Boron Fertilizers: Use, Challenges, and Slow-Release Sources (PDF)

BORON 2 (3), 111 - 122, 2017 ULUSAL BOR ARAŞTIRMA ENSTİTÜSÜ ISSN: 2149-9020...

BORON 2 (3), 111 - 122, 2017 ULUSAL BOR ARAŞTIRMA ENSTİTÜSÜ ISSN: 2149-9020 NATIONAL BORON RESEARCH INSTITUTE BOR BOR DERGİSİ DERGİSİ JOURNAL OF BORON AGROBOR CİLT/VOL SAYI/ISSUE YIL/YEAR 02 03 20 ÖZEL SAYI / SPECIAL ISSUE 17 JOURNAL OF BORON http://dergipark.gov.tr/boron Boron fertilizers: use, challenges and the benefit of slow-release sources – a review Fien Degryse* Fertilizer Technology Research Centre, School of Agriculture, Food and Wine, The University of Adelaide, PMB 1, Waite Campus, Glen Osmond, SA 5064, Australia, ORCID ID orcd.org/0000-0002-4875-2944 ARTICLE INFO ABSTRACT Article history: Boron (B) is an essential plant nutrient, but can be toxic when present in excess. Received 22 March 2017 Boron is usually present as an uncharged molecule (H3BO30) in the soil solution Received in revised form 28 July 2017 and is highly mobile in most soils. Deficiency of B is therefore quite common Accepted 19 October 2017 in high-rainfall environments, especially on sandy soils. Boron fertilizers are Available online 30 December 2017 commonly used to correct its deficiency in crops. The most commonly used Review Article fertilizers are soluble sodium borates (e.g., borax), but care should be taken with rates and placement of such B products, since elevated B concentrations may Keywords: result in seedling toxicity and yield reduction. Moreover, significant leaching losses Boron, of applied B may occur in high-rainfall environments. Slow-release B sources Toxicity, reduce both the risk of seedling toxicity and of leaching, and can provide adequate Deficiency, supply of B over a longer period. This may allow for lower B rates or less frequent Leaching, application compared to soluble fertilizers. The most commonly used slow-release Slow-release fertilizer B sources are sparingly soluble ores, such as colemanite. The limited data in the literature indicate that the release rate of B from slow-release sources in soil depends both on fertilizer characteristics and soil properties. However, more research is needed to predict the release rate of B from various B ores for given soil and climatic conditions. In recent years, slow-release coatings and matrices for N fertilizers have received considerable attention and these new technologies may potentially also be adopted for B-containing fertilizers. 1. Introduction fertilizers to overcome the aforementioned challenges. Boron (B) is an essential micronutrient, required for 2. Boron chemistry and mobility in soil normal growth and development of plants. Under normal soil conditions, B in soil solution is present as Boron is a member of the metalloid group and has in- boric acid (H3BO3), a non-ionized molecule. The reten- termediate properties between metals and non-metals. tion of H3BO3 in soils is weak, making it vulnerable to Essentially all B is in a trivalent (+3) oxidation state, leaching. Hence, B deficiency is most commonly ob- but unlike its neighbour Al, B does not exist as trivalent served in coarse-textured soils in high rainfall regions cation but generally forms covalent bonds. Boron. Boron toxicity occurs most commonly in arid or in soil solution is present mainly as boric acid (H3BO3) semi-arid regions because of high natural B levels or or at high pH also as borate (B(OH)4-), since the pKa is because of the addition of B with irrigation water. 9.24. Boron adsorption by soil components is gener- ally weak, and may occur on phyllosilicate clays , Boron fertilizers are used to correct B deficiency, but B oxides and hydroxides , carbonate minerals and fertilization can be challenging because of the narrow organic matter. window between deficiency and toxicity and the limited mobility of B within most plants. High B concentrations Boron adsorption is dependent on soil properties. The at seedling stage may result in seedling toxicity, while adsorption increases with increasing pH, reaching a leaching losses may result in insufficient B later in the maximum around pH 9. Keren et al. devel- season. Boron is notoriously known as the element for oped a phenomenological adsorption equation that which toxicity and deficiency may occur concurrently takes into account the effect of pH on B adsorption, in the same plant. This review discusses B fertiliza- which was also applied to describe B adsorption by tion, with particular focus on the use of slow-release whole soils. The