Sulphur nutrition management in Sub-Saharan Africa crop production: a systematic review

Sulphur (S) deficiency in Sub-Saharan Africa (SSA), driven by soil degradation and S-free fertilisers, threatens crop yield and protein quality. This systematic review synthesises four decades of studies (1980–2024) to assess soil S status, analysis methods, management challenges, and recommended rates for effective fertilisation to improve sustainable productivity. A systematic literature review was conducted following the preferred reporting items for systematic reviews and meta-analyses (PRISMA) framework to synthesise available evidence on S nutrient management in agricultural soils across SSA. The review revealed that S concentrations were generally higher in surface horizons compared to sub-surface layers, with vertical distribution influenced by soil texture, pedogenic processes, organic matter content, and fertiliser inputs. In highly weathered soils, S depletion was pronounced, contributing to widespread deficiencies across SSA’s agricultural landscapes. Analysis of S fertilisation practices showed a research cereal crop (s) emphasis, accounting for 65% of studies, followed by legumes with 25% and oilseeds with 10%. Most of the cereal studies have reported S application rates between 0 and 30 kg S/ha, with 71% of studies applying ≤20 kg S/ha. Legumes, by contrast, received higher rates (21–40 kg S/ha), typically through potassium sulphate or nitrogen-phosphorus-sulphur (NPS) blended fertilisers. Yield responses to S application varied significantly by crop type. Maize exhibited the higher yield increase, ranging from 20% to 260% depending on the fertiliser application rate, followed by wheat and rice. Legumes such as soybeans showed more modest increase of 25%, while oilseeds like canola and sesame responded minimally, even under higher S inputs. These findings underscore the need for crop- and site-specific S management strategies in SSA. The adoption of soil testing and decision-making frameworks such as the 4R nutrient stewardship (right source, rate, time, and place) is recommended to optimise crop yield and reduce environmental risks associated with nutrient mismanagement.

1 Introduction

The Sustainable Development Goal 2.2 aims to end all forms of malnutrition. However, many countries are not on track to achieve this target (Scott et al., 2020). Malnutrition remains a global challenge, particularly in regions where food security is compromised (Al-Worafi, 2023). Among the often-overlooked contributors to malnutrition and food insecurity is soil fertility, which directly influences the nutritional content of crops (Lehmann et al., 2020; Ramaswwamyreddy and Basavaraju, 2022). Soil infertility not only reduces crop yields but also diminishes the nutritional quality of food, disrupting overall food production. Therefore, a soil fertility approach is essential to improve both the productivity and nutritional value of food crops, thereby strengthening food systems (Havlin and Heiniger, 2020).

Sulphur (S) constitutes approximately 0.06–0.10% of the earth’s crust, making it the 13^th most abundant element and an essential nutrient for plant growth (Udayana et al., 2021). Mineralisation of soil organic matter provides about 95% of plant-available S (SO₄²^-) in most soils, contributing about 4–12 kg S ha^-¹ annually (Sharma et al., 2024; Weil and Brady, 2016). Plants can also absorb gaseous S (GS) from atmospheric hydrogen sulphide and sulphur dioxide (SO₂) (Telman and Dietz, 2019). S is involved in the synthesis of amino acids, proteins, and enzymes, which are critical for plant structure and physiological processes, including photosynthesis, respiration, and nutrient uptake, and form part of various vitamins, coenzymes, and secondary metabolites (de Mello Prado, 2021b; Havlin et al., 2017; Mengel and Kirkby, 2001).

Over the past decade, the atmospheric composition of SO₂ has been decreasing (Opio et al., 2021). Although this is positive in the environmental protection context, it negatively affects the amount of atmospheric GS taken up by plants (Narayan et al., 2022). At the same time, land degradation and climate change threaten soil organic matter, a key source of S (Das et al., 2024). This growing vulnerability of S sources underscores the need to integrate S into nutrient management programs alongside nitrogen (N), phosphorus (P), and potassium (K) (Sharma et al., 2024). S deficiency has become a global concern, with plant-available S in soils estimated to have declined by 34–86% (Sharma et al., 2024). Reduced atmospheric deposition, coupled with seasonal burning of vegetation and crop residues, further limits S availability for crops (Cassou, 2018; Harou et al., 2022).

