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Slow-release boron fertilisers: co-granulation of boron sources with mono-ammonium phosphate (MAP).

Introduction

Boron (B) is one of the essential micronutrients required for normal plant growth and cell development (Gupta 1979). Deficiencies as well as toxicities of B result in many anatomical and physiological changes in plants (Berger 1949). For the past 30 years, research on plant B nutrition has progressed significantly and the application of B fertiliser has become a standard practice in many B deficient regions. Boron fertilisers in the form of very soluble B compounds such as sodium borate or borax ([Na.sub.2][B.sub.4][O.sub.7] x 10[H.sub.2]O) and boric acid ([H.sub.3]B[O.sub.3]) and less soluble B sources such as colemanite ([Ca.sub.2][B.sub.6][O.sub.11]] x 5[H.sub.2]0) and ulexite (NaCa[B.sub.5][O.sub.9] x 8[H.sub.2]O) have been added to soil to maintain plant growth (Wear and Wilson 1954).

In any farming system, fertilisation represents one of the most important items contributing to the farm costs (Tissot et al. 1999). Separate application of pure B fertiliser is not cost-effective. Physical-mixture or bulk blending of B with other micronutrients is also an unattractive option, as it results in poor nutrient distribution in the field. For the bulk-blend fertilisers, the different components in the bags may separate with the finer product at the bottom and the bigger particles at the top, resulting in non-uniform nutrient distribution when spread. Granulation of micronutrients with macronutrient fertiliser allows for a single fertiliser application and a more even micronutrient distribution (Shorrocks 1997).

Granulation is a process of particle enlargement or agglomeration and is accomplished by the formation of inter-particle bonds between primary particles to form new entities called granules (Liu 2002; Sherrington 1968). Granulation processes are used extensively on powdered materials within pharmaceutical, food and agricultural industries to improve flow properties, strength, product appearance, shape and structural form (Mangwandi et al. 2013). Granulation can be used to solve different powder-flow problems, to ensure better results when mixing difficult powders, to reduce dust hazard problems, and to obtain controlled release of nutrients in fertilisers (Walker 2007).

Boron is only required by plants in small amounts, with recommended fertilisation rates ranging from 0.25 to 3.0kgB/ha, depending on crop requirement and method of application (Mortvedt and Woodruff 1993). With the low B application rates and the use of soluble B sources, B fertilisation has always been a problem, especially in high rainfall areas. Water soluble B fertilisers can provide high B concentrations in soil solution after application to soil, which can be hazardous to crop seedlings, while rapid leaching of B from the soil may result in an inadequate supply of B for plant uptake later in the growing season in high rainfall areas (Shorrocks 1997). Therefore, fertiliser sources with low water solubility and slow B release are necessary for the effective management of B fertilisation in humid regions. Slow-release fertilisers can meet the crop nutrient demand for the entire growth stage through a single application, thus saving spreading costs and time (Shaviv 2000). These slow-release fertilisers can be applied safely at planting, because of their slow-release characteristics. Up to now, research and development on B fertilisers, particularly on the co-granulation of slow-release B sources with other macronutrients, has received very little attention.

One of the B sources that has potential to be used as a raw material for slow-release B fertiliser is boron phosphate (BP[O.sub.4]). The relatively high B content (10% by weight) in BP[O.sub.4] compound and its low solubility make this compound an efficient slow-release B fertiliser source (Magda et al. 2010). Magda et al. (2010) reported the formation of BP[O.sub.4] by neutralising phosphoric acid ([H.sub.3]P[O.sub.3]) with ammonia (N[H.sub.3]) solution with the addition of sodium tetraborate pentahydrate ([Na.sub.2][B.sub.4][O.sub.7] x 5[H.sub.2]O) and calcining above 500[degrees]C. However, this material has not been tested for use as a slow-release B source for co-granulation with other macronutrients in compound fertilisers.

