An Eco-Friendly Polymer Composite Fertilizer for Soil Fixation, Slope Stability, and Erosion Control

In the Loess Plateau region, the poor structure and properties of loess slopes will cause many types of geological disasters such as landslides, mudflow, land collapse, soil erosion, and ground cracking. In this paper, an eco-friendly polymer composite fertilizer (PCF) based on corn straw wastes (CS) and geopolymer synthesized from loess was studied. The characterization by FT-IR of the PCF confirmed that graft copolymer is formed, while morphological analysis by scanning electron microscopy and energy dispersive spectroscopy showed that geopolymer and urea were embedded in the polymer porous network. The effects of PCF contents on the compressive strength of loess were investigated. The PCF was characterized in terms of surface curing test, temperature and freeze-thaw aging property, water and wind erosion resistance, and remediation soil acidity and alkalinity property, which indicates that PCF can improve loess slope fixation and stability by physical and chemical effects. Moreover, the loess slope planting experiment showed that PCF can significantly increase the germination rate of vegetation from 31% to 68% and promote the survival rate of slope vegetation from 45.2% to 67.7% to enhance biological protection for loess slopes. The PCF meets the demands of building and roadbed slope protection and water-soil conservation in arid and semi-arid regions, which opens a new application field for multifunctional polymer composite fertilizers with low cost and environmental remediation.


Introduction
The Loess Plateau is located in the middle and upper reaches of the Yellow River in Northwest China and covers a total area of about 648,700 km 2 . Loess soil is a typical type of aeolian silt, mainly formed by the sedimentation of wind-blown dust, which is sensitive to wind and water erosion [1,2]. Loess slopes have become extremely unstable due to the increasingly fragile environment and highly fragmented and complex terrain, steep slopes, and poor vegetation condition [3,4], and it leads to surface erosion of loess slopes and may even cause severe geological disasters, such as soil erosion, even mudslides and landslides [5]. In addition, the cities in Northwest China, such as Lanzhou City, have been accelerating urbanization to boost their economies and improve living standards [6], and human activities, such as transportation construction, agricultural irrigation, residential extension, and environmental depredation seriously decrease the stability of loess slopes [7,8] and result in soil erosion, which produces on-site soil and nutrient loss, vegetation cover reduction, and land degradation [9].
At present, loess slope protection technologies are mainly focused on theoretical studies, such as slope stability mechanisms [10], water-soil erosion control [11], biological protection by vegetation [12], and the root-soil system model [13]. In terms of practical materials and technology, various traditional treatment methods for loess slope protection 1%, 2%, 3%, 4%, and 5%, based on loess weight), and 5 mL tap water were mixed well. The mixture was put into a plastic mold and pressed tightly. Then, these plastic molds with samples were dried at room temperature for 24 h until the compact crumb structure was formed. The appearance of the loess columns containing different amounts of PCF (based on loess weight) is shown in Figure 1. Measurement was carried out using a press machine (Cangzhou JiluTest Instrument Co., Cangzhou, China) until columns cracked, and the value was recorded as the compressive strength.

Compressive Strength Property
Firstly, the loess column samples (1.50 cm in height and 2.65 cm in diameter) were prepared according to the previous study [25]. In detail, the soil columns mixed with different amounts of PCF were prepared as follows: a certain amount of loess, PCF (0%, 1%, 2%, 3%, 4%, and 5%, based on loess weight), and 5 mL tap water were mixed well. The mixture was put into a plastic mold and pressed tightly. Then, these plastic molds with samples were dried at room temperature for 24 h until the compact crumb structure was formed. The appearance of the loess columns containing different amounts of PCF (based on loess weight) is shown in Figure 1. Measurement was carried out using a press machine (Cangzhou JiluTest Instrument Co., Cangzhou, China) until columns cracked, and the value was recorded as the compressive strength. Figure 1. The appearance of the loess columns containing 0%, 1%, 2%, 3%, 4%, and 5% PCF (based on loess weight), respectively.