In Sub-Saharan Africa (SSA), trends in S application reveal critical gaps in nutrient management. Over 80% of smallholder farmers rely on nitrogen-phosphorus-potassium (NPK) fertilisers that contain little or no S (Johnson et al., 2023). Despite widespread fertiliser use, S is often excluded from government recommendations in many SSA countries, such as Tanzania (Hemesh, 2020; Michelson, 2017). The Limited adoption of S-containing fertilisers, such as superphosphates or ammonium sulphate, is driven by factors including higher cost, low availability and lack of awareness among farmers and extension agents (Johnson et al., 2023). Moreover, the environmental risks associated with poorly managed S fertiliser use remain underexplored in SSA, adding to the existing knowledge gap. Research on S nutrition in SSA remains fragmented, with limited region-specific recommendations that account for local soil types, crop types, and environmental conditions. This lack of integrated knowledge limits the development of effective strategies to mitigate S depletion, exacerbating hidden hunger and yield gaps. Exploring S nutrition management is therefore critical to optimise fertiliser use, enhance crop productivity and nutritional quality, and contribute to food security.

To address these challenges, this systematic literature review was conducted to synthesize existing knowledge on S nutrient management in agricultural soils of SSA. The review aimed to examine the current status of S in soils, its implications for crop productivity and environmental sustainability, and available recommendations for optimal S management. It also critically assessed soil sampling depths and extraction methods, which are essential for accurate evaluation of soil S status. Covering studies from 1980 to 2024, this review highlights the potential of improved S management as a strategy to strengthen food security in SSA.

2 Methodology

2.1 Literature search

The review utilised the preferred reporting Items for systematic review and meta-analysis (PRISMA) framework, as described by Page et al. (2021), for collecting and reporting information on S in agricultural soils in sub-Saharan Africa (SSA). This method is widely recognised for evaluating published systematic reviews through critical analysis (Hutton et al., 2015). To find and download journal articles and published reports, a literature search was conducted in four academic databases and search engines: Web of Science (http://apps.webofknowledge.com/), ScienceDirect (https://www.sciencedirect.com/), PubMed (https://pubmed.ncbi.nlm.nih.gov/), and Google Scholar (https://scholar.google.com/) The systematic search aimed at selecting fully published journal articles by carefully crafting search strings, ensuring that the majority of relevant articles for the review objective were included.

All search terms were based on predefined keywords related to the title, keywords, and abstract of the articles. To refine and hone the search results, specific keywords included (“Sulphur” OR “Sulfur”) AND (“fertiliz*” OR “fertilis*” OR “nutrition” OR “deficiency” OR “status”) AND (“crop production” OR “yield” OR “growth”) AND (“sub-Saharan Africa” OR names of specific countries like “Tanzania” OR “Kenya” OR “Nigeria”). The search was limited to English-language articles in the fields of agriculture, biological sciences, and plant sciences.

2.2 Article screening and classification

A total of 577 journal articles were identified through a search conducted to identify articles published between January 1980 to June 30, 2024. Of these, 156 articles were removed as duplicates, and the remaining 419 articles were further screened. The inclusion criteria used included (i) original research studies, (ii) studies that investigated soil S status and management in crop production, (iii) studies conducted within Sub-Saharan Africa (SSA) (iv) studies published within the time frame of 1980 – 2024, and (v) studies written exclusively in the English language. During the initial screening of titles and abstracts, 267 articles out of 419 articles were excluded for not meeting the inclusion criteria. Following a thorough filtering, 97 out of the 152 remaining articles were eliminated because the main text did not contain relevant information directly related to the review’s scope. In the end, 55 articles were included after a thorough full-text screening. The flowchart of the entire process from identifying the relevant literatures to making the final inclusion decision is illustrated in Figure 1.

Figure 1

Figure 1. The articles screening flowchart, based on the preferred reporting Items for systematic review and meta-analysis (PRISMA) approach modified by Page et al. (2021).

The included articles focused on various parameters, including the year of publication, the S extraction method used, the study country, as well as study target. Additionally, we assessed the target crop, S materials, and rates used to supply S for the crop production. The summary of the evaluated parameters is presented in Supplementary Table 1.

3 Results and discussion

3.1 Overview of the relevant articles

3.1.1 Spatial and temporal trend of sulphur researches

The spatial and temporal distributions of sulphur (S) studies across various regions in sub-Saharan Africa (SSA) are as illustrated in Figure 2. Over the past four decades, there has been a noticeable general increase in research focused on S and its impact on crop production. While many SSA countries began studying S in the 1990s, the number of studies has significantly increased since 2011 (Figure 2b). Nigeria has maintained a consistent focus on S management throughout all decades, whereas a significant number of the research (24% of the articles) has been concentrated in Ethiopia. Among the analysed articles, 58% (30 articles) investigated the role of S in plants, 27% (14 articles) focused on S in soils, and 15% (8 articles) examined both soil and plants. Additionally, 62% (32 articles) of the studies assessed how various crops respond to different S-containing fertilisers, while 38% (20 articles) evaluated the status of S in the soil (Figure 2c).

