Novel Slow-Release Multielement Compound Fertilizer with

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Novel Slow-Release Multielement Compound Fertilizer with Hydroscopicity and Moisture Preservation Boli Ni, Mingzhu Liu,* Shaoyu Lu¨, Lihua Xie, Xu Zhang, and Yanfang Wang State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu ProVince and Department of Chemistry, Lanzhou UniVersity, Lanzhou 730000, People’s Republic of China

To improve the utilization of fertilizer and water resources at the same time, a novel slow-release multielement compound fertilizer (SMF) with the function of water retention was prepared. The fertilizer nutrients (11.3% N, 9.6% P2O5, 6.1% K2O, and 0.76% Cu) were entrapped in the alginate matrix granules, and the sodium alginate-g-ploy(acrylic acid) (SA-g-PAA) superabsorbent polymer was used as a coating material. The structural and chemical characteristics of the product as well as its efficiency in slowing nutrient release and water evaporation in soil were examined. The degradation of the SA-g-PAA coating was assessed by examining the weight loss with incubation time in soil. Additionally, the nutrient release mechanism was proposed and the diffusional exponent n was calculated. These studies showed that the product with good slow-release and water-retention properties, being partially degradable in nature, could be expected to have wide potential applications in modern agriculture and horticulture. 1. Introduction In the past 40 years, global food production more than doubled and thus responded to the doubling of the world population, mainly through the use of high-yielding varieties of grain, massive increases in nitrogen (∼700%) and phosphorus (∼350%) fertilization, increased reliance on irrigation water, and a comparable increase in pesticide use. Although modern agriculture increased food production, it also caused extensive environmental damage, such as nitrate pollution of agricultural landscapes and groundwater resources and nitrogen- and phosphorus-driven eutrophication of terrestrial, freshwater, and near-shore marine ecosystems.1 Public concern about such challenges has increased the drive toward developing slow-release fertilizers (SRFs). SRFs have obvious and great advantages compared with common readily available fertilizers, such as a decreasing fertilizer loss rate, supplying nutrient sustainably, lowering application frequency, and minimizing potential negative effects associated with overdosage.2,3 It is important to mention that using SRFs has brought about innovation for entire cultivation systems. Shaviv and Mikkelsen4 proposed that SRFs can be generally classified into four types: (i) inorganic materials of low solubility, such as metal ammonium phosphates; (ii) chemically or biologically degradable low solubility materials, such as urea-formaldehyde; (iii) relatively soluble materials that gradually decompose in soil; and (iv) water-soluble fertilizers controlled by physical barriers (e.g., coating and matrix formation). Recently, hydrogel materials used as a matrix or coatings in slow-relsase formulations have attracted more and more attention.5-8 Superabsorbents are polymers with a network structure and an appropriate degree of cross-linking that can absorb a large amount of water. Because of their excellent characteristics, superabsorbent polymers have been successfully used as soil amendments in the agriculture and horticulture industries to improve the physical properties of soil with respect to increasing both its water-holding capacity and the nutrient retention of sandy soils to be comparable to that of silty clay or loam. * To whom correspondence should be addressed. Tel.: +86-9318912387. Fax: +86-931-8912582. E-mail: [email protected].

Superabsorbents potentially influence soil permeability, density, structure, texture, evaporation, and infiltration rates of water through the soil. In particular, superabsorbents can reduce irrigation frequency, improve nutrient retention in the soil, and increase the soil’s aeration and microbial activity.9,10 However, as the primary product of superabsorbents, the polyacrylate cannot be widely used because of its poor in degradability in soil and its accumulation over time to become a new type of pollution. Therefore, environmentally safe, degradable superabsorbent materials are expected to be used. Alginate is a typical polysaccharide obtained from brown algae and is used in many biomedical applications.11,12 This material is typically considered to be degradable and easily gels under gentle conditions with the addition of divalent cations.13,14 Alginate-based superabsorbent polymers could be prepared by graft polymerization with acrylic acid monomers onto a chain of alginate and subsequent cross-linking. The grafting polymers are expected to be widely used in agriculture and horticulture. Nitrogen (N), phosphorus (P), and potassium (K) are the three essential elements, and plants require large amounts of them for adequate growth. Micronutrients are also very important to the growth and development of plants, though plants use them in very small quantities. They protect plants from disease and improve the assimilation of other nutrient elements. Copper (Cu) is an essential micronutrient for plant growth. It can increase plants resistance to drought and disease caused by funguses. It also aids in the synthesis of chlorophyll and it is contained in the enzymes responsible for seed and fruit formation.15 In this study, double copper potassium pyrophosphate trihydrate was synthesized as the source of micronutrient copper fertilizer. In this study, we prepared a novel slow-release, superabsorbent system compounding a cross-linked alginate matrix with sodium alginate-g-poly(acrylic acid) polymers. The nutrient contents were entrapped in the alginate matrix granules, and the sodium alginate-g-poly(acrylic acid) superabsorbent polymers were used as an outer coating. This combination of synthetic and natural polymers can help maintain the water absorbence capacity of poly(acrylic acid) while partially improving its degradability. The system was expected to have slowrelease properties and absorb water and preserve moisture at