Remediation Soil Acidity and Alkalinity by PCF
Firstly, simulated loess solution was obtained by centrifugation: 200 g dry loess was soaked in 1 L distilled water for 24 h and centrifuged under 1000 rpm for 3 min, and the supernatant was obtained as simulated soil solution. The influence of PCF on pH values of soil was measured by different amounts of PCF in simulated soil solution. After the swollen PCF was filtered, the pH value of the filtrate was measured by a pH meter.
Then, the pH values of the simulated soil solution were adjusted with 0.1 mol/L HCl or NaOH aqueous solution. An amount of 0.1 g of PCF samples was immersed in 50 mL of soil solution with different pH values at room temperature. After the swollen PCF was removed, the pH value of the filtrate was determined by a pH meter.

Temperature of the Aging Property
In the thermal test, the loess column samples were exposed to an air-dry oven at 45 °C for 24 days. The compressive strength changes of the loess column samples were measured after different aging times, and the tests were carried out continuously and without interruption in the whole process.

Freeze-Thaw Resistance Performance
In a freeze-thaw cycle, the loess column samples were frozen at −15 °C for 2 h and thawed at 25 °C for 2 h, alternately, and the freeze-thaw test contained seven cycles. The compressive strength of the loess column samples was measured after different freezethaw cycle times.

Surface Curing Test
Firstly, a simulated loess slope was constructed as the following method: 10 kg loess (＜300 mesh, from Lanzhou South Mountain, Lanzhou City, Gansu Province, China) was piled up a simulated loess slope (33 cm length, 15 cm width, 15 cm height, and 45° gradient). An amount of 100 g loess and 5 wt% PCF were mixed and spread evenly on the simulated loess slope surface, and the blank group without PCF was compared as control under the same condition. Then, 350 mL tap water was sprayed on the slope surface and Figure 1. The appearance of the loess columns containing 0%, 1%, 2%, 3%, 4%, and 5% PCF (based on loess weight), respectively.

Remediation Soil Acidity and Alkalinity by PCF
Firstly, simulated loess solution was obtained by centrifugation: 200 g dry loess was soaked in 1 L distilled water for 24 h and centrifuged under 1000 rpm for 3 min, and the supernatant was obtained as simulated soil solution. The influence of PCF on pH values of soil was measured by different amounts of PCF in simulated soil solution. After the swollen PCF was filtered, the pH value of the filtrate was measured by a pH meter.
Then, the pH values of the simulated soil solution were adjusted with 0.1 mol/L HCl or NaOH aqueous solution. An amount of 0.1 g of PCF samples was immersed in 50 mL of soil solution with different pH values at room temperature. After the swollen PCF was removed, the pH value of the filtrate was determined by a pH meter.

Temperature of the Aging Property
In the thermal test, the loess column samples were exposed to an air-dry oven at 45 • C for 24 days. The compressive strength changes of the loess column samples were measured after different aging times, and the tests were carried out continuously and without interruption in the whole process.

Freeze-Thaw Resistance Performance
In a freeze-thaw cycle, the loess column samples were frozen at −15 • C for 2 h and thawed at 25 • C for 2 h, alternately, and the freeze-thaw test contained seven cycles. The compressive strength of the loess column samples was measured after different freeze-thaw cycle times.

Surface Curing Test
Firstly, a simulated loess slope was constructed as the following method: 10 kg loess (<300 mesh, from Lanzhou South Mountain, Lanzhou City, China) was piled up a simulated loess slope (33 cm length, 15 cm width, 15 cm height, and 45 • gradient). An amount of 100 g loess and 5 wt% PCF were mixed and spread evenly on the simulated loess slope surface, and the blank group without PCF was compared as control under the same condition. Then, 350 mL tap water was sprayed on the slope surface and placed in an outdoor environment, and the surface curing of different loess slopes was observed and photographed after 60 days. Meanwhile, the content of aggregate topsoil was determined by the dry-sieving method [26] as follows: 25 cm 2 topsoil of loess slope was removed, weighed (marked as W 0 ), and sieved (<0.25 mm). Then, the remaining topsoil was weighed again (marked as W 1 ), and the content of aggregate soil ratio (COAs%) of the loess slope was calculated from the below equation: 2.9. Washing Resistance Test 10 L/m 2 tap water was sprayed on a simulated loess slope treated with PCF, and another simulated loess slope without PCF was compared as control under the same amount of tap water. Then, photos were obtained from the water permeability of the simulated loess slope. In addition, the cover area and mass of loess washed down from the simulated loess slope were calculated and weighted after natural drying.

