Biogas Slurry as a Sustainable Organic Fertilizer for Sorghum Production in Sandy Soils: A Review of Feedstock Sources, Application Methods, and Agronomic Impacts

Abstract

Biogas slurry (BGS), a nutrient-rich by-product of anaerobic digestion, presents a promising opportunity for sustainable agriculture on sandy soils. This review explores the agronomic potential of using BGS for improving sorghum’s (Sorghum bicolor) productivity by enhancing soil fertility and the nutrient availability. It focuses on the sources and properties of BGS, its application methods, and their effects on the soil nutrient dynamics and crop productivity. The findings indicate that BGS improves the soil health and crop yields, offering an eco-friendly alternative to synthetic fertilizers, especially in resource-limited settings. Despite these benefits, research gaps persist, including the need for long-term field trials, the optimization of application strategies for sandy soils, and comprehensive economic evaluations. Additionally, concerns such as nutrient imbalances, phosphorus accumulation, and slurry composition variability must be addressed. This review recommends standardizing BGS nutrient profiling and adopting site-specific management practices to maximize its agronomic benefits and environmental safety. Integrating BGS into sustainable soil fertility programs could contribute significantly to achieving agricultural resilience and circular economy goals.

1. Introduction

Sandy soils, characterized by their low organic matter content, poor nutrient retention, and limited water-holding capacity, pose significant challenges to sustainable agriculture, particularly in arid and semi-arid regions [1,2]. In these fragile ecosystems, nutrient depletion and water scarcity severely constrain crop yields [3]. The challenges are further intensified by climate change, land degradation, and population pressures. Therefore, sustainable soil management strategies are urgently needed to improve fertility while preserving environmental resources. Organic amendments have emerged as a promising alternative to synthetic fertilizers, offering multiple benefits such as an improved soil structure [4], enhanced nutrient cycling [5], and increased resilience to drought stress [6].

Sorghum (Sorghum bicolor L. Moench), a hardy and drought-tolerant cereal crop, is extensively cultivated in marginal environments and serves as a vital source of food and fodder [7,8]. Despite its natural resilience to harsh conditions, sorghum’s productivity is frequently constrained by a low soil organic matter content [9], poor nutrient-holding capacities [10], and poor fertilization, particularly for smallholder farmers [2,11]. Fertilizer overuse, particularly concerning synthetic nitrogen (N), contributes significantly to the global greenhouse gas emissions (approximately 2.1%) worldwide, while the continued reliance on such inputs in sandy soil regions has become increasingly unsustainable due to their contribution to nitrate leaching, surface water pollution, soil acidification, economic strain, and broader environmental degradation [12,13]. Improving the soil organic matter and enhancing the nutrient profile of sandy soil through sustainable amendments presents a viable strategy for significantly increasing sorghum’s productivity under rainfed cultivation.

Biogas slurry (BGS), a nutrient-rich by-product of the anaerobic digestion of organic waste such as livestock manure, crop residues, food waste, and sewage sludge, has attracted increasing attention as a sustainable soil amendment [11,14]. Rich in essential nutrients such as nitrogen (N), phosphorus (P), potassium (K), and organic matter, BGS offers significant agronomic and environmental advantages for improving soil fertility, particularly in degraded sandy soils [15]. Its application is especially beneficial in sandy soils, where nutrient leaching and water loss are prevalent due to a low cation exchange capacity and high permeability [16]. Therefore, amending sandy soil with BGS can improve the physical, chemical, and biological soil properties.

Research has demonstrated positive outcomes from the use of organic amendments in sorghum production. For instance, Nie et al. [17] found that substituting 50% of chemical nitrogen with organic fertilizer significantly improved the soil organic matter and nutrient availability. Similarly, Samijan et al. [10] reported that combining organic and inorganic fertilizers and biofertilizers increased the sorghum yields by 21.38–36.06% compared to conventional fertilization in dryland areas. Biogas slurry amendments have also proven effective in other cropping systems. Kebede et al. [11] observed that combining 25% BGS with 75% chemical fertilizer produced the highest maize (Zea mays L.) grain yield of 7.09 t ha⁻¹. Likewise, potato (Solanum tuberosum) tuber yields increased to 27.6 t ha⁻¹ while wheat (Triticum aestivum L.) grain yields reached 3.85 t ha⁻¹ when BGS was applied [14]. While these findings are well documented for maize, wheat, and potatoes, the outcomes may also be relevant to sorghum despite species-specific differences in its nutrient uptake and root architecture.

Given the growing interest in sustainable agricultural intensification, it is essential to critically assess the existing knowledge on the application of BGS in sandy soil environments. This review aims to evaluate the existing literature on the use of BGS as a soil amendment to enhance soil fertility and sorghum’s productivity. Specifically, it seeks to identify strengths and limitations in the existing research and highlight knowledge gaps that warrant further investigation. This review focuses on the sources and properties of BGS, its drying and application methods, its integration into various cropping systems, and its impacts on the soil nutrient dynamics and crop productivity.

2. Challenges of Crop Cultivation in Sandy Soils

Sandy soils, containing more than 50% sand and less than 20% clay in the top 30 cm, significantly limit sustainable crop production due to their coarse texture and high porosity, especially in arid and semi-arid areas [18]. Globally, these soils cover approximately 4,990,200,000 hectares, accounting for 31% of the total land area (Figure 1). They are predominantly found in the northern regions of Europe, North America, and Asia, as well as in the southern parts of South America and Africa. These soils typically exhibit low amounts of organic matter (<1%) and a low cation exchange capacity (CEC < 5 cmol(+)/kg), which results in limited nutrient retention and poor structural stability [19]. The large pore spaces between sand particles promote rapid water infiltration and drainage, leading to frequent nutrient leaching, especially of mobile nutrients such as N, K, and S. According to [20], sandy soils can lose up to 50–70% of the applied N within the first 30 days of fertilization due to leaching, leading to nutrient deficiencies that suppress crop growth and yields. In the sandy soils of the Eastern Cape Province of South Africa, Mwakidoshi et al. [21] reported maize yields of less than 2 t/ha under non-amended conditions, significantly below the country’s average of 5.5 t/ha, highlighting the fertility limitations of these soils. Furthermore, their limited water-holding capacity (<10% volumetric water content at field capacity) makes crops highly susceptible to drought stress, especially during critical growth stages, resulting in poor root development, stunted biomass, and low productivity [22,23].

