.

(46) Sample Characterization

(47) Powder X-ray diffraction (XRD) was performed using a Bruker D8 Advance equipped with a Copper target scintillation detector and graphite monochromator with Cu K (=1.54 ) radiation. The 2 angle was scanned from 5 to 70 and the scanning rate was 3/minute. The Fourier transform infrared (FTIR) spectra were collected in the range of 4000-400 cm.sup.1 via a Fourier Transform Infrared-Attenuated Total Reflectance technique in a Nicolet 6700 FTIR spectrometer manufactured by Thermo Electron Corporation. SEM images were recorded in a JEOL JSM-6300 to investigate the morphology and particle sizes of the produced samples.

(48) The powder X-ray diffraction pattern of sample A, shown in FIG. 1A, uppermost pattern, was identified by comparison with the internationally accepted database of powder diffraction patterns, JCPDS (Joint Committee on Powder Diffraction Standards now administered by the International Centre for Diffraction Data) card 24-1460 as being zinc hydroxide nitrate according to the characteristic diffraction peaks that are marked with the Miller (hkl) indices, as seen in FIG. 1. The observed interlayer spacing for sample A was around 0.97 nm, which is in good agreement with literature reports (Hussein et al., 2009).

(49) The FTIR spectrum of sample A, as seen in FIG. 1B, uppermost spectra, further confirmed the compound as being zinc hydroxide nitrate. The sharp peak seen at 3573 cm.sup.1 is attributed to the stretching vibration of the OH bond associated with the zinc ion and is to be expected as zinc hydroxide nitrate contains a relatively high number of hydroxide groups. The broad band at 3448 cm.sup.1, as well as the peak at 1635 cm.sup.1, indicated the presence of water molecules in the interlayers and/or adsorbed on the molecule's surface. The shoulder seen at about 3300 cm.sup.1 is attributed to OH groups (from ZnOH and H.sub.2O) hydrogen-bonded with nitrate or water molecules. The intensive peak around 1367 cm.sup.1, the weak peaks around 1012 cm.sup.1, and the weak peak at 838 cm.sup.1 characterize various vibration modes of the nitrate group.

(50) According to the literature, a shoulder around 1430 cm.sup.1, relating to nitrate anions grafted to the hydroxide layer, should be observable. However, in this instance, the shoulder was not significant, probably indicating the nitrate group keeps its C.sub.3v symmetry. The band at 632 cm.sup.1 and the weak peak at 519 cm.sup.1 were due to bending of the ZnOH bond and the vibration of the ZnO bond resulted in a peak at 464 cm.sup.1. In this manner, the X-ray diffraction patterns and FTIR spectra allowed Sample A to be unequivocally identified as zinc hydroxide nitrate with a molecular formula of Zn.sub.5(OH).sub.8(NO.sub.3).sub.2.2H.sub.2O.

(51) Samples B and C gave a powder X-ray diffraction pattern, shown in FIG. 1A, middle and bottom, respectively, identical to the JCPDS card 36-1451, indicating the presence of wurtzite-structure zinc oxide. In the FTIR spectra of samples B and C, shown in FIG. 1B, middle and bottom, respectively, weak and broad bands at around 3372 cm.sup.1 were observed, which could be attributed to OH stretching of adsorbed water molecules. Vibration of the ZnO bonds was observed at around 500 cm.sup.1.

(52) Foliar Uptake of Samples A, B and C

(53) Capsicum (Capsicum annume L.cv. Giant Bell) plants were grown in a glasshouse with the temperature controlled at 25/20 C. (day/night). One week after germination, each capsicum seedling was transferred into a 3-L pot filled with potting mix. Basal nutrients were supplied to each pot by adding 5 g of Osmocote slow release fertilizer (NPK 16:9:12 plus micronutrient; Scotts Professional) per pot.

(54) Leaves from the 6-week-old plants were then cut at the base of their petioles. Petioles were immersed in Eppendorf tubes filled with a nutrient solution containing all basal nutrients except zinc. The tubes were inserted in holes at the bottom of Petri dishes. The leaf blades rested on moist filter paper to create approximately 100% relative humidity during the incubation process.

(55) The as-prepared leaf surfaces were then exposed to one of four different zinc sources being Samples A, B and C, described above, and a sample for comparison purposes. A commercial product, Activist 30% Zn (Agrichem Co. Ltd.), was applied as the comparison sample and some leaves were not exposed to any zinc-containing sample to thereby act as a control. Samples A, B and C were dispersed in deionized water to make homogeneous suspensions with the aid of ultrasonic treatment and employing the same surfactant as is found in Activist 30% Zn to ensure consistency between samples.

(56) The three synthesized zinc sample suspensions and the Activist 30% Zn were applied on separate adaxial leaf surfaces using a micropipette with droplet volume of approximately 5 L. The calculated loading amount of fertilizing compound on each leaf surface is displayed in Table 1. After application of the zinc-containing samples, the leaves were transferred into an incubator and incubated for three days with the temperature set at 25/20 C. (day/night). The light intensity on each shelf was greater than 170 mol/m.sup.2/s (TRISL model, Thermoline). The leaves were then harvested and all residual zinc compound on the leaf surface washed off by wiping the treated areas using clean moist cotton buds and then rinsing three times with triple deionized water. The leaves were then oven-dried at 68 C. for 48 hours before digestion with concentrated HNO.sub.3 and H.sub.2O.sub.2 using a microwave digestor (Milestone Inc). Foliar uptake of zinc was determined by comparison of the difference between the zinc concentration found in treated leaves and untreated leaves. Table 1 shows the results of the uptake study.

(57) The present invention provides for a foliar fertilizing compound demonstrating a number of improved properties. The morphology of the fertilizing compound particles is such that the surface area in contact with the leaf is maximized and the sheet-like nano-sized particles provide for limited mobility when applied to the leaf and allow good solubilization. The chemical composition of the compound is such that it sits within an optimal solubility range in water preventing rapid dissolution, which may result in phytotoxicity, but achieving a higher rate of dissolution than zinc oxides. This ensures an appropriate rate of controlled release of the desired element, thereby providing the plant with an immediate but long-lasting supply of nutrient with a single application. Further, consideration of the role charge can play in assisting with distribution of the fertilizing compound, as well as limiting the likelihood of its displacement from the leaf surface after application, has led to production of a fertilizing compound with an overall positive surface charge or potential in water. This interacts with the net negative charge presented by the leaf surface to give the advantages discussed.

(58) Throughout the specification, the aim has been to describe preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features. It will be appreciated by those of skill in the art that, in light of the present disclosure, various modifications and changes may be made in the particular embodiments exemplified without departing from the scope of the invention.