Wind Erosion-Resistance Test
Simulated loess slopes with different PCF contents (from 0 wt% to 5 wt%) were blown by simulating natural wind at a speed of 36 kilometers per hour (similar to level 6 of wind force), and the mass loss of slope topsoil was weighted and calculated before and after blowing.

Degradation Behavior of PCF in Soil
To examine the degradation behavior of the PCF, the dry weight loss of PCF was determined. Then, PCF was incubated in loess at a regular time interval, and then the samples were obtained, dried, and weighed. The ambient temperature and the soil humidity were maintained at room temperature and about 30% during the degradation experiment, and the degradation behavior was evaluated by weight change [27].

Influence of PCF on Seed Germination
In order to study the effect of PCF on the germination rate of vegetation, Cyperaceae for greening and biological slope fixation were chosen and cultivated at a density of 10 seeds per 25 cm 2 . A cultivated soil without PCF was compared as a control under the same condition. Then, the same amount of tap water was sprayed every day, and the germination of seeds was observed after a week of incubation.

Slope Planting Experiment
In brief, 100 g loess and 5 wt% PCF were mixed and spread on the surface of a simulated loess slope. A blank group without PCF was used as a control group. Afterward, 400 seeds of Cyperaceae were sown on simulated loess slopes, respectively and two simulated loess slopes were placed at room temperature. After germination, 100 mL water was sprayed every 2 days, and the germination of seeds and cracks on the slopes were observed and recorded after two months of incubation.

Preparation of PCF
The eco-friendly polymer composite fertilizer was prepared by graft polymerization and composite. CS was grafted with polyacrylic acid (PAA), and MBA and urea-loaded geopolymer were blended to form polymer composite gel. Then, PCF was obtained after drying, and the reaction mechanism, chemical structures and photographs of PCF are shown in Figure 2.

FT-IR Analysis
In the spectrum of CS, geopolymer, and PCF ( Figure 3), the characteristic absorption peak of cellulose structures in CS (-OH stretching vibration at 3409 cm −1 and asymmetric stretching vibration at 2938 cm −1 ) was obviously present and the characteristic absorption peaks of geopolymer (Si-O-Si antisymmetric stretch at 1027 cm −1 and bending vibration at 755 cm −1 due to the polycondensation of alternating Si-O and Al-O bonds) appeared in PCF. In addition, two absorption peaks at 1723 and 1665cm −1 corresponded to the -C=O stretching vibration and the C-N stretching vibration from urea, respectively. These results demonstrate that the CS has been successfully grafted to polymer chains, and geopolymer and urea have been successfully embedded in the polymer network.

FT-IR Analysis
In the spectrum of CS, geopolymer, and PCF ( Figure 3), the characteristic absorption peak of cellulose structures in CS (-OH stretching vibration at 3409 cm −1 and asymmetric stretching vibration at 2938 cm −1 ) was obviously present and the characteristic absorption peaks of geopolymer (Si-O-Si antisymmetric stretch at 1027 cm −1 and bending vibration at 755 cm −1 due to the polycondensation of alternating Si-O and Al-O bonds) appeared in PCF. In addition, two absorption peaks at 1723 and 1665 cm −1 corresponded to the -C=O stretching vibration and the C-N stretching vibration from urea, respectively. These results demonstrate that the CS has been successfully grafted to polymer chains, and geopolymer and urea have been successfully embedded in the polymer network.

FT-IR Analysis
In the spectrum of CS, geopolymer, and PCF ( Figure 3), the characteristic absorption peak of cellulose structures in CS (-OH stretching vibration at 3409 cm −1 and asymmetric stretching vibration at 2938 cm −1 ) was obviously present and the characteristic absorption peaks of geopolymer (Si-O-Si antisymmetric stretch at 1027 cm −1 and bending vibration at 755 cm −1 due to the polycondensation of alternating Si-O and Al-O bonds) appeared in PCF. In addition, two absorption peaks at 1723 and 1665cm −1 corresponded to the -C=O stretching vibration and the C-N stretching vibration from urea, respectively. These results demonstrate that the CS has been successfully grafted to polymer chains, and geopolymer and urea have been successfully embedded in the polymer network.