These biophysical constraints are further compounded by socio-economic factors. Many smallholder farmers in regions dominated by sandy soils, such as South Africa’s Eastern Cape Province, lack access to inputs like lime, compost, or commercial fertilizers that could mitigate the inherent deficiencies of sandy soils [24].

The crop yield is strongly influenced by the soil texture and composition. Sandy soils, characterized by a low CEC and poor water retention, often limit crop productivity unless appropriately amended. Research indicates that the application of BGS enhances the soil organic matter and nutrient-holding capacity, thereby improving crop yields [25,26]. For instance, under comparable management conditions, the yields in loamy soils can be 20–30% higher than those in sandy soils, primarily due to superior moisture and nutrient retention [22,27].

Although the biophysical limitations of sandy soils are well documented, significant gaps remain in the research (

Table 1. Studies on challenges of crop cultivation in sandy soils.

Issue	New Findings	Challenges	References
Poor water and nutrient retention in sandy soils	Compost + biofertilizers improve soil structure and nutrient availability. BGS enhances soil fertility, biological activity, and water retention.	Rapid organic matter depletion. Compost is costly and labor-intensive; BGS is affordable and renewable.	[25]
Soil salinity and nutrient imbalances affecting productivity	BGS buffers soil pH and improves nutrient availability. Stimulates beneficial microbial activity to mitigate salt stress.	Chemical fertilizers aggravate salinity. Organic alternatives often unavailable or unaffordable.	[5]
Low amount of soil organic matter and poor nutrient cycling	Tropical cover crops improve soil C content and nutrient cycling. BGS rapidly boosts amount of organic matter and essential nutrients.	Cover cropping takes years to improve soils. BGS offers immediate but complementary benefits.	[26]
Groundwater contamination from nitrate leaching in sandy soils	Olive pomace biomass slag reduces nitrate leaching. BGS provides slow-release N, reducing leaching risks and protecting water quality.	Sandy soils prone to nutrient leaching. Chemical fertilizers cause rapid nitrate loss.	[27]

). Long-term studies examining the cumulative effects of organic amendments like BGS on nutrient retention, the soil structure, and microbial diversity in sandy soils are scarce [28]. Most existing studies focus on short-term yield responses without evaluating the sustainability of repeated applications over multiple cropping cycles [11,14]. Furthermore, there is limited information on timing BGS application and optimizing BGS application rates in sandy soils, which differ considerably in their water-holding capacity and nutrient dynamics to heavier soils [5,28]. The interactions between slurry-derived nutrients and sandy soil microbial communities also remain poorly understood. Lastly, socio-economic analyses addressing the affordability and accessibility of organic soil amendments for smallholder farmers managing sandy soils are largely missing. Addressing these gaps is crucial for developing context-specific, resilient soil fertility strategies.

3. Sorghum as a Potential Underutilized Crop

Sorghum is one of the most versatile and resilient cereal crops, cultivated for over 8000 years. It exists in four main types—grain, sweet, forage, and biomass sorghum—each adapted to different uses and possessing different agronomic traits (

Table 2. Studies on the contribution of different types of sorghum to providing food security and poverty alleviation.

Sorghum Type	Grain Contribution	Forage Contribution	Biofuel Contribution	References
Grain Sorghum	Staple food in semi-arid regions. Dual-purpose breeding for grain and nutrition.	Residues (stover) serve as livestock feed.	Limited role in biofuel production.	[33]
Sweet Sorghum	Nutrient-rich grains for food. Suitable for various foods.	Stalks used as animal fodder.	High potential for use in bioethanol production due to its fermentable sugars. Grown on marginal lands.	[34,35]
Forage Sorghum	Not primarily used for grain production.	Cultivated mainly for livestock feed. Resilient during feed shortages.	Not commonly used for biofuel production.	[33]
Biomass Sorghum	Not typically used for grain.	Stover may be used as livestock feed.	High biomass yield makes it suitable for production of bioenergy, including bioethanol and electricity.	[34]

) [29]. Originating in northeastern Africa, specifically in regions such as Ethiopia and Sudan, sorghum has long served as a staple food for many communities across the continent and beyond [30]. Its domestication is believed to have occurred around 5000 years ago in eastern Sudan, with archeological evidence supporting its early cultivation around the Atbara and Gash Rivers [31]. Sorghum’s journey from Africa to other parts of the world was facilitated by ancient trade routes, spreading to India around 4000 years ago and later to China, becoming an integral part of various cuisines and agricultural practices [29,32]. In the United States, sorghum was introduced through the slave trade and later became a significant crop in regions like Kansas [29]. This historical dissemination of sorghum across varied climatic and soil zones, especially across arid and semi-arid regions, underscores its ecological plasticity and relevance for production in marginal, sandy soils.

Grain sorghum is a nutrient-rich crop that provides essential proteins (protein content averaging around 10–12%), carbohydrates, and fiber [36]. It is also a valuable source of minerals such as iron, zinc, and potassium, with approximately 3.5 mg of iron and 2.5 mg of zinc per 100 g of grain [37]. Sorghum is highly adaptable and can thrive in diverse environmental conditions, being drought-tolerant, heat-resistant, and capable of growing on a wide range of soils with pH levels between 5.0 and 8.5 [38]. This resilience makes it an ideal crop for use in arid and semi-arid regions. Sorghum plays a crucial role in providing food security and livelihoods, particularly in developing countries, where it serves as a staple food and animal feed and is used in the production of beer and biofuels [39]. Despite its potential, sorghum remains underutilized compared to other grains like maize and wheat [40].

4. Biogas Slurry: Production and Properties

4.1. Sustainable Soil Amendment

Biogas slurry (BGS) is a nutrient-rich by-product of anaerobic digestion, produced from organic materials such as livestock manure, agricultural residues, and food waste [41,42]. In this controlled biological process, organic compounds are decomposed by microbial communities under oxygen-free conditions, generating biogas, primarily methane, and leaving behind a liquid effluent known as biogas slurry. The nutrient composition of BGS varies significantly depending on the feedstock source and digestion conditions. For example, slurry derived from animal manure typically contains higher nitrogen concentrations than that from crop residues, which influences its fertilization potential and agronomic performance. Anaerobic digestion typically occurs at temperatures between 30 °C and 40 °C, with retention times of 30 days or more depending on the feedstock and environmental factors, and operates optimally under three thermal regimes—psychrophilic (<25 °C), mesophilic (35–42 °C), and thermophilic (>50 °C)—with mesophilic conditions being the most common in agricultural digesters [40,43]. The process of anaerobic digestion, through which biogas and BGS are produced, is depicted in Figure 2. This pathway involves the microbial breakdown of organic feedstocks such as animal manure, food waste, and crop residues in the absence of oxygen, resulting in the simultaneous generation of biogas (CH₄ and CO₂) and a nutrient-rich slurry suitable for agricultural use.

