AU2003203753A1 - A fertiliser - Google Patents

Description

Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

(ORIGINAL)

Name of Applicant: Actual Inventor: Address for Service: Invention Title: Christiaan de Vreeze Christiaan de Vreeze DAVIES COLLISON CAVE, Patent Attorneys, 1 Little Collins Street, Melbourne, Victoria 3000.

"A fertiliser" Details of Associated Provisional Application: No: PS1777/02 16 April 2002 The following statement is a full description of this invention, including the best method of performing it known to me: P:\OPER\MKR\SPECIHPS 1777-02-CAP.doc-I 6/04/03 -1- A fertiliser The present invention relates generally to a fertiliser and a method for producing a fertiliser. More particularly the present invention provides a granular fertiliser and a method of preparing same. Further the present invention provides a controlled or slow release fertiliser.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

BACKGROUND

Phosphorus is an essential inorganic plant nutrient involved in the formation of highenergy phosphate compounds such as ATP and ADP as well as the formation of nucleic acids, phospholipids and phosphorylation of sugars. Phosphorus fertilisers are second only to nitrogen fertilisers in importance for growing crops. Phosphorus encourages early rooting, general health, and larger yields. Without it, crops are thin and plants are stunted.

In soils P may exist in many different forms. In practical terms, however, P in soils can be thought of existing in 3 main forms: in solution, in the 'active' form and in the 'fixed' form.

The small amount of P in solution in soil is in the orthophosphate form (P043-) and this is the only form with any measurable mobility. Plants will only take up P in the orthophosphate form as dihydrogen phosphate and hydrogen phosphate ions (H 2 P0 4 and HP0 4 2 The H 2 P0 4 ion is more readily absorbed than the HP0 4 2 by most plants.

However, P in solution is highly susceptible to loss due to water run off.

P \OPE R\M KR\SPECI\PS 1777-02-CAP.do- 6/04/03 -2- The active P is in the solid phase which is relatively easily released to the soil solution, the water surrounding soil particles. Active P will contain inorganic phosphate ions that are attached (or adsorbed) to small particles in the soil, phosphate that has reacted with elements such as calcium or aluminium to form somewhat soluble solids, such as calcium hydrogen phosphate and organic P that is easily mineralised. As plants take up phosphate, the concentration of phosphate in solution is decreased and some phosphate from the active P form is released. A growing crop would quickly deplete the soluble P if it was not being continuously replenished from the active form.

Fixed P comprises inorganic phosphate compounds that are very insoluble and organic compounds that are resistant to mineralisation by micro-organisms in the soil. The inorganic phosphate compounds in this fixed P form are more crystalline in their structure and less soluble than the active P compounds. Such fixed P compounds include octocalcium phosphate, hydroxyapatite, crystalline variscite (an aluminium phosphate) and strengite (an iron phosphate).

Most of the phosphorus in present day fertiliser is made by treating rock phosphate with sulphuric and phosphoric acid to make superphosphate or with ammonia to form ammonium phosphates. The P in superphosphate is present as both orthophosphate and polyphosphate, however, polyphosphate ions rapidly convert to orthophosphate ions in the presence of soil water in days or less. The phosphate in fertilisers is initially quite water soluble and available. However, when the fertiliser phosphate comes in contact with the soil, various reactions begin occurring that make the phosphate less soluble and less available. As a particle of fertiliser comes in contact with the soil, moisture from the soil will begin dissolving the particle. Dissolving of the fertiliser increases the soluble phosphate in the soil solution around the particle and allows the dissolved phosphate to move a short distance away from the fertiliser particle. As phosphate ions in solution slowly migrate away from the fertiliser particle, most of the phosphate will react with the minerals within the soil to form compounds that are solids. The newly formed solids are relatively available to meet crop needs in the short term. However, gradually reactions occur in which the adsorbed phosphate and the easily dissolved compounds of phosphate P:OPER'MKRSPECI\PS 1777-02-CAP.do- 16/04103 -3form more insoluble compounds that cause the phosphate to be become fixed and unavailable. This process of available phosphorus being made unavailable to plants is called "phosphorus fixation." The fixation of P is partially the reason for the low efficiency ofP fertilisers.

