WO2005097947A1

WO2005097947A1 - A soil supplement that has the ability to improve the availability of applied phosphorus for uptake by plants

Info

Publication number: WO2005097947A1
Application number: PCT/AU2005/000501
Authority: WO
Inventors: Peter Warren English
Original assignee: Peter Warren English
Priority date: 2004-04-07
Filing date: 2005-04-06
Publication date: 2005-10-20

Abstract

The present invention is directed to a soil supplement and process for application of the supplement that improves plant uptake primarily of phosphorus that is either present in the soil, or applied to the soil in the form of granular and liquid forms of fertiliser at the time of planting of most crops. The supplement comprises a composition selected from the group consisting of olivine, serpentine, and wollastonite, and can be activated by the addition of acid and/or heat.

Description

A SOIL SUPPLEMENT THAT HAS THE ABILTY TO IMPROVE THE AVAILABILITY OF APPLIED PHOSPHORUS FOR UPTAKE BY PLANTS

FIELD OF THE INVENTION The present invention is directed to materials and processes (such as a soil supplement) that have the ability to improve plant uptake primarily of phosphorus that is either already present in soil, or applied to soils, typically in the form of granular and/ or liquid forms of fertiliser ' and usually at the time of planting of most crops. These materials can be used alone, or in conjunction with simple or multi nutrient phosphorus based fertilisers (phosphorus fertilisers).

BACKGROUND ART The use of phosphorus containing fertilisers to enhance the growth and yield of crops and plants is well known. Phosphorous is an essential element in the proper growth of plants. However, Australian soils are known to be phosphorous deficient. A problem experienced with phosphorus when it is applied to certain types of soils is that it can be tied up, fixed or adsorbed by certain compounds that are present in the soil, and the phosphorous becomes unavailable for plant uptake Phosphorus is particularly tied up by Al and Fe hydrous oxides and/or Ca and Mg carbonates in soils that range from clay to sandy in texture. In some soils the opposite occurs and the phosphorous is leached too readily from the soil before it can be taken up by the plants. Phosphorus is particularly leached down through the soil profile of sandy textured soils that do not contain these oxides or carbonates. Leaching of phosphorous also causes environmental damage by phosphorus entering waterways and promoting algal bloom therein. Much research has been directed to overcoming or at least reducing the tie-up of phosphorus particularly in clay and in sandy textured soils when a phosphorus containing fertiliser is applied. This is achieved by modifying or treating the fertiliser before application. As yet there have been few commercial products that are generally accepted by the agricultural and horticultural industries that have the ability to improve the availability of fertiliser phosphorus to plants, and none that have the ability to release phosphorus from the soil in the absence of fertiliser. The prior art describes several fertiliser compositions comprising known fertilisers in chemical combination with control-release matrices. JP 61-26382A describes a controlled release nitrogen (N₂), phosphorus (P) and potassium (K) fertiliser composition comprising powdered zeolite (silica and alumina complex), allophane (hydrous silicate of aluminium) and sources of N , P and K as aqueous solutions or dry powders. The N₂, P and K are adsorbed by the zeolite/allophane during the manufacturing process of the composition, and released slowly into the soil when applied thereto. US 5,695,542 describes a method of preparing a slow-release fertiliser, the method comprising heating an aqueous mixture of fertiliser; adding zeolite thereto and heating; cooling the mixture and adding a gelatinous organic coating. The gelatinous coating supposedly assists in the slow release mechanism. The composition is added to the soil as a single product. EP 1 070 690 A2 discloses the preparation of slow release nitrogenated fertilisers, comprising mixing a nitrogen compound and a metal silicate, particularly clays. The invention asserts that the slow release action is due to adsorption-absorption of N₂ in silicates, and an insoluble CaSO membrane which forms on the surface of the resultant discrete composition. WO 03/048076 Al discloses a method for production of a solid fertiliser containing zeolite and bentonite (mainly montmorillonite) with adsorbed fertiliser ingredients. The method comprises grinding zeolite ores; mixing them with aqueous solutions of N₂, H₂PO₄; K and trace elements, drying; and mixing the resultant product with bentonite grains. Once again, a discrete fertiliser composition is formed which is added as such to the soil. WO 03/061383 Al discloses a sustained release composition of a biologically active substance adsorbed onto a carrier, with a polysaccharide layer, KOH or NaOH, and a release regulator, being an organic acid. For agricultural compositions, the carrier can be a zeolite and the substance can be a plant growth regulator. The polysaccharide is essential to all compositions of this disclosure to ensure controlled-release of the active components. US 2004/0003636 Al discloses a method for making controlled-release ammonium phosphate fertiliser, comprising adding release control material, being zeolite, montmorillonite, or lignin, to an ammonium phosphate slurry; acidifying the mixture or not; condensing the mixture and granulating the product. Release controlled material can be added to the fertiliser slurry during manufacture, or to the dried fertiliser, which is further processed to the final product. A discrete fertiliser results, which is then added to the soil. A disadvantage with these known methods is that the fertiliser must be applied to the soil, so there is no mechanism to provide a phosphorous retaining/releasing benefit without adding fertiliser. However, it is not possible from the above methods to simply "separate" the phosphorous retaining/releasing benefit from the fertiliser as the two are interdependent on each other. Many of the methods are directed to the slow release of "actives" and do not teach any specific process for phosphorous. Therefore, there would be an advantage if it were possible to provide a soil supplement with greater flexibility of application. There would be an obvious economic yield advantage for improving the availability of phosphorus for crops and plants from soils, where a supplement can be added at any stage of planting or growth of crops, irrespective of the presence or absence of fertiliser. This would both increase the efficiency of added fertilizers, or reduce the amount of fertilizer needed for the same efficacy. It would also reduce the phosphorus available in the soil for leaching from the soil into waterways.