We attempted here to formulate slow-release B fertilisers by co-granulating BP[O.sub.4] compounds with the commonly used macronutrient fertiliser mono-ammonium phosphate (MAP). Boron phosphate compounds synthesised at 500 and 800[degrees]C for 1 h were selected for the co-granulation process with MAP. These two BP[O.sub.4] compounds were characteristically suitable and have potential to be used as raw materials for slow release B sources (Abat et al. 2014a). Three other commercially available B sources viz., borax, ulexite and colemanite were also co-granulated with MAP. All of the cogranulated products were tested for their suitability as a slow-release B fertiliser.

Materials and methods

Boron sources used in the co-granulation process

Five B sources were used as raw materials in the co-granulation process; two BP[O.sub.4] compounds, colemanite, ulexite and borax. The BP[O.sub.4] compounds were synthesised by mixing boric acid ([H.sub.3]B[O.sub.3]) and phosphoric acid ([H.sub.3]P[O.sub.3]) and heating the mixture at 500 or 800[degrees]C for 1 h (Abat et al. 2014a). The procedure has been modified from the method by Becher (1963). The acids used were analytical reagent (AR) grade (99.8% min) [H.sub.3]B[O.sub.3] (Merck) and 85% weight solution in water [H.sub.3]P[O.sub.3] (Acros Organic). The other three B sources were the commercially available colemanite (Active Micronutrient Fertilisers), ulexite (ChemSupply) and di-sodium tetraborate (borax) (BDH Analar).

Co-granulation of B sources with MAP

The equipment set up for the co-granulation process consisted of a stainless steel pan granulator, Masterflex air pump, peristaltic pump, Bosch heat gun and spray nebulizer (Fig. 1). The optimum conditions for the co-granulation process were as follows: (/) Masterflex air pump speed set at 8.6 mL/s, (ii) nebulizer spray rate at 0.12 mL/min, (iii) Bosch heat gun set at 500[degrees]C, and (iv) granulation speed at 1.0 rotation per min. Lignosulfonate (Lignobond DD from Lignotech, South Africa) was used as a binder. The quantities of B sources and MAP to obtain pre-determined B contents of 0.5%, 1% and 2% were thoroughly mixed and ground (<250 [micro]m) together using a grinder. The ground mixture (~20 g) was then transferred into a stainless steel laboratory scale pan granulator. The binder solution (~1 mL sprayed on 20 g ground mixture) was prepared by dissolving 1.03 g of lignosulfonate in 100 mL of deionised (DI) water and pumped using the peristaltic pump to the nebulizer at ambient temperature. The nebulizer, delivering an atomised spray, was positioned at an angle such that the binder was directed towards the tumbling materials rather than onto the pan surface while the pan granulator was rotating. The binder spray rate was adjusted depending on the moisture content of the mixture. A heat gun, set at a distance from the drum, was used to slowly and evenly dry the granules. This slow drying avoided localised over-heating and abrasion of granules and instead produced hard granules with lower porosity. The materials were rotated for ~15 to 20 min. Fines ([less than or equal to] 1 mm) were reground and fed back into the drum, after which the granules produced were poured into a container and dried overnight in an oven at 40[degrees]C. The granules then were sorted using a series of mesh size sieves. The granules were kept in an airtight container before analysis.

pH determination of the co-granulated products

One gram of each sample was weighed into a centrifuge tube and 10 mL of DI water was added. The mixture was shaken in an end-over-end shaker for I h. The solution was left standing for 30 min and then the pH was measured. Duplicate measurements were made.

Total elemental analysis of co-granulated products in aqua regia mixture

Total elemental analysis of the co-granulated products was determined by digesting ~0.1 g of each in an aqua regia (3 HCl: 1 HN[O.sub.3]) mixture. The solution was then analysed using inductively coupled plasma-optical emission spectroscopy (ICP-OES). All samples were analysed in triplicate.

