Role of polymeric coating on the phosphate availability as fertilizer

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Role of polymeric coating on the phosphate availability as fertilizer: insight from phosphate release in soil castor polyurethane coatings Diego Cruz, Ricardo Bortoletto-Santos, Gelton Guimarães, Wagner Luiz Polito, and Caue Ribeiro J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b01686 • Publication Date (Web): 22 Jun 2017 Downloaded from http://pubs.acs.org on July 5, 2017

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Journal of Agricultural and Food Chemistry

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Title: Role of polymeric coating on the phosphate availability as fertilizer: insight

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from phosphate release in soil castor polyurethane coatings

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Authors: Diego F. da Cruz¹; Ricardo Bortoletto-Santos; Gelton G. F.

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Guimarães; Wagner L. Polito; Caue Ribeiro*

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Number of Tables: 4

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Number of Figures: 5

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Corresponding author:

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Dr. Caue Ribeiro Embrapa Instrumentação Rua XV de Novembro, 1452 Phone: +55 (16) 2107 2915 Fax: +55 (16) 2107 2902 E-mail: [email protected]

ACS Paragon Plus Environment


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Role of polymeric coating on the phosphate availability as fertilizer:

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insight from phosphate release in soil castor polyurethane coatings

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Diego Fernandes da Cruz1, Ricardo Bortoletto-Santos2, Gelton Geraldo

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Fernandes Guimarães2, Wagner Luiz Polito1, and Caue Ribeiro2*

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(USP), Campus de São Carlos, Av. Trabalhador São-Carlense 400, Caixa

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Postal 780, Arnold Schimidt, São Carlos, São Paulo, 13566-590, Brasil.

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Phone/Fax: +55 (16) 3373 9976.

Instituto de Química de São Carlos (IQSC), Universidade de São Paulo

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Embrapa Instrumentação, Rua 15 de Novembro 1452, Centro, São Carlos, São

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Paulo, 13560-970, Brasil.

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Phone/Fax: +55 (16) 2107 2800.

Laboratório Nacional de Nanotecnologia para o Agronegócio (LNNA),

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* E-mail: [email protected]


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ABSTRACT

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The coating of fertilizers with polymers is an acknowledged strategy for

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controlling the release of nutrients and their availability in soil. However, its

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effectiveness in the case of soluble phosphate fertilizers is still uncertain, and

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information is lacking concerning the chemical properties and structures of such

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coatings. Here, an oil-based hydrophobic polymer system (polyurethane) is

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proposed for control of the release of phosphorus from diammonium phosphate

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(DAP) granules. This material was systematically characterized, with evaluation

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of the delivery mechanism and the availability of phosphate in an acid soil. The

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results indicated that thicker coatings can change the maximum nutrient

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availability towards longer periods like 4.5 to 7.5 wt% DAP coated that

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presented the highest concentrations at 336 h, compared to 168 h for uncoated

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DAP. In contrast, DAP treated with 9.0 wt% began to increase the concentration

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after 168 h until it results in maximum release at 672 h. These effects could be

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attributed to the homogeneity of the polymer and the porosity. The strategy

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successfully provided long-term availability of a phosphate source.

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Keywords: Castor oil; controlled release; fertilizer; phosphorus; polymers.



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INTRODUCTION

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Increased agricultural productivity to meet the demand for food, fiber, and

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energy production has driven the increased use of inputs such as mineral

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fertilizers.1 High doses of soluble fertilizers are required to supply the nutritional

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needs of plants grown on soils with low natural fertility, such as Oxisols.

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However, the acidity and high contents of iron and aluminum-based compounds

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in these soils reduce the efficiency of phosphate fertilizers, due to strong

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phosphorus (P) fixation, which decreases the availability of this nutrient to

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plants.2-4 Such losses can be minimized by controlling nutrient release and

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maintaining a temporally homogeneous availability of phosphate at moderate

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levels in soil.5-7

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One technique that can be employed to provide controlled delivery is the

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use of coatings that restrict the rate of solubilization of the fertilizer, hence

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altering the process of fixation in the soil.8-10 Considering the dynamics of

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phosphate assimilation by plants, the effect of such coatings can lead to the

