Ind. Eng. Chem. Res. 1997, 36, 869-873
869
Manufacture and Evaluation of Coated Triple Superphosphate Fertilizers Ma C. Garcı´a,* A. Vallejo, L. Garcı´a, and Ma C. Cartagena Departamento de Quı´mica y Analisis Agrı´cola, E.T.S.I. Agro´ nomos, Universidad Polite´ cnica de Madrid, Ciudad Universitaria, s/n, 28040 Madrid, Spain
With the aim of obtaining more efficient phosphate fertilizers than those traditionally used with calcareous soils, new coated slow-release fertilizers were produced at laboratory scale. Calcium triple superphosphate was used as the basic fertilizer and Kraft pine lignin for coating. The products obtained with a coating percentage of between 5 and 26% were evaluated by means of laboratory tests for solubility in water at a constant temperature as well as by carrying out physical tests regarding handling and transport. It was found that the rate of P contribution to the water solution is less in the coated products than in the uncoated superphosphate and that the rate constants of the nutrient release process decrease with increasing coating percentages of the fertilizers. The nutrient release in these coated products follows a kinetic equation of the first order. Finally, an incubation test of the experimental fertilizers was carried out in a calcareous soil for 3 mo. It was confirmed that available phosphorus increases with the application of coated phosphate fertilizers in inverse proportion to the coating percentage of the fertilizer. The greatest increase occurs with the product SPL-11 (11% coating) 30 days after its application. Introduction The amount of phosphorus that can be used efficiently by a crop treated with phosphorus fertilizers depends basically on the amount of nutrients available in the soil solution. Available P depends on various factors, among them the type of soil, the ionic species coming from the fertilizers, temperature, water content of the soil, etc. (Bolan et al., 1990; Gonza´lez et al., 1992; Hagin and Harrison, 1993). Phosphorus availability in calcaerous soils as a result of adding a fertilizer occurs immediately after application, and the plant is only able to make use of it for a short period of time (Mengel, 1985; Chand and Tomar, 1993) since it retrogrades rapidly due to fixation and precipitation mechanisms. Split fertilizer application throughout the crop is therefore recommended for this type of soil (Wolf et al., 1987; Shaviv et al., 1989). With the aim of checking whether the use of coated slow-release phosphate fertilizers achieves an effect similar to split or sequential additions, the behavior of a new type of phosphates fertilizer was studied in a calcareous soil and this is the subject of this study. The experimental fertilizers were calcium triple superphosphate coated with Kraft pine lignin and a mixture of three rosins (dimerized, natural, and esterified rosin). Calcium triple superphosphate is a conventional lowcost fertilizer with a high P content than simple superphosphate. Due to its acid pH, its application is very suitable for calcareous soils. Lignin is a residual product with adequate properties to be included in coating fertilizers (Wilkins, 1981). It is cheap, biodegradable, and insoluble in water. In addition, it contributes to increasing the organic matter content in the soil since its degradation leads to the formation of humic substances (Inbar, 1990). This substance has already been used with satisfactory results as a urea coating, together with a mixture of three rosins as adhesives (Garcı´a et al., 1996). In this study the same coating was applied to calcium triple superphosphate. The nutrient release processes of the products obtained were studied, as were their physical S0888-5885(96)00153-4 CCC: $14.00
properties. Finally, they were applied to a calcareous soil with high phosphorus retrogradation power in order to check whether its available phosphorus content increases with the application of these new fertilizers. A problem soil was chosen for its lack of response to phosphorus fertilization with conventional products so that any positive conclusion reached could be extrapolated with regard to other soils with lower phosphorus fixation power. Materials and Methods 1. Preparation and Determination of the Fertilizer Composition. A series of fertilizers with different coating percentages were prepared at laboratory scale, using the method suggested by Jime´nez et al. (1984). Granular calcium triple superphosphate was used as the base fertilizer. It was passed through a 2 mm screen and coated with Kraft pine lignin commercialized by Westvaco under the name