equation can be written in the *corresponding author: [email protected] 111  Degryse F. / BORON 2 (3), 111 - 122, 2017 form of the Langmuir equation: el the adsorption behavior of B in soils. The empiri- cal Freundlich model generally describes the sorption Kc isotherms well, but only applies for the soil and the s  smax  (1) conditions evaluated. Surface complexation models 1  Kc describe adsorption using an equilibrium approach, accounting for surface and aqueous speciation chang- with s the adsorbed B concentration (mol kg-1), c the B es as a function of pH and solution composition. The concentration in solution (mol L-1) and K a pH-depen- most commonly used surface complexation model to dent adsorption coefficient (L mol-1): describe B sorption in soils is the constant capacitance K BH  10 pH K a.K B model, which has been found to successfully describe K (2) B sorption over a range of soils. (1  10 pH K a )(1  K OH.10 ( pH14) ) Overall, the literature results indicate that B adsorption The constants KBH, KB and KOH represent empirically is weak in most soils. The strongest sorption is seen in defined affinity coefficients for binding of H3BO3 or soils with high clay content and high pH (8 or higher), B(OH)4- and OH- to B-specific adsorption sites and Ka but in soils with pH 7 or less, the retention is weak even is the hydrolysis constant of H3BO3 (value of 5.9×10- in soils with heavy texture. Given the weak adsorption 10 at 25 °C). The adsorption maximum, smax, is soil and hence high mobility of added B in most soils, one dependent and has been found in several studies to would expect soils in high rainfall environments to be correlate most closely with clay content and/or CEC. mostly depleted in B. However, most soils have a to- Following relationship was derived from literature data tal B concentration between 10 and 100 mg kg-1, but [12-14]: only a small part is in readily available form. Most of this B is in a sparingly soluble form, as several miner- smax (mol kg-1) = clay (%) × 0.14 + 0.34 (3) als in soils contain B as a structural component. The B content of phyllosilicates in particular is higher than Based on this relationship and typical affinity coeffi- that of most other minerals , explaining why often cients reported in the literature , estimated Lang- a positive correlation is seen between clay content muir parameters for soils with different pH and clay and B concentration of soils. For instance, in a dataset content were derived (Table 1; parameters converted with 17 European soils , there was strong corre- from mol- to mg-basis for easier comparison with most lation (r=0.89) between clay content and total soil B published values). The corresponding sorption iso- concentration. In general, soils rich in clay have higher therms are illustrated in Figure 1. A decrease in soil B concentration than sandy soils. Boron occluded pH below 7 is expected to have little effect on B sorp- in minerals may already have been present in the par- tion, whereas an increase in soil pH from pH 7 to 9 ent material, but may also originate from added B that strongly increases sorption. These values are only es- has become irreversibly sorbed. Several studies have timates, as other soil properties (e.g., organic matter, shown strong hysteresis in B sorption , which may type of clay minerals, etc.) may also affect B sorption, be related to incorporation of B into tetrahedral sites of but these sorption isotherms are useful to discuss the effect of soil properties on mobility of B. The Langmuir clay minerals. isotherm can be simplified to a linear relationship in As B retention is weak in most soils, B added in soluble the low concentration range (c calcium (magnesium) borates (colemanite, produced by melting silicates with powdered borates hydroboracite) > borosilicates (datolite, howlite). How- and their B concentration generally ranges between ever, not only inherent solubility, but also particle size 2 and 11%. Another slow-release source is BPO4, has a strong effect on the release rate of B in soil. which is produced from phosphoric acid and boric acid The solubility of B frits is variable, but generally low, so [52, 53]. In recent years, also a few slow-release B they need to be finely ground to be effective, and even fertilizers have been described in literature that use a then are more suited for maintenance fertilization than slow-release matrix with borax incorporated [54, 55]. for correcting severe deficiencies. There are not many studies that compared