Figure 2

Figure 2. Trend on S related studies across Sub-Saharan Africa (SSA) for the period between 1980 to June 2024 (a) Africa map showing sulphur (S) studies per SSA countries (b) A graph showing number of S articles published per each decade in SSA countries, and (c) A graph showing trend of S studies type over the past four decades.

The increase in soil S research across sub-Saharan Africa (SSA) since the 1990s is largely driven by nutrient depletion due to the soil degradation caused by climate change and unsustainable nutrient management (Abebe and Beyene, 2021; Lal and Stewart, 2019). Nigeria’s consistent studies into S management highlight the country’s long-standing recognition of S deficiency as a significant barrier to crop productivity, especially in its Savannah zone (Adetunji, 1992; Kang et al., 1981; Kwari et al., 2009). Ethiopia’s prominence (24% of studies) reflects its efforts in improving crop performance through the alternative use of S-containing materials in various crop plants (Getachew et al., 2017; Habtegebrial and Singh, 2009; Tehulie and Yimer, 2021).

The predominance of plant-focused studies (58% of articles) and the emphasis on crop responses to S-containing fertilisers (62% of articles) indicate some efforts to tackle S deficiencies, which have been worsened by the prolonged use of low-S nitrogen-phosphorus or nitrogen-phosphorus-potassium (NPK) fertilisers along with a decline in soil organic matter (Getinet et al., 2022; Harou et al., 2022). However, the relatively small proportion of studies analysing soil-S status (38%) highlights a research need, so as to understand long-term S cycling and availability, especially in regions with heavy rainfall and weathered soils that are susceptible to leaching (Ulén, 2020).

3.1.2 Soil sulphur extraction methods in Sub-Saharan Africa

The results showed significant methodological variation in soil S extraction across Sub-Saharan Africa (SSA) studies (Figure 3). Among the reviewed articles, 69% (36 studies) explicitly reported soil-S extraction protocols, while 31% omitted methodological details (Figure 3a). Of the reported methods, the 0.01 M monocalcium phosphate (Ca(H₂PO₄) ₂) extraction method dominated, accounting for 55% (20 articles), followed by Mehlich-III at 14% (5 articles) and monopotassium phosphate (KH₂PO₄) at 11% (4 articles). Less common methods included 0.015 M CaCl₂ (5%, 2 articles), water extraction, and 0.1M HCl (3% each, 1 article) (Figure 3b). Two studies that used electro ultrafiltration and direct analysis on dried soil samples applied ion chromatography and X-ray fluorescence method of analysis, respectively.

Figure 3

Figure 3. Percentage presentation of (a) reporting status and (b) the reported soil-S extraction methods by various studies in sub-Saharan Africa.

Variation in extraction methods has important practical implications for S-related soil research in SSA. The predominance of 0.01 M Ca(H₂PO₄)₂ aligns with its global acceptance for estimating SO₄²^--S in acidic to neutral soils (Bankole et al., 2022; Tabatabai, 1982), which are common across SSA. Its cost-effectiveness and simplicity make it ideal for low-resource laboratories. However, its reliance on SO₄²^- extraction overlooks organic S pools (Watkinson and Kear, 1996), which are critical in contributing about 95% of available plant-S in soils (Haneklaus et al., 2007; Tabatabai, 1982; Weil and Brady, 2016). Thus, imposing risk in misinterpreting S availability, especially in organic-rich soils, where mineralisation rates may affect crop uptake. The underuse of other methods, such as electro ultrafiltration and direct analysis, coincides with infrastructural limitations, favouring resource-rich institutions such as those used by Fischer et al. (2020) and Uloro and Mengel (1994). Conversely, the CaCl₂ extraction method is underutilised despite its relevance in efficiently extracting soil-S (Amuri et al., 2023; Ketterings et al., 2011). The omission of extraction methods in 31% of studies hinders study reproducibility and undermines data harmonisation for policy frameworks like national and regional fertiliser and soil health programs.