Once digestion is complete, the slurry is often separated into solid and liquid fractions using mechanical separators. The liquid portion, BGS, retains a high concentration of N, P, K, and various micronutrients, making it suitable for use as an organic soil amendment [45]. Further processing, such as filtration and stabilization, may be applied to enhance its storage stability and usability [46]. Integrating BGS into crop production practices offers a sustainable alternative to chemical fertilizers (

Table 3. Studies on application of BGS as sustainable soil amendment for improving soil fertility and crop productivity.

Issues	Soil Fertility	Crop Productivity	References
Overuse of chemical nitrogen fertilizers causing nutrient imbalance	Improved soil availability of N, P, K, and organic C; enhanced microbial diversity and activity	Increased maize biomass and plant height with partial substitution of chemical N with BGS	[46,49]
Low nutrient use efficiency in rice systems	Increased soil NH4+-N, NO3−-N, available P, and organic carbon; improved NPK uptake	Higher rice grain and straw yields, optimal at 30% substitution of N with BGS	[50]
Decline in soil fertility due to intensive fertilization	Increased dissolved organic C, SOC, and available N and P; improved bacterial diversity	Maintained or increased paddy yield with long-term BGS + NPK application	[15]
High nitrogen losses in dryland cropping systems	Increased N use efficiency and reduced N leaching in dry conditions	Sustained wheat and maize yields under partial or full BGS substitution	[51,52]
Soil nutrient depletion in potato–wheat systems	Increased soil pH, organic C, total N, available P, and micronutrients	Higher tuber yield in potatoes and increased wheat biomass under 75–100% BGS substitution	[14]
High fertilizer dependency in leafy vegetables	Enhanced nutrient uptake with fertigation of BGS	Comparable lettuce yields to those with 100% chemical fertilizer when using BGS + NPK fertigation	[53,54]

), reducing nutrient runoff and supporting resource-efficient farming [47,48].

In addition to its agronomic benefits, using BGS contributes to environmental sustainability. By reducing the reliance on chemical fertilizers, it mitigates issues associated with chemical runoff and soil degradation, thus promoting healthier ecosystems [47]. Moreover, BGS can help in managing agricultural waste effectively, turning potential pollutants into valuable resources. Its use not only improves the physical and chemical properties of the soil but also supports the sustainable intensification of crop production by providing a cost-effective alternative for farmers who can produce their own organic fertilizers from livestock waste, food waste, and crop residues [48]. Overall, the integration of BGS into farming systems offers a pathway toward more sustainable practices while enhancing the crop productivity and environmental quality.

4.2. Agronomic Impacts of Biogas Slurry

Biogas slurry offers significant agronomic benefits in different soils and cropping systems. It enhances soil fertility by increasing the organic carbon, available nitrogen, phosphorus, and cation exchange capacity (CEC) [15,33]. These improvements promote stronger root growth, boost microbial activity, and improve the soil structure, particularly in nutrient-poor sandy soils [1,15,44,53].

Moreover, BGS positively influences plants’ physiological traits, enhancing their chlorophyll content, photosynthetic efficiency, and drought resilience [2,54]). Field studies consistently report higher crop yields and nutrient use efficiency when BGS is applied, either alone or combined with synthetic fertilizers [51,55]. Its slow nutrient release pattern aligns well with the crops’ demand, reducing the risks of nutrient leaching and volatilization compared to synthetic inputs [44,47]. However, the agronomic outcomes can vary based on the feedstock type, soil conditions, and management practices [56], emphasizing the need for site-specific recommendations.

In monoculture systems with the continuous cultivation of a single crop, BGS mitigates soil fertility depletion and enhances the N use efficiency, with studies reporting maize and wheat yield increases of 20–25% when the partial substitution of synthetic fertilizers is practiced [51,52]. Crop rotation systems alternating crop species over the seasons, especially those involving legumes, benefit from using BGS through improved soil microbial diversity and enhanced N fixation, increasing the residual nutrient availability for subsequent crops [14]. Sequential cropping systems, where multiple crops are grown in one season, also benefit from BGS’s slow-release nutrients, sustaining a high cumulative yield over time [57].

More complex systems like intercropping (the simultaneous cultivation of two or more crops in the same field) and relay cropping (the sequential planting of a second crop before the first is harvested) similarly profit from BGS’s balanced nutrient profile and its capacity to stimulate beneficial microbial activity, improving the plant compatibility and resource use efficiency [58]. Even in agroforestry setups like alley cropping, BGS enhances the nutrient availability, litter decomposition, and soil moisture retention [59]. In intercropping systems, where the nutrient demand varies spatially and temporally between crop components, BGS offers a gradual nutrient release that supports continuous uptake. In relay cropping, the residual nutrients and improved soil conditions provided by BGS support the establishment of the second crop without additional fertilizer inputs, improving the system efficiency.

BGS has been shown to improve the nitrogen use efficiency and yield in both monoculture and rotation systems. For example, in maize monoculture, the grain yields increased from 3.1 t/ha (control) to 7.09 t/ha when 75% of inorganic N was replaced with BGS [43]. In crop rotations involving legumes, BGS contributes to nutrient cycling and microbial enhancement, providing residual effects for subsequent crops and reducing the need for synthetic inputs.

Despite these successes, the existing studies have limitations: a focus on cereal monocultures, a lack of long-term data, and the insufficient standardization of the BGS quality and application rates. To optimize the use of BGS, more research is needed on underutilized crops, the long-term soil health impacts, and practical application models for smallholders.

4.3. Effect of Biogas Slurry on Soil Nutrient Dynamics

Biogas slurry improves soil nutrient management by providing the gradual release of essential nutrients, thereby reducing leaching and enhancing nutrient retention, especially in sandy soils [53,60]. By increasing the organic matter and CEC, BGS improves the soil water-holding capacity and microbial activity [44]. Additionally, it raises the soil pH buffering capacity in acidic soils [47].

However, the nutrient release dynamics and composition of BGS depend on the feedstock type and digestion conditions [61,62]. While cattle manure-based slurry offers a balanced nutrient profile, pineapple waste-derived slurry contributes organic acids that boost microbial nutrient cycling.