of Indulin AT. An ethanol solution was used as an adhesive, consisting of a mixture of the same proportion of three types of rosin: polymerized, esterified, and natural rosin. These products were supplied by Unio´n Resinera Espan˜ola and were chosen based on the results obtained by Garcı´a et al. (1996) when they evaluated urea coated with lignin and rosins. The operation was carried out with 2 kg loads of calcium triple superphosphate in a coating drum at a rotating speed of 60 rpm and a revolving angle of 45°. Ground lignin was added with a particle size of under 20 µm by means of a vibratory spatula, alternating with adding the solution of the adhesive. The solution is sprayed with an automatic pistol at regular intervals. Excess solvent was removed through evaporation using a cold air current. The treatment is repeated as often as necessary until the desired coating percentage is reached in each case. In order to determine the composition of the fertilizers obtained, their components were separated first by means of selective solubilizing processes. The calcium triple superphosphate was solubilized in water, and its content was determined following the spectrophoto© 1997 American Chemical Society
870 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997
metric ammonium molybdate method (AOAC, 1990). The rosins were dissolved in benzene and evaluated together by means of UV-vis spectrophotometry. The same method was used to analyze the lignin content using dioxane as a solvent. 2. Fertilizer Evaluation by Means of Laboratory Tests. By submitting the experimental fertilizers to solubility tests in water and physical tests, the following was determined: 2.1. Rate Constants of the Phosphorus Release Process in Water at Constant Temperature. Released phosphorus of each fertilizer in the series was measured as a function of time at 25 °C. With this in mind, 0.5 g of each product and 10 mL of distilled water were put into a recipient (Wilkins, 1983). When the time corresponding to each test had passed, the samples were filtered and the P content of the filtrate was determined by the spectrophotometric ammonium molybdate method (AOAC, 1990). All the tests were carried out in triplicate, and the average value was taken as the result. By a simple linear correlation of the experimental results obtained, the kinetic order of the nutrient release process and its rate constant was determined for each case. 2.2. Physical Properties. According to the rules suggested by the Tennessee Valley Authority (1970) and the International Fertilizer Development Center (1986), the following properties were studied in some of the experimental fertilizers: grain size, crushing strength, apparent density, critical relative humidity, and humidity absorption rate. A microphotographic study of the coating was also carried out to study the surface and determine thickness. To this aim, fertilizer grains received a metalized coating using a SEM coating unit PS-3. The microphotographs were taken with a SEM “ISISx-25”. 3. Incubation Test. Four treatments were used in this test: a control treatment without phosphorus fertilization, calcium triple superphosphate without coating (SPO), and the experimental fertilizers SPL-5 and SPL-11. No fertilizers were used that had a coating percentage above 11% since the kinetic solubilization constants in water for these products are too small and it is possible that such slow nutrient release would cause them to be inadequate. A calcareous soil was used, characterized by the lack of response to phosphorus fertilization caused by its high phosphorus fixation power. The nutritional characteristics of the soil were obtained using the electroultrafiltration (EUF) technique (Dı´ez et al., 1985). The EUF technique predicts the short-term and long-term availability of P from soils (Ne´meth, 1979; Shimard and Sen Tran, 1993). It consists essentially of an electrodialysis process providing information about a considerable number of parameters regarding soil fertility by using a program of potentials, times, and temperatures applied to a soil suspension in water. Using the EUF method, the nutrient dosage was calculated on the basis of the fertilizer recommendations suggested by Wicklicky (Wicklicky and Ne´meth, 1981). The experiment was carried out in an incubation chamber at a controlled temperature of 20 °C. The experimental design involved random blocks with each treatment tested in triplicate. Pots without a leaching device were used, and 2 kg of soil was placed in each. During the experiment, the pots were watered to field capacity once a week.
