Evaluation and Improvement of Mine Site Quality

The Effect of Improving Slag Vegetative Substrate at the Dump of Open-pit Coal Mine in Rujigou, Ningxia, China

  • YANG Xinrui , 1 ,
  • SHI Changqing , 1, * ,
  • ZHAO Tingning 1 ,
  • HU Yang 2 ,
  • ZHANG Junjiao 3
  • 1. School of Soil and Water Conservation, Beijing Forestry University, Beijing 100083, China
  • 2. Huaxi District Natural Resources Agency, Guiyang 550025, China
  • 3. Yunnan Rural Science and Technology Service Center, Kunming 650021, China
*SHI Changqing, E-mail:

YANG Xinrui, E-mail:

Received date: 2022-08-20

  Accepted date: 2023-01-30

  Online published: 2023-07-14

Supported by

Key Research and Development Program of China(2017YFC0504406)

The Key Research and Development Program of Ningxia Hui Autonomous Region(2018BFG02002)


While promoting economic and social development of China, open-pit coal mining will also cause irreversible harm to the surrounding environment due to disturbing the topsoil. Therefore, ecological restoration measures are urgently required. Vegetation cover is an important part of mine ecological restoration work. Suitable soil structure and nutrient conditions are the basis of normal plant growth and development. In this study, the slag, sand, humic acid, diammonium phosphate (DAP) and urea were selected as the improved materials to adjust the structure and fertility of the slag and improve the slag into a suitable vegetative substrate for plant growth. From among various configurations of the vegetative substrate, the most suitable substrate for plant growth was filtered by comparing the physical and chemical properties and their effects on Alfalfa growth under different treatments. The results showed that the application of fertilizers could increase the nutrient content of the vegetative substrate and increase the pH, but the growth of Alfalfa was not optimal under the high fertilization treatment. The addition of sand could mitigate the structural problems of slag, and sand also showed a certain degree of regulation on fertility. Moreover, the addition of appropriate amounts of humic acid can enhance slag maturation and stabilize the chemical properties of the vegetative substrate. According to the results, the most optimal slag-sand-humic acid ratio for the vegetative substrate of the dump site of the Dafeng Open-pit Coal Mine is 18:1:1. The findings are expected to offer a reference for the improvement method of slag in this area, which alleviates the shortage of soil resources. It provides support for ecological environmental protection in soil source area, and also offers a new idea for the resource utilization of slag in the ecological restoration.

Cite this article

YANG Xinrui , SHI Changqing , ZHAO Tingning , HU Yang , ZHANG Junjiao . The Effect of Improving Slag Vegetative Substrate at the Dump of Open-pit Coal Mine in Rujigou, Ningxia, China[J]. Journal of Resources and Ecology, 2023 , 14(4) : 775 -783 . DOI: 10.5814/j.issn.1674-764x.2023.04.010

