WO1992006056A1

WO1992006056A1 - Fertilizer coating process

Info

Publication number: WO1992006056A1
Application number: PCT/AU1991/000459
Authority: WO
Inventors: Roderick David Bruce Lefroy; Graeme John Blair
Original assignee: The University Of New England
Priority date: 1990-10-05
Filing date: 1991-10-04
Publication date: 1992-04-16

Abstract

A process for producing coated fertilizer granules wherein water-soluble adhesive and an additive are added sequentially to fertilizer granules to form multiple layers of adhesive and additive.

Description

FERTILIZER COATING PROCESS TECHNICAL FIELD

The invention relates to a process for coating fertilizer granules for the inclusion of additives such as additional nutrients, herbicides and insecticides. BACKGROUND ART

Fertilizers are materials which supply mineral nutrients to soil. Some nutrients such as carbon, hydrogen, nitrogen, oxygen, phosphorus, sulphur, potassium, calcium and magnesium, are required in relatively large quantities while other nutrients such as iron, manganese, zinc, copper, molybdenum, boron and chlorine are only needed in small amounts. A balanced supply of all essential nutrients is required for healthy growth of plants. Four major fertilizers are readily and relatively cheaply available on the world market, these being urea, monoammonium phosphate, diammonium phosphate and triple superphosphate. As the requirements for nutrients vary greatly with the type of soil, climate and type of agriculture, incorporation of additional nutrients at the time of manufacture is not a practical proposition. A method for tailoring fertilizers for specific local requirements at or nearer the point of use is therefore required. The addition of herbicides and/or insecticides may also be desirable.

In the fertilizer industry, a method for coating granules involves chemical reaction between chemical compounds in the granules to be coated and additives to be affixed to the granules. This method is specific to certain fertilizers and certain additives so that the desired chemical reaction will occur. Australian Patent No. 601099 by HI-FERT PTY. LIMITED (hereinafter referred to as the "HI-FERT Patent") discloses a method of coating phosphate containing fertilizer granules for the incorporation of sulphur or micro-nutrients. The surface of the fertilizer granules is rendered tacky by addition of water or an aqueous solution of a salt selected from the group consisting of sulphates and phosphates of ammonium or potassium and mixtures thereof. Elemental sulphur or micro-nutrients are added thereafter and if water only were used in the first step of the process, a salt is now added selected from the group consisting of water soluble sulphates and phosphates of ammonium or potassium, water soluble sulphates of metallic trace elements and mixtures thereof. The coating process is designed such that there is chemical interaction of the chemical constituents in the granules with the chemicals employed in the establishment of the coating and some of the nutrients to be incorporated into the coating, thereby ensuring that the coating is an intimate part of the original granule.

Oil and wax have been used to stick nutrients onto the surface of fertilizer granules but the aliphatic and hydrophobic nature of the compounds prevents them from establishing a tenacious coat on the surface of the fertilizer granules.

Another method for coating granules involves using a binder to attach nutrients to fertilizer granules. US patent No 3353947 assigned to American Cyanamid Co discloses a method for preparing a coated granular fertilizer comprising mixing finely divided nutrient particles of size passing through a -100 mesh to -400 mesh Tyler screen (150μm to 38μm) , with granular fertilizer of size passing through a -5 mesh to +20 mesh screen (4000μm to 775μm)and adding an aqueous solution of conditioner and mixing until adhesion of nutrient to fertilizer is attained. The conditioner is selected from water-soluble sugars, alkali metal lignin sulfonates and water-soluble fertilizers. This patent discloses an alternative method whereby the fertilizer is admixed with conditioning solution prior to introduction of nutrient.

US patent No 3938469 also assigned to American Cyanamid Co discloses a method for preparing coated granular fertilizer wherein a falling curtain of granular fertilizer is sprayed with a binder and then mixed with finely divided nutrient in a mixing chamber. Application of binder to a curtain of fertilizer is considered to enhance the uniformity of coating by the binder and subsequently by the nutrient. DISCLOSURE OF THE INVENTION

The present inventors have found an alternative method that provides good adhesion of additives to fertilizer granules and ready release of additives when the coated granules are contacted with moisture in soil.