SUMMARY OF THE INVENTION It is an object of the invention to provide a soil supplement that can control phosphorus in the soil and make the phosphorus more available for plant uptake, and which may overcome at least some of the above-mentioned disadvantages or provide a useful or commercial choice. It is a further object of the invention to provide materials and methods which can render phosphorus in soil, or in soil with phosphorus fertilisers, available in a "plant available" form for a longer period of time, and thereby overcome the problem of phosphorus in the soil being "locked away" or leached away from the plant. In one form, the invention resides in a soil supplement for making phosphorus more available for uptake by plants, the soil supplement comprising olivine, serpentine, wollastonite, or a combination thereof. The supplement can be added to the soil in the absence of fertiliser, or before, simultaneously with or after addition of a phosphorus fertiliser, and can function to hold or desorb the phosphorus or phosphate ion present in the soil in a plant available form. It is preferred that the supplement according to the first form of the invention is activated. While not wishing to be bound by theory, it seems that the activation enables less supplement to be used. In a second form, the invention resides in a modified soil supplement formed by reacting a metal silicate material with an acidic solution to form a soil supplement, and further reacting the soil supplement with an alkali material and water to form the modified soil supplement. In a third form, the invention resides in a soil supplement comprising activated partially or completely serpentinised dunite, peridotite, harzburgite, wehrlite, Iherzolite, gabbro, basalt, calc silcates, or a combination thereof. This supplement can be added to the soil in the absence of fertiliser, or before, simultaneously with or after addition of a phosphorus fertiliser, and can function to hold or desorb the phosphorus or phosphate ion present in the soil in a plant available form. In a fourth form of the invention, there is provided a method of improving the growth rate and yield of plants, the method comprising adding a soil supplement to the soil at or before planting, the soil supplement comprising activated metal silicates, oxides, sulphates, or a combination thereof, the supplement being of the type that can bind and slowly releases phosphorus in a plant-available form. The supplement of the invention can be used in any type of soil, soil mixture, soil replacements and the like that would benefit from a better uptake of phosphorus, and it is considered that the supplement may be most useful in clay and sandy textured soils. An advantage with the supplement is that the amount of phosphorus that needs to be added to the soil can be reduced, as there seems to be reduced leaching of phosphorus from the soil and/or reduced "locking away" of the phosphorus in the soil. The plants that can benefit from the supplements will typically comprise crop plants such as sugarcane, wheat, barley, canola, cotton, rice, but it should be appreciated that no particular limitation should be placed on the type of plants that can benefit from the supplement, and the plants may include crops for use as fodder, crops for use as human food, crops for medicinal use, plants for any other type of use including medicinal use, flowers, fruit bearing plants, vegetables, bulbs, tubers, trees, shrubs, bushes, seedlings, and the like. With regard to all forms of the invention, the term "can be added" is to be interpreted broadly and may include a material that is coated or otherwise applied onto, over, or in association with a phosphorus fertiliser; a material that is added with a phosphorus containing fertiliser; a material that is mixed into a phosphorus fertiliser and the like. The term "can be added" can also extend to cover a situation where the material is applied to the ground prior to, together with, or even after a phosphorus fertiliser in such a manner that the material can function to hold the phosphorus or the phosphate ion in a plant available form for a period of time which may be longer than if the fertiliser is applied without the