1 Introduction

Recent years, the mining and utilization of coal resources plays an important role in the economic development and social progress of China. Coal resources are mainly mined through open-pit mining and underground mining, of which open-pit mining accounts for 43.12% of the country’s total coal production (Hu, 2020; Wang et al., 2022). However, with the high production, the frequency of natural disasters such as landslides and soil erosion are increasing (Ullah et al., 2018; Feng et al., 2019; Lu and Liu, 2021). The “Land Reclamation Regulations” clearly states that land damaged by surface excavation such as open-pit mining should be remediated by the relevant responsible authority by referring to the land reclamation standards. The main ecological restoration projects for open-pit coal mines in China are landform remodeling, land reclamation, and vegetation restoration (Lv et al., 2019). In such projects, the stipulated thickness of the soil cover is more than 30 cm, and the demand of the topsoil is huge (Wang et al., 2019). During the process of open-pit coal mining, the surrounding topsoil is disturbed and cannot be reused, and the slag at the dump site does not meet the requirements for health plant growth.
Moreover, the introduction of foreign soil is insufficient for plant growth. Therefore, the shortage of soil resources is serious at coal mining sites (Wang et al., 2019). In this context, the improvement of slag to a substrate suitable for plant growth under limited resource conditions and realizing the utilization of slag resources have become hot issues in ecological restoration works for coal mines.
For the vegetative substrate, it is the most critical to meet the needs of plant growth. Good structure and nutrient conditions are the basis of normal plant growth and development. Therefore, we need to improve the slag from the aspects of structure and fertility by adding improved materials. At present, commonly used materials for the improvement of slag in mining areas include water-retaining agents (Zhao et al., 2013), organic fertilizers, loess (Cao et al., 2020), sand, straw (Liu, 2009), animal manure, humic acid (Niu et al., 2008), and nanomaterials (Huang and Wang, 2016). Liu et al. (2021) improved the soil of the Shanxi-Shaanxi- Mongolian mine dump with weathered coal and arsenic sandstone by increasing the content of water-stable aggregates. Using organic fertilizers and loess to prepare a vegetative substrate for waste rock soil of iron ore dumps, Cao et al. (2020) investigated the effects of different ratios on the soil micromorphology of iron ore dumps, and found that the vegetative substrate could significantly increase the content of soil macroaggregates and improve the soil structure. Wang et al. (2014) combined humic acid and arbuscular mycorrhizae to improve soil quality in Shendong mining area, and reported that the conductivity, available phosphorus (AP), and acid phosphatase activity were significantly increased.
In this study, sand, peat humic acid, diammonium phosphate (DAP), and urea were used to improve the slag at the east outer dump of the Rujigou Dafeng Open-pit Coal Mine. The improvement effects of adding materials to the slag soil were explored by setting different sand contents and fertility gradients. Pot experiments were performed to comprehensively evaluate the effect of different vegetative substrates on the growth of Alfalfa, a typical plant in this area. Thereafter, the plant substrate ratio suitable for vegetation growth was screened out. The findings offer a reference for the improvement method of slag in this area, which alleviates the shortage of soil resources and reduces the consumption of high-quality soil. It provides support for ecological environmental protection in soil source area, and also offers a new idea for the resource utilization of slag in the ecological restoration.

2 Materials and methods

2.1 Study area

The study area, Dafeng Coal Mine, is located in the Rujigou mining area in the northern hinterland of the Helan Mountains in the Ningxia Hui Autonomous Region, with coordinates of 106°07′E and 39°04′N (Fig. 1). The area belong to a temperate continental climate. Its annual evaporation is greater than annual average rainfall, 167.5-188.8 mm, and annual average temperature is 9.16±0.46 ℃. The terrain is high in the west and low in the east, with many valleys, mostly alpine terrain, and exposed mountain bedrock. The soil is mainly fertilized by light lime-calcium soil and coarse bone soil. The vegetation is dominated by climatic xerophytic deciduous shrubs in the mountains, including Ulmus glaucescens Franch., Amygdalus mongolica (Maxim.) Ricker, Halogeton glomeratus (Bieb.) C. A. Mey., Artemisia scoparia, and Astragalus melilotoides. Due to the frequent mining activities in this area, large areas of surface vegetation have been destroyed, and the vegetation coverage is less than 10%.
Fig. 1 Location and topographic of the study area

