Article Cite This: J. Agric. Food Chem. 2018, 66, 5762−5769
pubs.acs.org/JAFC
Water- and Fertilizer-Integrated Hydrogel Derived from the Polymerization of Acrylic Acid and Urea as a Slow-Release N Fertilizer and Water Retention in Agriculture Dongdong Cheng,*,†,‡ Yan Liu,† Guiting Yang,† and Aiping Zhang§ †
National Engineering Laboratory for Efficient Utilization of Soil and Fertilizer Resources, National Engineering & Technology Research Center for Slow and Controlled Release Fertilizers, College of Resources and Environment, Shandong Agricultural University, Tai’an, Shandong 271018, China ‡ State Key Laboratory of Nutrition Resources Integrated Utilization, Shandong Kingenta Ecological Engineering Company, Ltd., Linyi, Shandong 276700, China § Institute of Agricultural Environment and Sustainable Development, Chinese Academy of Agricultural Sciences, Beijing 100081, China ABSTRACT: To reduce the preparation cost of superabsorbent and improve the N release rate at the same time, a novel lowcost superabsorbent (SA) with the function of N slow release was prepared by chemical synthesis with neutralized acrylic acid (AA), urea, potassium persulfate (KPS), and N,N’-methylenebis(acrylamide) (MBA). The order of influence factors on the water absorbency property was determined by an orthogonal L18(3)7 experiment. On the basis of the optimization results of the orthogonal experiment, the effects of a single factor on the water absorption were investigated, and the highest water absorbency (909 g/g) was achieved for the conditions of 1.0 mol urea/mol AA ratio, 100% of AA neutralized, K+, 1.5% KPS to AA mass fraction, 0.02% MBA to AA mass fraction, 45 °C reaction temperature, and 4.0 h reaction time. The optimal sample was characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Swelling behaviors of the superabsorbent were investigated in distilled water and various soil and salt solutions. The water-release kinetics of SA in different negative pressures and soils were systematically investigated. Additionally, the maize seed germination in various types of soil with different amounts of SA was proposed, and the N could release 3.71% after being incubated in distilled water for 40 days. After 192 h, the relative water content of SA-treated sandy loam, loam, and paddy soil were 42, 56, and 45%, respectively. All of the results in this work showed that SA had good water retention and slow N-release properties, which are expected to have potential applications in sustainable modern agriculture. KEYWORDS: integrated water and fertilizer, superabsorbent, urea, acrylic acid
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INTRODUCTION Water is one of the main factors that improve crop production. However, due to rising water and irrigation costs, drought and water shortages have been plaguing world agricultural development. According to estimates, 84% of the cultivated area in the 1 440 000 acres of the world’s arable land was dry cropland, and the loss of agricultural production caused by drought and water shortages every year in the world amounts to more than the sum of the losses caused by other factors;1 hence, it is important and necessary to use water resources efficiently. In the past few years, the study of superabsorbents (SAs) as water management materials for agricultural and horticultural applications has received increasing attention.2−4 Practical applications have also indicated that superabsorbent polymers display a promising future in applications of increasing the survival rate and simultaneously alleviating the drought stress effects on the plants in arid and semiarid areas.5−7 At the same time, nutrients are other important factors which limit the growth and yield of crops.8 Nitrogen (N) is a key element in plant nutrition and greatly affects the crop yield. However, the utilization rate of N-based fertilizer is relatively low and accounts for only about 30−50%.9 Inefficient nitrogen absorption increases farmers’ input costs and brings about © 2018 American Chemical Society