material. The supplement may also be used without any addition of a phosphorus fertiliser if the soil contains sufficient phosphorus and there is an advantage in making the phosphorus more available to plants using the supplements which is the subject of the present invention. With regard to all forms of the invention, the term "plant available form" includes that the phosphorus or phosphorus ion is either adsorbed, absorbed or otherwise held by or released from the supplement in a form which is suitable for uptake by the plant. Referring to the first form of the invention, the supplement may comprise at least one of each material but preferably comprises at least two of the materials. If more than one material is used, the amount of each material may be between 5%-95%. The amount of each material may depend on the cost of the material, the availability of the material, the reactivity of the material, handling problems that may be presence with a particular material, the type of soil to which the supplements will be added, the type of plant and the like. The materials may be granulated or otherwise reduced in particle size either prior to mixing or together. The particle size will depend on the ability of the material to make the phosphorus into a more plant available form. It is considered that the material should be granulated or otherwise reduced in particle size to less than 1 mm, and this range may be between 0.001 mm-1 mm and typically approximately 0.1 mm. Olivine is the name for a series of minerals between two end members, fayalite and forsterite. Fayalite is the iron rich member with a pure formula of Fe₂SiO . Forsterite is the magnesium rich member with a pure formula of Mg₂SiO₄. The two minerals form a series where the iron and magnesium are substituted for each other without much effect on the crystal structure. Fayalite, due to its iron content has a higher index of refraction, is heavier and has a darker color than forsterite. Otherwise they are difficult to distinguish and virtually all specimens of the two minerals contain both iron and magnesium. For simplicity sake and general public recognition, they are often treated as one mineral, olivine. Olivine is found in ultramafic igneous rocks and marbles that form from metamorphosed impure limestone. Mafic is a word that is used to define igneous rocks with a high iron and magnesium content. The olivine minerals have a high melting point and are the first minerals to crystallize from a mafic magma. Some ultramafic rocks can be composed of almost all olivine and these are called dunites or peridotites. Notable occurrences of olivine are numerous and include the ancient source of Zagbargad Island in the Red Sea off the coast of Egypt; Mogok, Myanmar (formerly known as Burma); South Africa; Ural Mountains, Russia; Kohistan, Pakistan; Norway; Sweden; France; Minas Gerais, Brazil; Eifel, Germany; Chihuahua, Mexico; Ethiopia; Victoria, Australia; China and Salt Lake Crater, Oahu, Hawaii; North Carolina; New Mexico and Peridot Mesa, San Carlos Apache Reservation, Gila County, and Arizona, USA. Serpentine, (Mg,Fe)₃Si₂O₅(OH) , magnesium iron silicate hydroxide, is a major rock forming mineral and is found as a constituent in many metamorphic and weather igneous rocks. It often colors many of these rocks to a green color and most rocks that have a green color probably contain some serpentine. Serpentine is actually a general name applied to several members of a polymorphic group. These minerals have essentially the same chemistry but different structures (polymorphs). Serpentine's structure is composed of layers of silicate tetrahedrons linked into sheets. Between the silicate layers are layers of Mg(OH)₂. These Mg(OH)₂ layers are found in the mineral brucite and are called brucite layers. The stacking of the brucite layers is the main reason for the multiple polymorphs. The stacking is not perfect and has the effect of bending the layers. In most serpentines, the silicate layers and brucite layers are more mixed and produced convoluted sheets. In the asbestos varieties the brucite layers and silicate layers bend into tubes that produce the fibers. Notable occurrences of serpentine include Nal Antigorio, Italy; Russia;