2.2 Materials

Slag was collected from three randomly selected sites in the east outer dump of the Dafeng Coal Mine. The slag was mixed with the test materials and the physical and chemical properties of the mixture were measured: 4.98% organic matter (OM), 16.5 mg kg‒1 alkali-hydrolyzable nitrogen (AN), 25.77 g kg‒1 total nitrogen (TN), 128 mg kg‒1 available potassium (AK), 25.77 g kg‒1 total potassium (TK), 3.57 mg kg‒1, available phosphorus (AP), 0.04% total phosphorus (TP), 51.32% clay, 13.58% silt, 35.10% sand, and pH of 8.43. Due to the properties of the slag with high clay content and low elements of nitrogen and phosphorus, we selected sand to improve the structure of the residue, and selected common fertilizers DAP and urea to improve the fertility of the residue. Peat humic acid has the characteristics of good ventilation performance, increase soil aggregate viscosity, and maintain water and fertilizer (Fortun et al., 1990). Therefore, peat humic acid was used to promote the formation of aggregate structure, improve the nutrient utilization rate in muck, and make fertilizer supply to be stable. Sand was collected from the surrounding sandy land of the mining area, at three sampling points in sparsely populated and undisturbed areas. After thorough mixing, samples were collected to determine their physical and chemical properties: 0.011% OM, 24.6 mg kg‒1 AN, 0.061 g kg‒1 TN, 247 mg kg‒1 AK, 27.5 g kg‒1 TK, 1.5 mg kg‒1 AP, 0.038% TP, 1.55% silt, 98.45% sand, and pH of 7.24. Peat humic acid was used to ripen soil produced by Inner Mongolia Letu Environmental Protection Technology Co., Ltd, containing 21.5% OM, 634 mg kg‒1 of AN, 11.1 g kg‒1 of TN, 252 mg kg‒1 of AK, 22.8 g kg‒1 of TK, 169.4 mg kg‒1 of AP, 169.4 mg kg‒1 of TP, 0.296% TP, and pH of 6.34. DAP produced by Yunnan Yuntianhua Co., LTD was used as a base fertilizer, with TP (calculated as phosphorus pentoxide) of 47.10%, AP (calculated as phosphorus pentoxide) of 45.31%, and TN of 18.92%. Urea produced by Inner Mongolia Erdos United Chemical Co., LTD was used as the top dressing, with TN of 45.11%.

2.3 Test design

2.3.1 Study on the ratio of vegetative substrates

The slag (passed through diameter 1 cm soil sieve), sand, and peat humic acid were fully mixed according to a certain mass ratio, and 9 vegetative substrates were configured, as shown in Table 1. After watering and settling, the vegetative substrate was air-dried and passed through a 2 mm soil sieve.
Table 1 Ratio of vegetative substrates
Vegetative substrate Slag (%) Sand (%) Peat humic acid (%)
S1 100 0 0
S2 95 0 5
S3 90 5 5
S4 85 10 5
S5 80 15 5
S6 75 20 5
S7 70 25 5
S8 65 30 5
S9 60 35 5
According to the national grading standard table of soil nutrient content, the target values of low, medium, and high levels of fertilization were set with medium, upper middle, and high contents of TN and AP, respectively. Combined with the background values of diammonium and urea nutrients, the amount of fertilization was calculated according to Formulas (1)-(3), and four fertilization levels (low fertilization, medium fertilization, and high fertilization) were set, as shown in Table 2.
Table 2 Fertilization level of vegetative substrates
Fertilization level Diammonium (g kg‒1) Urea (g kg‒1)
No fertilization 0.00 0.00
Low fertilization 0.01 0.41
Medium fertilization 0.02 1.24
High fertilization 0.06 2.35
In total, nine kinds of vegetative substrates were set with 4 fertilization levels formed, a total of 36 treatments, as shown in Table 3. By comparing the effects of different fertility levels on the properties of the same substrate and the differences in the properties of different substrates under the same fertilization level, the effects of different additive materials on the properties of slag were analyzed. Each treatment was applied on vegetative substrates with a total weight of 2 kg. The substrate was placed in a nutrient bowl, watered and settled, and air-dried for measuring particle size composition, TN, AP, and pH. Three replicates were set for each treatment.
$Sl_n =Tar_n-S_n $
$Ur_ar =Sl_TN/Ur_TN $
where Sl_n is the slag lacks nutrients, mg kg-1; Tar_n is the target nutrients, mg kg-1; S_n is the slag nutrients, mg kg-1; DAP_ar is the DAP application rate, mg kg-1; Sl_AP is the slag lacks AP, mg kg-1; DAP_TP is the TP in DAP, mg kg-1; Ur_ar is the urea application rate, mg kg-1; Sl_TN is the slag lacks TN, mg kg-1; Ur_TN is the TN in urea, mg kg-1.
Table 3 Experimental treatments
Treatments Vegetative substrate Fertilization level Treatments Vegetative substrate Fertilization level
1 S1 No fertilization 19 S5 Medium fertilization
2 S1 Low fertilization 20 S5 High fertilization
3 S1 Medium fertilization 21 S6 No fertilization
4 S1 High fertilization 22 S6 Low fertilization
5 S2 No fertilization 23 S6 Medium fertilization
6 S2 Low fertilization 24 S6 High fertilization
7 S2 Medium fertilization 25 S7 No fertilization
8 S2 High fertilization 26 S7 Low fertilization
9 S3 No fertilization 27 S7 Medium fertilization
10 S3 Low fertilization 28 S7 High fertilization
11 S3 Medium fertilization 29 S8 No fertilization
12 S3 High fertilization 30 S8 Low fertilization
13 S4 No fertilization 31 S8 Medium fertilization
14 S4 Low fertilization 32 S8 High fertilization
15 S4 Medium fertilization 33 S9 No fertilization
16 S4 High fertilization 34 S9 Low fertilization
17 S5 No fertilization 35 S9 Medium fertilization
18 S5 Low fertilization 36 S9 High fertilization