many environmental problems. It is a major challenge to improve the efficient use of nitrogen, to reduce farmers’ input costs, and to decrease the environmental impact of N losses while maintaining the crop yields. These shortcomings can be overcome by using slow-release fertilizers (SRFs), which release fertilizers to plants gradually at a certain rate to coincide with the plant’s nutritional requirements while simultaneously reducing fertilizer loss.10,11 Water scarcity, environmental pollution caused by the excessive application of fertilizers, and the high costs of irrigation and fertilizers demand a greater increase than ever in the grain yields of crops with less water and less fertilizer in China.12 Thus, there is an increasing need to develop water-saving and N-fertilizer-efficient technologies for the economically and environmently friendly production of crops. The water holding capacity and nutrient retention of sandy soils can be improved via the combination of superabsorbent and SRFs. The aeration and microbial activity of soil can be increased, the influence of water-soluble fertilizers Received: Revised: Accepted: Published: 5762
February 14, 2018 May 9, 2018 May 21, 2018 May 21, 2018 DOI: 10.1021/acs.jafc.8b00872 J. Agric. Food Chem. 2018, 66, 5762−5769
Article
Journal of Agricultural and Food Chemistry on the environmental will be mitigated, and the frequency of irrigation can be lowered at the same time.13 Currently, multifunctional slow-release fertilizer with water conservation and nutrient release is prepared by coating a water-absorbing resin in the uncoated fertilizer.14−16 However, the coating process is limited by the surface characteristics of fertilizer granules, coating material, and production process, which is bound to result in complex processes, rising costs, and the limiting of their use in most high-value crops.17 Therefore, it is necessary to prepare materials with retention and fertilizerrelease functions using other means. Chemical-reaction grafting is a simple material multifunctional method, and if the graft reaction between the fertilizer and water absorbent is carried out, it will solve the cost problem caused by the expensive process and materials. Nowadays, most super water absorbents (SA) are obtained by solution or inverse-suspension polymerization techniques from acrylic acid and its salts and acrylamide (AM).18 The above raw materials are expensive, and acrylamide is harmful to human health.19 It is well known that superabsorbent can absorb 100 times its own weight because of the chemical or physical cross-linkings of individual polymer chain.20−22 The highly water-absorptive polymer possesses a linear structure, which contains many strong water-absorption groups including −COO−, −OH, and −NH2.21−23 Therefore, increasing the number of water-absorbent groups is an important way to increase the water absorption of a material. Urea is a low-cost material with an amide structure. It has excellent hydrophilicity and is also a major source of crop nitrogen. The price of urea is just 1/10 to 1/15 that of acrylic amide, showing potential application for water-retention materials with a nitrogen sustained-release function. However, urea is a hydrophilic small molecule that can be quickly dissolved in water. Therefore, it should be modified to be employed as a highly water-absorbent resin. Herein, a low-cost super-absorbent hydrogel (SA) was prepared by a cross-linking reaction of acrylic acid (AA) and urea with potassium sulfate and N,N’methylenebis acrylamide (MBA) as an initiator and crosslinker. The synthesis conditions and properties of SA were studied to determine the optimal synthesis parameters. This work not only innovated the theory and method of “water and fertilizer integration” but also achieved the simultaneous slow release of water and fertilizer so as to achieve the goal of water and fertilizer cooperation in farmland.
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Figure 1. Synthesis schematic of SA. the oxygen-free nitrogen gas was bubbled into the solution for 30 min. Then, an aqueous solution with a certain amount of KPS was added. The oil bath was kept at a target temperature to complete the polymerization process in the prescribed reaction time. After the reaction, the product was dried to constant weight at 85 °C. All samples used for tests were ground into particles with diameters of between 40 and 70 mm. Determination of Water Absorbency. A series of weighed dried samples (0.1 ± 0.0001 g) were immersed in distilled water to achieve swelling equilibrium at room temperature (about 30 min). Then, swollen samples (M2) were filtered through a 100-mesh nylon net and suspended for 10 min until there was no free water on the surface. The equilibrium water absorbency (Qeq, g/g) was calculated according to the following equation Q eq =
M 2 − M1 M1
(1)
where Qeq (g/g) is the water absorbency per gram of dried sample and M1 (g) and M2 (g) are the weights of the dry and the swollen samples, respectively. Measurement of Water Retention in Different Soil Types. The three types of soils are sandy loam, loam, and paddy soil (Table1).