10.1021/ie9019769  2010 American Chemical Society Published on Web 04/22/2010

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the same time. In addition, low-water-soluble compound double copper potassium pyrophosphate trihydrate was used as the source of micronutrient copper fertilizer, which could decrease the copper loss rate and the resulting serious environmental hazards, compared with those of water-soluble copper fertilizers. The research has been ongoing to prepare a multifunctional fertilizer that can be used in agroindustry. 2. Experimental Section 2.1. Materials. Sodium alginate (SA, the viscosity of a 2% solution is 3200 mPa · s at 25 °C) was obtained from Qingdao Haiyang Chemical Co. (China). Acrylic acid (AA, chemical grade, Beijing Eastern Chemical Works, Beijing, China) was distilled at reduced pressure before use. N,N′-Methylene bisacrylamide (NNMBA) was recrystallized from 95% ethanol prior to use. Cupric sulfate pentrahydrate and potassium pyrophosphate trihydrate (analytical grade, Sinopharm Chemical Regent Co., Ltd., Shanghai, China) were used as purchased. All other chemicals were analytical grade and used as received. 2.2. Synthesis of Double Copper Potassium Pyrophosphate Trihydrate (K2Cu3(P2O7)2 · 3H2O). Double copper potassium pyrophosphate trihydrate was synthesized according to a previously reported procedure.16 Briefly, the cupric sulfate solution (0.1 M) was added to the potassium pyrophosphate solution (0.1 M) at a constant flow rate of 1 mL/ min, up to a Cu2+/P2O74- molar ratio of 1:1. During the synthesis, the temperature was maintained at 25 ( 1 °C. The suspension was left for 24 h at 25 °C for aging, and then the precipitate was filtered and vacuum dried for 48 h. 2.3. Preparation of SA-g-PAA Superabsorbents. A series of samples with different amounts of cross-linker and SA were prepared according to the following procedure. A certain amount of SA was first dissolved in 20 mL of distilled water under vigorous stirring in a four-necked flask equipped with a stirrer, thermometer, and gas inlet tube. The solution was stirred and deaerated with nitrogen for 15 min. Then the solution of 5.0 g of AA (partially neutralized by an 8 mol/L NaOH solution), ammonium persulfate (APS), and NNMBA was added to the flask dropwise and stirred constantly. The water bath was heated slowly to 75 °C and maintained at this temperature for 3 h. Finally, the resulting polymers were dried, milled, screened, and stored for further use. 2.4. Preparation of Slow-Release Mutielement Compound Fertilizer (SMF). Dried K2Cu3 (P2O7)2 · 3H2O powder (0.25 g), KH2PO4 (10 g), and urea (8 g) were placed into a glass beaker and stirred with 30 mL of 3% (w/w) SA solution. The mixed solution was then added dropwise to a 5% (w/w) CaCl2 aqueous solution and stirred constantly. The drops immediately turned into granules (about 4 mm in diameter) because the SA in the drop was cross-linked with Ca2+ at once. The granules were placed on a pan with SA-g-PAA powder (below 110 mesh) and shaken. In this manner, SA-g-PAA powder could adhere to the surface of the granules. The process was completed until a compact homogeneous coating formed on the fertilizer granules. The coated granules were dried in an oven at 70 °C to obtain the final products. 2.5. Fourier Transform Infrared Spectroscopy. Fourier transform infrared (FTIR) spectroscopy was carried out with a KBr disk using a Nicolet Nexus 670 FTIR spectrometer. In the case of sodium alginate-g-poly(acrylic acid), the dried products were ground into powder and then extracted in a Soxhlet extractor with a solution of acetic acid and ethylene glycol (2:3 v/v) for 72 h.