Studies in sandy soil systems recommend that the total nitrogen inputs, including those from biogas slurry and mineral sources, should generally not exceed 150–250 kg N ha⁻¹ to maintain high yields while minimizing nitrate leaching and preserving the carbon sequestration potential [28,63]. Exceeding this threshold often results in diminishing returns and greater environmental losses. Excessive nitrogen inputs, regardless of the source, increase the risk of nitrate leaching into the groundwater, nitrous oxide (N₂O) emissions, and surface water eutrophication, posing serious environmental and public health risks [64,65]. High nitrogen loading can also disrupt the soil structure by lowering the pH and affecting the aggregate stability, particularly in sandy soils with a low buffering capacity, leading to compaction or reduced water infiltration in deeper layers [65,66,67].

Achieving the optimal yield while minimizing the environmental risks requires the careful balancing of nitrogen inputs. Blending BGS with mineral fertilizer at moderate rates enhances nutrient synchronization and uptake. Integrated nutrient management using a recommended proportion of 50 to 75% of mineral N and BGS has been shown to maintain or improve yields while significantly reducing nitrate losses and N₂O emissions [43,65].

Despite these benefits, repeated BGS applications may cause nutrient imbalances, particularly phosphorus accumulation, posing environmental risks [63]. The current research often overlooks long-term nutrient build-ups, field-based nutrient dynamics, and slurry standardization. Addressing these through rigorous, multi-year trials is critical for developing sustainable application guidelines. To manage potential phosphorus build-ups, best practices include regular soil testing, rotating phosphorus-demanding crops, applying BGS based on the agronomic need, and using conservation buffers or subsurface application methods to reduce the runoff risk.

The agronomic and environmental benefits of biogas slurry (BGS) application are illustrated in Figure 3, which highlights improved nutrient cycling, enhanced soil fertility, increased organic matter, a reduced reliance on synthetic fertilizers, and the mitigation of environmental pollution.

Despite its many advantages, BGS also presents notable limitations, as summarized in Figure 4. These include the variability in the nutrient composition depending on the feedstock, the risk of nutrient imbalances (particularly phosphorus accumulation), the potential for nitrate leaching if overapplied, odor and pathogen concerns, and logistical challenges in handling and transport.

4.4. Effect of Biogas Slurry on Crop Productivity

Biogas slurry enhances crop productivity by supplying essential nutrients and improving the soil’s physical and biological conditions. It increases the chlorophyll content, boosts the photosynthetic efficiency, and promotes root development, enabling crops to better withstand drought and nutrient stress [44,54].

Multiple studies have confirmed that biogas slurry (BGS) application, particularly when integrated with the application of synthetic fertilizers, significantly increases crop yields, with the wheat grain yield rising by approximately 23% following the 40 t/ha application of BGS and the maize yield increasing from 3.1 t/ha (control) to 7.09 t/ha when BGS application was combined with the use of 75% inorganic nitrogen [43,64]. The slurry also enhances the soil microbial diversity, suppresses soil-borne pathogens, and improves plant resilience [14,65]. However, ensuring optimal application rates is crucial, as excessive BGS can lead to nutrient imbalances, ammonia volatilization, and phosphorus build-ups [60,63].

Although its positive effects are well documented, gaps remain in long-term field research on underutilized crops, system-specific responses, and the economic viability of BGS use in smallholder farming. Further studies focusing on these areas are essential for scaling up sustainable biogas slurry use.

4.5. Biogas Slurry Application Methods

BGS can be applied to soil using a variety of techniques, each with distinct operational characteristics, advantages, and limitations. Surface broadcasting (splash plate or hose distribution on the soil surface) is the simplest method, requiring minimal equipment, and thus suited to smallholders or farms without specialized machinery [43]. Broadcast application covers the entire soil surface, exposing a large area to the air, which causes substantial ammonia volatilization and nutrient runoff [14,64]. Its advantages are its low cost and high speed of application, but major drawbacks are its high nitrogen losses (100% baseline emissions) and odor.

Shallow incorporation (also called shallow injection) involves placing slurry just below the soil surface (typically 2–5 cm deep) without fully covering the band [60,65]. This method substantially reduces ammonia loss and odor by lowering the slurry’s exposure to the air and improves the uniformity of nutrient placement [43,52]. It is more equipment-intensive than broadcasting but still viable in many mechanized systems. Shallow injection is commonly used on both arable land and grassland. Its pros include large reductions in NH₃ volatilization (about 70% less than that with broadcast application) [66] and a more even N distribution. Its cons include higher labor and fuel requirements (custom equipment) and the potential disturbance of shallow-root crops if not timed properly.

Subsurface (deep) injection uses tine or coulter rigs to inject slurry 5–20 cm deep into the soil [43]. This method achieves the greatest reductions in gaseous losses (up to ~90% less NH₃ emissions vs. broadcasting) [54,66] and virtually eliminates surface runoff. By placing nutrients below the surface, it improves retention; however, deep placement may increase the risk of nutrients leaching to the groundwater due to deeper ammonium and nitrate movement [45,54]. Injection requires heavy machinery and is generally used on large, tilled fields (pre-planting). It is less suitable for use in no-till systems or grassland unless performed before sowing. The equipment costs and soil compaction risk are its main cons, though it effectively retains N and reduces odor.

Trailing hose/dribble bar and trailing shoe banding are compromise methods where hoses or shoes attached to a moving tank apply slurry in narrow surface bands close to the ground [54,66]. A trailing hose (dribble bar) lays slurry on the soil at a set height, while trailing shoes (low-emission applicators) part the vegetation and deposit slurry at the soil level [54]. Both greatly reduce ammonia volatilization compared to broadcasting (trailing hose ≈ 30% NH₃ loss reduction; trailing shoe ≈ 60%) [44]. They are suited to use in areas with low-emission regulations and can be used on both arable land and grassland. The trailing shoe’s advantages are efficient application on taller swards and further odor control [67], whereas hoses are simpler but may coat the vegetation. Their pros include reduced N loss and odor and the ability to use them on growing crops without inducing complete soil disturbances. Their cons include moderate equipment costs and the fact that they still leave slurry near the surface (higher NH₃ loss than that with injection). These methods improve the nutrient use efficiency but require a tractor and tank.