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delivery of smaller amounts of phosphate, with fixation in the soil components

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being reduced due to lower anion migration. This effect was studied by

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Figueiredo et al. (2012), who observed better performance in terms of yield and

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total dry matter production of maize, compared to the use of uncoated

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fertilizers.11 Garcia et al. (1997) reported that pine lignin coatings increased the

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availability of P in the soil and reduced fixation.12 The coating of diammonium

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phosphate (DAP) with rosin has also been shown to provide control of P fixation

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in the soil, with coated fertilizer enabling the maintenance of P availability for

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longer periods.13-14


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Previous studies have not described the structures of coatings, and have

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not correlated the chemical properties of coated materials with agronomic

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outcomes. Hence, there is a need for improved information concerning suitable

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coatings designed specifically for agricultural applications. Many of the

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materials currently available employ coatings developed for other applications,

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which have been tested for use with phosphate without detailed investigation of

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their chemical characteristics.

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A recent study about our group described the kinetics of N release in a

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biodegradable oil-based polymeric coating system.15 However, there are

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several concerns about the effectiveness of this strategy to non-volatile

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fertilizers, such as DAP. Especially for phosphate fertilizers, the main drawback

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for efficient use is related to the reaction against Fe or Al sources, widely

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present in tropical soils – which leads to very low soluble compounds

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immobilizing the phosphate. Our findings show that the slow DAP release has a

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role in the reduction of its phosphate immobilization by a continuous nutrient

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supply, avoiding local saturation and consequently optimizing the fertilizer

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usage. Therefore, despite the paper investigates the effect of different coating

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thickness in this fertilizer release, the main goal of this paper is show that

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polyurethane (PU) coatings may be an effective strategy for phosphate-based

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fertilizers management, which is a considerable advance for the chemical

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processing of such products.

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In addition, the present work also proposes the use of an oil-based

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hydrophobic polymer system to control the release of phosphorus from DAP

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granules. A systematic characterization of this material was performed, and

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evaluation was made of the delivery behavior and availability of phosphate in an



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Oxisol. The findings contribute to an improved understanding of the effects of

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coatings on phosphate fertilizers, supporting future strategies for better fertilizer

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management.

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MATERIALS AND METHODS

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Preparation of materials

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Granules of diammonium phosphate (DAP), typically 3 mm in diameter,

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were coated using a castor oil-based polyurethane resin system (here denoted

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PU). DAP was chosen due to its high phosphate content, high solubility, and

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granular morphology, which enabled it to be employed as a model for different

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soluble granular phosphate fertilizers. The preparation of the coating followed

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the methodology of Bortoletto-Santos et al. (2016), and the resin was

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synthesized using commercial MDI (4.4'-diphenylmethane diisocyanate, Bayer).

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The oil:MDI mass ratio used was 60:40, and the quantities were calculated from

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the mass of DAP. The resin was dispersed over the granules using a metal

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turntable coater with 25 cm inner side flaps, at 30 rpm rotation, under a flow of

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air heated at 145-155 ºC.

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The quality of the coatings was first evaluated by deposition onto glass

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slides, with selection of the conditions that resulted in the most visually

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homogeneous layers. The coatings were prepared at mass ratios from 1.5 wt%

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to 9.0 wt% (in steps of 1.5 wt%). For example, in the process employing 1.5

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wt% coating, 15 g of polymer were added to 1.0 kg of DAP. The coating

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process involved only one stage for each percentage prepared, with the

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required mass of resin being added to the metal dish coater containing the


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granules using a needleless syringe. The same procedure was performed for

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each of the coatings.

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After preparation of the material, the actual coating percentage was

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determined in order to check for any significant difference from the theoretical

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coating percentage. For this, a 0.5 g sample of the coated granules was

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comminuted and solubilized in distilled water for 15 min with the aid of an

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ultrasonic stirrer. The solution was then filtered to obtain only the coating, which

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was dried in an oven at 35 °C for 48 h, followed by weighing. The results are

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shown in Table 1.