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2.6. Component Analysis of SMF. The nitrogen content in the SMF was determined with a 501 ammonia-selective electrode. The phosphorus, potassium, and copper contents in the SMF were determined by an inductively coupled plasma (ICP) instrument (American TJA Corp., model IRISER/S). 2.7. Measurement of Water Absorbency of SA-g-PAA. The accurately weighed superabsorbents (0.2 g, 40-90 mesh) were immersed into a certain amount of tap water or soil solution and were allowed to soak at ambient temperature for 60 min. The swollen polymers were filtered and weighed. The water absorbency (WA) was calculated using eq 1 WA )

M - M0 M0

(1)

where M and M0 refer to the weight of the swollen and dried superabsorbents, respectively. 2.8. Slow Release Behavior of SMF in Soil. The sandy soil used in this study is representative of the area of Lanzhou, which lies in northwest of China and is a dry, semidesert region with an average annual precipitation of 300 to 600 mm. To study the slow-release behavior of SMF in soil, the following experiment was carried out: 1 g of SMF was well mixed with 200 g of dry soil (below 26 mesh) and kept in a 200 mL glass beaker that was properly covered and incubated for different periods of time at room temperature. Throughout the experiment, the soil samples were maintained at 30 wt % water-holding capacity by periodically weighing and adding tap water if necessary. After 1, 2, 5, 10, 15, 20, 25, and 30 days, the remaining granules in the beaker were picked out and then dried at 70 °C to a constant weight to be estimated for the contents of N, P, K, and Cu. The SMF without superabsorbents was used as a control. 2.9. Measurement of the Water Retention of SMF in Soil. To measure the water-retention behavior of soil with and without SMF, a glass beaker containing 250 g of dry soil (below 26 mesh) was used. The glass beaker was 7 cm in diameter and 12 cm in height. SMF (2 g) was placed 6 cm beneath the surface of the soil in the form of a spot application, and then 165 g of tap water was slowly added to the beaker and weighed (marked M1). A control experiment without SMF was also carried out. The beaker was maintained at room temperature and weighed every 2 days (marked Mi) over a period of 24 days. The water evaporation ratio (WE%) of the soil was calculated from eq 2: (M1 - Mi) × 100 (2) 165 2.10. Degradation of SA-g-PAA Superabsorbents in Soil. Degradation of the superabsorbents was monitored by the dry weight loss. The SA-g-PAA superabsorbents with different amounts of SA used in this experiment were prepared in a tube. They were cut into disks and then dried. The dried disks (9-10 mm in diameter and 0.45-0.65 mm in thickness) were buried 6 cm beneath the surface of the soil in a plastic box at ambient temperature. The soil moisture was kept at 20-25%. After 10, 20, 30, 40, 70, and 100 days, the disks were taken out, washed with distilled water and vacuum dried to a constant weight. The percentage of degradation (PD%) of the superabsorbents was calculated from eq 3 WE% )

PD% )

(W0 - Wt) × 100 W0

(3)

where W0 and Wt are the weight of superabsorbent disks before and after degradation, respectively.

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Figure 1. Morphology of the dry (a) and swollen (b) SMF samples. Table 1. Characteristics of SMF characteristics

value

nitrogen content phosphorus (P2O5) content potassium (K2O) content copper content diameter of dry sample SA-g-PAA coating percentage