2.3.2 Potted trials of Alfalfa

Ningxia is one of the main areas of Alfalfa in China. With developed stems, leaves, roots and nodules, Alfalfa can fix nitrogen and improve soil structure, so it’s suitable for soil and water conservation. The long life, wide adaptability, high yield and only 30-40 days from sowing to harvest are the reasons as a test plant. In each treatment, 5.7 kg of each evenly mixed substrate mixture was placed in a pot with a hole at the bottom and a holder. The planting pot had an inner height of 23 cm, lower inner diameter of 17.6 cm, and upper inner diameter of 23.7 cm. After applying the basal fertilizer and filling the pots, 30 Alfalfa seeds soaked in warm water for 8 h and then sown in each pot. The surface of the seeds was covered with a small amount of slag. Before emergence of the seedling, the soil was irrigated to saturation to keep the seeds moist. After emergence, the soil was watered very 3 days. No watering and pipe protection was performed within a week after urea topdressing.

2.4 Physicochemical properties and effect on Alfalfa growth

According to the USDA particle size classification (1951), the slag particles were divided into clay grain (<0.002 mm), silt grain (0.002-0.05 mm), and sand grain (0.05-2 mm) (Wu and Zhao, 2019). The percentage of each particle size was determined using a Malvern laser particle sizer (Mastersizer 3000, Malvern, UK). Ph was determined using the potentiometric method, TN was determined using the Kjeldahl method, and AK was determined using NH4OAC extraction-flame photometry.
The seedling emergence rate was determined after the seedling emergence stabilized, which occurred on the eighth days after sowing. The seedling emergence rate is the ratio of the number of seedlings to the total number of seeds sown. Seedling height was measured using a steel ruler after the plant growth stabilized, which occurred 21 days later. Seedling height is the height from the base of the root neck of the seedling to the growth point. The plants matured after 35 days, and they could then be harvested for the measurement of total biomass. The Alfalfa plants were harvested using the whole root method. After washing, they were fixed (105 ℃) and dried (80 ℃) to constant mass, and the total biomass and rhizome ratio were determined.

2.5 Data analysis

The software Microsoft Excel 2011 was used for basic statistical analysis of the data, Rstudio was used for drawing, and SPSS 25.0 was used for the analysis of significance and principal component analysis of the data.