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Physical and chemical characteristics of the coatings

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Scanning electron microscopy (SEM)

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Morphological characterization of DAP and the polymer coatings was

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performed by SEM, using a JEOL JSM 6510 microscope coupled to an energy

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dispersive X-ray (EDX) analysis system (Thermo Scientific). For analysis, the

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samples were dispersed on a carbon ribbon attached to the surface of a metal

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disc (stub), followed by coating with gold in an ionization chamber (MED 020,

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Bal-Tec). The images were acquired using the following conditions: 200 or 500

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magnification levels, 10 kv tension and secondary detector.

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Water release assays

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Water release assays were used to determine the rate of fertilizer release

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according to time, at room temperature. The experimental conditions followed

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the methodology described by Bortoletto-Santos and Ribeiro (2014), in which a

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beaker containing a known mass of the coated material was immersed in a



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larger beaker containing water, under constant stirring, hence ensuring

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homogeneous release of DAP. This arrangement ensured that the release was

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only due to diffusion through the coating, and not the mechanical action of the

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stirrer.16

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Phosphorus determination

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The determination of phosphorus followed the methodology presented by

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Plotegher and Ribeiro (2016), involving the preparation of a mixed reagent (25

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mL of a 25% solution of sulfuric acid + 5.5 mL of 9.5% ammonium molybdate

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solution + 0.6 mL of 3.25% antimony and potassium tartrate solution), together

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with solutions of ascorbic acid (7%) and citric acid (2.2%).

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A 5 mL volume of the diluted sample was mixed with 0.2 mL of citric acid

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solution + 2 mL of ascorbic acid solution + 2 mL of the mixed reagent solution.

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This mixture was heated at 50 °C for 10 min, forming a blue complex that was

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analyzed at a wavelength of 800-900 nm using a UV-Vis spectrophotometer

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(1601PC, Shimadzu).17

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Phosphorus release in soil

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The release of phosphorus into soil from the DAP coated with different

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proportions of PU polymer was evaluated in an incubation experiment. DAP

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granules, with or without coating, were applied to an Oxisol collected from the

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surface layer (0-20 cm) of an agricultural soil in São Carlos (São Paulo State,

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Brazil). Samples of soil with particle size smaller than 2 mm showed the

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following characteristics: 433 g.kg-1 (sand), 35 g.kg-1 (silt), and 532 g.kg-1 (clay),

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according to soil texture analysis using the pipette method; equivalent moisture


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200 g.kg-1; pH (H2O) 5.18, measured with a glass electrode; potential

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extractable acidity 2.6 cmolc H + Al kg-1; available P 0.7 mg.kg-1; remaining

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phosphorus (P-rem) 2.1 mg.L-1; organic carbon 7.56 g.kg-1, measured by the

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Walkley-Black method; and cation exchange capacity (CEC) 4.23 cmolc.kg-1.18-

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Samples (20 g) of soil were incubated in Petri dishes (5.5 cm diameter)

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with DAP granules, with or without a polymeric coating, using a P:soil ratio of

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1:2000 g.g-1. After incorporation of the granules, soil moisture was standardized

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to 80% of water-holding capacity and was maintained constant during the

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incubation period, with the addition of groundwater whenever necessary.

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The samples were incubated at a controlled temperature of 25 °C for 24,

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72, 168, 336, and 662 h. After each incubation period, the remainder of the DAP

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granules was removed and the soil was dried at 40 °C. The phosphorus

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remaining in the granules was indirectly estimated from the remaining N

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content, which was measured by mass spectrometry (CHN-S/O 2400 Series II

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Elemental Analyzer, PerkinElmer). The available phosphorus in the soil was

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determined by Mehlich-1 extraction using a soil:extractant ratio of 1:50 (g.mL-1),

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with quantification as described by Embrapa (1979).