Fertigation (application of slurry via irrigation) uses irrigation systems (e.g., drip or sprinkle lines) to apply the filtered liquid fraction of BGS directly to crop roots. Recent studies (e.g., subsurface drip fertigation) have shown that this method can dramatically improve the N use efficiency and reduce N losses [44,54,66]. In an organic maize trial, pure slurry fertigation cut the cumulative N losses (NH₃, N₂O, leaching) by over 50% relative to those with pre-sowing broadcast application [28,53,67]. Fertigation requires clean slurry (to avoid emitter clogging) and an irrigation infrastructure, making it more feasible for use in intensive or horticultural systems than for smallholders. Its pros are its precise nutrient delivery and minimal surface emissions; its cons are its high technical complexity, filtering needs, and dependency on irrigation capabilities.

Foliar spraying (diluted BGS) involves spraying a highly diluted slurry (often a ~1:5 slurry-to-water ratio) onto foliage. This method is mainly used at a small scale or in horticulture to supply nutrients, especially micronutrients, or to correct deficiencies. Foliar application bypasses some soil losses but carries risks of leaf phytotoxicity if it is too concentrated, the potential spread of pathogens, and damage to application equipment. Empirical advice suggests applying it in cool conditions to reduce volatilization [63,67]. Because the evidence for it is limited, foliar BGS is supplementary; it is seldom used as a primary N source due to its low N concentration and practical difficulties.

Dewatered or pelletized BGS refers to the solid fraction separated from slurry (via presses or filters) and dried/compacted into a granular form [55,67]. Dewatered solids can be broadcast or further processed. Pelletizing stabilizes the nutrients and organic matter, facilitating transport, storage, and application with standard fertilizer spreaders [54,68]. The pros of this form include its reduced volume (lower transport cost), weaker smell, and longer shelf-life. However, drying/pelletizing requires additional processing infrastructure and energy, which increases the cost. Some nutrient losses (especially of ammonia) can occur during drying. Suitable scales for its use are larger operations or co-operatives where centralized processing is viable; pellets can be used in both the cultivation of field crops and horticulture as a NPK-rich amendment.

Each application method’s suitability is context-dependent. Broadcasting and shallow incorporation are adaptable to many scales but with a trade-off of higher emissions. Injection and banding methods (hose/shoe) are preferred in intensive, larger-scale systems or where regulations demand low emissions [43,67]. Fertigation suits high-value crops and well-irrigated farms [66]. Foliar spraying is niche and usually small-scale. The use of dewatered/pelletized BGS is the most relevant where transport or high-volume handling is a constraint; it allows for digestate’s use as a granular fertilizer (suitable across crop types). The impact on the soil and plant nutrients varies: subsurface methods keep more ammonium in the soil (lower NH₃ loss but may elevate the leaching risk) [14,65,68], whereas surface methods release more N into the air. Overall, low-emission methods improve nutrient retention and reduce the volatile losses, whereas simpler methods provide ease of application but at an environmental cost.

A comparative overview of the common biogas slurry application methods, including their operational characteristics, agronomic benefits, limitations, and suitability for different farming systems, is presented in

Table 4. Summary of BGS application methods and their agronomic implications.

Method	Description	Pros	Cons	Suitability	References
Broadcasting (surface)	Spread slurry across soil surface using splash plate or hose.	Simple; low equipment needs; fast application.	High N losses (NH3 volatilization), odor, and runoff; non-uniform placement.	Smallholders; when no other means.	[14,43,64]
Shallow incorporation	Shallowly mix slurry into soil (2–5 cm deep) immediately after broadcasting.	Reduces NH3 loss (≈70% reduction vs. broadcasting); reduces odor; better N utilization.	Requires additional pass or specialized implement (higher labor/fuel requirements).	Mechanized fields; grassland and arable land.	[43,52,60,65,66]
Subsurface injection	Inject slurry 5–20 cm deep using tine/coulter rigs; slurry ends up buried below surface.	Max NH3 loss reduction (≈90% vs. broadcasting); minimal odor/runoff; retains more N in soil.	High capital cost; soil disturbances and compaction; risk of deep nutrient leaching.	Large, tilled fields; pre-planting.	[43,45,54,66]
Trailing hose/shoe	Hoses or shoes deposit slurry in narrow bands at ground level (20–30 cm apart).	Low emissions: substantial NH3 loss reduction (hose: ~30%; shoe: ~60%); works with standing crops; reduces odor.	Equipment and tractor needed; slurry still on surface (higher NH3 emissions than with injection); shoe systems slower to empty.	Arable land (hose) and grassland (shoe).	[44,54,66,67]
Fertigation	Apply filtered slurry via irrigation (e.g., drip lines) to root zone.	High efficiency: synchronizes N supply, cutting NH3, N2O, and leaching loss by ~40–70% vs. broadcasting; precise delivery.	Requires filtration to avoid clogging; costly irrigation infrastructure; mainly for high-value or irrigated crops.	Drip-irrigated fields; horticulture.	[28,44,53]
Foliar spray	Spray highly diluted slurry on crop foliage (commonly ~1:5 dilution ratio).	Can quickly address nutrient deficiencies; usable on growing plants; low technology requirements (use of hand-held sprayer possible).	Risk of leaf burn, disease, and equipment corrosion; low N supply capacity; benefits not well-quantified.	Small-scale/horticulture trials.	[63,67]
Dewatered/pelletized	Separate slurry solids and dry/press into granules.	Reduced volume for transport; stable (reduced odor); can be applied with fertilizer spreaders; shelf-stable.	High processing cost and energy required; nutrient losses (NH3) during drying; may need binding additives; less equal N:K balance (solids rich in P).	Large farms or co-ops; all crop types.	[55,68]

. This summary highlights the trade-offs between low-cost, high-loss techniques such as surface broadcasting and more efficient but infrastructure-dependent methods like subsurface injection and fertigation.

4.6. Sources of Biogas Slurry

Biogas slurry originates from various organic feedstocks processed in anaerobic digesters [66]. The composition and nutrient quality of the slurry depends on the source material [67,68], digestion conditions [69], and post-treatment processes [60]. The primary sources of BGS include animal manure, crop residues, food waste, and sewage sludge [62,70].

Animal manure is one of the most used sources of BGS due to its high organic matter content and nutrient availability [28,71]. Studies indicate that cattle manure provides a balanced nitrogen-to-carbon ratio, making it highly suitable for biogas production and subsequent slurry application [72,73]. However, variations exist between different livestock sources, with poultry manure exhibiting a higher nitrogen content but also posing risks of ammonia volatilization [74,75].