SEM and EDS Analysis
In Figure 4, PCF shows a porous network structure and rough surface caused by the blend of PAA and geopolymer to form a porous polymer network structure. Meanwhile, the surface elements of PCF were measured by EDS analysis, and the uniform presence of C, N, O, Si, and Al is observed, which is consistent with the loess-based geopolymer, corn straw, and urea. It indicates the composite of the polymer, the geopolymer and urea In PCF.

SEM and EDS Analysis
In Figure 4, PCF shows a porous network structure and rough surface caused by the blend of PAA and geopolymer to form a porous polymer network structure. Meanwhile, the surface elements of PCF were measured by EDS analysis, and the uniform presence of C, N, O, Si, and Al is observed, which is consistent with the loess-based geopolymer, corn straw, and urea. It indicates the composite of the polymer, the geopolymer and urea In PCF.

Compressive Strength Property
The compressive strength is an important parameter to evaluate the cohesion and the deformation resistance between soil particles, especially in the loess slopes, where there are large severe landslides and mudslides due to water and soil erosion. To assess the effect of the content of PCF on the loess slope-fixing property, the compressive strength of the loess columns with the different PCF contents (from 1% to 5%, based on the weight of dry loess) was evaluated, and shown in Figure 5. It can be seen that the compressive strength of the soil columns increased with the increase of PCF contents. Compared with the blank group, the compressive strength was increased from 0.89 MPa to 1.82 MPa. It may be due to the swelling of PCF in the loess column sample after absorbing water, which makes the macromolecules chains of polymer in PCF enwrap and entangle the surrounding loess particles and form physical binding between PCF and loess particles.

Compressive Strength Property
The compressive strength is an important parameter to evaluate the cohesion and the deformation resistance between soil particles, especially in the loess slopes, where there are large severe landslides and mudslides due to water and soil erosion. To assess the effect of the content of PCF on the loess slope-fixing property, the compressive strength of the loess columns with the different PCF contents (from 1% to 5%, based on the weight of dry loess) was evaluated, and shown in Figure 5. It can be seen that the compressive strength of the soil columns increased with the increase of PCF contents. Compared with the blank group, the compressive strength was increased from 0.89 MPa to 1.82 MPa. It may be due to the swelling of PCF in the loess column sample after absorbing water, which makes the macromolecules chains of polymer in PCF enwrap and entangle the surrounding loess particles and form physical binding between PCF and loess particles.  Figure 6a shows the effects of different content of PCF on the pH value of loess solution. It can be seen that the pH value of loess solution decreases slightly with increasing PCF. Overall, the addition of PCF has little impact on the weakly acidic simulated loess solution (pH = 6.85).

Remediation Loess Acidity and Alkalinity by PCF
In addition, in order to study the regulating effect of PCF on different pH values of loess solution, the factors of PCF on the acidity and alkalinity of loess are investigated, as shown in Figure 6b. When the pH value of the simulated loess solution is lower than nine,  Figure 6a shows the effects of different content of PCF on the pH value of loess solution. It can be seen that the pH value of loess solution decreases slightly with increasing PCF. Overall, the addition of PCF has little impact on the weakly acidic simulated loess solution (pH = 6.85).

Thermal Aging Property
The effect of thermal aging on the compressive strength of the loess column samples with 1% and 2% PCF is shown in Figure 7a. It can be observed that the compressive strength of samples increased firstly and then decreased gradually with increasing aging time. It may be because the compressive strength of samples improved with increasing cross-linking density of the PCF in the early stage. However, due to the degradation of PCF under excessive and prolonged heating, the compressive strength decreased slightly in the subsequent test, which indicated that the PCF can maintain stability and excellent thermal stability [29].