The rates and products of these "fixation" reactions are dependent on such soil conditions as pH, moisture content, temperature, and the minerals already present in the soil as well as the form of P applied to soil. The degree of P fixation in soils is regulated to a large extent by pH. Reactions that reduce P availability occur in all ranges of soil pH but can be very pronounced in acidic soils. In acidic soils (especially with soil pH less than 5.5) the first products formed are amorphous Al and Fe phosphates, as well as some Ca phosphates.

Since calcium phosphates are relatively more water-soluble than aluminium phosphates, in strongly acid soils, most of the P is bound and not released. Thus phosphorus availability is greatest at a neutral pH due to a reduction in the conversion of active phosphate to fixed

P.

Soil acidity is commonly induced by over-application of superphosphate preparations. The application of ammonium-based fertilisers, such as ammonium sulfate, can also increase soil acidity. As a result, the pH level of many soils is acidic and pH increase, or sweetening of the soil to a more neutral pH, enhances the commercial returns to the land user. The sweetening of soil enhances P access and the ability of plants to grow, since low pH reduces both viability and activity of most commercial crops. In particular, sweetening aids in reducing aluminium and iron toxicity. While the traditional approach has been to separately apply P and a sweetening agent, a P fertiliser which is capable of sweetening the soil pH to between 6.5 and 7.0 would provide increased access to available P and generally improve crop health.

While P solubility of fertilisers is traditionally assessed by solubility in water, the solubility of P in citrate solution may provide a more useful reference of P available to the P plant. A high degree of water solubility, as found in the superphosphates, is undesirable in a fertiliser, since at first rain or contact with soil the fertiliser may immediately P:\OPER\MKR\SPEC\PS I 777-02-CAP.doc-16/04/03 -4solubilise, and then become fixed, requiring repeated application of the fertiliser to ensure continued supply of available P to the plant. Citrate solubility is believed to be highly desirable as this provides P in the plant rhizosphere where the plant itself manufactures citrate ions. Thus fertilisers with water insolubility but citrate solubility may provide P in a form available to the plant without the need for reapplication and the undesirable runoff caused by high levels of water solubility.

The reaction of limestone or other forms of lime with superphosphate has been proposed in W092/12945 and AU20813/53 to provide phosphatic fertilisers with increased plant availability. However, these reactions, which are allowed to proceed to completion, produce Ca 3

PO

4 and hydroxyapatite (Ca 3 (P0 4 2 Ca(OH) 2 which are generally insoluble and inaccessible to plants. AU12689/97, a modification of W092/12945, teaches the additional use of trace elements, which are not involved in the reaction, to provide further plant nutrients. AU 13066/88 teaches the reaction of a calcined product, rock phosphate and acid to produce an dicalcium phosphate from a tricalcium phosphate. However, the product pH is between 8.5 and 12, indicating the formation of predominantly Ca 3

PO

4 Another approach that has been tried to increase fertiliser P availability is the use of liquid fertilisers. However, as a result of fixation, liquid fertilisers do not have increased availability or agronomic value over dry fertilisers. While a liquid phosphorus fertiliser may be completely water soluble (completely plant available) when manufactured, it does not remain this way very long after it is applied to the soil. When applied to the soil, most water soluble phosphorus fertilisers are converted to the less soluble forms of phosphorus.

There is generally adequate water in the soil to access dry fertilisers. However, liquid phosphorus fertilisers cost more than an equal amount of phosphorus in the dry form because liquids require greater processing and a higher purity to prevent settling or "gunking out" in the liquid formulations. Suspension fertilisers use clay to help suspend fertiliser particles and impurities, thus permitting the use of less pure phosphorus sources.

Agronomic value of suspensions, liquid and dry phosphorus fertilisers are considered equal. Suspensions are usually priced more nearly like dry phosphorus fertilisers than P:'OPER\MKR\SPECIPS I 777-D2-CAP.do- 16/O4/03 liquid forms. However, they are more difficult to apply without special equipment that provides agitation.