4.6.1. Crop Residues

The most widely used biogas slurries are generated from crop residues, including straw, husks, and stalks. The primary challenge associated with crop residues is their high lignocellulosic content [76,77], which slows down anaerobic digestion and reduces the biogas yield [78]. Some researchers advocate for using pre-treatment methods such as enzymatic hydrolysis or microbial inoculation to improve the digestibility [79,80], but others argue that these methods are costly and not widely scalable [81,82]. Additionally, concerns persist about the availability of crop residues, as their removal for bio-digestion may affect the soil organic matter levels and reduce the long-term agricultural productivity [79].

4.6.2. Food Waste

Food waste has emerged as a promising but underutilized source of BGS [83]. Rich in readily degradable organic compounds, food waste slurry has been shown to enhance the microbial activity in the soil and improve crop yields [84,85]. However, the heterogeneity in food waste’s composition presents challenges in ensuring nutrient consistency [86,87]. Most studies emphasize the need for controlled feedstock selection [74,88], while others suggest that natural variability can be managed through co-digestion strategies [65,70] where food waste can be co-digested with cattle manure and sewage sludge to increase the biogas production [89,90,91].

4.6.3. Sewage Sludge

Sewage sludge is another significant source of BGS, particularly in urban areas. While it contains essential nutrients beneficial for soil fertility, it also contains heavy metals and contaminating pathogens that limit its widespread use [70,76]. Regulatory frameworks in different regions impose varying restrictions on sewage sludge application, leading to inconsistencies in the research findings [43,65,92]. Some research has shown the effectiveness of decontamination techniques such as composting and biochar treatment [92,93], while others have cautioned that residual contaminants may still pose risks to human health and the environment [79,94].

4.6.4. Livestock Manure

Livestock manure is an important source of BGS due to its numerous benefits. It serves as a nutrient-rich fertilizer, providing essential nutrients like nitrogen, phosphorus, and potassium, which enhance soil fertility and crop yields [95]. The use of BGS supports sustainability by reducing the reliance on synthetic fertilizers, thereby mitigating environmental issues such as water pollution and greenhouse gas emissions [96]. Additionally, it offers a cost-effective solution for farmers by utilizing waste to produce fertilizer, lowering costs and supporting local economies [94]. The anaerobic digestion process also aids waste management by reducing the harmful components in manure and minimizing water pollution [75]. Furthermore, the biogas produced during digestion provides a clean energy source, contributing to sustainable energy practices [96]. Overall, livestock manure is crucial for biogas slurry production due to its nutrient content, environmental benefits, cost-effectiveness, waste management capabilities, and role in sustainable energy generation.

The literature demonstrates that BGS feedstocks offer diverse nutrient profiles beneficial for soil fertility improvement. Animal manure-based slurry is consistently identified as being reliable due to its balanced nutrient composition. However, feedstocks like food waste and sewage sludge present significant weaknesses: namely, variability in their nutrient concentrations and potential contamination risks. Moreover, the high lignocellulosic content of crop residues will remain a key limitation for large-scale biogas production unless effective and affordable pre-treatment technologies are developed. A notable gap is the lack of standardized nutrient profiling across feedstocks, which hinders the determination of precise recommended rates for slurry application in different soil types and cropping systems.

The nutrient composition of BGS varies significantly depending on the type of feedstock and digestion conditions used, as shown in

Table 5. Typical nutrient composition of biogas slurry derived from different feedstocks.

Feedstock Type	Total N (g/kg)	P (g/kg)	K (g/kg)	Organic Matter (%)	Special Characteristics	Full Reference Citation
Cattle manure (biogas slurry digestate)	21.4	13.7	7.0	–	Neutral pH (~7–8); low heavy metal content. N in digestate usually mostly NH4+.	[97]
Poultry manure (liquid digestate fraction)	50	20	30	–	Very high in ammonium N; liquid fraction contains N–P–K at ~5–2–3% (i.e., 50–20–30 g/kg). Phosphorus mostly in solids; moderate Cu/Zn content.	[98,99]
Crop residues (maize silage digestate)	5.3	0.8	5.8	6.9	pH ~7.8; high organic C (VSs ~90% of TSs); low N:P ratio (~6.6:1); N mainly NH4+ after digestion.	[100]
Food waste digestate	8.0	2 (est.)	6 (est.)	~15 (est.)	High NH4+ fraction; nutrient contents vary widely with waste composition. Typically contains moderate K and C content and low heavy metal content.	[101]
Sewage sludge digestate (municipal)	13	5 (est.)	3 (est.)	30 (est.)	High in nutrients (esp. P and N) and organic solids; often elevated levels of heavy metals (Cd, Pb, etc.) and pathogens.	[102]
Mixed crop-livestock residue digestate	8 (est.)	2 (est.)	6 (est.)	35 (est.)	Combines manure and plant feedstock traits: moderate N content (lower than that of pure manure), moderate C:N ratio (~12–15), neutral pH. Often co-digestion improves nutrient balance.	[102]

. Animal-based feedstocks tend to produce slurry with higher nitrogen and phosphorus concentrations compared to crop residues or food waste, influencing the slurry’s agronomic effectiveness and application rate.

4.7. Forms of Biogas Slurry

BGS exists in a liquid or dry state, each with distinct characteristics and agronomic benefits [47]. The composition of BGS varies depending on the type of digester used, the feedstock composition [103], and the post-digestion processing methods [43,65]. While liquid BGS provides readily available nutrients, dry forms offer improved storability and gradual nutrient release, thereby influencing their suitability for different agricultural applications [44,104].

4.7.1. Liquid Biogas Slurry

Liquid BGS is a nutrient-rich effluent produced during anaerobic digestion, characterized by its high moisture content and readily available plant nutrients [47,103,105]. Liquid BGS is commonly applied through irrigation systems, foliar spraying, or direct soil incorporation, allowing for effective nutrient delivery [28]. Liquid BGS is particularly beneficial due to its high nutrient solubility, which facilitates immediate uptake by crops [106]. The presence of N, P, and K in liquid slurry has been linked to enhanced microbial activity in soils, which contributes to improved nutrient cycling [86]. Liquid slurry tends to be more beneficial in sandy soil due to its rapid infiltration [44,86].

Despite these advantages, studies have documented several limitations associated with liquid BGS application. One of the primary concerns is its high ammonia concentration, which, if it is not properly diluted, can lead to leaf scorching and inhibited plant growth [82,86]. Moreover, research has identified transportation challenges due to its high water content, which increases the handling costs and limits its use in regions lacking irrigation infrastructure [104]. Additionally, environmental studies have raised concerns regarding nitrate leaching, particularly in high-rainfall areas, where excessive application can lead to groundwater contamination and eutrophication [15,107].