Freeze-Thaw Resistance Performance
Loess is widely distributed in the northwest of China, where repeated seasonal freeze-thaw cycles damage soil structures [30]. Therefore, the influence of freeze-thaw resistance performance of PCF on loess slope fixation and stability was tested and the results are shown in Figure 7b. It could be seen that the compressive strength of loess column samples with 1 wt% and 2 wt% PCF decreased slightly with increasing freeze-thaw cycles, which may contribute to the destruction on the surface of the loess column samples brought by multiple freeze-thaw cycles. Moreover, the compressive strength of the loess column samples mixed with 1 wt% PCF reduced only 0.1% after seven freeze-thaw cycles, which suggests that the PCF has a good freeze-thaw resistance performance to cope with temperature changes in loess slopes. In addition, in order to study the regulating effect of PCF on different pH values of loess solution, the factors of PCF on the acidity and alkalinity of loess are investigated, as shown in Figure 6b. When the pH value of the simulated loess solution is lower than nine, it can be adjusted to near neutral or weakly acidic after being treated with PCF. When the simulated loess solution pH > 9, PCF had no obvious regulating ability, which indicates that PCF did not interfere with the original alkaline loess environment. It is because PCF has large amounts of -COO − groups, which can react with H 3 O + of the loess solution [28]. Therefore, PCF can regulate the pH value of soil, especially acid soil, and it can be applied in environmental remediation on loess slopes as a kind of amendment.

Thermal Aging Property
The effect of thermal aging on the compressive strength of the loess column samples with 1% and 2% PCF is shown in Figure 7a. It can be observed that the compressive strength of samples increased firstly and then decreased gradually with increasing aging time. It may be because the compressive strength of samples improved with increasing cross-linking density of the PCF in the early stage. However, due to the degradation of PCF under excessive and prolonged heating, the compressive strength decreased slightly in the subsequent test, which indicated that the PCF can maintain stability and excellent thermal stability [29]. samples with 1 wt% and 2 wt% PCF decreased slightly with increasing freeze-thaw cycles, which may contribute to the destruction on the surface of the loess column samples brought by multiple freeze-thaw cycles. Moreover, the compressive strength of the loess column samples mixed with 1 wt% PCF reduced only 0.1% after seven freeze-thaw cycles, which suggests that the PCF has a good freeze-thaw resistance performance to cope with temperature changes in loess slopes.

Freeze-Thaw Resistance Performance
Loess is widely distributed in the northwest of China, where repeated seasonal freezethaw cycles damage soil structures [30]. Therefore, the influence of freeze-thaw resistance performance of PCF on loess slope fixation and stability was tested and the results are shown in Figure 7b. It could be seen that the compressive strength of loess column samples with 1 wt% and 2 wt% PCF decreased slightly with increasing freeze-thaw cycles, which may contribute to the destruction on the surface of the loess column samples brought by multiple freeze-thaw cycles. Moreover, the compressive strength of the loess column samples mixed with 1 wt% PCF reduced only 0.1% after seven freeze-thaw cycles, which suggests that the PCF has a good freeze-thaw resistance performance to cope with temperature changes in loess slopes.

Surface Curing Test
It is well known that surface curing can effectively prevent the slope from runoff. The surface curing of different loess slopes is shown in Figure 8. It can be seen clearly that the loess slope without PCF (Slope A) is uneven and has a lot of large and deep cracks, but the surface treated with PCF (Slope B) is solid and smooth with fewer cracks due to the adhesion and connection of polymer chains in PCF after swelling. In addition, the COAs of different samples in loess slopes were measured and the results are shown in Table 1. The COAs of loess slopes treated with PCF are 91.64% and much higher than the blank group (76.85%), which protects loess slopes against wind and rain erosion. Furthermore, the effects of application methods on the COAs of loess were also studied comparatively. Table 2 shows that the COAs of dry and wet loess by the soil-mixing method are higher than that of the spray-seeding method. Therefore, it can be concluded that the application of PCF could retard evaporation, retain moisture, and effectively prevent soil erosion on loess slopes over a long period of time in arid areas. The COAs of loess slopes treated with PCF are 91.64% and much higher than the blank group (76.85%), which protects loess slopes against wind and rain erosion. Furthermore, the effects of application methods on the COAs of loess were also studied comparatively. Table 2 shows that the COAs of dry and wet loess by the soil-mixing method are higher than that of the spray-seeding method. Therefore, it can be concluded that the application of PCF could retard evaporation, retain moisture, and effectively prevent soil erosion on loess slopes over a long period of time in arid areas.  The content of PCF is 1 wt%.