11.3% 9.6% 6.1% 0.76% 4.0-5.0 mm 33-37%

3. Results and Discussion 3.1. Morphology and Characteristics of SMF. Figure 1 shows the morphology of the dry and the swollen samples. As shown in Figure 1b, the structure of SMF was the core/shell structure. Its core contained nutrients in the Ca2+-cross-linked SA matrix, and the outer coating consisted of cross-linked SAg-PAA superabsorbent polymers. The characteristics of SMF are presented in Table 1. 3.2. FTIR Analysis of PAA, SA, and SA-g-PAA. The FTIR spectra of PAA, SA, and SA-g-PAA are shown in Figure 2. PAA (Figure 2a) has a strong carboxyl stretching absorption band at 1724 cm-1. Some characteristic peaks in the SA spectrum (Figure 2b) can be ascribed to 892 cm-1 for the glycosidic bonds’ stretching vibration, 1030 cm-1 for C-O(H) stretching vibration, and 1615 and 1420 cm-1 for the -COOasymmetric and symmetric stretching vibrations, respectively. It is noted that in the spectrum of SA-g-PAA (Figure 2c) a characteristic PAA peak (1734 cm-1 for CdO stretching of the -COOH group) is observed. Meanwhile, there exist characteristic SA peaks (1034 cm-1 for C-O(H) stretching, and 877 cm-1 for the glycosidic bonds), indicating the grafting reaction of AA on SA. Scheme 1 shows the possible mechanism of the grafting reaction. The sulfate anion radicals produced from the thermal decomposition of APS abstract hydrogen from the hydroxyl groups of the polysaccharide substrate to form corresponding alkoxy radicals on the substrate. The alkoxy radicals in active centers on the substrate initiate the polymerization of AA and lead to a graft copolymer.17 3.3. Effect of NNMBA Content on Water Absorbency. One of the important properties of SMF is the water absorbency due to the coating of SA-g-PAA superabsorbent polymers. To improve the water absorbence capacity of the products, the reaction parameters were optimized. The water absorbency as a function of NNMBA content was investigated for cross-linked SA-g-PAA superabsorbents, as shown in Figure 3. As we can see from Figure 3 (reaction conditions include the neutralization degree of AA, 60%; initiator, 0.5 wt %; SA, 10 wt %; reaction time, 3 h; and reaction temperature, 75 °C), the maximum absorbency is achieved at 0.08 wt %. The water absorbency increases when the cross-linker content is lower than 0.08 wt %. This is due to incomplete conversion of the cross-linking process, leading to the presence of PAA homopolymers, which do not contribute to water absorption. When the cross-linker content is larger than 0.08 wt %, a further increase in the crosslinker content results in the cross-linking degree increasing and the same for the number of elastically effective chains, which consequently causes a decrease in the water absorbency. These results are in accordance with Flory’s network theory.18

Figure 2. FTIR spectra of PAA (a), SA (b), and SA-g-PAA with 10% SA (c).

3.4. Effect of SA Content on Water Absorbency. The effect of SA content on the water absorbency of the superabsorbents is shown in Figure 4 (reaction conditions include the neutralization degree of AA, 60%; initiator, 0.5 wt %; cross-linker, 0.08 wt %; reaction time, 3 h; and reaction temperature, 75 °C). The maximum water absorbency (374 g/g) is achieved when the SA content is 10 wt %. The water absorbency increases when the SA content is below 10 wt %, and decreases with higher content of SA. The reason for the change can be explained as follows: when the SA content is low, the AA monomer is superfluous in the reaction system. The superfluous AA turns out to be a homopolymer, which in turn results in an increase in soluble materials at fixed cross-linking density.19 When the amount of SA is larger than 10 wt %, the viscosity of the reaction mixture increases, which hinders the movement of the reactants. Thus, the grafting ratio and the molecular weight of the grafted PAA chains decrease, resulting in a decrease in water absorbency. 3.5. Effect of Ions in Soil on Water Absorbency. SA-gPAA superabsorbents are ionized hydrogels with swelling behavior that depends on the characteristics of the chemical structure and the medium. Considering that the SMF granules that we prepared are applied in soil, the effect of ions on water absorbency is investigated in soil solutions (pH 8.2) with different concentrations. Different amount of dry soil (below 70 mesh) were added to 200 g of tap water with stirring for 30 min and then were allowed to sit undisturbed. The supernatant solution was decanted and used as the swelling medium. Figure 5 shows the effect of the concentration of the soil solution on water absorbency. As shown in Figure 5, the water absorbency decreases with the increase in the concentration of soil solution. Therefore, the presence of ions in the swelling medium has a profound effect on the absorbency behavior of the superabsorbents. According to Donnan equilibrium theory, osmotic pressure is the driving force for the swelling of the superabsorbents. In soil solution with a higher concentration, the osmotic pressure difference between the polymeric network and the external solution decreases, resulting in a decrease in the water absorbency. In addition, the penetration of counterions (such as Na+, K+, Mg2+, and Ca2+ in soil solution) into the polymeric network makes the screening effect of them on anionic group (-COO-) more evident,20 which also decreases the water absorbency of the superabsorbents. Furthermore, in the case of multivalent

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Scheme 1

Figure 5. Effect of ions in soil on water absorbency. Figure 3. Effect of NNMBA content on water absorbency.