3 Results

3.1 Effects of different fertility levels on the physical and chemical properties of plant substrates

The mechanical composition can directly reflect the physical structure of the vegetation substrate. It is an important indicator for evaluating the quality of the substrate. As shown in Fig. 1, the proportion of clay grains in the vegetative substrate increased significantly with the increase of fertilization level (R2=0.078, P=0.0033, Fig. 2a), the proportion of silt grains showed a significant quadratic function relationship with the increase of fertilization level (R2=0.098, P=0.0044, Fig. 2b), and the proportion of sand grains decreased significantly with increasing fertilization level (R2=0.06741, P=0.0066, Fig. 2c). These results show that the use of DAP and urea can significantly affect the mechanical composition of vegetative substrates. Compared with the no fertilization and low fertilization levels, the percentages of clay grains, silt grains, and sand grains fluctuated widely under the medium and high fertilization levels. Changes in the TN content, AP content, and pH value of each vegetative substrate under different fertilization levels are shown in Fig. 2. The content of TN, AP, and pH value were significantly positively correlated with the fertilization level (R2=0.72, P<0.0001; R2=0.29, P<0.0001; R2=0.31 P<0.0001, Fig. 2d, e, f). According to these results, the use of DAP and urea can significantly increase the content of TN and AP in the vegetative substrate, and improve the pH value.
Fig. 2 Physical and chemical properties of vegetative substrates at different fertility levels

Note: “No” means no fertilization; “Low” means low fertilization; “Medium” means medium fertilization; “High” means high fertilization.

On the whole, by increasing the level of fertilization, the structure of the vegetative substrate can be improved to a certain extent, and the nutrient content of the substrate can be increased, but the pH value of the substrate may also increase to a certain extent.

3.2 Comparison of physical and chemical properties of different vegetative substrates

Excessive clay grain content in slag is one of the important factors inhibiting plant growth. As shown in Fig 3, the proportion of clay grain and silt grain in the vegetation substrate decreased significantly with increasing sand content (R2=0.63, P<0.0001; R2=0.41, P<0.0001, Fig. 3a, b). However, there was a significant positive correlation between the proportion of sand grains and sand content (R2=0.63, P<0.0001, Fig. 3c). This shows that appropriate amounts of sand can help improve the structure of the vegetative substrate. Changes in the content of TN, AP, and pH value of the vegetation substrate with the amount of added sand are shown in Fig. 3. The content of TN and sand content of the vegetative substrate showed a quadratic function relationship with an opening downward (R2=0.071, P<0.05, Fig. 3d). The content of TN reached the maximum value at 15% added sand content and 5% humic acid content. There was a significant negative linear relationship between the content of AP and the amount of sand added to the substrate (R2=0.049, P<0.05, Fig. 3e). Nevertheless, the AP content was more stable. The pH value and the amount of added sand in the substrate showed a significant upward quadratic function relationship (R2=0.23, P<0.0001, Fig. 3f), pH reached the minimum value at 5% added sand content and 5% humic acid content. These results show that a certain proportion of humic acid and sand can affect the pH value of the vegetative substrate.
Fig. 3 Physicochemical properties of different vegetative substrates
On the whole, the structural problems of slag can be ameliorated by adding sand, which can also regulate the chemical properties of slag to a certain degree. If the amount of sand addition exceeds 15%, the substrate structure will become too loose, which will affect the retention of fertilizers. Moreover, appropriate addition of humic acid can increase the nitrogen content in the substrate while curing the soil and improve the stability of phosphate fertilizers, thus improving their performance.