Washing Resistance Property
The washing resistance property of loess slopes with PCF was investigated. Figure 9 shows that there is no obvious change in the loess slope treated with PCF after spraying with tap water. However, the surface of the loess slope without PCF has some cracks and there is a lot of loess washed downhill from the slope. Meanwhile, the cover area and mass of loess washed down were measured, and the results are shown in Table 3. It can be

Samples COAs
Slope A 76.85% Slope B 91.64% Table 2. Effects of application methods on the COAs.

Washing Resistance Property
The washing resistance property of loess slopes with PCF was investigated. Figure 9 shows that there is no obvious change in the loess slope treated with PCF after spraying with tap water. However, the surface of the loess slope without PCF has some cracks and there is a lot of loess washed downhill from the slope. Meanwhile, the cover area and mass of loess washed down were measured, and the results are shown in Table 3. It can be clearly seen that both the cover area and mass of loess washed down from the slope treated with PCF are significantly lower than that of the slope without PCF. It may be due to the swelling of PCF after absorbing water (the swelling ratio is 181.58 g/g in distilled water) and the enhancement of soil particle coherence after adding long-chain macromolecules, which help prevent soil and water erosion and improve the washing resistance property of loess slope.

Wind Erosion-Resistance Property
Wind erosion-resistance properties have a great influence on PCF application in the arid and semi-arid region of the Loess Plateau, especially for loess slope fixation and stability. Wind erosion-resistance tests were carried out, and the results are shown in Figure 10. With the increase in PCF contents, the mass loss of surface loess in slope reduced from 23.14-3.53%. It shows that applying PCF can improve the wind erosion-resistance performance of loess slopes and reduce the amount of wind erosion of soil.

Degradation Behavior of PCF in Soil
The degradation behavior of PCF was assessed by examining the weight loss in soil at room temperature, as shown in Figure 11. The degradation ratio of PCF gradually increased and reached 12.8% after 30 days. It is observed that PCF shows sustained release behavior and long-term slope protection performance due to the slow and continuous biodegradation in loess.  Figure 12 showed photographs of the germination of Cyperaceae seeds after 7 days. From the figure, we can see that there was an influence on the germination of seeds treated with PCF (germination rate: 68%) compared with the blank group (germination rate: 31%), which may be due to the promotion of seed growth by regulating and releasing of water and nitrogen in PCF.

Slope Planting Experiment
Slope planting experiments were carried out to determine the applicability of PCF as a nitrogen fertilizer. Cyperaceae seeds were exposed to bare loess Slope A and loess Slope B with PCF (B), and their growth is shown in Figure 13. Furthermore, the survival rate of vegetation and the damage to the slope are counted and recorded in Table 4. It can be seen clearly that the Cyperaceae involved with PCF show significant improvement in the growth of vegetation compared with the blank group after 60-days of experiments. Meanwhile, compared with Slope B with PCF, there are a lot of long and deep cracks on the surface of Slope A without PCF. To evaluate the long-term effect of slope fixation, the simulated loess slopes for planting experiments after 60 days were placed outdoors without intervention from July to November, and the change in loess slopes was shown in Figure 11. Degradation of PCF in soil during 30 days. Figure 12 showed photographs of the germination of Cyperaceae seeds after 7 days. From the figure, we can see that there was an influence on the germination of seeds treated with PCF (germination rate: 68%) compared with the blank group (germination rate: 31%), which may be due to the promotion of seed growth by regulating and releasing of water and nitrogen in PCF.  Figure 12 showed photographs of the germination of Cyperaceae seeds after 7 days. From the figure, we can see that there was an influence on the germination of seeds treated with PCF (germination rate: 68%) compared with the blank group (germination rate: 31%), which may be due to the promotion of seed growth by regulating and releasing of water and nitrogen in PCF.