Figure 4. Effect of SA content on water absorbency.

cations, “ionic cross-linking” at the surface of the SA-g-PAA superabsorbents causes an appreciably decrease in water absorbency.21 3.6. Slow-Release Behavior of SMF in Soil. Figure 6 shows the slow-release behavior of nitrogen (N), phosphorus (P2O5), potassium (K2O), and copper (Cu) from SMF without and with superabsorbents in soil at room temperature. As shown in the Figure, the release rate of N, P, and K from SMF without

superabsorbents is much greater than that of SMF. In particularly, the release rate of N, P, and K from SMF without superabsorbents (Figure 6A) is more than 87 wt % within 1 day. In contrast, those released from SMF (Figure 6B) are comparatively low: about 53.3, 43.2, and 20.1 wt % for N, P, and K, respectively. For SMF, the release rate of N is higher than that of P, K, and Cu, 76.1 wt % within 5 days. As a neutral organic molecular, urea cannot be adsorbed easily by the charged SA-g-PAA and SA hydrogels, so it would quickly dissolve in the soil solution after immersion in soil. However, compared with the untreated urea granules, of which 98.5 wt % of N was released within 12 h,22 the N in SMF possesses a preferable slow-release property. The nutrient K has a lower release rate than N and P, 73.2 wt % within 30 days. This could be the result of electrostatic interaction between K+ and negatively charged -COO- in the SA-g-PAA and SA networks. The nutrient Cu in K2Cu3(P2O7)2 · 3H2O of SMF without and with superabsorbents has an excellent slow-release property (6.5 and 4.7 wt % within 15 days, respectively), which is available in the soil so as to be assimilated by plants for a long period of time as a result of the solubility equilibrium of K2Cu3(P2O7)2 described by the follow equation: K2Cu3(P2O7)2 h 2K+ + 3Cu2+ + 2P2O47

(4)

The nutrient-release mechanism of SMF in soil can be described as follows: (1) The SA-g-PAA superabsorbent coating was swollen in a soil solution and then transformed into a

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Figure 7. Water-retention behavior of soil without SMF (a) and with SMF (10% SA in SA-g-PAA) (b). Table 2. Diffusional Exponent n and Correlation Coefficients r for Nitrogen (N), Phosphorus (P2O5), Potassium (K2O), and Copper (Cu) Released from SMF in Soil nutrients n r

Figure 6. Slow-release behavior of nitrogen (N) (a), phosphorus (P2O5) (b), potassium (K2O) (c), and copper (Cu) (d) from SMF in soil: (A) without superabsorbents and (B) with superabsorbents (10% SA in SA-g-PAA).

hydrogel after being added to soil. (2) The absorbed water continued to diffuse into the cross-linked network of SA, dissolving the soluble urea and potassium dihydrogen phosphate and the slightly soluble double copper potassium pyrophosphate trihydrate. (3) Nutrients were released into the soil through the dynamic exchange of free water. To gain insight into the release mechanism, the following diffusion model described by the Ritger-Peppas equation23 was applied to fit the experimental data Mt ) ktn M∞

(5)

where Mt/M∞ represents the fractional nutrient release at time t, k is a kinetic constant, and n is a diffusional exponent characterizing the release mechanism. If n e 0.5, then the nutrients diffuse and are released following Fickian diffusion. When n ) 1, case II transport occurs, leading to zeroth-order release. When the value of n is between 0.5 and 1, non-Fickian transport is observed. Table 2 shows the values of n and correlation coefficients r for nitrogen (N), phosphorus (P2O5), potassium (K2O), and copper (Cu) released from SMF in soil. The n value for N, P, and K is in the range of 0.17 to 0.32, indicating Fickian diffusion transport trends. The n value for Cu is 0.77, indicating that the nutrient-release mechanism is non-Fickian-type transport and is a result of both the diffusion and decomposition of K2Cu3(P2O7)2 · 3H2O. 3.7. Water-Retention Behavior of Soil with SMF. Another principal property of the SMF that we prepared is its water-