3.3 Effect of the vegetative substrate on Alfalfa growth

In this study, a typical plant Alfalfa was selected as the test object, and principal component analysis was performed to evaluate the effect of different vegetative substrates on Alfalfa growth. To this end, total biomass (TB), root-to-shoot ratio (RSR), seedling height (SH), and seedling emergence rate (ROE) were selected as the evaluation indicators of plant growth (Zhang et al., 2013). Principal components (PC) with initial eigenvalues ≥1 was extracted and denoted as PC1, and the cumulative variance contribution rate was 56.283%. According to the component load of each index in the extracted PC1, the weight of each evaluation index was calculated, as shown in Table 4. The Z-score method was used to standardize the data. The PC1 score and comprehensive score of Alfalfa growth are expressed as Formulas (4) and (5):
y1 = 0.491x1 + 0.488x2 + 0.553x3 + 0.464x4
$Y = 56.283y1/100 $
Table 4 Component load and weight of principal component analysis
Indicator PC1 component load Index weight Initial
Cumulative variance explained rate (%)
TB 0.736 0.491 2.251 56.283
RSR 0.732 0.488
SH 0.829 0.553
ROE 0.697 0.464
where y1 is the score of PC1; Y is the comprehensive score of Alfalfa; x1 is the total biomass, g; x2 is the root-to-shoot ratio; x3 is the seedling height, cm; x4 is the emergence rate, %.
According to Formulas 4 and 5, the comprehensive score of the effect of the vegetative substrate on Alfalfa growth under each treatment was calculated (Table 5). The three treatments of “S8 low fertilization”, “S3 no fertilization”, and “S3 low fertilization” was found to promote Alfalfa growth in the vegetative substrate, although the difference was not large.
Table 5 Comprehensive score of the effect of vegetative substrate on Alfalfa growth
Vegetative substrate Score Rank Vegetative substrate Score Rank
S1 no fertilization ‒0.094 23 S1 medium fertilization 0.184 18
S2 no fertilization 0.858 7 S2 medium fertilization 0.625 11
S3 no fertilization 1.096 2 S3 medium fertilization 0.921 5
S4 no fertilization ‒0.013 21 S4 medium fertilization ‒0.236 25
S5 no fertilization 0.834 8 S5 medium fertilization ‒0.220 24
S6 no fertilization 0.032 20 S6 medium fertilization 0.542 12
S7 no fertilization ‒0.093 22 S7 medium fertilization ‒0.630 28
S8 no fertilization 0.129 19 S8 medium fertilization ‒0.862 31
S9 no fertilization ‒0.618 27 S9 medium fertilization ‒0.699 29
S1 low fertilization 0.899 6 S1 high fertilization 0.222 16
S2 low fertilization 1.037 4 S2 high fertilization 0.186 17
S3 low fertilization 1.049 3 S3 high fertilization ‒0.906 32
S4 low fertilization 0.225 15 S4 high fertilization ‒0.971 33
S5 low fertilization 0.688 10 S5 high fertilization ‒0.859 30
S6 low fertilization 0.446 13 S6 high fertilization ‒0.585 26
S7 low fertilization 0.829 9 S7 high fertilization ‒1.569 34
S8 low fertilization 1.105 1 S8 high fertilization ‒1.761 35
S9 low fertilization 0.309 14 S9 high fertilization ‒2.101 36
Among them, the “S8 low fertilization” treatment showed the strongest positive effect on Alfalfa growth with the substrate ratio of 13:6:1 for slag: sand: humic acid, 0.01 g kg-1 of DAP, and 0.41 g kg-1 of urea. But owing to the excessive sand ratio, the substrate structure became too loose, fertility was lost, and the slag resources could not be fully utilized. Under “S3 no fertilization” treatment, the growth effect score of Alfalfa was 1.947, second only to “S8 low fertilization” treatment. Under this treatment, the proportions of clay, silt, and sand in the slag were properly adjusted, although fertility levels did not significantly increased, the addition of peat humic acid to promote the formation of aggregate structure, improve the utilization efficiency of nitrogen and phosphorus elements in the matrix and stability, the substrate is slightly alkaline, suitable for alfalfa growth, and full use of the slag without fertilizer use, low cost. Therefore, the “S3 no fertilization” treatment was selected as the optimal ratio for the improvement of the vegetative substrate in the study area. The most optimal substrate ratio is 18:1:1 for slag: sand: humic acid, without the application of additional fertilizer.

4 Discussion

Soil quality plays a crucial role in the ecological restoration of mines and vegetation restoration. The physical and chemical properties of soil can well characterize soil quality, and structure and fertility directly control the growth and development of plants. Compared with the traditional method by introducing foreign soil, the approach of improving the soil with sand and applying appropriate amounts of fertilizer provides a new way for the ecological and vegetation restoration of mining areas, which would greatly improve resource utilization.