4.7.2. Solid/Dry Biogas Slurry

Dry BGS, on the other hand, is a nutrient-rich by-product of anaerobic digestion that has undergone dewatering processes such as sun drying, mechanical pressing, or thermal treatment, resulting in a reduced moisture content and improved storage stability [88]. Dry BGS has been extensively examined for its advantages regarding its transportability and provision of long-term soil conditioning [28]. Dry BGS has been shown to act as a slow-release organic fertilizer, improving the soil structure and sustaining crop yields over extended periods [75,76]. Studies comparing soil types suggest that dry slurry is particularly effective in clay soils, where it reduces compaction and enhances aeration [44,86].

Despite its benefits, research highlights certain limitations associated with dry BGS utilization. The drying process often results in the loss of volatile nitrogen compounds [86], thereby reducing its overall fertilization efficiency compared to that of a liquid slurry [76,108]. Moreover, studies emphasize the importance of immediate application after drying to prevent further nutrient loss, particularly decreases in the ammonium content [109,110,111]. Additionally, findings suggest that while dry slurry enhances the soil organic matter content, its use may require supplementation with additional fertilizers to meet the short-term crop nutrient demand effectively [66].

A growing body of literature has examined the environmental implications of BGS application [49,75]. While liquid slurry poses risks related to nutrient leaching and water contamination, dry and composted forms offer more environmentally stable alternatives [43,68]. However, challenges remain regarding the optimization of the nutrient availability in these forms [80]. Research suggests that integrating different BGS forms into crop rotations and soil fertility programs can offer a balanced approach [82,94], leveraging the immediate benefits of liquid slurry while ensuring long-term soil improvement through the use of dry and composted forms [66,106].

The current research highlights that both liquid and dry biogas slurries offer complementary benefits to soil fertility management (

Table 6. Comparison of liquid vs. dry biogas slurry for agricultural application.

Form	Moisture Content	Nutrient Availability	Application Methods	Transportation and Storage	Environmental Risks	Cost	Best Suited for	Full Reference Citation
Liquid	Very high (>85–90% water; typically <15% dry matter)	High immediate nutrient availability; N mainly present as ammonium (NH4+); good levels of P and K	Applied via tankers using splash plate, dribble bar, or injection systems	Requires large storage tanks/lagoons; high transport cost due to bulk volume	Higher ammonia volatilization and nitrate leaching if not well managed	Lower processing cost but higher logistics cost	Ideal for nearby fields, short-distance hauling, and high-N-demand crops	[47]
Dry	Low moisture content (>15–20% dry matter); often pelletized	Lower immediate N availability (mostly organic N); higher organic matter and P content; slower release	Applied using solid manure spreaders or incorporated with tillage	Easier to store and transport; stackable; reduced volume	Lower risk of leaching/runoff; possible dust emissions	Higher processing cost (drying/separation), lower transport cost	Suitable for long-distance transport, organic systems, or long-term soil improvement	[102]

). Liquid BGS is advantageous for rapid nutrient delivery but poses significant environmental risks in high-rainfall areas, while dry BGS ensures better nutrient conservation over time but requires careful management to minimize the volatilization losses. A major strength of the literature is the clear identification of different application contexts for each form. Nevertheless, the weaknesses include a lack of comparative field trials under real-world farming conditions, especially on sandy soils. Moreover, few studies systematically assess the economic feasibility of slurry form choices for smallholder farmers. This creates a critical gap in the practical recommendations for slurry management based on the crop type, soil texture, and climate conditions.

4.8. Techniques for Drying Biogas Slurry

The drying of biogas slurry is crucial for improving its storage, transportation, and application efficiency. Different biogas slurry drying techniques, including natural drying, mechanical dewatering, solar drying, and thermal treatment, have been explored to convert liquid slurry into a more manageable solid form [63,112]. However, the choice of the drying method significantly influences nutrient retention and the overall efficiency of the slurry as an organic fertilizer. Some researchers advocate for low-cost drying methods such as solar and natural drying [54,107,112], while others emphasize the efficiency of high-tech solutions like thermal and mechanical drying [56,108].

While some studies highlight the benefits of using biogas slurry in reducing the synthetic fertilizer dependency [83,106], others caution about the potential soil degradation and pollution risks [113,114]. The long-term effects of repeated slurry applications on microbial diversity and soil health remain unclear, necessitating further longitudinal studies. Additionally, discrepancies exist regarding the role of pre-treatment methods in improving the slurry drying efficiency. While some advocate for microbial inoculation and enzymatic treatments to enhance nutrient retention [76,79], others argue that these approaches introduce additional complexities and costs [80,115].

4.8.1. Natural Drying

Natural drying is one of the oldest and most cost-effective techniques, involving the exposure of slurry to sunlight and wind to evaporate the moisture. While effective in reducing the bulk volume, this method has limitations, including prolonged drying times and nutrient losses due to volatilization and leaching [14,89]. Some researchers advocate for controlled drying conditions to minimize these losses [14,75,116], while others argue that natural drying is unsuitable for large-scale applications due to its dependency on the weather conditions [15,68,94].

4.8.2. Mechanical Dewatering

Mechanical dewatering uses presses, centrifuges, and filter beds to separate the solid and liquid fractions of the slurry. This technique has gained popularity due to its ability to rapidly reduce the moisture content and enhance the handling properties [82]. However, concerns about the energy requirements and costs associated with mechanical dewatering have been raised [104,117]. Conflicting research findings exist on the efficiency of different mechanical drying systems, with some studies reporting significant nutrient preservation, while others highlight nitrogen loss through ammonia volatilization [69,118,119].

4.8.3. Solar Drying

Solar drying has been promoted as a sustainable and energy-efficient method for slurry dehydration. This technique utilizes solar radiation to evaporate moisture, reducing the slurry volume while retaining essential nutrients [68,104]. Research indicates that solar drying enhances nitrogen and phosphorus retention compared to open-air drying [52,65,120]. However, variability in the solar exposure and seasonal fluctuations affect the drying efficiency [77]. Some authors propose hybrid drying systems combining the use of solar energy with mechanical aids [85,121], but others contend that these systems are economically unfeasible for smallholder farmers [14,21,69].