The present invention provides a process for producing coated fertilizer granules comprising the steps of a) spraying fertilizer granules with a water-soluble adhesive to establish a layer of adhesive on said granules, b) adding a first portion of an additive to coat said granules, c) spraying the fertilizer/additive granules again with the water-soluble adhesive to establish a further layer of adhesive thereon, d) adding a further portion of additive to coat said fertilizer/additive granules, and e) maintaining sufficient movement of the granules throughout the process until the adhesive is substantially dry so as to prevent agglomeration. Steps c) and d) may be repeated to form third and subsequent layers of additive. A heating step may be .included after the final addition of additive to promote drying of the adhesive. Heating steps may also be included after one, some or all of the intermediate additions of additive.

Fertilizer granules which may be used include urea, monoammonium phosphate, diammonium phosphate and triple superphosphate.

The additive may include nutrients such as elemental sulphur, molybdenum, copper, zinc or boron and may include herbicides and insecticides. The amount of and nature of the additive may be selected to suit the soil type, climate and type of agriculture in connection with which the coated granule is to be used.

Each layer of additive is preferably about 0.1-10% by weight of the weight of the fertilizer, more preferably 0.1-5% and most preferably 0.1-3% by weight. The total amount of additive is preferably 0.2 to 30% by weight, more preferably 0.2 to 20% by weight and most preferably 0.2 to 10% by weight.

The adhesive is water soluble and selected such that the additives will be readily released into the soil. Suitable adhesives are polyvinyl alcohol and sodium lignosulphonate. The amount varies with adhesive type and viscosity and application is preferably in short bursts to avoid agglomeration. The coating of the granules comes about by physical bonding between the granule, adhesive and additives, rather than a chemical bonding. The term "physical bonding" as used herein implies that binding occurs without new chemical compounds being formed during the binding process.

It has been found that the multiple layering of adhesive and additive enhances adhesion to the fertilizer granules and improves the rate of release into soil in the presence of moisture. The present invention- is also directed to a coated fertilizer granule when produced by the method described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a graph of percent of coated granule lost against rotating time in minutes comparing coated granules of the present invention with those of the HI-FERT Patent.

Fig. 2 is a graph of percent release of sulfur from coated granules against time in hours comparing coated granules of the present invention with those of the HI-FERT Patent.

BEST MODE FOR CARRYING OUT THE INVENTION

So that the invention may be more clearly understood, there follows an example of preparation of fertilizer granules coated with elemental sulfur and comparative examples of the effectiveness of the coated fertilizer granules of the present invention. EXAMPLE; PRODUCTION OF SULFUR COATED FERTILIZER GRANULES

The coating apparatus consisted of: a) a coating pan (285mm diameter and 195mm depth), b) adhesive spray (a galvanized thick-walled pipe connected to a spray solenoid and a compressed air supply (5000kPa)), c) drier (which supplies 45°C, during drying process), d) electrical connections to a computer to control the coating operation, e) a hood of a large exhaust fan.

The apparatus was arranged as described by Scott, J M, (1986), "Seed Coating as an Aid to Pasture Establishment", PhD Thesis, University of New England. The adhesive was a low viscosity polyvinyl alcohol

(Gelvatol 40-10) diluted in hot distilled water in a glass container. The adhesive concentration was 12%(w/v).

180g of TSP (triple superphosphate granules) and 20g of fine particle elemental S (particle size <- lOOμrn) was used. Before the coating operation was started, the adhesive reservoir was filled with adhesive. The filling valve was closed and pressurised to the desired pressure (5000kPa) . A command to the computer commenced the coating process and the coating steps were as follows: Step 1 a) place the TSP granules into the rotating pan, b) spray adhesive (5x0.4ml), c) solid addition (add approx 5g of elemental S into the pan) , d) wait for 1 minute, e) spray adhesive (5x0.4ml), f) solid addition (add approx 5g of elemental S), g) wait for 1 minute h) drying process (approx 5 minutes) and then continue to step 2. Step 2 a) spray adhesive (5x0.4ml) b) solid addition (add approx 5g of elemental S), c) wait for 1 minute, d) spray adhesive (5x0.4ml), e) solid addition (add approx 5g of elemental S), f) wait for 1 minute, g) drying process (approx 15 minutes), h) the end of the coating process.