Table 1. Soil Texture
MATERIALS AND METHODS
Materials. Acrylic acid (AA, chemical purity, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was used after vacuum distillation. Potassium persulfate (KPS) and N,N’-methylenebis(acrylamide) (MBA, chemical purity, Tianjin Kermel Chemical Reagent Co., Ltd., Tianjin, China) were used as received. NaOH, KOH, and urea (analytical grade, Tianjin Yongda Chemical Co., Ltd., China) were used as received. Other agents were all analytical grade, and the solutions were prepared in distilled water. Preparation of Superabsorbent. Figure 1 shows the scheme for the preparation of the low-cost superabsorbent. A series of superabsorbents with different water-absorption capacities were prepared as follows: Initially, 100 mL of AA was neutralized with a 20 wt % KOH or 20 wt % NaOH solution in a 1 L glass beaker under ice-bath conditions and mixed thoroughly. Second, an appropriate amount of urea and N,N’-methylenebis(acrylamide) (MBA) were added to the beaker while stirring until they dissolved. After that, all of the mixed solution was transferred into a 250 mL three-necked flask equipped with a mechanical stirrer, nitrogen line, and reflux condenser. The reactor was immersed in an oil bath. Before adding the initiator,
soil types
field capacity (%)
clay (g/kg)
sand (g/kg)
silt (g/kg)
loam sandy loam paddy soil
17.72 6.27 28.83
185 61 341
191 793 321
624 146 338
SA (0.5 g) and dry soil (50 g, below 10 mesh) were fully mixed and placed in a plastic cup, and then 417 mL of distilled water was slowly dripped into the beaker and weighed (W1). The controlled experiment was also carried out (W2) without SA. The beaker was kept at 25 °C and weighed (Wt) at selected time points (0, 24, 48, 72, 96, 120, 168, and 192 h). All of the water-holding tests were conducted three times, and the average value was used for plotting. The water retention (WR %) of soil was calculated from eq 2
WR% =
Wt − W2 × 100% W1 − W2
(2)
where WR% is the soil water retention, W1 and Wt are the total masses of 0.5 g of SA, soil, the plastic cup, and the water content of the different treatments before and after soil culture, respectively, and W2 is the total mass of the control (no SA). Measurement of Water Retention at Different Negative Pressures. Dried SA (0.1 g) was maintained in 250 mL of distilled water for swelling equilibrium at room temperature. The swollen 5763
DOI: 10.1021/acs.jafc.8b00872 J. Agric. Food Chem. 2018, 66, 5762−5769
Article
Journal of Agricultural and Food Chemistry
Table 2. Factors and Levels of the Orthogonal Experimenta
sample was weighed (W) and then placed in a pressure membrane instrument and weighed at various negative pressures. The waterretention ratio at different negative pressures was calculated by eq 3
WR% = 1 −
W − Wp W
× 100%
(3)
where WR% is the water retention percentage and Wp is the mass of the sample at negative pressure p. Water Absorbency in Different Salt Solutions. The water absorbency in different salt solutions (KH2PO4, K2SO4, and NH4Cl) at concentrations of 0.2, 0.6, and 1.0% was tested according to the same procedures described in Section 2.3. Characterization. FTIR spectra were recorded on a NEXUS-470 series FTIR spectrometer (Thermo Nicolet, NEXUS). The samples were initially dried under vacuum until they reached a constant weight. Later on, the dried samples were ground into powder, mixed with KBr powder, and then pressed to make a pellet for FTIR characterization. SEM images were taken on JSM-5600LV equipment after the sample was coated with a gold film for good conductivity. Growth Experiments. Sandy loam, loam, and paddy soil ( B > F > A > C > D > G. The MBA to AA mass fraction is the most influential factor, the percentage of AA neutralization degree followed, and then the reaction temperature, urea to AA mole ratio, cationic species, KPS to AA mass fraction, and reaction time are last. According to the orthogonal experiment results, the best combination of synthesis conditions is A2B3C2D2E1F1G2, and the corresponding conditions are as follows: 1.0 mol/mol urea to AA mole ratio, 100% of AA neutralized, K+, 2.5% KPS to AA 5764
DOI: 10.1021/acs.jafc.8b00872 J. Agric. Food Chem. 2018, 66, 5762−5769
Article
Journal of Agricultural and Food Chemistry
Table 3. Orthogonal L18(3)7 Experiments of Urea, the Percentage of AA Neutralized, Cationic Species, KPS, MBA, Reaction Temperature, and Reaction Timea sample no.