Table 1. Description of the Composition of the Base Triple Superphosphate Fertilizer Products Expressed as Percent of Their Components fertilizer
coating
phosphorus
lignin
total rosin
lignin/total rosin
SP-0a SPL-5 SPL-8 SPL-11 SPL-19 SPL-26
0.00 5.00 8.00 11.05 18.98 26.01
19.60 18.62 18.03 17.43 15.87 14.50
3.50 5.62 7.96 13.29 18.20
1.50 2.32 3.09 5.69 7.81
2.33 2.44 2.57 2.33 2.33
a
SP-0: triple superphosphate without coating.
In order to evaluate the evolution of available P throughout the experiment, samples were taken before and after fertilization during 3 mo. Monitoring P availability in the soil was achieved using the electroultrafiltation (EUF) technique (Ne´meth, 1979; Dı´ez et al., 1985). After passing a dried 5 g soil sample through a 1 mm screen, it was subjected to the EUF process in a Vogel 724 using the following program:
Fraction I: Fraction II:
30 min, 20 °C, 200 V, 15 mA 5 min, 80 °C, 400 V, 150 mA
In the first fraction, extraction takes place at a constant voltage of 200 V and a maximum current of 15 mA. In the second fraction, the voltage was raised to 400 V and 150 mA, which causes a progressive increase in the force of the electric field. A total of 5 g of soil, to which 50 mL of water was added, was placed in the central chamber. This soil to water ratio (5/50) was kept constant during extraction, since the water filtered through the ultrafilters located in front of the electrodes is automatically replaced. Filtration speed, with -0.2 bar vacuum pressure, was on the order of 5 mL min-1. In the extracts, P was determined spectrophotometrically using the ammonium molybdate method (AOAC, 1990) by means an automatic analyzer Model AII (Technicon, Spain). Results and Discussion 1. Composition of the Fertilizers and Evaluation by Laboratory Tests. Table 1 gives the composition of all the experimental fertilizers with their components in percentage. The abbreviation SPL used for the coated fertilizers refers to the basic superphosphate and to the lignin in the coating. It is followed by a number which represents the coating percentage of each fertilizer. The lignin/total rosin ratio is between 2.3 and 2.5, which is the best order of magnitude for this kind of coating (Garcı´a et al., 1996). Figure 1 shows the percent of P released in water as a function in terms of time at 25 °C of all the coated fertilizers and of the uncoated calcium triple superphosphate. As can be observed, the nutrient solubilitation rate in water is much slower in the case of coated fertilizers than in uncoated superphosphate. SPL-5, the product with the lowest coating percentage, still contains 51% of nonsolubilized P after 15 days. The P solubilization rate in H2O decreases in all cases as the coating percentage grows. However, for coating percentages above 19%, the solubilitation rate hardly varies, although the coating is increased. The pattern of release achieved by a controlled-release system can be explained by different kinetics orders (Baker, 1987).
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 871 Table 2. Results Obtained by Simple Linear Correlation for SPL Fertilizers between ln I0′ and the Time in Days (n ) 15)
a
fertilizer
K1 (days-1)
ln I0′
ra
SPL-5 SPL-8 SPL-11 SPL-19 SPL-26
0.034 0.028 0.023 0.019 0.015
4.34 4.42 4.48 4.55 4.55
-0.83 -0.89 -0.92 -0.96 -0.95
Significant at P < 0.001.
Table 3. Values of the Release Rate Constant and the Coefficient of Correlation Obtained by Simple Linear Regression for Series ULIIIa and ULIIIAb Fertilizers (n ) 15) Figure 1. Phosphorus release in water, as a function of time, at a constant temperature of 25 °C for SPL fertilizers with different coating percentages.
fertilizer
K1 (days-1)
rc
ULIII-16 ULIII-22 ULIII-29 ULIIIA-18 ULIIIA-34
0.059 0.035 0.025 0.046 0.015
-0.98 -0.97 -0.99 -0.99 -0.98
a A series of fertilizers containing urea, lignin, polymerized, esterified, and natural rosin. b A series of fertilizers containing urea, lignin, polymerized, esterified, and natural rosin and linseed oil. c Significant at P < 0.001.
