The Fusion Impact of Compost, Biochar, and Polymer on Sandy Soil Properties and Bean Productivity

: Two of the most signiﬁcant issues confronting arid and semi-arid countries are soil degradation and the need to reclaim sandy soils and improve their properties to enhance the agricultural area and ensure food security. Many attempts to improve sandy soil properties have been attempted using soil amendments, but further research is needed to explore the combined impact of cost-effective amendments. To that purpose, we investigated the impact of various soil amendments, including single and combination applications of synthetic Super Absorbent Polymer (SAP), compost, and biochar, on sandy soil physiochemical characteristics and bean ( Vicia faba L.) production and quality throughout three growing seasons. In a randomized complete block design with three replicates per treatment, different treatments such as control (without application), lower dose of SAP (SAP1), higher dose of SAP (SAP2), biochar, compost, SAP1 plus biochar, SAP1 plus compost, SAP2 plus biochar, SAP2 plus compost, and biochar plus compost were used. The combined treatments, such as SAP2 plus biochar (T8), SAP2 plus compost (T9), and biochar plus compost (T10), improved soil phys-iochemical characteristics and crop production signiﬁcantly. Application of T10 decreased soil bulk density by 15%, 17%, and 13% while increasing soil available water by 10%, 6%, and 3% over the ﬁrst, second, and third growing seasons, respectively, compared to untreated soil (T1). The application of treatment (T9) surpassed other treatments in terms of yield, quality, and economic return, signiﬁcantly increasing the seed yield by 24%, 26%, and 27% for the ﬁrst, second, and third season compared with untreated soil. The higher rate of polymer combined with compost could be considered a cost-effective soil amendment to improve sandy soil productivity in arid and semi-arid regions.


Introduction
According to estimates, 70% more food will be needed due to the world's rapid population growth in order to support the expected population of 9.6 billion people by 2050, with the majority of this need coming from developing nations and Africa [1,2].In order to overcome this obstacle, it is necessary to boost crop production across the board, particularly in developing countries.Legumes are a key component of agricultural production systems and a major source of proteins and minerals, contributing mainly to human diets and farming systems [3,4].Domestic Vicia faba production in Egypt fell three decades ago.Egypt's self-sufficiency in the crop was destroyed by a combination of political actions and environmental changes, resulting in the country's severe dependency on imports today.However, domestic bean (Vicia faba L. c.v. Nubaria 2) production is growing more slowly than needed to meet domestic consumption demands.Because of this, the faba bean food gap continues to grow [5].In fact, the difference widened significantly, growing by roughly 891% from 73 thousand tonnes in 2000 to about 650 thousand tonnes in 2016 [6].To close the gap, bean production must be increased both horizontally and vertically, which requires high quality soils.
Soil quality can be described as the techniques used to improve the physical, chemical, and biological properties of soil [7].Sandy soil is one of the most difficult challenges facing agricultural production, due to its characteristics of low fertility, low water holding capacity, extensive erosion, and high evaporation, requiring much attention to improve soil quality [8].There have been numerous efforts made to enhance the characteristics of sandy soil, including applications of both organic [9,10] and inorganic amendments [11].Among such amendments are biochar [12], vermicompost [13], recycled agricultural wastes [14], and synthetic Super Absorbent Polymer (SAP) [15].Compost has long been used as an organic soil amendment to improve physiochemical and biological characteristics [16,17].It can also improve soil fertility, structure, increase soil water holding capacity, and decrease erosion and runoff in sandy soils [8].Biochar, which is made from pyrolyzed wastes that have a longer residence period than unpyrolyzed wastes, increases soil fertility by affecting the physiochemical and biological characteristics of the soil [18,19].Many studies have shown that biochar has a good impact on soil characteristics and crop output [20][21][22].Depending on the kind, dosage, and soil type, biochar features such as pH, increased surface area, cation exchange capacity, and nutrient content positively influence soil parameters and ultimately improve soil fertility [23].The feedstock from which biochar is created, as well as its makeup and the type of soil to which it will be applied, all have a significant impact on the biochar application rate [22].The combined effect of composts, biochar, and polymers on the physical and chemical characteristics of soil has not yet been studied in detail, confirming the importance of the current study.Understanding how changed composts alter soil structure and, subsequently, soil physical quality through soil micromorphological studies may be helpful.
Polymers are huge molecules made up of monomers, which are repeating units.Typically, polymerization of monomers results in the formation of polymers, which differ from monomers in both their physical and chemical characteristics.There have been reports of using both natural and synthetic polymers to stabilize soils [24].Recently, there has been a focus on the use of synthetic polymers (SA) to stabilize soil properties and reduce deterioration in arid and semi-arid soils [25,26], but further research is needed to encompass a wider range of soils and settings.Therefore, the main objectives of this work are to (I) investigate the individual and combined effects of compost, biochar, and polymer in low and high rates on the physiochemical characteristics of sandy soil, and (II) study the response of bean yield and yield components to the application of these amendments in sandy soil conditions.