Zimbabwe; Switzerland; North Carolina, California, Rhode Island and Arizona, USA and Quebec, Canada. Wollastonite is a common mineral in skarns or contact metamorphic rocks. Wollastonite forms from the interaction of limestones, that contain calcite, CaCO₃, with the silica, SiO₂, in hot magmas. This happens when hot magmas intrude into and/or around limestones or from limestone chunks that are broken off into the magma tubes under volcanoes and then blown out of them. It forms according to the following reaction: CaCO₃ + SiO₂ → CaSiO₃ + CO₂ Wollastonite is named after the English chemist and mineralogist W. H.

Wollaston (1766 - 1828). Its actual mineralogical name is wollastonite - IT. The IT is for the Triclinic symmetry of the most common and first described wollastonite mineral. The reason the IT is needed is to distinguish it from the much more rare wollastonite - 2M, also known as parawollastonite. These minerals are polymorphs, which means that they have the same composition, CaSiO₃, but different structures. All specimens named simply wollastonite are most likely wollastonite - IT. Notable occurrences include Willsboro and other sites in New York, Texas, California and Franklin, New Jersey, USA; the volcano Monte Somma, Vesuvius, Italy; Perheniemi, Finland; Banat, Rumania; Saxony, Germany; Chiapas, Mexico; Greece; China; Ontario and at the Jeffrey Mine, Asbestos, Quebec, Canada and Tremorgio, Switzerland.

In the second form of the invention, the metal silicate material of the supplement preferably comprises iron silicate, magnesium silicate and/or calcium silicate. When reacted to form the fertiliser supplement, the cations (magnesium and calcium) are also considered useful as a plant supplement. The metal silicate may be selected from basic rocks, ultra basic rocks, ultra mafic rocks, or calc-silicate rocks (wollastonite rich). Suitably, the metal silicate is in the form of a crushed solid material such as crushed rock. To facilitate reaction with an acid, it is preferred that the metal silicate has a high surface area and therefore has been finely crushed for instance ranging in size from 3 mm, to fines preferably less than 0.25mm, most preferably minus 0.1mm. In all forms of the invention, "activated" refers to a modification of the substrate which results in enhanced activity with respect to the adsorption/adsorption of phosphorous or phosphate ion, and slow release thereof. Activation can be achieved by any suitable means, including chemical means such as oxidation or reduction, or mechanical means such as heating and/or compression. Activation is preferably by addition of acid to the substrate or heating of the substrate. Most preferably, activation comprises a combination of acid addition and heating of the substrate. With regard to the acid activation, it appears that a better product is obtained if the reaction is carried out in a dry to moist solid state. It is therefore preferred that the liquid components of the reaction are kept to a minimum such that the reaction is not carried out in an aqueous solution or slurry but is instead carried out in a substantially solid state . A preferred acid is sulphuric acid which is preferably in the form of a concentrated sulphuric acid. A concentration of sulphuric acid of between 50%- 100% by weight is most preferred. Sulphuric acid is preferred as it results in the creation of cation sulphate such as magnesium and/or calcium sulphate which are useful to plants. It is considered that other acids may be used instead of or with sulphuric acid, these including nitric acid, phosphoric acid, hydrochloric acid or any other acid considered appropriate by one of skill in the art. The mixing ratio of the metal silicate material to the acid is preferably such that the solid metal silicate material is in the larger amount to ensure that the reaction is carried out in a substantially solid or dry state. Mixing ratios may be between 2:1 to 10:1 of metal silicate material to the acid, but are most preferably 4:1 to 7:1. It is considered that a solid state reaction results is an acidolysis process, as opposed to a hydrolysis process which would occur if the reaction was carried out in a substantially aqueous manner. When activation is by heating, the metal silicate material can be preheated such that most or substantially all the water of crystallisation is removed from the material prior to reacting with the acid. If the metal silicate is a rock containing olivine and/or serpentine, the rock may be preheated to a temperature of between 350°C-1500°C, with the proviso that the temperature is kept below a temperature which would degrade the rock. If required, the rock may be cooled prior to being reacted with the acid. The rock may be crushed either prior to or after the heating step. In a most preferred form, the metal silicate is activated by both heat and acid treatment.