Table 4. Crushing Strength, Bulk Density, and Percent H2O Absorbed of Prepared Fertilizers
Figure 2. Linear correlation of the experiment results for the first kinetic order of the nutrient release process.
Phosphorus released by each of the coated fertilizers can be represented by a kinetic equation of the first order (Collins, 1983):
ln I ) -K1t + ln I0 where I ) percentage of nonsolubilized P at time t, t ) time (days), I0 ) percentage of initial P (ln I0 ) 4.6), K1 ) first-order release rate constant. With a simple linear regression analysis of the ln I values in terms of time (Figure 2), the values of the slopes (K1) and ordinates at the origin (ln I0′) were obtained for each of the fertilizers as well as their correlation coefficient (r) (Table 2). The level of signification obtained in the correlation is good in all cases (P < 0.001). As calculated values of the intercept were for some products smaller than 4.6, ln I0′ was defined as the intercept of the regression equations obtained. The interpretation of this behavior was that, due to the nature of the coatings, there may have been a variable amount which was immediately solubilized, and, therefore, first-order kinetics were not strictly followed. The values of the rate constants decrease when the coating percentage of the fertilizer grows. However, in the products with more coating, the decrease of the constant is very small. As from a coating of 19%, the growing thickness of the coating hardly modifies the value of the solubility rate constants of the product. When the results of urea with the same coating were studied (Garcı´a et al., 1996) (Table 3), it was confirmed that the nutrient release rate equation through the coating consisting of lignin and a mixture of natural, dimerized, and esterified rosin can be adjusted to a first-
fertilizer
crushing strength (kg‚grain-1)
apparent density (g/cm3)
SP-0 SPL-5 SPL-11 SPL-19
3.60 3.91 4.32 4.85
0.96 0.97 0.99 1.01
% H2O absorbed (RH 95%; T ) 20 °C) 1 day 2 days 3 days 5.2 4.1 3.7 3.0
10.5 8.0 6.2 4.1
15.7 10.8 8.5 4.7
order pattern when the place of urea is taken by calcium triple superphosphate. However, the magnitude of rate constant changes because this depends on the basic fertilizers since the rate constants (K1) of the SPL products are smaller than those of the ULIIIA products sealed with linseed oil. For this reason, no additional sealing agent was used with the SPL fertilizers to reduce their release rate, as was done with the ULIII fertilizers which contain urea. This rate difference in the nutrient release can be explained if it is taken into account that urea solubilizes in water at a much greater rate than calcium triple superphosphate and that the diffusion rate of urea is higher than that of the ion H2PO4-. 2. Physical Properties. 2.1. Grain Size. The grain size of fertilizers is expressed in percentages retained in each sieve. In all the cases, grain size was between 2 and 4 mm in accordance with the European regulation (Berquin and Burko, 1974; Kelly, 1974) and Spanish legislation which stipulates that 85% of the fertilizer be between 1 and 4 mm (Bol. Of. Estado, 1994). 2.2. Crushing Strength. Crushing strength (Table 4) increases directly and proportionally to the coating percentage of the fertilizer. The crushing strength of the product SPL-19 is 25% greater than that of uncoated superphosphate. In all cases it is greater than 2.3 kg/ pellet, in accordance with TVA recommendations (TVA, 1970). 2.3. Apparent Density. The apparent density (Table 4) increases as the fertilizer coating grows, and in all cases its values agree with those suggested by USDA (1977) and Hignett (1985). 2.4. Critical Relative Humidity (RH) and Humidity Absorption Rate. Humidity absorption (Table
872 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997
Figure 3. Surface of a SPL-19 pellet (19% coating) cut through the middle and magnified 200 times. Table 5. Coating Thickness of Some SPL Fertilizers fertilizer
thickness (µm)
extreme values (µm)
SPL-11 SPL-19 SPL-26
95 172 193
83-106 162-188 160-211
4) begins with an RH value of 95% (Hoffmeister, 1979). The amount of H2O absorbed by the coated fertilizers is lower than that of uncoated superphosphate and decreases when the fertilizer coating increases. The water absorption rate of the product stored in a watersaturated atmosphere decreases when the coating thickness of the fertilizers grows. To be more specific, the water percentage absorbed as a function of time changes from 3.35 in the product SPL-5 to 0.85 in SPL-19. Therefore, the results obtained in the physical properties study of the experimental fertilizers prove that they are stable with adequate conditions for handling and transport. 2.5. Microphotographic Coating Study. The microphotographs of the fertilizers obtained for this study made it possible to determine the thickness and homogeneity of their coating. The top and bottom values and the average thickness of the fertilizer coatings are given in Table 5. Thickness increases in proportion to the coating increase of the fertilizer. Figure 3 shows the appearance of the coating of the fertilizer SPL-19, magnified 200 times, while Figure 4 is a SPL-11 pellet cut in half and magnified 36 times. As can be seen in the figures, the coatings look compact and quite homogeneous. There are no external cracks or fissures that would give the nutrient direct and easy access to the exterior.