Study Location, Soil, and Weather Characteristics
The experiment was carried out in the Baltim area of Kafrelsheikh province in Egypt's north (Figure 1).The soil texture is sandy loam, with low fertility and water retention (Table 1).The region is located at the north of Egypt with low temperature and moderate precipitation (Figure 2), proving that soil properties are the main limiting factor in crop production.The study site represents the first agroclimatic zone in Egypt, which characterizes the moderate climate.

Experimental Design and Treatments
The field experiment, which included ten treatments, was set up in a randomized complete block design with three replicates per treatment.Individual plots included compost, biochar, SAP polymer, and their mixtures as: T1: control (without addition); T2: SAP1 polymer at 5 g per hull (SAP1); T3: SAP2 polymer at 10 g per hull (SAP2); T4: biochar 5.88 tonnes per acre (B); T5: compost 11.75 tonnes per acre (C); T6: SAP1 + B; T7: SAP1 + C; T8: SAP2 + B; T9: SAP2 + C; and T10: B + C. The local cultivar Nubaria 2 was sown on 9 November in three successive growing seasons (2020/2021, 2021/2022, and 2022/2023).The area of the field experiment was 450 m 2 (36 lines + 9 lines free between all treatments); the distance between each line of 30 cm, with one seed placed per hull, and the distance between the holes along the line was 25 cm.Amendments of biochar and compost, as well as phosphorus, were given during seedbed preparation.Phosphorus fertilizer was applied at 60 kg P ha −1 as ordinary Ca-superphosphate, and K was applied at 120 kg K ha −1 as potassium sulphate.Nitrogen (N) was applied at 72 kg N ha −1 as ammonium sulphate in three equal splits: 30, 45, and 60 days after sowing.SAP polymer was added after the line was prepared.faba beans were sown on 9 November in three seasons at 168 kg ha-1 using a dry planting by hand method.At the harvest stage, after 142 days, soil and plant samples were collected for analysis.

Soil and Plant Measurements
Before and after harvesting, soil samples were taken and subjected to normal procedures for chemical and physical analysis [27,28].The pressure membrane method was used to test the soil moisture field capacity (FC) and permanent wilting point (PWP) at 0.1 and 15 bars, respectively.Using the pipette method, the particle size distribution of soil samples was measured.Using a cylindrical, sharp-edged core sampler, the bulk density of the soil was calculated [29].The modified wet combustion method was used to measure the amount of soil organic carbon [30].Conventional techniques were used to extract and determine the amount of N, P, and K in the soil [31].During the three growing seasons, the contents of NPK in faba bean seeds were determined, along with the plant height (cm), weight of 100 seeds (g), HI (%), seed, straw, and biological yields (kg ha −1 ).

Economic Return
Total cost, total return, and net return were computed based on the change in the exchange rate between the Egyptian pound and the US dollar between 9 November (planting date) and 30 March (harvest date) during three growing seasons.

Statistical Analysis
Regression analysis of soil physiochemical properties under different treatments and over different growing seasons, as well as pair plot visualization of all crop features under all treatments, were performed in Python using the seaborn and matplotlip packages [32].Principle Component Analysis (PCA) for all crop features and treatments was conducted using the Factoextra in R language.Analysis of variance and least significant differences (LSD) (p ≤ 0.01) were performed according to [33] using the agricolae package in R studio.

Soil Physiochemical Properties Subjected to Amendment Application
Figure 3 indicates that amendments had a favourable influence on most soil characteristics, revealing that the addition of compost enhanced CEC, soil organic carbon, and soil nitrogen content significantly throughout the first and second seasons, while the soil phosphorus and potassium concentration were excessive only in the first season.Biochar, on the other hand, increased soil phosphorus and potassium content during the last two seasons (2nd and 3rd), as well as CEC values and soil nitrogen content during the third season.Furthermore, both SAP dosages (SAP1 and SAP2) exhibited a negligible to minor effect on soil chemical characteristics.The lowest increase in values was recorded when no amendments were received.The maximum CEC and SOC values were obtained by combining biochar and compost application (T10).All the combinations, particularly those with biochar, boosted the NPK content of the soil.