The soil supplement may contain a percentage of mono silicic acid as well as amorphous silica depending on the amount of moisture in the soil supplement or processed soil supplement after it has been granulated or pelletised. In the second form of the invention, the soil supplement may be reacted with an alkali material and a minimum of water such that the pH of the modified or processed fertiliser supplement is in the order of 6-8 to provide a more pH neutral substance. It is believed that this modified or processed fertiliser supplement contains activated silicates and amorphous silica that can adsorb or otherwise become associated with phosphorus from solution in such a manner that the phosphorus remains in a plant available form for a longer period. The alkali material may be selected from the group consisting of: potassium bicarbonate; potassium carbonate; sodium carbonate; calcium hydroxide; potassium hydroxide; sodium hydroxide; magnesium hydroxide; calcium carbonate; magnesium carbonate; and calcium carbonate. A convenient source of calcium carbonate is limestone. A convenient source of magnesium carbonate is magnesite. A convenient source of calcium carbonate and magnesium carbonate is dolomite. The ratio of mineral silicate to alkali can be from 5:1 to 20:1, but is preferably from 10:1 to 14:1. The soil supplement and the modified or processed soil supplement can be in the form of an additive to normal phosphorus fertilisers. It can be compounded with phosphorus fertilisers, coated onto phosphorus fertilisers or simply mixed with phosphorus fertilisers. A preferred fertiliser is granular phosphorus fertiliser. Useful fertilisers include, but are not limited to, triple-superphosphate, phosphoric acid, mono- ammonium phosphate, di-ammonium phosphate, and ammonium polyphosphate. Typically, the soil supplement is granulated or pelletised. A typical granule size range can be between 1 mm- 10 mm. although this can vary depending on the type of applications, the type of soil, the type of plants, the application machinery and the like.

The soil supplement can be added to the soil in any suitable amount, and in any suitable combination with phosphorus fertilisers. Suitably, 5-500 kg per hectare of the supplement can be added to the soil, but preferably 15-50 kg per hectare is added. The ratio of supplement to fertiliser, when a combination is desired, can be any number from

0 to 1, but is suitably between 0.1 and 0.5, and preferably 0.2 to 0.4. It is found that the soil supplement provides improvements to plant growth if the supplement is added to the soil with or without the phosphorus fertiliser at planting. The supplement can be applied after planting but it appears that this may provide a less beneficial effect to the plant. Suitably, the method can comprise addition of a soil supplement as described above, that is comprising activated magnesium silicates, oxides and sulphates, iron silicates, oxides and silicates, calcium sulphates and carbonates, to soil with or without fertiliser, at the time of planting crops. For the purposes of the third form of the invention, dunite is an igneous, plutonic rock, of ultramafic composition, with coarse grained or phaneritic texture. The mineral assemblage is typically greater than 90% olivine with minor pyroxene and chromite. Dunite is the olivine rich endmember of the peridotite group of mantle derived rocks. Dunite and other peridotite rocks are considered to represent the Earth's mantle. Dunite is rarely found within continental rocks, but where it is found, it typically occurs at the base of ophiolite sequences where slabs of mantle rock from a subduction zone have been thrust into continental crust by obduction during continental or island arc collisions (orogeny). Peridotite is a dense, coarse grained ultrabasic rock, consisting mainly of the minerals olivine and pyroxene. Peridotite is also a group of mantle derived igneous rocks. They all are ultramafic or ultrabasic meaning the contain less than 45% silica and are high in iron and magnesium.