Application of powdered fertilisers has also been tried in an effort to improve P availability. In one such effort, superphosphate and calcium carbonate was reacted with a substantial amount of magnesium oxide (US 1953419) to form a fine dry powder containing a large amount of magnesium oxide. While such fine fertilisers may provide quick availability of nutrients, their application to soil is difficult in windy conditions due to wind losses and uneven distribution. Further, wind redistribution may cause foliage damage, uneven distribution or pollution of waterways. To address these problems, granular or slow release fertilisers have been produced in an effort to provide continual availability of P.

Current production of granular triple superphosphate from rock phosphate uses phosphoric acid. The reaction mixture, a slurry, is sprayed onto recycled fertiliser fines in a granulator. Granules grow and are then discharged to a dryer, screened, and sent to storage. The problem with these fertilisers is that because of the high water solubility of superphosphate, the granules dissolve at contact with water, so the application of granulated triple superphosphate provides no more available P than regular powdered triple superphosphate, it is just easier to apply. Granulation aids application by reducing wind loss and allowing for more appropriate, even distribution. However, granules must be durable to withstand handling and sufficiently dry and stable to remain in the granulated form. In an effort to produce a granulated NP fertiliser, AU12689/97 and W092/12945 describe a process for pelletising limestone and superphosphate using a binding agent and pelletiser. The only binding agent disclosed is sugar, and as a result, the pellets soon absorb moisture and clump under normal conditions. These granulation methods have been ineffective in producing a granulated fertiliser which is capable of increasing P availability.

P:\OPER\MKR\SPECI\PS 1777-02-CAP.doc- 16/0403 -6- Controlled release fertilisers, also called slow release fertilisers, make nutrients available over time. One benefit is the delayed release of nutrients over time, reducing nutrient losses due to leaching by limiting the amount of P in soluble form at any one time. Urea coated with sulfur was one of the original manufactured control release fertilisers. The slow deterioration of the sulfur coating on the urea granules on contact with water meant the urea was not all available at application. More recently various types of plastic coated fertiliser have been manufactured. Additionally, there have been manufactured materials that solubilise slowly such as urea-formaldehyde and magnesium ammonium phosphate.

The cost of manufacturing these materials is higher than that of readily soluble fertilisers.

However, the benefits in terms of continuous feeding, economies of single application and reduced likelihood of run off make these materials economical in many situations.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge.

SUMMARY OF THE INVENTION In work leading to the present invention, the inventor sought to provide a P fertiliser which is easy and economic to use and has high P availability to plants, but limited water solubility. In one aspect of this work, the inventor has developed a novel granulated fertiliser with low water solubility and enhanced citrate solubility. The inventor has also developed a method of producing a granular P fertiliser which over time releases P in an available form to plants.

In one embodiment, the present invention provides a granulated fertiliser comprising MgHPO 4 and/or CaHP0 4 In a preferred embodiment, the granulated fertiliser comprises CaHPO 4 (calcium hydrogen phosphate).

In yet another embodiment the present invention provides a granulated fertiliser further comprising magnesium sulphate (MgSO 4 and/or calcium sulphate (CaSO 4 P:'OPERWl KR\SPECIPS 1777-02-CAP.doc- 16/04/03 -7- In another embodiment of the present invention, the granulated fertiliser further comprises MgCO 3 and/or CaCO 3 In yet another embodiment the granulated fertiliser further comprises NH 4

H

2 P0 4 and/or

(NH

4 2 HP0 4 In a preferred embodiment the granulated fertiliser further comprises low levels of Ca 3

PO

4 and hydroxyapatite. Preferably the amount of (Ca 3

PO

4 hydroxyapatite) is less than more preferably less than 40%, 30%, 20%, 10% or 5% by weight.

In another aspect, the present invention provides a method of providing phosphate to a plant comprising applying the granulated fertiliser in the immediate vicinity of the plant.

In yet another aspect, the present invention provides a method of providing phosphate to soil comprising applying the granulated fertiliser to soil.

In yet another aspect, the present invention provides a method of providing phosphate and modifying the pH of soil comprising applying the granulated fertiliser to soil.