Faba Bean Yield and Traits Subjected to Amendment Application
The soil amendments had a significant effect on bean seed yield and yield components (p < 0.001), and this effect differed significantly across all treatments (Table 2 and Compost application reduced soil BD by an average of 13.77% and 15.82% in seasons 1 and 2, respectively, while biochar application reduced it by 11.68% in the third season.It was reduced by an average of 18.05%, 20.51%, and 18.61% in each season using biochar +compost (T10) or SAP2 + biochar (T8).The application of SAP polymer had a significant impact on increased soil moisture content, such as WP, FC, and AW, with the application of SAP2 in respective seasons increasing WP, FC, and AW by 14.19%, 23.43%, and 29.50% in the first season, 20.73%, 27.44%, and 32.03% in the second season, and 19.95%, 29.80%, and 36.23% in the third season.SAP1 caused increases of 14.03%, 19.95%, and 23.84%; 11.83%, 22.83%, and 30.35%; and 18.87, 24.29, and 27.84 in the first, second, and third seasons, respectively.A combination of treatments resulted in an increase in soil moisture content, outperforming other treatments.Application of SAP polymers used with both organic additions (biochar and compost) enhanced soil moisture content.On the other hand, for WP, FC, and AW, there were no significant changes between all the investigated combinations (Figure 3).According to the findings, the residual effect of compost diminished in the third season.The effect of biochar, on the other hand, showed an increasing curve during the second season, particularly in the soil content of elements, due to increased CEC values, while the effect of adding both polymer dosages on soil moisture content was obvious.

Faba Bean Yield and Traits Subjected to Amendment Application
The soil amendments had a significant effect on bean seed yield and yield components (p < 0.001), and this effect differed significantly across all treatments (Table 2 and Supplementary Figures S1-S8).The treatment of SAP2 plus biochar outperformed other treatments, with highly significant differences in seed yield, straw yield, plant height, and seed weight when averaged over three growing seasons.There are no significant differences between SAP2 + Biochar (T8), SAP2 + Compost (T9), and Biochar + Compost (T10), confirming that any treatment from them could be adopted as long as the biggest economic profit is achieved.The treatment (T10), on the other hand, revealed the best nutritional status in bean seeds (N, P, and K) with highly significant differences from the other treatments.faba bean yield and yield attributes responded positively to the fusion application of biochar, compost, and SAP polymer.The pair plot showed positive correlation between all traits except HI, which showed negative correlation with all traits (Figure 4).To properly grasp the correlations between soil amendment treatments, yield, and their qualities, the PCA analysis (loadings and scores) was applied (Figure 5).Yield, yield attributes, and qualities correlated positively with the treatments SAP2 + Biochar (T8), SAP2 + Compost (T9), and Biochar + Compost (T10), and negatively with the trait HI.This confirms the importance of applying SAP2 + Biochar (T8), SAP2 + Compost (T9), and Biochar + Compost (T10) in improving sandy soil properties and maximizing yield and quality in these conditions.However, as there are no significant differences between these treatments (SAP2 + Biochar (T8), SAP2 + Compost (T9), and Biochar + Compost (T10)), further economic analysis is required to select the optimum treatment that achieves the highest return for farmers.

Economic Returns
According to the results in Table 3, the expenses of adding amendments were high in the first season 2020/2021 compared to the control; therefore, the net return favoured the control over the new treatments.The difference was clear for all treatments compared to the control in the two subsequent seasons, as net profits increased by 136.93% and 59.66% with the addition of SAP 10 g + 2.5 tonne biochar, followed by 2.5 tonne biochar + 5 tonne compost (134.42% and 57.54%) in 2021/2022 and 2022/2023, respectively.Finally, the treatment of SAP2 plus compost (T9) may be chosen as the most cost-effective treatment with the highest net return in the third season.The costs of the raw materials in 2020 were 11.75 ton compost ha −1 = USD 236.67; 5.88 ton Biochar ha −1 = USD 285.71; 1.60 ton SAP ha −1 = USD 304.76 and 1.92 ton SAP ha −1 = USD 609.52.