3.2 Available guide for the soil sulphur interpretation

Soil sulphur (S) levels are interpreted relative to the extraction method used, as each extractant varies in its ability to extract different S fractions, and associated crop responses to applied S. In Tanzania, for example, Amuri et al. (2023) reported a critical concentration range of 4.1–4.8 mg S/kg for S extracted using 0.01 M CaCl₂ with the SoilDoc method, providing a localised benchmark for deficiency assessment. In Ethiopia, a critical range of 20–80 mg/kg following extraction by Mehlich-III has been proposed by the Ethiopia Soil Information System [EthioSIS], 2014, as reported by (Lelago et al., 2016). For the widely used 0.01 M Ca(H₂PO₄)₂ method, Horneck et al. (2011) proposed the following classification: < 2 mg S/kg as very low, 2–5 mg/kg as low, 5–20 mg/kg as medium, and >20 mg/kg as high. In tropical contexts, Landon (1991) recommended slightly higher threshold values, classifying a critical range of 6–12 mg S/kg as adequate for various extraction methods. Furthermore, Peverill et al. (1999) suggested a general sufficiency range of 5–10 mg S/kg that could apply to multiple methods, including both 0.01 M Ca(H₂PO₄)₂ and KCl extractions. These ratings provide essential context for interpreting soil S data and identifying potential deficiencies, based on the specific extraction method used. However, for efficient and accurate interpretation, there is a pressing need to align critical values with both the extraction method and geographic context.

3.3 Sulphur fertility status in Sub-Saharan Africa agricultural soils

Sulphur (S) status in soils varies with soil depth. Thus, to determine general S fertility status in soils, this study categorised sampling depth into two groups (shallow topsoil sampling: 0–15 cm, and deeper sampling: 0 – 20/30 cm). A total of 14 studies were conducted at the 0–15 cm depth, with the majority located in West Africa, notably in Nigeria and Ghana, between 1987 and 2022. Studies with shallow-depth sampling are comparatively rare in East and Southern Africa, with only one example from Malawi (Weil and Mughogho, 2000). In contrast, the 0–20/30 cm category dominates the literature, comprising 34 studies spread across East, Southern, and West Africa. This deeper sampling approach has been prevalent from the early 1990s through to 2024, reflecting a widespread emphasis on deeper sampling S dynamics.

Of the 52 reviewed articles, 31 reported soil S status, covering 38 experimental sites across Sub-Saharan Africa (SSA). A regional assessment of soil S status revealed distinct regional variability (Table 1). In East Africa (n = 19), the majority of sites (63.2%, n = 12) were classified as having low S levels, with a mean extractable S concentration of 4.38 mg kg^-¹. Medium and high S levels accounted for 26.3% (n = 5; mean = 8.23 mg kg^-¹) and 10.5% (n = 2; mean = 74.15 mg kg^-¹) of sites, respectively. In West Africa (n = 17), low S was observed at 47.1% of sites (n = 8; mean = 3.14 mg kg^-¹), followed by medium S (41.2%, n = 7; mean = 9.28 mg kg^-¹) and high S (11.8%, n = 2; mean = 31.95 mg kg^-¹). In Southern Africa (n = 2), both sites fell within the medium S category, with a mean S concentration of 8.69 mg kg^-¹. Overall, low S status was the most prevalent across the SSA fields, representing 55.3% of all observations. Ratings were determined using interpretation ranges for most commonly used extraction reagents based on Landon (1991) and for the Mehlich-III thresholds by Ethiosis (2014), as cited in (Lelago et al., 2016).

Table 1

Table 1. Summary of rating for status of the extractable soil sulphur reported in various sites in Sub Saharan Africa.