In another embodiment the present invention provides a method of producing a granulated fertiliser comprising the steps of reacting a phosphatic material; a first material comprising calcium carbonate and/or magnesium carbonate; and a second material comprising calcium sulphate or ammonium sulphate; for a time and under conditions suitable to form a granulated fertiliser.

In yet another embodiment, the invention provides a method of producing a granulated fertiliser comprising the steps of preparing a mixture of: P:\OPER\MKR\SPECI\PSI777-02-CAP.doc-16/04/03 -8a phosphatic material; and a first material comprising calcium carbonate and/or magnesium carbonate; and adding water and a second material comprising ammonium sulphate to the mixture in discrete or continuous increments.

In yet another embodiment, the invention provides a method of producing a granulated fertiliser comprising the steps of preparing a mixture of: a phosphatic material; a first material comprising calcium carbonate and/or magnesium carbonate; and and water added in discrete or continuous increments; and adding a second material comprising calcium sulphate hemihydrate to the mixture.

DETAILED DESCRIPTION The granulated fertiliser of the invention comprises MgHPO 4 and/or CaHPO 4 in proportions dependent upon the proportions of Mg and Ca in the material utilised to make the fertiliser. The fertiliser of the present invention is manufactured from magnesium and/or calcium carbonate. The carbonate is preferably provided as limestone or dolomite or a combination of the two. Since dolomite contains both magnesium carbonate and calcium carbonate, its use will produce a fertiliser with both magnesium and calcium phosphates. Preferably the composition of the fertiliser is at least 20% (MgHP0 4 CaHPO 4 and more preferably it is at least 30%, 40%, 50%, 60%, 70%, 80% or 90% by weight of (MgHPO 4 CaHPO 4 In addition, the presence of calcium and/or magnesium carbonate in the fertiliser is a result of the excess of calcium carbonate and/or magnesium carbonate in the reactions utilised to make the fertiliser. Preferably the composition of the fertiliser is less than 70%, more preferably less than 50%, 40%, 30%, 20%, 10% or 5% by weight of (MgCO 3 CaCO 3 P:\OPER\MKR\SPECI\PS 777-02-CAP.doc- 1604/03 -9- While magnesium and/or calcium sulphate serves little purpose as a fertiliser, it is present in the granulated fertiliser, as a by product of the reaction process which is used to form the fertiliser. It is preferably present in the fertiliser in a proportion in less than 30%, more preferably less than 20%, 10% or 5% by weight.

The presence of NH 4

H

2

PO

4 and/or (NH 4 2 HP0 4 in the granulated fertiliser provides N to the soil. Diammonium phosphate and monoammonium phosphate are highly water soluble and diammonium phosphate has an acid effect upon the soil similar to anhydrous ammonia. Therefore large amounts of these materials are not always desirable, depending upon the soil condition. Preferably (NH 4

H

2

PO

4

(NH

4 2 HPO4) are present in an amount of less than 30%, more preferably less than 20%, 10% or 5% by weight. The ammonium phosphates may add some strength to the granule and contribute to the availability of phosphate ions.

The fertiliser of the present invention may advantageously elevate soil pH towards neutrality. The acidity of soil may have been induced by previous over-application of various superphosphate preparations. The modification of the soil pH to neutrality generally makes more phosphate available for uptake by the plants. Thus the present invention may both provide available phosphate and improve soil pH in a single application. Preferably the pH of the granulated fertiliser is not strongly acidic. More preferably the pH is not less than 6.0 and more preferably not less than 7.0. Most preferably the pH is in the range 7.0 to 7.4.

The novel fertiliser may be modified by addition of chelated or unchelated trace elements, plant nutrients, macronutrients, herbicides and or fungicides. Plant nutrients include potassium calcium, magnesium, and sulfur. However, addition of Ca and Mg is not usually necessary because the contents of soil are generally sufficient for most plant species. Trace elements include various forms of iron, manganese, zinc, copper, boron, molybdenum, and chlorine. Examples of suitable trace element containing compounds that may be included in the fertilisers of the invention include sodium borate, sodium molybdate and zinc sulphate. Such compounds may be added in production as solutions at P:\OPER\M KR\.SPECI\PSI 777-02-CAP.doc. 16/04/03 the conclusion of ammonium sulphate spraying. For example, amounts of trace elements within the fertiliser of in the order of 100ppm may be appropriate.