Determination of water soluble B and phosphorus (P) in the co-granulated products

About 200 mg of each co-granulated product was weighed into a tube. Twenty mL of DI water was added into the tube and equilibrated by shaking on an end-over-end shaker for 24 h. The suspension then centrifuged for 30 min at 3500 relative centrifugal force (RCF). Five mL of supernatant was taken out from the suspension, filtered (0.45 pm Sartorius filter) and kept for determination of B and P by ICP-OES. All samples were analysed in duplicate.

Dissolution of B from co-granulated B sources using a column perfusion technique

The kinetics of B release from the co-granulated B fertilisers at 1.0% B content was determined using a column perfusion technique. The column was packed with acid washed sand (Sigma Aldrich). Glass wool was placed at the bottom of the column to obtain an even distribution of leaching solution and to prevent sand loss during leaching. Twenty grams of acid-washed sand was packed into the column. One gram of co-granulated B source was wrapped in a 0.5 mm mesh size cloth and placed on the surface of the sand and then covered with another 10 g of acid-washed sand. Glass wool was placed on the sand to minimise surface disturbance during leaching and to create an even distribution of leaching solution over the whole surface. Ten mM calcium chloride (Ca[Cl.sub.2]) was used as the leaching solution and pumped at a flow rate of 2 mL/h using a peristaltic pump from the bottom of the sand column upwards through the column. The leachate was collected at 5 h intervals. The remaining B material and sand above the fertiliser (top 10 g sand) were removed carefully from the column at the end of the dissolution process, air-dried and digested to check the mass balance for B. The concentrations of B and P in the leachate and digests were measured by ICP-OES. All samples were run in duplicate.

Boron release from the co-granulated products in soil column study

The kinetics of B release from the co-granulated products with 1.0% B content was also determined in a soil column study. Granular MAP was included as a control (no B) treatment. A column without any fertiliser was also included.

Sixty grams of Mt Compass soil was packed into a leaching column (60 mL Removable Luer Lock syringe; internal diameter of 3 cm, height of 11 cm) to an approximate bulk density of 1.2 g/[cm.sub.3] and height of 7 cm. Mt Compass is a coarse-textured soil and has low concentrations of available (hot-water extractable) B (Table l).The base of the column was covered with a thin layer of glass wool and a thin layer (0.5 cm) of acid-washed sand. Soil was added to the column to a height of 6 cm. Eight granules of co-granulated B fertilisers (corresponding to ~0.2 g fertiliser; 2 mg B) were wrapped in a mesh cloth (0.5 mm mesh size) and placed on the soil and covered with ~1 cm of soil. All treatments were replicated three times.

The column was saturated by slowly pumping Dl water from the bottom of the column using a peristaltic pump set to a flow rate of 10 mL/h until the soil was saturated. The column was brought to field capacity by applying vacuum through a 25 [cm.sub.3] syringe, drawn to the 12 [cm.sub.3] mark, after allowing for gravity drainage for ~30min. The top of the column was covered with glass beads to minimise moisture loss. The columns were weighed and left to stand at room temperature.

The moisture content was maintained at field capacity, correcting for column weight change on a weekly basis. The columns were leached with ~one pore volume (21.25 mL) of DI water every week for four weeks. A vacuum suction was applied using 25 [cm.sub.3] syringes to extract the solution from the column. The volume of the leachate was recorded and filtered through a 0.45 pm Sartorius membrane filter. The pH of the leachate was measured after every leaching. The leachate was acidified with 10 [micro]L of concentrated HN[O.sub.3] and stored at 4[degrees]C before analysis. After four weeks, the fertiliser granules were recovered from the soil column by carefully removing the soil around the mesh cloth. The recovered granules were digested in aqua regia mixture. The soil below the granules was collected and mixed homogenously. A subsample was digested in aqua regia and the B concentration in the digests was analysed using ICP-OES.