Members of the peridotite family include: dunite, harzburgite, Iherzolite, pyroxenite. These peridotite rocks are usually subducted back into the mantle in subduction zones. However, they can be emplaced into or overthrust on continental crust during continental collisions (orogenies) or island arc collisions by the process of obduction. The occurances of these peridotites along with other mafic gabbros and basalts within continental crust are referred to as ophiolites. The rocks of the peridotite family are uncommon at the surface and are highly unstable. Many, if not most, surface outcrops have been highly altered by retrograde metamorphism to serpetinites in which the pyroxenes and olivines have been converted to green serpentine along with amphibole minerals. This hydration reaction involves considerable increase in volume with concurrent deformation of the original textures. Harzburgite is composed of olivine, enstatite, and minor chromite; Iherzolite of olivine, enstatite, diopside, and minor chromite and/or pyrope garnet, and pyroxenite of orthopyroxene and/or clinopyroxene, with lessor amounts of olivine, garnet, and spinel. Gabbro consists mainly of pyroxene and plagioclase, with greater amounts of pyroxene than plagioclase or equal amounts of both. Olivine is often present, as well as grains of iron ore (magnetite and/or ilmenite) and bronze-coloured biotite. It is found as plutons and similar large bodies, but not as large as those of granites. Also as large sheets, often containing valuable ore deposits (e.g. Lake Superior deposits). Gabbro is found in Scotland, Scandinavia, Canada, England, Germany, USA (New England, New York , Minnesota, California and lesser amounts in other states). Basalt is composed of pyroxene and plagioclase, with pyroxene appearing in greater amounts than plagioclase or equal amounts. Olivine is also often present, as well as grains of iron ore (magnetite and/or ilmenite) and bronze-coloured biotite. It may contain olivine or pyroxene nodules brought up from depth. Basalt forms as lava, flows, sills and dykes associated with volcanoes. It is found worldwide, but particularly in Canada (Lake Superior has vast copper deposits), Greenland, India (Deccan traps), Iceland, Scotland, USA (Montana, western states). Calc-silicate is a metamorphic rock consisting mainly of calcium-bearing silicates, such as diopside and wollastonite, and formed by metamorphism of impure limestone or dolomite; associated with skarn-type mineral deposits. While not wishing to be bound by theory, it appears that the material keeps the phosphorus in a plant available form for a longer period of time than otherwise would be possible by forming some type of "association" with the fertiliser and/or the phosphorus and/or the phosphate ion. The "association" may comprise the ability of the material to adsorb some or all the soil or fertiliser phosphorus, thereby holding the phosphorus or phosphate ion in a plant available form. By this means the phosphorus is neither locked up by the soil nor leached from the soil. It is also believed that the process of carrying out the synthesis of the soil supplement in a substantially solid moist state, activates the metal silicates and oxides that can adsorb phosphorus from solution, keeping it in a labile state in the soil, ready to be taken up by plants. However, no particular limitation should be placed on the method by which the fertiliser supplement may be formed. In order that the invention may be more readily understood and put into practice, one or more preferred embodiments thereof will now be described. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Having broadly described the invention above, non-limiting examples of the supplement, their synthesis, and their efficacy, will now be given.