Coated granules were produced in a similar way using sodium lignosulfonate as the adhesive, at a concentration of 10% w/v.

The strength of the coated fertilizer which was made using the device described above, was tested in a friabilator and leaching bed. The agronomic effectiveness of this product was investigated under flooded and non-flooded rice and under pasture conditions. COMPARATIVE EXAMPLE 1 - MECHANICAL TEST A 30cm diameter and 4cm deep of perspex cylinder ( "friabilator") was set up to rotate at 25rpm on its central axis in a vertical plane (Scott, 1986). During each rotation the material placed in the friabilator was lifted by a small plate attached to the edge of the cylinder, dropped in a distance of approximately 15cm and then rolled for approximately 3/4 of the circumference until the plate was encountered again.

Four replicates of 20g sample of each coated fertilizer granules [TSP-S with PVA (PVA), TSP-S with sodium lignosulfonate (LS) and TSP-S manufactured by

Hi-Fert Pty Ltd, Portland, Victoria, Australia (HF) ] were tumbled separately in the friabilator. Rotation was stopped after 1, 2, 4, 8, 16 and 32 minutes. At each of these times the material was sieved through a 20 mesh (841μm) screen. The fine material was removed, weighed and the coarse material returned for further testing. Four samples of each fertilizer were tested.

The amount of particles lost from the HF granules increased with time, and the rate and extent of loss differed from LS and PVA. No significant differences were observed between the PVA and LS adhesives. The highest particle loss was recorded in the HF (Fig. 1). COMPARATIVE EXAMPLE 2 - LEACHING EXPERIMENT

2g samples of each coated fertilizer [TSP-S with PVA (PVA), TSP-S with sodium lignosulfonate (LS) and TSP-S by HI-FERT (HF) ] were placed in a 4cm diameter plastic vial between nylon mesh (500μm) . Droplets of deionized water were pumped onto the fertilizer at the rate of 20ml min^" through three 1.5mm internal diameter tubes. The solution and fine material ran to waste. The coated fertilizer remaining between the nylon mesh was collected after 3, 6, 12, 24, 36, 48 or 96 hours, dried at 30°C, weighed and analyzed for total and elemental S. The experiment was repeated three times. The amount of elemental S remaining in the coated granules decreased with time. The loss of elemental S from the PVA and LS did not differ significantly and was greater than the loss from the HF (Fig. 2).

COMPARATIVE EXAMPLE 3 - SULFUR SOURCES FOR FLOODED AND NON FLOODED RICE

An S-deficient granitic Aquic Haplustalf soil from Uralla, NSW, was collected from an unfertilized pasture site, air-dried and passed through a 3mm sieve before being used in the experiment. 6.6kg of the soil was placed in each pot.

Two nested experiments were undertaken in a heated glasshouse at the Department of Agronomy and Soil Science, University of New England, Armidale, NSW, Australia.

The components of the experiment were S source and S rate, flooded and non-flooded soil conditions and two consecutive crops. Each component consisted of three replicates of a randomized block design.

The first experiment was a study of plant response to S application rate, where S was applied at rates of 0, 5, 10 and 20 kg S ha~ as gypsum and yield response determined at active tillering (AT) 59 days after transplanting (dat), maximum tillering (MT, 89 dat) and maturity (M, 144 dat) . These treatments were applied to pots which were maintained under both non-flooded (field capacity) and flooded conditions (Table 1) .