various 6.2. Fertilizer rates and application slow-release B fertilizers and because different stud- ies often use different methods, comparison between The B requirement of crops varies considerably, so studies is often not possible. Abat et al. assessed recommended rates are crop-dependent. Also the the solubility and release rates of colemanite, ulexite, manner of application, B status of the soil and B and of BPO4 synthesized at different temperatures fertilizer source should be considered. In general, (Table 3). They found that ulexite was about 10 times recommended rates range from 0.25 to 3 kg ha-1 more soluble than colemanite, in agreement with ther-. Boron fertilizer can be either soil-applied or as modynamic solubility calculations using published a foliar spray. Soil applications are generally used on solubility products. The solubility of the BPO4 com- field crops, but foliar applications are commonly used pounds decreased with increasing synthesis tempera- to correct deficiencies in fruit trees. In general, ture. Colemanite and ulexite had an alkaline reaction foliar fertilization has been found to be more effective in water and their solubility increased by acidifying the than broadcast soil application [59, 60], but repeated solution to pH 5, whereas the BPO4 compounds had application may be necessary because of B immobility an acid reaction in water and their solubility increased within the plant and judicious use is required to 116  Degryse F. / BORON 2 (3), 111 - 122, 2017 Table 3. The solubility of finely ground boron sources in water without pH adjustment or when pH is adjusted to 5. The BPO4 compounds were synthesized by heating the reaction mixture of H3PO4 and H3BO3 at various temperatures for 1 h or 24 h (Results from ref ). Unadjusted pH Adjusted to pH ~ 5 Boron source -1 -1 B (mg L ) pH B (mg L ) a a Sodium borate (borax) 5736 9.37 5892 a Ulexite 2733 9.29 4385 Colemanite 246 9.35 3507 BPO4 500 C 1 h 15.8 2.3 200 500 C 24 h 11.2 2.5 140 800 C 1 h 5.3 3.4 25 800 C 24 h 4.1 3.4 20 1000 C 1 h 1.9 3.7 5.0 1000 C 24 h 0.2 4.4 0.5 a : The mineral was completely dissolved (undersaturated solution) avoid toxicity. Soil-applied B fertilizer can be cm. It is estimated that at ten days after application, either banded or broadcast. Banding is usually more concentration close the granule (6 to reduce the risk of seedling toxicity. cm) would be in the deficient range (Figure 4a) and that it would take around a year for the fertilizer B to Boron fertilizers may exist as single-compound fertiliz- be evenly spread in the topsoil. These issues can to er, e.g., granular borax, in which case they are usually some extent be resolved by combining B with a mac- co-blended with a macronutrient fertilizer. However, ronutrient fertilizer. When a macronutrient fertilizer there are several disadvantages of using a blend. is enriched with a B source to reach a concentration Segregation may occur during handling or application, resulting in an uneven field distribution of the B fertil- of 0.5% B in the fertilizer, a rate of 1 kg B ha-1 corre- izer. Also, given the high B content and low B fertilizer sponds to an inter-distance between granules around rates needed, the number of B-containing granules 5 cm, and the concentrations around the granule are for a given area is relatively low when using a single- expected to be in the adequate range for most crops compound fertilizer. This may result both in higher risk (Figure 4b). Micronutrients can be combined with the of seedling toxicity, for seedlings close to a granule, macronutrient fertilizer either by incorporation during and deficiency, for plants further away from the fertil- granulation or compaction, or as a coating post-granu- izer application point. For instance, at a rate of 1 kg lation. Coatings provide more flexibility than incorpora- B ha-1 and with granules of circa 50 mg weight, the tion to obtain specific grades, but care must be taken inter-distance between granules would be around 24 to ensure the coating is homogeneous and adheres Figure 4. Estimated concentration profile of B around a fertilizer granule 10 days after application for a granule containing (A) 5 mg easily soluble B or (B) 0.25 mg easily soluble B. Note the different scales of the y-axes. The concentration profiles were modelled using the analyti- cal solution for spherical diffusion from a point source , assuming a bulk density of 1.43 kg L-1, volumetric water content of 0.25, tortuosity factor of 0.19, Kd of 0 (no sorption) and diffusion coefficient in water of 1.12x10-5 cm2 s-1. 