Example I: Soil Supplement from Acid Activation of Dunite A 50g sample of dunite (crushed to a particle size of minus 0.10 mm) and 5g of the alkali material (limestone, finely ground to minus 0.1mm), is well mixed and reacted in an exothermic reaction with lOg of sulphuric acid (98% by weight sulphuric acid) and lOg of water until reacted. The small amount of water and the use of concentrated acid results in a solid state reaction, as opposed to an aqueous solution or slurry. The product can be used as a soil supplement and contains activated magnesium and iron oxides and silicates, amorphous silica, magnesium sulphate and calcium sulphate. The products of this reaction typically have a pH of 6.5. They now have the ability to adsorb phosphorus or the phosphate ion from solution.

Example 2: Soil Supplement from Acid and Heat Activation of Dunite The metal silicate in this example is a dunite rock which has been preheated to a temperature of 600°C for 15 minutes and then crushed to a particle size of about minus 0.10 mm. The acidic solution is sulphuric acid solution (98% by weight sulphuric acid). The dunite can be completely or partially serpentinised and may have up to 10% free moisture (H₂0) added to it after it has been crushed to about minus 0.10 mm. A 50g sample of dunite is mixed and reacted in an exothermic reaction with 8g of the sulphuric acid. The products of this reaction typically have a pH of 3-4. The products are then mixed with 4g of alkali material consisting of about 2g of calcium carbonate and about 2g of potassium bicarbonate, and about 4g of water, until reacted. The product can be used as a soil supplement and contains activated magnesium and iron oxides and silicates, amorphous silica, magnesium sulphate and calcium sulphate and can be applied directly to soils after it has been granulated or can be added with or coated onto known phosphate fertilisers. The pH of the material is typically 7-8.

Example 3: Efficacy of Soil Supplement in Laboratory Samples The following experiment shows how phosphate (P) is first adsorbed and then desorbed by the material made by the process of the invention. To 0.1 g of acid activated material, and dual activated material (heat of 400°C and acid), both prepared as per the invention, was added either 50 or 100 μg P in liquid solution, corresponding with a loading of 500 or 1000 μg P/g of each source respectively. The mixtures were allowed to react by shaking and then allowing them to stand for 24 hours. The solid was filtered from the solution and dried. This material thus contained the adsorbed P, and the amount of P left in the solution was measured. Table 1 below shows the amount of P first adsorbed by the material used and then the amount desorbed over 4 desorption cycles, over a period of 30 days. Desorption was achieved by adding 20 mL of 0.01 M CaCl₂ to the dried solid material in a tube and shaking for 1 hour prior to centrifugation and analysis for P in supernatant using malachite green. The material was allowed to dry completely between desorption cycles. The amount of P left in the entrained solution after 30 days, accounts for <2 μg. Over the cycle of a crop that is in the ground for 2-12 months it can be expected that a large amount of the applied P will be made available to the soil solution and consequently any plants growing in this environment. The experiment shows that the dual activated (heat and acid) material gives the P back to the soil solution more readily. This is an advantage in some applications, for example crops grown in short life cycles (horticulture). The acid activated material is appropriate for crops grown over longer cycles (cereals, oil seeds and sugarcane). Table 1 Amount of P adsorbed (μg P/g) and subsequently desorbed (μg P/g) in 4 desorption cycles. Acid Activated Material Acid & Heat Activated Material P added (μg/g) 500 1000 500 1000 Adsorbed 490 933 43: 767

1^st desorption 5 38 46 67 2^nd desorption 5 38 19 34 3^rd desorption 24 42 29 44 4^th desorption 49 22 54 28 Example 4: Growth Response Rates The processed soil supplement of Example 1 was applied to the soil using barley both with and without the planting phosphorus fertiliser (of conventional type - DAP). The supplement was added at a rate of 50 kg per hectare. A root mass increase became clearly evident from emergence, and at 60 days after planting, the root mass response was estimated to be 20-50% greater with the treatments that had the soil supplement added to the phosphorus fertiliser. There was no visible difference in growth of the treated and untreated crop, until harvest. The treated area was noticeably darker at harvest and yielded 26% greater than the untreated areas. A person skilled in the art will understand that there are many other rocks which contain olivine and/or serpentine. Rocks with lower percentage of olivine, such as partially or completely serpentinised peridotites, harzburgites, wehrlites, iherzolites or basalts may be substituted for dunite in the above examples.