The treatments in the major experiment consisted of the factorial combination of 10 fertilizers including a control and three times of harvesting (AT, MT and M) . The fertilizer treatments included six commercial S-containing fertilizers: gypsum (G) , elemental sulfur (ES) of particle size -^ 0.01mm, urea-S melt (US) manufactured by Cominco Ltd, Calgary, Alberta, Canada, sulfur coated urea (SCU) manufactured by Tennessee Valley Authority, National Fertiliser Development Center, Muscle Shoals, Alabama, USA, TSP-S HI-FERT (HF) manufactured by Hi-Fert Pty Ltd, Portland, Victoria, Australia and S-bentonite (SB) manufactured by DegraSul Fertiliser Production Ltd, Calgary, Alberta, Canada. Tnese fertilizers were compared with three TSP-S coated fertilizers made using a rotating drum seed-coating device in accordance with the example. Slack wax (SW), Sodium lignosulfonate (LS) and polyvinyl alcohol (PVA) were used as adhesive materials to bind elemental S (with particle size less than 0.01mm) to the surface of TSP granules. The S was applied at the rate of 10% of total TSP-S weight. Each of the nine products was applied at a rate of 10kg S ha~ .

Table 1 Source and Rate of Applied Fertilizer

Form of Source Treatment Application Fertilizer Code kg S mg product ha-¹ pot"¹

Nil Nil ^GO

5 89 10 178 20 356

10 330 10 330 10 330

10 330

10 165 10 206

10 33

10 37

Data were analyzed by analysis of variance using the NEVA computer program (Burr, E J, (1980), "Neva User's Manual", University of New England Computer Centre, Armidale, NSW) . At each harvest, data from the S source and S rated as well as flooded and non-flooded components of the experiment were analyzed separately. Data of control (G_Q) and G._β treatment from the S rate experiment were combined with that of the other S sources for analyses. Basal nutrients were thoroughly mixed with the soil prior to potting. Fertilizer K, P and Mg were applied in two equal applications 14 and 2 days before transplanting and fertilizer N was applied one day before transplanting and re-applied 21 days after transplanting (50% of the total fertilizer) . Other basal fertilizers (Zn, Cu, Mo and B) were applied 2 days before transplanting. Carrier free Ca 35S0₄ from Amersham Australia Pty

Ltd, was diluted with distilled water to give a solution containing 1.08 MBq ml^" . For the flooded experiment the solid was placed in the pots and flooded with distilled water and incubated for two weeks prior to transplanting to allow the equilibration of 35S with the native S in the soil. Puddling was also conducted three days after flooding. The soil was incubated for two weeks in a plastic bag. After incubation the- soil was placed in the pots and both the non-flooded and flooded pots placed in the glasshouse. The treatments listed in Table 1 were incorporated into the soil two days before transplanting. Seeds of IR 43 rice (Oryza sativa L) , which is commonly cultivated under flooded and non-flooded conditions, were germinated and grown for 2 weeks in quartz sand. One healthy seedling was transplanted to each pot. For the flooded treatment, the depth of water was maintained at approximately 5cm above the soil surface and the soil was dried one week before the end of the experiment. For non-flooded rice, the water status of soil was maintained at or near field capacity by weighing. The temperature in the glasshouse was maintained between 20 and 35 C throughout the experiment. In this study, two successive crops were grown with - li ¬

the same water regime to investigate the initial and residual effects of the fertilizer application under flooded and non-flooded conditions. No S was added to the second crop but basal nutrients were re-applied at the same rate as the first crop. The plant parameters recorded included tiller number and at maturity panicle number; dry weight of stem + leaf and root and grain, grain per panicle and 100 grain dry weight and percentage of empty grain; harvest index and S uptake into stem + leaf, grain and root.

At harvest, the whole plants were cut approximately lcm above the soil surface. The tops were separated into grain and stem + leaf. All soil from each pot was pushed out of the pot and laid in a plastic tray and the roots removed and washed. The soil was then thoroughly mixed and approximately 500g sample of soil taken, air-dried, ground and passed through a 2mm sieve and analyzed for total extractable S (Till, A R, McArthur, G S, Rooke, R L, (1984), "An automated procedure for the simultaneous determination of sulphur and phosphorus and radioactivity in biological samples", Proc of Sulphur-84 ' , pp649-60), acetone extractable elemental S (Shedley, C D, (1982), "An Evaluation of Elemental Sulfur as a Pasture Fertilizer", PhD Thesis, University of New England), Ca(H₂P0₄)₂ extractable S (Barrow, N J, (1967),

"Studies on extraction and on availability to plants of adsorbed plus soluble sulfate". Soil Science, 104, 242-9) and the S in the extracts determined on an ICP-AES and