Figure 4. SPL-11 pellet (11% coating) cut through the middle and magnified 36 times. Table 6. Values of Available Phosphorus (mg of P‚kg-1) Obtained for the Different Treatments in the Incubation Test treatment
time (mo)
available phosphorus (mg of P‚kg-1)
control
0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3
1.7 1.8 2.1 0.9 1.7 1.8 1.8 2.1 1.7 3.1 2.4 3.7 1.7 5.8 5.6 4.0
SP-0
SPL-5
SPL-11
3. Incubation Test. For efficiency evaluation of the coated fertilizers, these were applied to a soil lacking in phosphorus, and then the available phosphorus was analyzed for a period of 3 mo. Table 6 gives the values for available P in milligrams of P for each kilogram of soil, obtained by using the EUF technique for the three fertilizers applied and the control treatment. As can be observed, very similar values are found for the triple superphosphate (SP-0) and the control treatment in the first 2 mo. In the third month, the value for the control treatment is lower. This means that P available in the soil does not increase after adding uncoated superphosphate. After adding coated
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 873
fertilizers, however, in 3 mo higher values of available phosphorus are obtained than those of the control treatment and SP-0. The highest increase appears in the first month, when the soil treated with SPL-5 and SPL-11 increased by 0.14 and 0.41 mg of P‚kg-1 if compared with the initial phosphorus availability. The efficiency of these products increases with the coating percentage. This confirms that coated fertilizers can be more efficient than uncoated triple superphosphate in calcareous soils lacking in phosphorus. By applying them, it is possible to mitigate the loss of this nutrient, caused by fixation processes in this kind of soil. Conclusions New kinds of slow-release fertilizers can be obtained by coating calcium triple superphosphate with Kraft pine lignin. The release rate of the P content under the coating depends on the thickness of the latter. The overall nutrient release process may be adjusted to a first-order kinetic equation. By using these products in calcareous soils which lack phosphorus caused by fixation problems, it was possible to increase available P after 3 mo. It confirms the efficiency of this kind of fertilizer as well as the importance of their use. Literature Cited AOAC. Official Methods of Analysis, 15th ed.; Association of Official Analytical Chemistry, Inc.: Arlington, VA, 1990. Baker, R. W. Controlled release biologically active agents; WileyInterscience: New York, 1987. Berquin, Y.; Burko, J. Hot spherodizer. Process and complex fertilizers. Recents developments. Proceedings of the 24th Annual Meeting of the Fertilizer Industry Round Table, 1974, 10-112. Bol. Of. Estado 1994, No. 167, 22576. Bolan, N. S.; Hedley, M. J.; Harrison, R.; Braith-Waite, A. C. Influence of manufacturing variables on characteristics and the agronomic value of partially acidulated phosphate fertilizers. Fert. Res. 1990, 13, 223-239. Collins, R. L. Design parameters. In Controlled-Release Technology. Bioengineering Aspects; Das, K. G., Ed.; Wiley: New York, 1983; pp 15-22. Chand, T.; Tomar, N. K. Effect of soil properties on the transformation of phosphorus in Alcaline Calcareous Soils. J. Indian Soc. Soil Sci. 1993, 41, 56-61. Dı´ez, J. A.; Cadahı´a, C.; Ga´rate, A.; Revilla, E. Estudio de la dina´ mica de nutrientes mediante electroultrafiltracio´ n como base de la fertilizacio´ n; Malquisa: Madrid, 1985. Garcı´a, M. C.; Dı´ez, J. A.; Vallejo, A.; Garcı´a, L.; Cartagena, M. C. Use of Kraft Pine Lignin in Controlled-Release Fertilizer Formulations. Ind. Eng. Chem. Res. 1996, 35, 245-249. Gonza´lez, E.; Socias, M.; Villafranca, M.; Valverde, A.; Ferna´ndez, M. Phosphate sorption by Almeria Soils. J. Agric. Food Chem. 1992, 40, 2284-2289.