A (mol/mol)
B (%)
C
D (%)
E (%)
F (°C)
G (h)
Qd (g/g)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
1(0.4) 1(0.4) 1(0.4) 2(1.0) 2(1.0) 2(1.0) 3(1.6) 3(1.6) 3(1.6) 1(0.4) 1(0.4) 1(0.4) 2(1.0) 2(1.0) 2(1.0) 3(1.6) 3(1.6) 3(1.6)
1(70) 2(85) 3(100) 1(70) 2(85) 3(100) 1(70) 2(85) 3(100) 1(70) 2(85) 3(100) 1(70) 2(85) 3(100) 1(70) 2(85) 3(100)
1(Na+) 2(K+) 3(Ca2+) 1(Na+) 2(K+) 3(Ca2+) 2(K+) 3(Ca2+) 1(Na+) 3(Ca2+) 1(Na+) 2(K+) 2(K+) 3(Ca2+) 1(Na+) 3(Ca2+) 1(Na+) 2(K+)
1(0.5) 2(2.5) 3(5.0) 2(2.5) 3(5.0) 1(0.5) 1(0.5) 2(2.5) 3(5.0) 3(5.0) 1(0.5) 2(2.5) 3(5.0) 1(0.5) 2(2.5) 2(2.5) 3(5.0) 1(0.5)
1(0.02) 2(0.10) 3(0.20) 2(0.10) 3(0.20) 1(0.02) 3(0.20) 1(0.02) 2(0.10) 2(0.10) 3(0.20) 1(0.02) 1(0.02) 2(0.10) 3(0.20) 3(0.20) 1(0.02) 2(0.10)
1(45) 2(55) 3(65) 3(65) 1(45) 2(55) 2(55) 3(65) 1(45) 2(55) 3(65) 1(45) 3(65) 1(45) 2(55) 1(45) 2(55) 3(65)
1(3.0) 2(4.0) 3(5.0) 3(5.0) 1(3.0) 2(4.0) 3(5.0) 1(3.0) 2(4.0) 1(3.0) 2(4.0) 3(5.0) 2(4.0) 3(5.0) 1(3.0) 2(4.0) 3(5.0) 1(3.0)
157.7 171.5 28.6 51.8 142.3 699.1 64.1 317.0 189.1 40.2 41.2 665.2 246.5 226.6 164.5 52.7 231.2 94.7
a
A, Urea to AA mole ratio; B, the percentage of AA neutralized by a 20% KOH solution; C, cationic species; D, KPS to AA mass fraction; E, MBA to AA mass fraction; F, reaction temperature; G, reaction time; and Qd, the water absorbency in the distilled water.
Table 4. Analysis of the Orthogonal L18(3)7 Experimenta K1b K2 K3 Rc influencing order optimal combination
A
B
C
184.1 255.1 158.1 97.0
102.2 188.3 306.9 204.7
139.3 230.7 227.4 91.4
D
A2
B3
C2
E
213.9 382.8 237.1 129.0 146.4 82.2 90.7 300.6 E>B>F>A>C>D>G D2 E1 A2B3C2D2E1F1G2
F
G
238.9 228.4 129.0 109.9
152.7 233.4 211.6 80.7
F1
G2
a
A, Urea to AA mole ratio; B, the percentage of AA neutralized by a 20% KOH solution; C, cationic species; D, KPS to AA mass fraction; E, MBA to AA mass fraction; F, reaction temperature; and G, reaction time. bK1 = (∑water absorbency in the distilled water of a single factor)/4 cR = max K1 − min K1.