The weighted-mean dry raw-material density of the granulated fertiliser is preferably 3.00 g/cc and in a preferred embodiment it is 2.55g/cc. The bulk density of the granulated fertiliser of the invention is preferably less than 1.5g/cc and more preferably about g/cc. From mathematical considerations on packing efficiency, estimated granule density is preferably less than about 2.0g/cc and more preferably about 1.25 g/c. On the basis of these estimates, the granules have on average at least about 25% and more preferably of its volume as voids. There may be many voids in each granule, which give rise to a honeycomb effect in the granule. Whilst the voids may initially contain largely CO 2 in time the CO 2 equilibrates with atmospheric gases. The voids may hold water, since the granules are stable in water.

In one embodiment, the granulated fertiliser, by its properties of granulation, porosity and content of calcium hydrogen phosphate, is capable of reducing the frequency or quantity of phosphatic fertiliser applied to the soil, compared to conventional superphosphate.

The method of producing a granulated fertiliser provided by the invention involves the combination of a material comprising calcium carbonate and/or magnesium carbonate with a phosphatic material to form a mixture.

As used through out the specification and the claims which follow, the term "phosphatic material" is to be understood as encompassing material which comprises phosphate ions.

The phosphatic material is preferably calcium dihydrogen phosphate, which may be in the form of superphosphate, preferably triple superphosphate. Superphosphate is generally sourced from rock phosphate.

The ratio of calcium carbonate or magnesium carbonate to phosphatic material is dictated by stoichiometric considerations. Preferably the carbonate is present in excess, to allow the product to elevate soil pH.. The carbonate and phosphatic material are mixed together.

P:NOPERM KRSPECIPI 777-O2-CAP.dO 16/0403 -11 Preferably the materials are uniformly distributed within each other by mixing in the dry state. Where dry ingredients are used they are preferably ground to equal sized particles preferably less than 200 microns in diameter, since increased surface area increases the reaction rate. Additive such as powdered sulphur and/or coal fines for example in amounts of 0.5% to preferably about by weight may also be incorporated within the dry mix.

Ammonium sulphate or calcium sulphate hemihydrate may be provided in the anhydrous state or as an aqueous solution (including in a slurry form). If they are anhydrous, there must be a source of water to allow the reaction to proceed. Anhydrous calcium sulphate dihydrate or aqueous ammonium sulphate are preferred. Potassium sulphate may also be added to produce a NKP (nitrogen, potassium and phosphate containing) or KP (potassium and phosphate containing) fertiliser.

The rate of the reactions is also dependent on sulphate concentration. The reaction appears to be first order, that is, the rate depends on sulphate concentration only. Generally, the lower the sulphate concentration, the smaller the granules that are generally produced. The sulphate is preferably in the range from 0.2% to 15% of the total mass of the sulphate, carbonate and phosphatic material. More preferably the sulphate is in the range of 0.5 to 10% of the total mass and most preferably 8% of the total mass.

When aqueous ammonium sulphate is used it is preferably added in discrete or continuous increment and not all at once, but the time period will be related to the concentration and scale. When calcium sulphate hemihydrate is used, the water may be added to the mixture in discrete or continuous increments and mixed for a period, then the calcium sulphate is added and mixed for a further period. A suitable period may preferably be more than minutes and preferably less than 4 hours. The water or aqueous ammonium sulphate is preferably added by spraying over the mixture. The addition of the aqueous ammonium sulphate or water all at once generally results in poor granule formation and excess fines.

It is also desirable to control the reaction temperature by the gradual addition of the sulphate or water. The higher the reaction temperature the more rapidly the reaction P:\OPER/M KR\SPECI\PS 1777-02-CAP.do-I 604,03 12proceeds to abstract water and the more viscous the mixture becomes. This makes it more difficult to control the spread of granule size. It is preferable to remain within the temperature band of 15 to 400 Celsius for the process.