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The P-rem value of each sample was expressed as a percentage relative

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to the P applied as DAP. The recovery profile of available phosphorus in the soil

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was calculated using means and standard deviations. Differences between

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mean values were determined by analysis of variance (ANOVA), and

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differences between treatments were compared using Fisher’s test (p 1, we assume that this indicates a strong influence of the

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coating on retarding the release, which is gradually allowing the diffusion

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(accelerating the process) by opening pores or degrading the structure. The n

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values close to 1 to uncoated and 1.5 wt% coated sample indicate that these

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samples are typically solubilizing and diffusing in the medium, i.e., the 1.5 wt%

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sample probably have significant uncoated parts where the dissolution takes

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place without any barrier. The similar n values for the other PU coated samples

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(≈ 2) indicate that the coating is probably complete in the samples, imposing a

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continuous diffusion barrier for phosphate dissolution. It is noteworthy that these

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samples differ mainly in k values, indicating that this process velocity is

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governed by coating thickness; and in t0, which may be interpreted as the

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necessary time to the coating attain equilibrium to the solution. These results

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also show that PU coated samples are changing their structure during release,

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probably by the interaction with water opening pores or void spaces.

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In this case, the similar n and k values for 6.0 and 7.5 wt% PU coated

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samples indicate that the diffusion is continuously occurring through the coating,

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whereas the strong deviation observed for 9.5 wt% PU coated sample indicate

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that the coating is changing its structure during release. Similar results were

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observed for urea-coated granules, showing that the chemical interaction of PU

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coating to water is almost the same. The adjust constants in modified Ritger-

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Peppas model were depicted in Table 2.

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A soil incubation experiment was conducted to evaluate the effectiveness

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of the castor oil-based PU resin for DAP coating in order to control the release



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of phosphorus. Figure 4 and Table 3 shows a plot of the remaining P content of

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the DAP granules after different incubation periods. The results indicated that

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the treatments could be classified into three groups. The first "fast release"

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group showed total P release in the first 24 h after incorporation of the fertilizer

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in the soil, and included the untreated DAP and the treatments using 1.5 wt%

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and 3.0 wt% coatings (Table 3). The second "slow release" group showed

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release of around 60% during periods of between 168 h and 672 h, and

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included the DAP treatments using 4.5 wt%, 6.0 wt%, and 7.5 wt% PU. The

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third "extended release" group consisted of the DAP coated with 9.0 wt% PU,

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which showed only 6.0% release of P in the first 168 h, and 95% release after

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672 h. These results were in agreement with the release of P into water from

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coated DAP (Figure 2), reflecting a positive correlation between the release

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tests performed using water and soil. As suggested by the release tests using

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water (Figure 2), the poor release control exhibited by DAP coated with 1.5 wt%

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or 3.0 wt% PU could be attributed to the presence of imperfections (cracks) in

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the coating, showing that 3.0 wt% PU was not sufficient to adequately cover the

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granules. However, further increase in the proportion of PU increased the

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thickness of the coating, thereby reducing the imperfections. Hence, the 9.0

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wt% PU treatment, which resulted in the greatest thickness of coating,

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prevented any significant release of P for up to 168 h after incorporation into the

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soil. However, after this time, the degradation of the coating by the action of

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microorganisms present in the soil was intensified, leading to an increase in the

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quantity of pores (cracks) in the coating and simultaneously increasing the

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release of phosphorus.


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Figure 5 shows the recovery profile of available phosphorus in the soil for

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different periods of incubation of the DAP granules. The soil phosphorus

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concentration reflected the profile of release from the granules, and the

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treatments could be grouped in the same way as for the data shown in Figure 6.

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For the uncoated DAP and the granules coated with 1.5 wt% and 3.0 wt% PU,

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the maximum concentration of available phosphorus in the soil was observed at

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168 h. However, the DAP treated with 4.5 wt%, 6.0 wt%, and 7.5 wt% PU

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presented the highest concentrations at 336 h, reflecting slower release. On the

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other hand, in the case of the DAP treated with 9.0 wt% PU, the concentration

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started to increase after 168 h and then continued to increase until the end of

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the incubation period.

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Table 4 shows the average values for P available in the soil during the

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incubation period. Due to the rapid and similar release of P from the uncoated

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DAP and the granules coated with 1.5 wt% and 3.0 wt% PU, the available P

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content of the soil exceeded 53 mg.kg-1 up to 24 h. For the treatment using DAP

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coated with 4.5 wt% PU, there was a significant increase in available P content

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from 72 h of incubation. In the case of the treatments with DAP coated with 6.0

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wt% and 7.5 wt% PU, there were increases in the soil P content from 168 h,

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while the use of DAP coated with 9.0 wt% PU resulted in an increase from 336

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h of incubation (after 14 days).