N

P

K

Cu

0.17 0.98

0.18 0.93

0.32 0.92

0.77 0.98

retention capacity. Figure 7 shows the water-retention behavior of soil with and without SMF. It can be found that the water transpiration ratio of soil without SMF had reached 35.2 wt % on the 10th day whereas that of soil with SMF was 29.7 wt %. After 20 days, the water content of the soil without SMF was 28.5 wt % whereas that of soil with SMF was 34.8 wt %. The reason is that the soil with 0.8 wt % SMF granules can absorb more water than soil without SMF and allows the absorbed water to be released slowly when the soil moisture decreases. At the same time, fertilizer nutrients could also be released slowly with water. The results indicate that the addition of SMF to soil could obviously increase the water retention of the soil. Similar observations have been reported by others.10,24,25 Consequently, the product could improve the utilization of water and fertilizer nutrients efficiently and is expected to have great potential applications in arid areas. 3.8. Degradation of SA-g-PAA Superabsorbents. Weight loss is a simple evaluation of the degradation progress of polymers.26,27 In this work, the degradation of SA-g-PAA was monitored by examining the weight loss of superabsorbents with incubation time in soil at ambient temperature, as shown in Figure 8. A decrease in weight with time demonstrates the degradability of the hydrogels. It has been shown that the type of degradable link and the structure of the network play important roles in the control of the degradation behavior.28 In this study, the connectivity of the SA-g-PAA hydrogel network was maintained by the glycosidic bonds of the sodium alginate molecules and the alkyl linkages formed via CdC polymerization (shown in Scheme 1). Therefore, degradation of the hydrogel is caused by the breakage of the glycosidic bonds of the sodium alginate molecules and the ether bonds of grafted PAA. During the initial stage of degradation (10 days), the breakage of a small number of glycosidic bonds and ether bonds cannot damage the whole hydrogel network, but the lattice size of the networks will enlarge. As a result, the swelling ratio of the superabsorbents increases, resulting in adsorption for ions

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of China (grant no. 20090211110004) and the Gansu Province Project of Science and Technologies (grant no. 0804WCGA130). Literature Cited

Figure 8. Degradation of SA-g-PAA superabsorbents with different amounts of SA: (a) 0, (b) 4, (c) 10, and (d) 16%.

and soil particle increases. This may explain why there is a weight increase in the first 10 days for all cases. The percentage of degradation (PD%) increases with increasing SA content. During the period investigated, the sample with 16% SA lost weight faster than the others whereas the weight loss of pure PAA (0% SA) and the sample with 4% SA was almost unchanged during the course of degradation in soil. As shown in Figure 8, after 100 days, the final PD values were -14.7, 1.8, 13.7, and 25.5% for samples with 0, 4, 10, and 16% SA, respectively. It is understandable that a higher content of SA will form more glycosidic bonds and ether bonds, consequently leading to greater weight loss. These results indicate that the SA-g-PAA superabsorbent material is partially degradable and can be used as a coating for the fertilizers to alleviate the environmental pollution that is caused by the conventional nondegradable superabsorbent polymer materials. 4. Conclusions A slow-release multielement compound fertilizer (SMF) was prepared in the laboratory. The nutrients (H2NCONH2, KH2PO4, and K2Cu3(P2O7)2 · 3H2O) were entrapped in the alginate matrix granules, and the cross-linked sodium alginate-g-poly(acrylic acid) (SA-g-PAA) superabsorbent polymer was used as a coating. The product contained 11.3% nitrogen, 9.6% phosphorus (via P2O5), 6.1% potassium (via K2O), and 0.76% copper. The product had good slow-release properties: nutrients N, P, and K had release values of 95.1, 87.3, and 73.2% after being incubated in the soil for 1 month, respectively. In particular, micronutrient Cu in the product had a release value of only 9.2% within 30 days, which is available in the soil for assimilation by plants for a longer period of time. The waterretention experiment showed that the product could significantly improve the water-holding capacity and decrease the moisture evaporation of the soil. Moreover, the partially degradable superabsorbent materials used as coatings can alleviate the pollution that was caused by conventional nondegradable superabsorbent polymer materials. All of the results of the present work indicate that the SMF may be expected to have wide potential applications in modern agriculture and horticulture. Acknowledgment We gratefully acknowledge the financial support of the Special Doctorial Program Fund of the Ministry of Education

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ReceiVed for reView December 12, 2009 ReVised manuscript receiVed April 3, 2010 Accepted April 13, 2010 IE9019769