As the reactions proceed at the desired rate, the material begins to change from a very soft plastic mass to a friable mass. Mixing can be continued while this occurs. Monitoring of the viscosity of the product (by routine methods) is required to establish the appropriate time to cease mixing and allow the product to granulate. If the product is mixed too long it may form solid "marbles" or lumps too big for use as granules. If it is not mixed sufficiently, it will not granulate, but produce fines or dust.

Viscosity can be determined by many methods. One simple, but effect method is a measurement of the degree of slump in an inverted cone. This can be conducted by filling a cone (of preferably 30cm height and 10cm radius at the mouth) with the material, ensuring the material is level with the top of the cone, inverting the cone onto a horizontal surface and removing the cone, waiting a short time of approximately 10 seconds then measuring the difference between the height of the cone and the pile of material. When the material has a slump of approximately 10% or less of the cone height, it may be discharged from the mixing device. However, once the slump reaches delay in discharging the product may result in the inability of the product to granulate.

Another useful test is the "hand" test which is done as the mixing process continues. A handful of material is taken. When squeezed, if the shape of the handmark is retained then the slump is ready to be measured as described above. Of course, more technical and complex methods of measuring the viscosity of the material will be known to the person skilled in the art, to enable timing to be determined with accuracy.

When the product reaches the desired viscosity, it can then be discharged onto a granulation surface, such as vibrating table placed at an angle to the horizontal, or onto any conventional device, which permits and facilitates the formation of the granules.

Preferably the granulation surface is an inclined vibrating or rotating surface. Preferably P:OPER\M KRSPECII 1777-02-CAP.doc- 16/04103 13the process further includes a drying step, using conventional drying equipment. From this point the material is dry and may be discharged into a bulk store or into containers as finished product.

Preferably the granule size (diameter) is between 0.5 and 5 mm. For agricultural considerations, the optimum granule size lies between 1.5 and 3.5 millimetres with fines constituting less than five per cent by weight of the product mass. Granule size is dependent on reaction time, material and ambient temperature, ambient relative humidity, period of vibration of the granulation surface and angle of the surface to the horizontal.

In a preferred embodiment the method is adapted to a continuous process. It is also suited to automation so that the product can be produced continuously and hence enhance production capacity.

Without wishing to be bound by theory, it is proposed that as a result of the combination calcium and/or magnesium carbonate with a phosphatic material and calcium or ammonium sulphate, multiple reactions ensue. To illustrate these reactions, the following example and discussion utilises calcium dihydrogen phosphate as the phosphatic material, a material comprising calcium carbonate and aqueous ammonium sulphate: Reaction I is the formation of calcium hydrogen phosphate from calcium dihydrogen phosphate: Ca(H 2

P

4 2 CaCO 3 n H 2 0 2 CaHPO 4

CO

2

H

2 0 I The calcium dihydrogen phosphate reacts with water and calcium carbonate to produce calcium hydrogen phosphate, carbon dioxide and more water. The carbon dioxide is released slowly and generates micro-cavities in the eventual product. This porosity of the product is of benefit in the field as it permits water penetration and hence the movement and transport of beneficial ions in the plant rhizosphere. It is noted that in this reaction ifn moles of water are added then n+l moles of water are produced.

P:\OPER\MKR\SPECI\PS1777-02-CAP.doc- 1604/03 14- If permitted to continue the above reaction would result in the formation of calcium phosphate and then hydroxyapatite, by virtue of the water present. These two materials and very hard, insoluble and make the contained phosphorus unavailable to the plant.

Thus it is critical that the formation of large amounts of these compounds is prevented by the formation in situ of calcium sulphate from calcium ions and ammonium sulphate and in reaction II and the subsequent absorption of water in reaction III: Ca 2 (NH4) 2 S0 4 NH4 Ca SO 4 1 II CaSO 4 2H 2 0 CaSO 4 .2H 2 0 III The reaction of sulphate ions with calcium ions present in the heterogeneous reaction mixture produces calcium sulphate which reacts with the water from the first reaction and, over the time of reaction, forms the dihydrate of calcium sulphate and hence removes water, consequently giving rise to the self-granulation, texture and hardness of the product.