Hagin, J.; Harrison, R. Phosphate rocks and partially-acidulated phosphate rocks as a controlled release P fertilizers. Fert. Res. 1993, 35, 25-31. Hignett, T. P. Fertilizer manual; Martinus Nijhoff and Dr. Junk Publishers: Dordrecht, The Netherlands, 1985. Hoffmeister, G. Physical Properties of Fertilizers and Methods for Measuring Them; TVA Bulletin 4-147; Tennessee Valley Authority: Muscle Shoals, AL, 1979; pp 13-18. Inbar, Y. Humic substances formed during the composting of organic matter. Soil Sci. Soc. Am. J. 1990, 54, 1316-1323. International Fertilizer Development Center. Manual for Determining Physical Properties of Fertilizers; TVA: Muscle Shoals, AL, 1986. Jime´nez, S.; Cartagena, MC.; Vallejo, A.; Castan˜eda, E. Procedimiento para obtener fertilizantes de liberacio´n lenta. Spanish Patent 536567, 1984. Kelly, W. J. Solids handling and metering in an NPK prilling plant. Proc. Fert. Soc. London 1974, 141. Mengel, K. Dynamics availability of major nutrients in soils. Adv. Soil Sci. 1985, 2, 65-115. Ne´meth, K. The availability of nutrients in the soil as determined by electroultrafiltration (EUF). Adv. Agron. 1979, 31, 155187. Saviv, A.; Shachar, N.; Hagin, J. Kinetics of phosphorus reactions in calcareous soils. Commun. Soil. Sci. Plant Anal. 1989, 20, 465-482. Shimard, R. R.; Sen Tran, T. Evaluating plant-available phosphorus with the Electro-Ultrafiltration Technique. Soil Sci. Soc. Am. J. 1993, 57, 404-409. Tennessee Valley Authority (TVA). Procedures for Determining Physical Properties of Fertilizers; Special No. S-444; TVA: Muscle Shoals, AL, 1970. USDA. Fertilizer Specifications; Small Business Memo. No. 773; Agency for International Development, Office of Small Business: Washington, DC, 1977. Wicklicky, L.; Ne´meth, K. Dungunsoptimierung mittels, EUFBoenuntersuchung bey der Zuckrrube. Sonderdruck Band. 1981, 982-988. Wilkins, R. M. Lignins as a formulating agents for controlled release in agriculture. Br. Polym. J. 1981, 15, 177-78. Wilkins, R. M. Determination of Release Rates. In ControlledRelease Technology. Bioengineering Aspects; Das, K. G., Ed.; Wiley: New York, 1983; pp 143-173. Wolf, J.; Wit, C. T.; Janssen, B. H.; Latwell, D. J. Modeling longterm crop response to fertilizers phosphorus. Agron. J. 1987, 79, 445-451.
Received for review March 18, 1996 Revised manuscript received September 20, 1996 Accepted November 18, 1996X IE960153O
X Abstract published in Advance ACS Abstracts, January 15, 1997.