Results

Physical appearance of the co-granulated products and granule size

The granules produced were spherical in shape and regular in size. The standard required specification for fertiliser granules is that at least 85% of the granules have sizes between 1.0 and 4.0 mm (Rico et al. 1995). About 90 to 95% of the granules obtained from the various co-granulated products were of the size range between 1.0 and 4.0 mm, with ~90% between 2.35 and 3.35 mm. About 5% of the granules were less than 1.0 mm in mesh size. The fraction greater than 4.0 mm only accounted for-1-2%.

pH of co-granulated products

The solution pH of the MAP without added B was 4.3. The solution pH of the MAP fertiliser co-granulated with BP[O.sub.4] ranged from pH 4.2 to 4.4. For the fertilisers co-granulated with colemanite, ulexite and borax, which have a basic reaction, the solution pH was between 5.0 and 5.5.

Total B and P in the co-granulated products

The B concentrations obtained in most of the co-granulated products were close to the target B contents ([less than or equal to] 10% difference from target). The P concentrations in the co-granulated products ranged from 20 to 24% (data not shown).

Water soluble B in the co-granulated products

For the co-granulated ulexite and borax products, >90% of B was water-soluble (Table 2). For co-granulated colemanite, the B solubility ranged from 74 to 81% of the total B content (Table 2). The solubility of B in the co-granulated products containing BP[O.sub.4] synthesised at 500[degrees]C was between 16 and 26% of the total B content. The solubility of B in co-granulated BP[O.sub.4] synthesised at 800[degrees]C was only -6% of the total B, irrespective of the total B content (Table 2).

Kinetics of B release in column perfusion technique

The dissolution of B from the co-granulated ulexite and borax was rapid and B was almost completely leached out after -48 h (Fig. 2a). For co-granulated colemanite, 95% of total B was released after ~72 h. Boron release after 600 h of column dissolution was ~17% for the co-granulated BP[O.sub.4] synthesised at 500[degrees]C and ~7% for that synthesised at 800[degrees]C (Fig. 2a). The concentration of B in the leachates of co-granulated ulexite, borax and colemanite were high at the beginning (137, 189 and 89 mg B/L, respectively), and strongly decreased over time until the concentration was almost below detection limit (~10 ppb) (Fig. 2b). The B concentrations in the leachate of the cogranulated BP[O.sub.4] products gradually decreased until they reached near constant values of 0.02 and 0.01 mg/L, for the co-granulated BP[O.sub.4] 500 and 800[degrees]C, respectively (Fig. 2b).

Soil column study

The pH of the leachate from the first leaching ranged from 5.19 to 5.56 (Fig. 3), but then gradually increased to ~6.10 and 6.50 after two leaching events (Fig. 3). The changes in the pH were mainly determined by the MAP fertiliser addition. The majority of added P was rapidly leached out because of the low P retention of the soil.

The B released from the soil followed a similar trend as observed in the sand column study. Boron release from cogranulated ulexite was the fastest, with -97% of added B leached over the four weeks (Fig. 4a). This was followed by cogranulated borax (75% B leached) and co-granulated colemanite (58% B leached). For co-granulated BP[O.sub.4] synthesised at 500[degrees]C, 16% of added B was leached after four weeks, while only 4% was released from the co-granulated BP[O.sub.4] synthesised at 800[degrees]C (Fig. 4a). The initial concentration of B in the leachates was high for co-granulated ulexite, borax and colemanite and much lower for the co-granulated BP[O.sub.4] products (Fig. 4b).

Only ~2 to 3% of added B was recovered at the end of experiment in the residual granules of co-granulated ulexite, borax and colemanite (Table 3). A substantial amount of B was retained in the soil treated with co-granulated borax (10%) and co-granulated colemanite (33%). For the granules containing BP[O.sub.4], 41 % of B was recovered in the granules of co-granulated BP[O.sub.4] synthesised at 500[degrees]C for 1 h and 31% of B was recovered from the soil. For the granules of co-granulated BP[O.sub.4] synthesised at 800[degrees]C for 1 h, 44% of B was recovered in the granules and 38% in the soil (Table 3).