35 S determined by Liquid Scintillation Counting. The oxidation rates of elemental S were determined from the difference between elemental S added and elemental S remaining in the soil as determined by acetone. The organic S content (OS) was estimated by subtraction the amount of elemental S remaining in the soil plus extractable sulfate sulfur- from the total S content of the soil. The plant materials were dried at 80°C until a constant dry weight was achieved and ground to pass 1mm screen. A subsample of each plant component was taken (0.2g), digested with 70% HC10₄ and 30% H₂0₂ (Anderson, D L and Henderson, L J, (1986), "Sealed chamber digestion for plant nutrient analyses", Agronomy Journal,

78, 937-9) and measured for total S in an ICP-AES and

35 S content determined by Liquid Scintillation Counting.

After the first crop harvested, the soil was mixed thoroughly and returned to the same pot. The pots were kept in the glasshouse for two weeks. For the flooded rice the soil was flooded with distilled water and for the non-flooded the soil moisture content was maintained at or near field capacity. One week prior to the first crop being harvested, the same variety of rice seeds were germinated in quartz sand. One healthy seedling was transplanted to each pot two weeks after the first crop was harvested. The experimental procedures and measurements were the same as in the first crop, except that the crop was only harvested at maturity. FLOODED RESULTS Total Fertilizer S Uptake

In the first crop, the highest recovery of fertilizer S in the whole plant was recorded in the G,_n treatment. Among the TSP-S treatments, highest recovery was recorded in the PVA and LS. A lower but similar rate was observed with the HF and SW. Among the treatments, SB resulted in the lowest rate although this diα not differ significantly from that of SCU (Table 2). In the second crop, the G-₀ treatment resulted in the lowest recovery of fertilizers, although this was similar to that of PVA, LS and ES. The highest recovery was recorded in the SB treatment, although this did not differ significantly from that of SCU (Table 2). Table 2 The Initial and Residual Effects of S Source on the Recovery of Fertilizer S in the Whole Plant Under Flooded Conditions

Source Fertilizer S uptake (% applied) of S Crop 1 Crop 2

GlO 51.6 a 14.3 e

PVA LS HF SW

US SCU

ES SB

Data followed by the same letter in the same column are not significantly different (P 0.05) according to DMRT (Duncan's Multiple Range Test).

NON-FLOODED RESULTS

Total Fertilizer S Uptake

In the first crop, the G,₀ treatment resulted in the highest recovery of fertilizer S in the whole plant, although this was similar to that of PVA (Table 3).

Among the TSP-S treatments, PVA resulted in a higher recovery than did other treatments, but did not differ from that of LS. A lower recovery was observed from HF and SW treatments. There was no significant difference in total fertilizer S uptake by the whole plant recorded among the N-S treatments. The lowest recovery was recorded in the SB treatment (Table 3).

In the second crop, the G,_fi treatment resulted in a significantly lower total fertilizer S uptake by the whole plant than did HF, SW, SCU and SB, but did not differ from the other treatments. In grain, the recovery of fertilizer S from G,_Q was similar to that of other treatments, except HF and SB (Table 3) .

Among the TSP-S treatments, HF produced the highest recovery of fertilizer S in the whole plant. Similar results were observed from SW and LS and also for PVA and LS (Table 3). There was no significant difference in recovery of fertilizer S in grain among the TSP-S treatments.

Table 3 The Initial and Residual Effects of S Source on Recovery of Fertilizer S in the Whole Plant Under Non-Flooded Conditions

Source Fertilizer S uptake (% applied) of S Crop 1 Crop 2

PVA LS HF SW

US SCU

ES SB

Data followed by the same letter in the same column were not significantly different (P" 0.05) according to DMRT.