(When magnesium sulphate is present, reaction III is also: MgSO 4 nH20 MgSO 4 .nH 2 0, where n is less than 8) The reactions I and II take place simultaneously, although reaction II is quicker. As Ca2+ becomes available from the superphosphate and water is produced from reaction I, reaction II begins slowly and then hastens as the CaSO 4 formed absorbs and reacts with water to form CaSO4.2H 2 0. This reaction is slightly exothermic. The gradual addition of the aqueous ammonium sulphate to the reaction mixture delays reaction II and allows better control of the reactions.

The reactions exemplify le Chatelier's principle to force conversion of Ca(H 2 P0 4 2 to CaHPO 4 by abstracting water so that CaSO 4 CaS04.2H 2 0 and the mixture simultaneously begins drying and granulating. The process also halts further irreversible conversion, in the presence of chemically unbound water, of calcium hydrogen phosphate into calcium phosphate and hydroxyapatite, both of which lack utility in agriculture, by virtue of insolubility and unreactivity, and constitute an economic loss to the manufacturer and to the buyer..

P:\OPER\MKR\SPECI\PSI777-02-CAP.doc-1604103 15 Reactions IV and V produce ammonium phosphates: NH4 H 2 P0 4

NH

4

H

2 P0 4

IV

2NH 4 HP0 4 2

(NH

4 2 HP0 4

V

These reactions form ammonium phosphates by the interaction of the free ammonium ion from the ammonium sulphate and free phosphate ions. The small amount of ammonium phosphates formed may add some strength to the granules and contribute to the availability of phosphate ions.

Thus, in a preferred embodiment, highly water-soluble calcium dihydrogen phosphate is converted into water insoluble but citrate soluble calcium hydrogen phosphate. Citrate solubility is highly desirable as this delivers phosphorus in the plant rhizosphere where the plant itself manufactures citrate ions.

It is to be recognised that the present invention has been described by way of example only and that modifications and/or alterations thereto which would be apparent to persons skilled in the art, based upon the disclosure herein, are also considered to fall within the spirit and scope of the invention.

The invention will now be further described, with reference to the following non-limiting examples.

EXAMPLES

Example 1 kg of CaCO 3 and 25kg of triple superphosphate, with 48% P 2 0 5 analysis, are reduced to 150 micron particle size and mixed in the dry state.

P:IOPER\MKR\SPECI\PS 1777.02-CAP.dc- 16/04/03 -16of (NH 4 2

SO

4 is dissolved in 8.0 L of water and sprayed over the dry mix over a period of 40 minutes while mixing. An extra 20 minutes of mixing in a cement mixer rotating at about 1 Hz is then allowed, until the degree of slump is measured as 10%. The product is then dispensed onto a vibrating table. The temperature at discharge is 400C.

The granules formed are approximately 2.5mm 1 mm in diameter. The pH of the granules as measured by placing 10g in 100ml water is 7.2. The granules comprise about CaHPO 4 and about 30% CaCO 3 and the remainder is binder (CaSO 4 .2H 2 0) and ammonium salts and unreacted superphosphate. The grounds are readily broken with manual force, but sufficiently hard to maintain form on repeated handling.

Example 2 Grapevines A and B of 5 years old were observed for 1 year.

In year 1 yield from A 8.0kg, from B 8.7kg.

In year 2, A was provided with 15g of the fertiliser granules of example 1 spread over 1m 2 at stem.

Yields A: 12.4kg B: 9.3kg In Year 3 no more product was added Yields A: 11.9kg B: Example 3 2 identical rows of onions were planted. One row was given the fertiliser granules of Example 1: Yield from row without product: 6.4kg Yield from row with product: 11.3kg P \OPER\MKR\SPECITPSI 777-02-CAP.doc- 16/O403 -17- Example 4 Leaching in brickies' sand: All triple superphosphate was leached from the sand within 3 months.

Only 20% of the phosphate of the invention was leached in the same period.

Phosphate determined by method of Fiske and Subbarao (The colorimetric determination of phosphorus. J Biol Chem 1925; 66: 375).