electrostatic repulsion of contiguous −COO− groups limited the free movement of bonds and weakened the storage capacity of the polymers’ microspore structure. The water absorbency of hydrogels was reduced due to the resulting dense network structure. Effect of Reaction Temperature on Water Absorbency. With the other parameters unchanged, the influence of reaction temperature from 25 to 75 °C was investigated. Figure 2D shows that the water absorbency increased significantly when the temperature increased from 25 to 45 °C and then decreased relative to the temperature further increasing. When the reaction begins, the chain formed by the polymerization is a short chain, which is bad for water absorption. On the other hand, a long reaction time will lead to an increase in the degree of polymer cross-linking. Multiple branched chains were formed in the network structure, and they tangled with each other to produce small holes in three-dimensional networks, obstructing the expansion of the polymer. Hence, too long or too short of a reaction time resulted in a low water absorbency.28 Effect of the Ratio of Urea to AA on Water Absorbency. Different components and monomer ratios have an obvious influence on the properties of superabsorbent resin. To increase the absorption performance and reduce the
manufacturing costs of the superabsorbent, urea was introduced as a nonionic water-absorbent monomer. At the same time, another monomer, AA, was used to improve the water absorbency ability (Qeq). It can be seen in Figure 2E that the maximum value of Qeq was obtained when the mole ratio of urea/AA was 1.6. An initial increase of Qeq was apparently observed when the mole ratio was less than 1.6. On the contrary, the Qeq decreased when the mole ratio was greater than 1.6. Such a phenomenon was caused by the cooperation effect of the two monomers. Urea can improve the water ability of the superabsorbent, which results in the increase in Qeq in water in the initial stage. However, AA has a stronger hydrophilicity than urea, and the Qeq decreased with the urea further increasing. Effect of the Initiator Amount on Water Absorbency. The water absorption initially increased and then decreased with the increase in initiator dosage (Figure 2F). The maximum absorbency in distilled water could reach 909 g/g. It was found that the amount of initiator had an obvious effect on the water absorbency of hydrogels, which was consistent with the relationship between the initiator concentration and the average chain length during the polymerization process. AA monomers could be polymerized because the KPS initiator could generate many free-radical reactive sites. When the dosage of KPS 5765
DOI: 10.1021/acs.jafc.8b00872 J. Agric. Food Chem. 2018, 66, 5762−5769
Article
Journal of Agricultural and Food Chemistry
Figure 2. (A) FTIR spectra of SA, urea, and MBA and the influence of different factors on water absorption; (B) quality ratio of MBA/AA(g/g), (C) neutralization degree of AA (%), (D) temperature, (E) urea/AA (mol), and (F) initiator content.
stretching vibration. For SA, the characteristic peaks of urea were observed (1669 cm−1 for the −CO stretching of −CONH2 and 1160 cm−1 for the bending of the amide bands), and the active hydrogen of −NH2 in 3455 cm−1 was obviously weakened. The appearance of these bands confirms the formation of SA material. Morphological Analyses. In order to identify the surface and internal structure of SA, the scanning electron microscope (SEM) technique has been applied. The SEM image of SA (Figure 3a) showed that a large number of pore structures with sizes lower than 0.1 mm were uniformly distributed in them. These porous structures were very favorable for the absorption and retention of water. Besides, it could be found that the interior was filled with three-dimensional polymeric networks in SA (Figure3b). The highly porous structures with interlinked channels formed by urea, AA, and MBA are beneficial to more water molecules diffusing into the network of SA, leading to a higher swelling ratio.
initiator was low, the SA superabsorbent with a threedimensional structure was formed, and the water absorbency was improved. However, excess KPS initiator accelerated AA self-polymerization, but it also reduced the amount of reaction between AA and urea. In the meantime, with the increase in the amount of KPS initiator, the three-dimensional structure of SA became denser, resulting in a decrease in water absorbency. From the above single-factor experiments, it can be concluded that the optimal conditions with maximum water absorbency (909 g/g) were found as follows: 1.0 mol/mol urea to AA mole ratio, 100% of AA neutralized, K+, 1.5% KPS to AA mass fraction, 0.02% MBA to AA mass fraction, 45 °C reaction temperature, and 4.0 h reaction time. FTIR Measurements. Figure 2A shows the FTIR spectra of urea, SA, and MBA. The characteristic peaks in the urea spectra are ascribed to the following: 3440 and 1687 cm−1 could be assigned to the −NH2 and CO stretching of −CONH2. For MBA, the peak at 3300 cm−1 corresponded to the CC 5766
DOI: 10.1021/acs.jafc.8b00872 J. Agric. Food Chem. 2018, 66, 5762−5769
Article
Journal of Agricultural and Food Chemistry
Figure 3. SEM micrographs of (a) surface and (b) internal structure.
Water Retention at Various Negative Pressures. The water absorbed and transported by the plant was powered by root pressure and the negative pressure of transpiration.29 Therefore, only the water absorbed in a certain negative pressure range is effective for plants, and not all water can be absorbed by plants. Figure 4 displays the relationship between
Figure 5. Water absorbency in different salt solutions.
monovalent cationic (K+) solution, and the sensitivity of superabsorbent to various cations was PO43− < SO42− < Cl−.28 Water Retention in Various Soils. The water retention property of superabsorbent used in agriculture and horticulture is a key property for soil water conservation. It has positive effects on improving soil quality, raising the survival rate of seedlings, and promoting the growth of plants. In order to invest the water retention effect of SA in different kinds of soil, experiments on a water-retaining agent were carried out in three types of soil (sandy loam, loam, and paddy soil). Figure 6
Figure 4. Water retention at various negative pressures. (Water absorbency in distilled water: A, 909 g/g; B, 885 g/g; and C, 835 g/g).