Discussion

In a previous study, we determined the kinetics of B release from pure ulexite, borax, colemanite and BP[O.sub.4] compounds synthesised at 500 and 800[degrees]C for 1 h using the same column dissolution technique (Abat et al. 2014a). As a pure compound, the kinetics of B released from BP[O.sub.4] compound synthesised at 500[degrees]C for 1 h was rapid and almost similar to those of pure ulexite and borax. Boron released from pure colemanite was slow. Thus, the kinetics of B released from these pure B sources in decreasing order was BP[O.sub.4] 500[degrees]C ~ulexite ~borax > colemanite ~BP[O.sub.4] 800[degrees]C. This contrasts with the order of solubility found for the B sources co-granulated with MAP, which is: ulexite -borax > colemanite > BP[O.sub.4] 500[degrees]C>BP[O.sub.4] 800[degrees]C,

The differences between the pure B sources and cogranulated products can be explained by the conditions imposed by the MAP carrier. As a pure source, colemanite has high pH and low solubility. However, when co-granulated with MAP, the lower pH and the high P concentrations, which reduce the [Ca.sup.2+] concentration through Ca-P precipitation, increased the solubility of colemanite (Abat et al. 2014a). This limits the use of these minerals (colemanite and ulexite) in slow-release B fonnulations with fertilisers such as single superphosphate (SSP), triple superphosphate (TSP) or MAP. This limitation does not apply to the BP[O.sub.4] compounds.

The concentration of B released from co-granulated BP[O.sub.4] products in the soil column experiment was below the toxicity level of 5 mg B/L for most crops (Nable et al. 1997). These results indicate the potential of co-granulated BP[O.sub.4] products as seedling-safe slow-release B fertiliser sources. Because of the slow release, the BP[O.sub.4] products also showed a longer tail in the leaching curve, which suggests that they could sustain optimal plant growth for a longer period under leaching conditions, which was confirmed in a pot experiment with two consecutive canola crops (Abat et al. 2014b).

The recovery results indicate that a significant amount of the B was retained in the soil (Table 3). The one week period between each leaching would have contributed to fixation of B by the soil. The initial concentration of B released from cogranulated BP[O.sub.4] products is lower; therefore, B does not 'flush" through the soil and has time to be retained. With the more soluble B sources, the high B concentration at the beginning and the low pH would result in negligible B retention. Several studies have indeed pointed to fixation of B in soil. Eguchi and Yamada (1997) conducted a long-term field experiment (3 and 15 years) using slow-release B fertiliser on three soil types (diluvial, granitic and volcanic ash) in an area with an average precipitation of more than 2000 mm. After 15 years of cultivation, 40--60% of added B had leached from the topsoil and 10% was absorbed by plants. Of the 30-40% left in the topsoil, most B was present in a fixed form (Eguchi and Yamada 1997). Mortvedt (1968) carried out a study to determine the availability of B in borax incorporated with various macronutrient fertilisers with B contents ranging from 0.2 to 10%. The recovery of B from the soil treated with these products increased with the increasing concentrations of water soluble B in the products but decreased with time, indicating fixation of B in the soil (Mortvedt 1968). The fixation reactions of B prevent short-term B leaching from the soil, but with time, this 'fixed' B may be desorbed back into the soil solution for plant uptake at a later stage. Alternatively, the large percentages of B recovered in the soil for the BP[O.sub.4] products may not be due to fixation of B released from the granules, but due to particulate movement. As the leaching continued the granule would have disintegrated as P and N were released. The fertiliser fine particles may have moved through the mesh cloth with the mass flow and hence have been recovered in the soil, rather than at the point of application.

Conclusions

Co-granulated products of MAP with BP[O.sub.4] synthesised at 500 and 800[degrees]C for 1 h show potential as slow-release B fertilisers. Boron release from the co-granulated BP[O.sub.4] was slow and the initial B concentration around the granule is likely to be safe for most crop seedlings. By contrast, B release from cogranulated ulexite, borax and colemanite was rapid with initial B concentration in fertiliser leachates exceeding the toxicity level for most crops. The application of slow release formulations will allow a relatively high B addition without inducing toxicity to plants immediately following application. Further research is underway to compare the toxicity effect of these co-granulated fertilisers on crop seedlings and to determine the availability of B to crops.