4.1 The properties of plant substrate and the effect of plant growth with different fertilizer

This study showed that the application of small amounts of DAP and urea can optimize the structure of the vegetative substrate, increase the content of TN and AP, and increase the pH value. Some scholars have found that proper fertilization can increase the proportion of aggregates in soil, improve the stability of soil aggregates, and thus improve the soil structure; it can also increase the content of nutrients such as TN and AP in the soil to varying degrees and slow down soil acidification (Wu et al., 2017; Liu and Sun, 2019; Zhang et al., 2021), this is consistent with the results obtained in this study. However, the application of moderate and large amounts of fertilizers will lead to unsatisfactory growth of Alfalfa, which indicates that excessive fertilization will affect plant growth and development. This may be attributable to the much higher concentration of nutrients in the aqueous solution in the substrate than that in the plants, resulting in a water potential gradient between the plants and the soil. Under such circumstances, the water in the plant seeps back into the substrate, resulting in water shortage in the plant (Bucci et al., 2003). It is also possible that the pH value of the vegetative substrate increased due to fertilization, exceeding the suitable pH range for the growth of Alfalfa (Wei et al., 2013).

4.2 The role of peat humic acid

The conclusion of this study that appropriate addition of humic acid can increase the nitrogen content in the vegetation substrate and improve the stability of AP which is consistent with the findings of previous studies. Previous studies have shown that the application of humic acid can modify the carbon-nitrogen ratio in the soil, promote the activity of microorganisms in the soil, accelerate the maturation of raw soil, and promote the growth and development of plants (Khan et al., 2018). Liu et al. (2021) pointed out that the application of peat and humic acid can enhance the stability of soil aggregates and improve soil fertility within a certain period of time. Liu (2008) showed that the application of humic acid can improve the nitrogen utilization rate and increase the content of AP in soil. However, as this study only used peat humic acid as a soil curing agent, the improvement effect of humic acid on vegetation substrates was not explored in depth.

4.3 Control of physicochemical properties by plant substrate structure and proportion

This study found that the sand content in the vegetative substrate is closely related to the structural changes of the substrate. Some studies have shown that the addition of soil to slag can improve soil tightness and increase total porosity, thereby optimizing the structure of the slag, which is conducive to the improvement of air and heat conditions and the utilization rate of soil nutrients (Yu et al., 1998). Taking the particle size composition of loam as the standard, soils with higher sand grain proportions have better aeration and water permeability, but low fertility; in contrast, soils with higher clay grain proportions have worse aeration and low permeability, but greater ability to retain water and fertilizers. Therefore, it is important to configure a scientific and reasonable ratio of substrate components. In this study, the optimal ratio of the vegetative substrate was found to be 18:1:1 for slag: sand: humic acid. Compared with the sand-soil ratio of 1:1 proposed by Yu et al. (1998), the addition of an appropriate amount of humic acid to the sand slag structure in this study would accelerate the maturing of raw soil and promote the optimization of the structure of the vegetative substrate. At the same time, it increases the nutrient content in the substrate, which plays a key role in the growth and development of plants.

5 Conclusions

(1) The application of fertilizers could improve the vegetative substrate structure to a certain extent, and increase the nutrient content and the pH value, but the growth of alfalfa under high fertilization treatment is not optimal.
(2) Sand can improve the structure of the vegetative substrates and affect the fertility effect, but excessive sand will lead to a loose structure, which is not conducive to the retention of fertilizers.
(3) An appropriate amount of humic acid can make the fertilizer effect more stable.
(4) Among these vegetative substrates, the substrate ratio with the best growth effect of Alfalfa was slag: sand: humic acid=13:6:1, DAP 0.01 g kg-1, urea 0.41 g kg-1. Under the conditions of comprehensively considering the growth effect of Alfalfa, the maximum utilization of slag resources and the improvement cost, the optimal plant substrate ratio was recommended as slag: sand: humic acid=18:1:1.
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