117  Degryse F. / BORON 2 (3), 111 - 122, 2017 well to the granule. In general, co-granulation is most seeded and grown for 12 days, after which the area commonly used for large-tonnage products, whereas of the non-vegetated zone was determined (Figure 5). coating is mainly used to add micronutrients to spe- No B toxicity was observed around granules with 2% B cialty fertilizers. as BPO4 synthesized at high temperature. Ulexite and borax showed the highest toxicity. Colemanite also 7. Advantages of slow-release fertilizers showed considerable toxicity when cogranulated with MAP, but not when cogranulated with MOP. This effect Sodium borates are most commonly used as a B fertil- of macronutrient carrier could be explained by lower izer source, but they are highly soluble, which increas- pH and high P concentrations around the MAP gran- es both the risk of seedling toxicity and of leaching ule, which result in lower Ca2+ activity because of the losses, which may result in deficiency later in the sea- precipitation of Ca phosphates, resulting in enhanced son. Slow-release fertilizers release the nutrients at a dissolution of colemanite. The visual toxicity ef- slower rate than fertilizers in which the nutrient is read- fects corresponded well with the chemical analysis ily available and hence extend the availability to the of the soil in concentric circles around the soil, which plant. Ideally, the release of a slow-release source showed that toxicity roughly occurred when HWB con- should be slow enough to protect against leaching and centrations exceeded 5 mg kg-1 (Figure 5). Mortvedt seedling toxicity, but fast enough to provide nutrients in a reasonable time frame for crop growth. and Osborn also pointed out that high concentra- tions around boronated granules may result in seed- 7.1. Less risk of seedling toxicity ling toxicity, as they found that concentrations near granules with 2% B as a soluble B source were in the In a recent study , toxicity of several B sources co- toxic range for canola even at 8 weeks after applica- granulated with MAP or MOP was assessed using a tion. They suggested toxicity could be prevented by newly developed method. A granule was placed in the decreasing B content of the granule or by using a less centre of a soil-filled Petri dish, canola was densely soluble B source. The lower risk of toxicity with slow- Figure 5. (Left) Pictures (original and after image processing) of Petri dishes with a granule of MAP +2% B (top) or MOP + 2% B (bottom) applied in the centre, at seven days after fertilizer application and plant seeding. (Right) The hot water-extractable B (HWB) concentrations in the same Petri dishes at 12 days after fertilizer application in the soil at 7.5-15 mm or >15 mm from the fertilizer application site. The dashed line indicates a HWB concentration of 5 mg kg-1, which is considered to be toxic for many species. Results from Ref.. 118  Degryse F. / BORON 2 (3), 111 - 122, 2017 release sources has also been demonstrated in field ingly soluble borosilicate. Winsor compared reten- trials. For instance, Winsor found that the yield tion of borax and colemanite of various particle sizes of hairy indigo was reduced at borax rates of 22.4 kg in a sandy soil (Figure 6). He found that after 3 weeks ha-1 (corresponding to 2.6 kg B ha-1), which was attrib- (25 cm rainfall), only 9% of the B added as borax was uted to B toxicity, as visual toxicity symptoms were ob- recovered in the top 15 cm. Also fine colemanite (80% was still time. This is illustrated in Figure 7, which shows the retained in the soil for the colemanite and BPO4 com- HWB concentration in the top soil at different times af- pounds respectively. Saleem et al. assessed ter fertilizer application to a sandy soil. For borax leaching from borax, powdered and granular coleman- and fine colemanite, HWB concentrations were ini- ite in column studies, and found that leaching losses tially high, with fine colemanite even showing higher were greatest for the borax treatment and lowest for B concentrations and greater injury to native plants the granular colemanite. In a field leaching study on a than borax, because B applied as borax got quickly loamy sand , borax leached out of the topsoil very leached to the subsoil due to heavy rains in the first rapidly, while howlite leached slowly, with concentra- weeks. The HWB concentration declined rapidly for tions in the topsoils changing little over a 12-months these two sources to