COMPARATIVE EXAMPLE 4 - SULFUR AND PHOSPHORUS SOURCES FOR PASTURES

PVC tubes, 16cm in internal diameter and 13cm deep, were driven into the S-deficient, unfertilized pasture sites of granitic soil at Uralla, NSW. The cores with the intact pastures were then removed from the soil, capped at the base and, for the leached treatments, the cores were packed with special leaching bases. The cores were then transported into the glasshouse and maintained at a moisture content of field capacity by daily weighing and watering with distilled water. The original grasses were cut approximately 0.5cm above the soil surface. The soil was then oversown with perennial ryegrass (Lolium perenne L) and white clover (Trifolium repens L) seeds. Black shade screens were used to cover the pots in order to facilitate seed germination. The pots were kept in the glasshouse with daily temperature ranging from 15 to 25°C. Prior to the experiment, analysis of the soil samples was undertaken in the laboratory. The calcium monophosphate extractable sulfate of this soil was 7.8ppm, total S content was 106ppm and soil pH was 5.0 (1:5 H,0) . Basal nutrients were diluted in distilled water and applied 14 days before the main treatments were applied. On the same day, carrier free 35SO., obtained from Amersham Australia Pty Ltd, was diluted with distilled water to give a solution containing 2.93MBq ml^" . A syringe was used to apply 5ml of the radioactive solution to the surface of the soil. Thus, at the beginning of the experiment the specific radioactivity (SR) of the soil-available sulfate pool was the same in each pot. From this period up to the fertilizer application, the water content of the soil was maintained at field capacity by weighing.

A randomized block design experiment with three replicates was conducted in semi-controlled glasshouse conditions, of the Department of Agronomy and Soil Science, the University of new England, Armidale, NSW, Australia.

The treatments (Table 4) consisted of the factorial combination of two S sources (elemental S and gypsum), two P sources (triple superphosphate and rock phosphate), two methods of S and P application (S mixed with granulated triple superphosphate or rock phosphate and S coated onto with granulated triple superphosphate or rock phosphate) . One non-fertilized treatment (G_Q) was included in the experiment in order to calculate the percentage of S in plant derived from the fertilizer. The treatments were differentiated under two water conditions (non-leached treatment, with the soil-moisture content maintained^' at or near field capacity by weighing, and leach treatment, with 25% of excessive water from field capacity added every time of watering) . The rock phosphate used in this experiment was derived from North Carolina, USA, with 14% P content. The rock was finely ground prior to the experiment with particle size less than 400μm. The granulated rock phosphate and coated fertilizers were made using the same device and procedure as in the previous examples. The rock phosphate granule size was made similar to the TSP granulates by sieving. Polyvinyl alcohol was used as an adhesive material to bind elemental S with granulated TSP or granulated rock phosphate. Each fertilized treatment received the same amount of S and P at the rate of 20kg S ha^" (40mg elemental S pot^" or 216mg gypsum pot^" ) and 40kg P ha^" (402mg TSP or 574mg rock phosphate).

A comparison of different TSP-S sources (Table 5) was also investigated. Therefore another set of small experiments was included to study the effectiveness of S sources under non-leached pasture conditions. This experiment consisted of six treatments: TSP-S with PVA, TSP-S with slack wax, TSP-S with sodium lignosulfonate (these fertilizers contained 9.05% elemental S and 90.95% TSP) which was manufactured by Hifert Fertiliser Ltd, Victoria, Australia, TSP mixed with gypsum and control. An additional 42mg granulated TSP was required to be added to the TSP-S Hifert treatment to adjust the level of applied P. These treatments were replicated three times. Data of TSP-S with PVA, TSP + gypsum and control were derived from the non-leached treatment in the main experiment. The same computer program as previously used, was used in this experiment to analyze the data. Data from control in the main experiment were included in the analysis of variance. Data presented throughout this study are the mean of S x P x L interaction due to the non-significant difference recorded between coated and mixed fertilizers at each observation time.

Table 4 Treatments and Treatment Codes for Pasture

Table 5 Source and Rate of S and P Applied under Non-Leached Conditions

Treatment Treatment S & P Application Code S P product kg ha^-1 kg ha"¹ mg pot^-1

Two weeks after the application of basal nutrients and 35S, the treatments were applied. The fertilizers were spread on the surface of the soil. The soil was watered every day. For the non-leached treatments, the soil was watered at or near field capacity (measured by weight) and for leached treatments an excess of 25% distilled water was added to each pot every watering time. The leachates were collected and weighed every two days and stored in plastic bottles. The bottles were stored in a cold room. The S and P content of leachates as well as radioactive determination were conducted every harvest, using ICP-AES and Liquid Scintillation Counting respectively.