Acknowledgements

The first author thanks the University of Adelaide for the scholarship to enable her to pursue her PhD and the Sarawak State Government for study leave. The authors also thank Bogumila Tomzcak, Deepika Setia, Colin Rivers, Ashleigh Broadbent and the staff of CSIRO Land and Water for their advice and technical support.

References

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Abat M, Degryse F, Baird R, McLaughlin MJ (20146) Responses of canola to the application of slow-release boron fertilizers and their residual effect. Soil Science Society of America Journal doi: 10.2136/ sssaj2014.07.0280

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http://dx.doi.org/10.1071/SR14128

Margaret Abat (A,C), Fien Degryse (A), Roslyn Baird (A), and Michael J. McLaughlin (A,B)

(A) Adelaide University Fertilizer Technology Research Centre, School of Agriculture, Food and Wine, University of Adelaide, PMB 1, Glen Osmond, SA 5064, Australia.

(B) CSIRO Sustainable Agriculture Flagship, CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia.

(C) Corresponding author. Email: margaretabat@yahoo.co.uk

Received 12 April 2014, accepted 15 August 2014, published online 6 August 2015

Table 1. Physical and chemical properties of Mt Compass soil

Properties                                 Value/concentration

pH (Ca[Cl.sub.2])                                 5.30
Conductivity (dS/m)                               0.041
Phosphorus, P (mg/kg) (A)                         9.00
Potassium, K (mg/kg) (A)                           30
Sulfur, S (mg/kg) (A)                             3.70
Exchangeable calcium, Ca (cmol(+)/kg)             1.45
Exchangeable magnesium, Mg (cmol(+)/kg)           0.28
Exchangeable sodium, Na (cmol(+)/kg)              0.04
Exchangeable potassium, K (cmol(+)/kg)            0.03
Exchangeable aluminium, Al (cmol(+)/kg)           0.02
Hot water extractable boron, B (mg/kg)            0.20

(A) P and K extracted using Colwell method;
S extracted using KC140 method.

Table 2. Percentage of water soluble B relative to total B
in the fertiliser, for the different B sources that were
co-granulated with MAP at a rate of 0.5 1 or 2% B

Values in parentheses are standard deviation (s.d.) of
two replicates

Properties                               Percentage water-soluble B (%)

                              0.5% B      1.0% B      2.0% B

Co-granulated ulexite         104 (1)     99 (7)      106 (3)
Co-granulated borax           93 (3)      96 (3)      105 (1)
Co-granulated colemanite      81 (10)     77 (3)      74 (5)
Co-granulated BP[O.sub.4]     16 (1)      21 (2)      26 (2)
  500[degrees]C 1 h
Co-granulated BP[O.sub.4]    6.5 (0.0)   5.4 (0.8)   5.3 (0.3)
  800[degrees]C 1 h

Table 3. Mass balance for the soil column experiment:
percentage B recovered in the leachates (four pore volumes
in total), or in the granules or the soil at the end of the
experiment

The values are means of triplicate samples

Product name                 Cumulative B leached    Boron recovered
                              (as % of B added)     from granules (%)

Co-granulated ulexite                 97                   1.9
Co-granulated borax                   75                   1.8
Co-granulated colemanite              58                   2.9
Co-granulated BP[O.sub.4]             16                   41
  500[degrees]C
Co-granulated BP[O.sub.4]            4.0                   44
  500[degrees]C

Product name                 Boron recovered   Total
                              from soil (%)     (%)

Co-granulated ulexite             0.12          99
Co-granulated borax                9.7          87
Co-granulated colemanite           33           94
Co-granulated BP[O.sub.4]          31           88
  500[degrees]C
Co-granulated BP[O.sub.4]          38           86
  500[degrees]C
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Author:Abat, Margaret; Degryse, Fien; Baird, Roslyn; McLaughlin, Michael J.
Publication:Soil Research
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Geographic Code:1USA
Date:Aug 1, 2015
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