negative pressure and the water retention of SA. With the increase in negative pressure, the moisture absorbed by SA is gradually released. The water retention values of A, B, and C were 42.6, 29.9, and 62.8 wt % at 0.1 MPa, respectively. When the negative pressure was more than 0.3 MPa, the downward trend in the water retention of A, B, and C became slow with the increase in negative pressure. According to the literature,30 the optimum soil water potential is less than 0.8 MPa for plant growth. It can be found that the effective water content, that is, the water that can be used by plants for A, B, and C, were 94.3, 95.6, and 72.4 wt %, respectively. Considering the above, product B has excellent water-retention properties, an important application valve, and broad application prospects in agriculture. Water Absorbency in Different Salt Solutions. The effect of various ions on water absorbency can be concluded from Figure 5. The water absorption rate significantly decreased when the salt concentration of the solutions increased from 0.2 to 1.0% and gently decreased when the concentration rose higher than 0.6%. For ionic hydrogels, the additional cations caused an anion−anion electrostatic repulsion, which led to a decrease in osmotic pressure between the external solution and the polymer network. Therefore, the swelling decreased.31 Second, the order of water absorbency of the hydrogel in the salt solutions followed KH2PO4 > K2SO4 > NH4Cl when the solution concentrations were lower than 1.0 wt %. This was due to the water absorbency in the polyatomic monovalent cationic (NH4+) solution decreasing more than that in a single atom
Figure 6. Daily water retention property of sandy loam, loam, and paddy soil amended with SA.
presents the water absorbency of the SA in sandy loam, loam, and paddy soil, representing the typical soil types in different regions. It can be found that the addition of SA to sandy loam, loam, and paddy soil could significantly improve the waterholding capacity and water-retention properties of soil. After 192 h, the relative water contents of sandy soil, loam, and paddy soil treated with SA were 42, 56, and 45%, respectively. However, the relative water contents of soils (sandy loam, loam, and paddy soil) without SA were only 2, 18, and 4%, respectively. Effect of SA on Maize Germination. SA has a significant influence on the development of maize seeds. Measurements of the seeding lengths showed that the SA treatment with 0.2% less content promoted root growth (Figure 7B). The average seedling lengths of maize seeds treated with the SA were significantly larger than those of the control groups, not only the sandy loam group but also the loam and paddy soil groups. 5767
DOI: 10.1021/acs.jafc.8b00872 J. Agric. Food Chem. 2018, 66, 5762−5769
Article
Journal of Agricultural and Food Chemistry
Figure 7. Effect of SA Content in Different Types of Soil on Seedling Height (A) and Root Length (B) in Maize.
days. This indicated that the N in SA was released very slow and could be applicable to crops with a long duration of fertilizer. Currently, the most slowly releasing fertilizer was prepared by coating common granule fertilizer to isolate the moisture outside the membrane and reduce water dissolution. SA was prepared through chemical synthesis to slow-release dissolution. The mechanism of SA was totally different from that of coated fertilizer. The nutrient release mechanism of SA could be illustrated as follows: (1) under the action of the waterabsorbing group, the SA adsorbed water molecules and became slowly swollen after it was added to distilled water. Thereafter, the three-dimensional network structure of SA became larger, and the pores were filled with free water until the maximum water-absorption rate was reached. (2) The urea molecule was gradually hydrolyzed from the molecular chain and dissolved in the three-dimensional network pores. The dissolved urea would be slowly diffused out of the network pores through the dynamic exchange of free water.
Among all of the groups, the 0.2% treatments had the longest seedling height (Figure 7A), which was almost twice that of the 0 and 0.5%. Besides, it can be found that when the soil SA content exceeded 1%, maize seeds germinated in only loam soil (Figure 7). SA is a new type of water-retention slow-release fertilizer containing a large amount of nitrogen. SA releases nitrogen slowly after it is applied to the soil. Therefore, more nitrogen would be release into the soil solution when the amount of SA increased, and the high nitrogen concentration of the soil inhibited the growth of crop roots. These results indicated that a low concentration of SA (