Harvesting was conducted within four-to-eight-week intervals depending on the growth rate of pastures. At harvesting plant tops were cut approximately 2cm above the soil surface. The pastures were then separated into the grass and clover. These plant materials were then oven-dried at 80 C until the constant weight was achieved, ground and digested with the mixture of perchloric acid and hydrogen peroxide (Anderson and Henderson,1986) . The plant digests were then determined for the S and P content using ICP-AES, and for radioactivity using scintillation counting.

After the fourteenth harvest (24 months) the pastures were cut in the same way as previously. The soils were pushed out and roots were taken. The soils were then mixed thoroughly and air-dried for further analysis. Soil analysis included total extractable S (Till et al, 1984) and Ca(H₂P0₄)₂ extractable S (Barrow, 1967). The percentage of S in plant derived from the fertilizer was calculated by the same method as in the previous examples. Total Fertilizer S Uptake Gypsum resulted in the highest recovery of fertilizer S in pastures at weeks 4 and 8. No differences were observed between PVA and LS and also between HF and SW (Table 6). At weeks 12 and 17, highest recovery was recorded in the PVA and LS treatments, whilst the lowest recovery was recorded from gypsum. The recovery of fertilizer S did not differ between HF and SW. The ranking at weeks 24 and 28 as well as 36 and 44 was HF = SW > LS = PVA> G. The cumulative recovery of fertilizer S was highest in the gypsum treatment. SW resulted in the lowest recovery, although this was similar to that of HF. A non-significant difference was also recorded between PVA and LS (Table 6). Table 6 The Effect of S Source on the Recovery of Fertilizer S in Pastures

Treatment Harvest Times (weeks)

4 + 8 12 + 17 24 + 26 36 + 44 Total

Data followed by the same letter in the same column are not significantly different (P > 0.05) according to DMRT.

INDUSTRIAL APPLICABILITY

The present invention is useful for enhancement of readily available fertilizers by the addition of a wide range of desired additives. It will be appreciated that the method of coating is suitable for a broad range of additives and fertilizers.

From the aforegoing description, it should be apparent that the invention encompasses an advantageous advance in the art or at least a commercial alternative to the prior art. Further, it should be clear that the invention may be embodied in other specific forms without departing from the spirit or the essential characteristics thereof. The present embodiments are therefore, to be considered in all respects as illustrative and not restrictive.

Claims

1. A process for producing coated fertilizer granules comprising the steps of a) spraying fertilizer granules with a water-soluble adhesive to establish a layer of adhesive on said granules, b) adding a first portion of an additive to coat said granules, c) spraying the fertilizer/additive granules again with the water-soluble adhesive to establish a further layer of adhesive thereon, d) adding a further portion of additive to coat said fertilizer/additive granules, and e) maintaining sufficient movement of the granules throughout the process until the adhesive is substantially dry so as to prevent agglomeration.

2. A process according to claim 1 which includes a heating step to promote drying of the adhesive on the granules after at least a first portion of the additive has been used to coat the granules.

3. A process according to claim 2 wherein the heating step is included intermediate steps b) and c) and/or after step d) to promote drying of the adhesive.

4. A process according to claim 1 wherein steps c) and d) are repeated sequentially at least once.

5. A process according to claim 1 wherein the additive is a nutrient.

6. A process according to claim 5 wherein the nutrient is selected from the group consisting of elemental sulfur, molybdenum, copper, zinc and boron.

7. A process according to claim 5 wherein each portion of nutrient is from 0.1 to 10% by weight of the weight of the coated fertilizer granules.

8. A process according to claim 7 wherein each portion of nutrient is about 0.1 to 5% by weight of the weight of the coated fertilizer granules.

9. A process according to claim 1 or claim 5 wherein the fertilizer is selected from the group consisting of urea, monammonium phosphate, diammonium phosphate and triple superphosphate.

10. A coated fertilizer granule prepared by a method according to any one of the preceding claims.