Claims (8)

18- THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A granulated fertiliser comprising MgHPO 4 and/or CaHPO 4 2. The fertiliser of claim 1 further comprising MgSO 4 and/or CaSO 4 3. The fertiliser of either claim 1 or claim 2 further comprising MgCO 3 and/or CaCO 3 4. The fertiliser of any one of claims 1 to 3 further comprising NH 4 H 2 P0 4 and/or (NH4) 2 HP0 4 The fertiliser of any one of claims 1 to 4 comprising MgHPO 4 and/or CaHP0 4 in a total amount of at least 20% by weight. 6. The fertiliser of any one of claims 1 to 4 comprising MgHPO 4 and/or CaHPO 4 in a total amount of at least 50% by weight. 7. The fertiliser of claim 5 comprising at least 20% by weight of CaHPO 4 8. The fertiliser of claim 6 comprising at least 50% by weight of CaHPO 4 9. The fertiliser of any one of claims 1 to 8 comprising MgCO 3 and/or CaCO 3 in a total amount of less than 40% by weight. 10. The fertiliser of any one of claims 1 to 8 comprising MgCO 3 and/or CaCO 3 in a total amount of less than 10% by weight. 11. The fertiliser of any one of claims 1 to 10 comprising MgSO 4 and/or CaSO 4 in a total amount of less than 10% by weight. P:\OPCR\MKR\SPECI\PSI 777.02-CAP.do- 16,04/03 -19- 12. The fertiliser of any one of claims 1 to 8, 10 and 11 comprising NH 4 H 2 P0 4 and/or (NH 4 2 HP0 4 in a total amount of less than 10% by weight. 13. The fertiliser of any one of claims 1 to 8 and 10 to 12 comprising Ca 3 PO 4 and/or Ca 3 (P0 4 2 Ca(OH) 2 in a total amount of less than 10% by weight. 14. A granulated fertiliser comprising: MgHPO 4 and/or CaHPO 4 in a total amount of at least 50% by weight; MgCO 3 and/or CaCO 3 in a total amount of less than 20% by weight; SMgSO 4 and/or CaSO 4 in a total amount of less than 20% by weight; NH 4 H 2 PO 4 and/or (NH4)HPO 4 in a total amount of less than 20% by weight; Ca 3 PO 4 and/or Ca 3 (P0 4 2 Ca(OH) 2 in a total amount of less than 20% by weight. The granulated fertiliser of claim 14 further comprising one or more of chelated or unchelated trace elements, plant nutrients, macronutrients, herbicides, fungicides and additives. 16. A method of producing a granulated fertiliser comprising preparing a mixture of a phosphatic material; and a first material comprising calcium carbonate and/or magnesium carbonate; and adding water and a second material comprising ammonium sulphate and/or calcium sulphate hemihydrate in discrete or continuous increments. 17. The method of claim 16 wherein and are mixed together in a dry state. 18. The method of claim 17 wherein and are in the form of particles of substantially uniform diameter. P:\OPER\MKR\SPECI\PS 777-02-CAP.doc- 1604/03

19. The method of claim 18 wherein the particles are less than 200 microns in diameter. The method of any one of claims 16 to 19 wherein the phosphatic material comprises calcium dihydrogen phosphate.

21. The method of any one of claims 16 to 20 wherein the phosphatic material is superphosphate.

22. The method of claim 21 wherein the superphosphate is rock phosphate derived.

23. The method of any one of claims 16 to 22 wherein the second material is added in an anhydrous state, with separate addition of water.

24. The method of any one of claims 16 to 22 wherein the second material is added as an aqueous solution or slurry. The method of any one of claims 16 to 24 wherein the granules produced are between 0.5 and 5 mm in diameter.

26. A granulated fertiliser produced by the method of any one of claims 16 to

27. A method of providing phosphate to a plant which comprises applying to the immediate vicinity of the plant a granulated fertiliser of any one of claims 1 to DATED this 16th day of April, 2003 Christiaan de Vreeze By DAVIES COLLISON CAVE Patent Attorneys for the Applicant