Discussion
The current study clearly demonstrated that the combined use of biochar, polymer, and compost improved soil quality and plant growth in sandy soil conditions, rather than using the single effect as performed before [34,35].Application of SAP polymer recently showed a significant improvement in soil properties and crop production [36], but further application in different environments and soils, particularly sandy soils, is required.In the current study, the combined application of polymers, particularly at the highest rate with compost and biochar, as well as the combination of compost and biochar, showed significant improvements in sandy soil properties and crop production.Some studies showed the importance of SAP polymer in improving clay soil properties [37], reducing fertilizer loss [38], and increasing soil content of N, P, and K, which is required for the nutrient demand during plant growth and development [39,40].Other research emphasized the value of SAP polymer in sandy soils, demonstrating that SAP polymer treatment increased soil water content [41].However, integrating SAP polymer with other amendments has been given less attention so far, emphasizing the novelty of the current research.Here, the combined application of SAP2 polymer with either biochar (T8) or compost outperformed other single and low-rate SAP treatments in terms of improving soil physiochemical properties and bean yield and quality.This might be explained by SAP's three-dimensional, cross-linked structure, which can take in and hold as much water as 400 times its weight [42,43].Its hydrophilic functional groups, like hydroxyl, carboxyl, amide, and sulfonic groups, have excellent adsorption and complex capacities.When the molecular chain swelled beneath the three-dimensional cross-linked structure, water moisture entered the internal network easily and formed a water-blocking layer between soil particles, which could inhibit moisture from moving from the soil surface to the atmosphere or to the rock layer of slopes, but made it move horizontally, or to the place that had little SAP.As a result, soil treated with SAP can absorb more water than untreated soil, and it also permits the water to be released gradually when the soil moisture level drops [44].Application of SAP polymer not only increases soil water holding capacity, but also improves other physiochemical properties in the sandy soil.The chemical hydrophilic groups and network structure of the SAP hydrogel molecule may be responsible for this.The water molecules were charged by weak connections, which could result in a good water exchange between the polymer and the soil.However, combining SAP polymer with biochar or compost outperformed a single application of SAP polymer in terms of enhancing soil characteristics and crop yield.This is primarily explained by the fact that biochar decomposes more slowly in the soil than compost because it contains more stable organic carbon molecules than compost [45,46].In contrast, the compost's decaying organic elements promote the development of soil microorganisms and boost the activity of soil enzymes, increasing soil organic matter and improving the qualities of sandy soil [47].This demonstrates the importance of combining compost and biochar with SAP polymer in enhancing the physiochemical characteristics of sandy soils and increasing crop production.Nevertheless, further research is required to use these combinations in different environments with summer crops rather than the current winter crop.Application of SAP polymer with compost and biochar in diverse environments, including moderate and high temperatures with summer crops, will give a realistic insight into using cost-effective amendments in improving degraded soils and crop production to accomplish food security and nutrition goals.Furthermore, the effect of these materials on soil microbial activity and greenhouse gas emissions should be considered as a future research direction.On the other hand, including these resources into decision support tools such as crop models [48] will enable them to be used at scale in a cost-effective manner.

Conclusions
The combination of biochar (its shelf life in the soil is 25 years) + compost (its shelf life is 3 years) led to an improvement in the biological activity of the soil, which was reflected in the chemical properties and bulk density of the soil with reduced carbon emissions.From the perspective of economic returns, the use of this mixture gave an economic return like the addition of polymers to the soil (its shelf life is from 6 to 7 years) and, thus, reduced the total costs of adding the polymers alone or in combination with other treatments.In conclusion, the treatment of SAP2 plus compost (T9) could be adopted as the best economic treatment, achieving the highest net return in the third season.However, applying these treatments to a single crop is a study restriction that necessitates more research into employing such amendments in crop rotation and on diverse crops.

Figure 1 .
Figure 1.The study location of the field experiment at Baltim city, Kafr El-Sheikh governorate, Egypt.

Figure 1 .
Figure 1.The study location of the field experiment at Baltim city, Kafr El-Sheikh governorate, Egypt.

Table 1 .
The physiochemical properties of the study area before cultivation.

Table 1 .
The physiochemical properties of the study area before cultivation.

Table 2 .
Statistical analysis of bean yield and traits after being subjected to different treatments of soil amendments averaged over three growing seasons.

Table 2 .
Cont.Different letters to show statistical significance.For all variables with the same letter, the difference between the means is not statistically significant.If two variables have different letters, they are significantly different.

Table 3 .
Economic returns under different growing season.