Example 5: Field Trials The supplement was applied as granules mixed with granules of either Mono

Ammonia Phosphate (MAP) or Di Ammonium Phosphate (DAP), being two forms of granular phosphorus fertiliser used in broadacre cropping throughout Australia and other countries. 80DAP means that 80kgs of DAP per hectare was applied at planting and 25Sv means 25kgs of the supplement was first mixed with either the DAP or MAP and then both were applied at a rate of 105 kgs per hectare. The results are summarised in Figure 1. From Figure 1, it is clear that the supplement statistically increased the yield by 7% in the 80DAP+25Sv rate over the standard P application rate of 80 DAP. The other result shows that the supplement can substitute for DAP and still gives a 3% increased yield over the standard application of 80DAP, with 25% less P. Figure 2 illustrates the yield results in canola: Figure 2 shows that the 60MAP+30Sv rate has given a 23% increase in yield over the control rate of 90MAP showing that the same total weight of fertiliser can be applied, but having 30% less P. Figure 3 illustrates the increased yields in oats: Figure 3 shows that the 55DAP+15Sv rate of fertiliser has given a 10% increased yield with 27% less P applied to the soil. All of the above results show that the supplement can replace some of the standard phosphorus fertiliser application and still give the same or increased yield. This has two beneficial outcomes. Firstly, it means that farmers can obtain increased yields using the same weight of fertiliser, and secondly it is environmently important that less phosphorus is applied to many farming situations. For example, it will mean less run off of phosphorus from farms into creeks and rivers thus protecting the environment from algal blooms that result from increased phosphorus levels in these waterways, and subsequently dams and oceans. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the present invention without departing from the spirit or scope of the invention as broadly described. The above examples are therefore to be considered in all respects illustrative and not restrictive.

Claims

1. A soil supplement to make phosphorous more available to plants, the supplement comprising a composition selected from the group consisting of olivine, serpentine, and wollastonite.

2. The soil supplement of claim 1 wherein the composition is activated prior to use by contacting the composition with an acid.

3. The soil supplement of claim 2 wherein the acid is sulphuric acid.

4. The soil supplement of claim 3 wherein the acid concentration is from 50 to 100% sulphuric acid by weight.

5. The soil supplement of claim 2 wherein the mixing ratio of composition to acid is between 2:1 and 10:1.

6. The soil supplement of claim 1 wherein the composition is activated prior to use by heat.

7. The soil supplement of claim 6 wherein the temperature of thermal activation is from 350°C to 1500°C, with the proviso that the temperature does not cause degradation of the composition.

8. The soil supplement of claim 6, additionally comprising an alkaline material.

9. The soil supplement of claim 8 wherein the alkaline material is selected from the group consisting of limestone, magnesite, dolomite, potassium bicarbonate; potassium carbonate; sodium carbonate; calcium hydroxide; potassium hydroxide; sodium hydroxide; magnesium hydroxide; calcium carbonate; magnesium carbonate; and calcium carbonate.

10. A fertilizer supplement comprising the soil supplement according to claim 1 , and a fertiliser.

11. The supplement of claim 10 wherein the fertiliser is a phosphorus-containing fertiliser.

12. A soil supplement to make phosphorous more available to plants, the supplement comprising a composition selected from the group consisting of dunite, peridotite, harzburgite, wehrlite, Iherzolite, gabbro, basalt, and calc silcates.

13. A method of improving the growth rate and yield of plants, the method comprising adding a soil supplement to soil at or before planting of the plants in the soil, the soil supplement comprising a composition selected from the group consisting of metal silicates, oxides, and sulphates, with the proviso that the supplement binds and slowly releases phosphorus in a plant-available form.

14. The method of claim 13 wherein a phosphorus-containing fertiliser is added to the soil before, during or after addition of the soil supplement.