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The soils treated with uncoated DAP or DAP coated with 1.5 wt% and

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3.0 wt% PU showed maximum contents of available phosphorus after 168 h,

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with values exceeding 100 mg.kg-1. However, these contents decreased to

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about 70 mg.kg-1 by the end of the incubation period, corresponding to

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reductions of between 38.4 and 43.3 mg.kg-1. This could be attributed to the



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high P adsorption potential of the colloids present in the Oxisol used in the

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incubation experiment, as indicated by its low pH (5.18), high acidity potential

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(2.6 cmolc.kg-1), low available P content (0.7 mg.kg-1), and low P-rem value (2.1

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mg.L-1).23

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Smaller decreases of available phosphorus were observed for the

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treatments using DAP coated with 4.5 wt%, 6.0 wt%, and 7.5 wt% PU, with

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reductions of between 13.8 and 23.3 mg.kg-1. The lower phosphorus adsorption

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associated with these treatments could be explained by the shorter contact time

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of P with the soil, due to its slow release from the DAP granules. In contrast, the

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treatment using DAP coated with 9.0 wt% PU, with greater thickness of the

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coating, showed an increase in P release up to 672 h of incubation, with an

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available P content of 95 mg.kg-1. The latter finding could be attributed to the

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extended release of P from the DAP granules, which provided higher levels of P

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for longer periods after fertilization of the soil.

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In summary, the findings demonstrated that the coating of a soluble P

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source (DAP) with PU had an appreciable effect on the availability of

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phosphorus in an acid tropical soil. This effect was influenced by the coating

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thickness, since the use of thin coatings resulted in poor surface quality due to

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the irregularity of the granules, with the formation of cracks in the coating

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causing accelerated release of phosphorus. The results showed that thicker and

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more homogeneous coatings provided satisfactory control of P release, leading

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to extended availability of P in the soil over longer periods.

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ACKNOWLEDGMENTS


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The authors thank to financial support given by CAPES, FAPESP (grant

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#2016/09343-6),

CNPq

(402287/2013-4),

SISNANO/MCTI,

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Embrapa AgroNano research network for the support provided.

FINEP

and

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CORRESPONDING AUTHOR

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*Embrapa Instrumentação (CNPDIA), Rua XV de Novembro 1452, São Carlos,

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SP, 13560-970, Brazil.

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Phone: +55 16 2107 2915; E-mail: [email protected]

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Ritger, P.L.; Peppas, N.A. A simple equation for description of solute release

Novais, R.F.; Mello, J.W.V. Relação solo-planta. In: Fertilidade do Solo, Eds.

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TABLES

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Table 1. Actual coating amount as a percentage of the total weight, after drying. Sample DAP 1.5 wt% DAP 3.0 wt% DAP 4.5 wt% DAP 6.0 wt% DAP 7.5 wt% DAP 9.0 wt%

Experimental percentage (%) 1.9 ± 0.2 3.2 ± 0.3 4.5 ± 0.2 6.0 ± 0.2 7.5 ± 0.3 8.8 ± 0.4

406 407

Table 2. Release kinetic parameters according to the Ritger-Peppas model for

408

phosphorus released. Material

k (h-1) x 10-3

n

t0 (hours)

50.9 34.7 0.36 0.39 0.07 0.07 0.005

0.98±0.01 0.96±0.09 2.24±0.10 1.90±0.05 1.82±0.20 1.79±0.13 1.88±0.06

0 0 2 3 5 5 24

DAP (uncoated) DAP 1.5 wt% DAP 3.0 wt% DAP 4.5 wt% DAP 6.0 wt% DAP 7.5 wt% DAP 9.0 wt% 409 410

Table 3. Fertilizer phosphorus recovery as P-remaining from granules of DAP

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or DAP-coated applied to the soil after different incubation periods. Treatment

P-remaining (%) after incubation period (hours) 0 24 168 672

DAP (uncoated) DAP 1.5 wt% DAP 3.0 wt% DAP 4.5 wt% DAP 6.0 wt% DAP 7.5 wt% DAP 9.0 wt% 412

*

413

(Tukey’s test, P

Role of polymeric coating on the phosphate availability as fertilizer

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