Slope Planting Experiment
Slope planting experiments were carried out to determine the applicability of PCF as a nitrogen fertilizer. Cyperaceae seeds were exposed to bare loess Slope A and loess Slope B with PCF (B), and their growth is shown in Figure 13. Furthermore, the survival rate of vegetation and the damage to the slope are counted and recorded in Table 4. It can be seen clearly that the Cyperaceae involved with PCF show significant improvement in the growth of vegetation compared with the blank group after 60-days of experiments. Meanwhile, compared with Slope B with PCF, there are a lot of long and deep cracks on the surface of Slope A without PCF. To evaluate the long-term effect of slope fixation, the simulated loess slopes for planting experiments after 60 days were placed outdoors without intervention from July to November, and the change in loess slopes was shown in

Slope Planting Experiment
Slope planting experiments were carried out to determine the applicability of PCF as a nitrogen fertilizer. Cyperaceae seeds were exposed to bare loess Slope A and loess Slope B with PCF (B), and their growth is shown in Figure 13. Furthermore, the survival rate of vegetation and the damage to the slope are counted and recorded in Table 4. It can be seen clearly that the Cyperaceae involved with PCF show significant improvement in the growth of vegetation compared with the blank group after 60-days of experiments. Meanwhile, compared with Slope B with PCF, there are a lot of long and deep cracks on the surface of Slope A without PCF. To evaluate the long-term effect of slope fixation, the simulated loess slopes for planting experiments after 60 days were placed outdoors without intervention from July to November, and the change in loess slopes was shown in Figure 14. Compared with treatment by PCF, there are more deep cracks on the surface, slight slope sliding, and descent for the blank group. It indicates that PCF not only improves loess slope fixation and stability due to the physical and chemical effects of polymer chains and soil particles but also promotes slope vegetation growth as nitrogen fertilizer to enhance biological protection for loess slopes.
Polymers 2022, 14, x 13 of 16 Figure 14. Compared with treatment by PCF, there are more deep cracks on the surface, slight slope sliding, and descent for the blank group. It indicates that PCF not only improves loess slope fixation and stability due to the physical and chemical effects of polymer chains and soil particles but also promotes slope vegetation growth as nitrogen fertilizer to enhance biological protection for loess slopes.    Figure 14. Digital photographs of loess slopes exposed outside without intervention after slope planting experiment.

Conclusions
An eco-friendly polymer composite fertilizer (PCF) based on graft copolymerization of corn straw with acrylic acid and a composite of loess-based geopolymer has been developed, and its loess slope fixation, stability, and erosion control have been evaluated. When the content of PCF was increased from 0% to 5%, the compressive strength was increased from 0.89 MPa to 1.82 MPa. The PCF has not only good heat resistance, washing resistance, freeze-thaw resistance, wind erosion resistance, and surface curing properties, but it also can regulate the pH value of soil to a neutral environment, which will be very useful for loess slope fixation and stability by physical and chemical coefficients. Moreover, PCF can significantly promote the germination rate from 31% to 68% and the survival rate of slope vegetation from 45.2% to 67.7% as a nitrogen fertilizer to enhance biological protection for loess slopes. It is believed that this low-cost and eco-friendly polymer composite fertilizer has significant potential to be applied in slope protection and long-term remediation.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. Figure 14. Digital photographs of loess slopes exposed outside without intervention after slope planting experiment.

Conclusions
An eco-friendly polymer composite fertilizer (PCF) based on graft copolymerization of corn straw with acrylic acid and a composite of loess-based geopolymer has been developed, and its loess slope fixation, stability, and erosion control have been evaluated. When the content of PCF was increased from 0% to 5%, the compressive strength was increased from 0.89 MPa to 1.82 MPa. The PCF has not only good heat resistance, washing resistance, freeze-thaw resistance, wind erosion resistance, and surface curing properties, but it also can regulate the pH value of soil to a neutral environment, which will be very useful for loess slope fixation and stability by physical and chemical coefficients. Moreover, PCF can significantly promote the germination rate from 31% to 68% and the survival rate of slope vegetation from 45.2% to 67.7% as a nitrogen fertilizer to enhance biological protection for loess slopes. It is believed that this low-cost and eco-friendly polymer composite fertilizer has significant potential to be applied in slope protection and long-term remediation.