4.8.4. Thermal Treatment

Thermal treatment, including direct heating and pyrolysis, is another approach to drying biogas slurry. This method effectively eliminates pathogens and reduces the odor issues associated with slurry application [56,108]. Thermal drying requires significant energy inputs, making it less viable for regions with limited access to affordable fuel sources [60]. Studies have also raised concerns about potential alterations in the organic matter composition during high-temperature processing, which may affect the long-term soil benefits of the slurry [76,122].

While multiple drying techniques for biogas slurry have been studied, each presents trade-offs between its cost, nutrient preservation, energy use, and scalability. A key strength of the literature is the diverse exploration of low-tech and high-tech solutions adapted to different farming systems. However, significant weaknesses remain: natural and solar drying methods, while affordable, are often unsuitable for use in regions with an unpredictable climate, while high-energy methods are largely impractical for smallholders. Critically, there is a lack of long-term field evaluations comparing the agronomic performance of slurry dried using different methods. Additionally, few studies assess the carbon footprint implications of different drying techniques, representing a notable research gap in climate-smart agriculture.

5. Synthesis of Literature Findings

The reviewed literature consistently demonstrates that BGS is a promising sustainable soil amendment capable of enhancing soil fertility, improving the soil structure, boosting microbial activity, and increasing crop productivity across various cropping systems. A recurring pattern across the studies is the ability of BGS to provide a slow-release source of essential nutrients, particularly nitrogen, phosphorus, and potassium, thereby reducing the need for synthetic fertilizers. Additionally, a common theme is the role of BGS in improving water retention and the cation exchange capacity in sandy and marginal soils, leading to enhanced drought resilience and better nutrient cycling. The research findings further emphasize the environmental benefits of BGS use, such as reduced chemical runoff and waste recycling, contributing to circular economy and climate-smart agricultural initiatives.

Across the reviewed literature, biogas slurry consistently improved the nutrient availability, soil organic matter, and crop yields, especially in low-fertility or sandy soils. However, the degree of improvement varied based on the feedstock type, soil properties, and application strategy. The integrated use of BGS with mineral fertilizers was found to be more effective than using BGS alone. Major limitations include the variation in the BGS nutrient composition, overapplication risks, and logistical constraints. Long-term studies have been limited, especially on sandy soils and under diverse cropping systems. Standardizing nutrient profiling and application guidelines remains a key priority for researchers and practitioners.

Despite the consistent positive outcomes, several weaknesses and limitations emerged through examining literature. First, many studies focus on the short-term soil and crop responses, with a limited assessment of the long-term ecological impacts, such as phosphorus accumulation, nutrient imbalances, or potential microbial shifts. There is also a tendency to concentrate on a narrow range of major cereal crops like maize and wheat, while underutilized crops such as sorghum, particularly in sandy soil conditions, remain underexplored. Furthermore, the significant variability in the BGS nutrient content based on the feedstock type and processing methods is often overlooked, making it difficult to standardize the application rates and agronomic recommendations. Economic analyses assessing the cost–benefit ratio of BGS compared to that of conventional fertilizers are largely absent, particularly for smallholder farming contexts.

These observations reveal critical research gaps that need to be addressed to optimize the use of BGS for sustainable agriculture. Long-term, system-specific field trials evaluating the cumulative effects of BGS on soil health, nutrient cycling, and crop yields are urgently needed. Studies should also diversify beyond major cereals to include underutilized crops like sorghum and legumes and consider various soil textures, especially sandy soils. Moreover, standardized methods for characterizing the BGS composition and nutrient availability must be developed to support precise, site-specific application recommendations. Integrating agronomic evaluations with economic and environmental assessments will be essential to fully realize the potential of BGS in advancing sustainable, resilient, and profitable farming systems.

6. Conclusions and Recommendations

This review highlights the potential of BGS as an effective and sustainable soil amendment for improving sorghum’s productivity in sandy soils. The evidence from the literature consistently shows that BGS enhances the soil organic carbon, nutrient availability, cation exchange capacity, and water retention, leading to improved plant growth and yield performance. The integrated use of BGS with inorganic fertilizers offers synergistic effects, optimizing the nutrient availability and promoting more sustainable crop production systems. Furthermore, the recycling of organic waste through BGS production aligns with circular economic principles and contributes to climate-smart agriculture.

However, significant research gaps remain. The long-term effects of repeated BGS applications on the soil nutrient balance, microbial ecology, and potential environmental risks such as nutrient leaching have not been thoroughly studied, particularly in sandy soils. The variability in the BGS nutrient composition based on the feedstock type further complicates application recommendations, underscoring the need for standardized characterization protocols. Additionally, limited research exists on the response of underutilized crops like sorghum to different BGS formulations under varying soil and climatic conditions.

Considering these findings, it is recommended that farmers managing sandy, nutrient-depleted soils consider the combined application of BGS and inorganic fertilizers to sustainably enhance crop productivity. Policymakers should support the development of biogas infrastructure and create enabling policies to promote the use of BGS in agriculture. Extension services should focus on educating farmers about optimal application methods tailored to specific soil textures, crop types, and local climatic conditions.

Future research should prioritize long-term, field-based studies assessing the cumulative soil and crop responses to BGS applications, with special attention to sandy soils and a broader range of crops beyond major cereals. Economic analyses evaluating the cost-effectiveness and scalability of BGS use, especially for smallholder farmers, are also critical to support informed decision-making and widespread adoption. Future guidelines should prioritize the standardization of key parameters such as the total nitrogen (N), phosphorus (P), potassium (K), the carbon-to-nitrogen (C:N) ratio, the organic matter content, and micronutrients like iron (Fe) and zinc (Zn). The consistent reporting of these values will enhance comparability and facilitate site-specific nutrient management. Going forward, the sustainable use of BGS will require advancements in nutrient profiling, feedstock standardization, and integration with precision agriculture tools. Research into slow-release formulations, dewatered forms, and site-specific application protocols will be critical for optimizing its use in different agroecological zones. Policy incentives and farmer training are also essential to promote its safe and effective adoption.

Overall, this review highlights the strategic importance of biogas slurry (BGS) in advancing sustainable agriculture, particularly in resource-constrained and semi-arid regions where conventional inputs are costly or ineffective. By recycling organic waste into a nutrient-rich amendment, BGS offers an integrated solution that supports soil health, improves crop productivity, and aligns with global goals for ensuring food security, climate resilience, and a circular economy. Its wider adoption, especially in sandy soil agroecosystems, can play a vital role in building resilient farming systems under changing climatic conditions.