S concentrations in soil profiles varied significantly with depth, showing distinct vertical distribution patterns across sites and soil types (Table 2). S levels in the topsoil (0–20 cm) ranged broadly from 1.0 to 51.0 mg/kg. Low values were observed in Typic Paleudalf profiles in semi-deciduous rain forest of Ofin, Ghana, with 1.0–4.0 mg/kg, while high concentrations were recorded in Typic Haplorthox (38.0 mg/kg) and Oxic Tropaquent (51.0 mg/kg) of Ankasa and Kwabeng profiles, respectively (Acquaye and Beringer, 1989a). Uloro and Mengel (1994) reported 26.4 mg/kg in the 0–25 cm layer in Loam soils of Kalisha, Ethiopia, while Kalonga et al. (2024) reported 14.2 mg/kg at 0–20 cm and 13.95 mg/kg at 20–50 cm at the Maasai landscape of Arusha, Tanzania, indicating low Mehlich-III S levels. In the Ethiopian rift valley, Haplic Nitisols had low S levels; at 25–60 cm depth was 1.4 mg/kg, while at 60–102-cm depth was 1.0 mg/kg (Itanna, 2005). In contrast, Calcaric Fluvisols recorded elevated values: 33.6 mg/kg at 0–29 cm and 46.4 mg/kg at 29–75 cm. Similarly, Haplic Solonetz profiles showed 7.6 mg/kg in the Ah horizon (0–24 cm) and 33.2 mg/kg in Bm (24–56 cm), with progressively higher values in deeper clay horizons. S concentrations were highly variable in deeper layers, ranging from 0.2 to 288.0 mg/kg. In profiles like the Vitric Andosol and Haplic Nitisol, very low S levels were recorded (e.g., 0.2–1.0 mg/kg between 60 and 140 cm) (Itanna, 2005). However, Haplic Solonetz showed extraordinarily high values in Bt and Cm horizons: 288.0 mg/kg (56–118 cm), 107.2 mg/kg (118–122 cm), and 203.4 mg/kg (122–136 cm). Loam soil in Kalisha, Ghana, exhibited high S levels from 25 to 93 cm, ranging between 23.6 and 32.0 mg/kg (Uloro and Mengel, 1994).

Table 2

Table 2. Variation in sulphur levels in the soil profile as reported across Sub-Saharan African countries. Note: S – soil sulphur (mg/kg), Nr – not reported, Soil Hrzn – soil horizon.

The increased focus on deeper sampling aligns with agronomic research prioritising cereals (maize, sorghum) and legumes (soybean, common beans), whose root systems often extend >15 cm deep (Supplementary Table 1). Determining S in deeper soil is critical for these crops, as grain filling and nitrogen fixation depend on sustained S availability beyond surface layers. The observed regional differences in soil S status reflect underlying variations in agroecological conditions, soil parent material, land use intensity, and fertiliser management across SSA. The predominance of low S concentrations in East and West Africa soils is consistent with the inherently low S reserves of many weathered tropical soils and may be further exacerbated by continuous cropping, low use of S-containing fertilisers, and nutrient depletion (Tabatabai, 1982).

The low mean S levels remain below agronomically sufficient thresholds in East and West Africa regions (4.07 mg/kg in East Africa and 3.87 mg/kg in West Africa), highlighting a widespread risk of S deficiency that could limit crop productivity (Haneklaus et al., 2007; Landon, 1991). Conversely, the high S values of 74.15 mg/kg and 31.95 mg/kg occurred in a few East and West African sites, respectively. These observed high S values suggest localised enrichment, potentially due to site-specific factors, for instance, deposition from volcanic ash such as in parts of Kenya (Omenda, 2011), manure or compost applications, or mineralogical contributions from the parent rock (Haneklaus et al., 2007; Weil and Brady, 2016). The medium S levels in Southern Africa, though based on limited data, may reflect inherent soil characteristics or more effective nutrient management practices (Ngezimana and Agenbag, 2014; Ngezimana and Agenbag, 2015). However, these findings highlight the need for more comprehensive representative data to enable more robust conclusions. Overall, the spatial heterogeneity in soil S levels underscores the need for site-specific diagnostics and the integration of S into balanced fertilisation strategies. These findings also emphasise the importance of providing extraction methods used for accurate compilation or comparisons to scale to support informed nutrient management recommendations. Soil S stratification with depth is primarily shaped by pedogenic processes, texture, and leaching intensity (Weil and Brady, 2016). Surface horizons typically exhibit high soil S concentrations (Table 2), which may be attributed to organic matter inputs and fertiliser additions (Acquaye and Beringer, 1989a; de Mello Prado, 2021a). Subsurface layers often retain moderate S levels, as seen in Calcaric Fluvisols and Haplic Solonetz (Table 2), where limited leaching or higher clay content restricts vertical S movement. In contrast, sharply reduced S levels (to values less than 1.0 mg/kg) in Haplic Nitisols reflect the influence of organic matter accumulation in surface horizons of highly weathered profiles (Itanna, 2005). Exceptionally high S accumulation (up to 288 mg/kg) in the Bt horizon of clay-rich Haplic Solonetz is likely due to SO₄²^- illuviation and restricted drainage. Data from Tanzania show similarly low Mehlich-III S in both topsoil (14.2 mg/kg) and subsoil (13.95 mg/kg), potentially due to S depletion from continuous cultivation and limited organic inputs (Kalonga et al., 2024).

Organic matter stabilises S through microbial mineralisation–immobilisation cycles, which decline with depth (Das et al., 2024). Moreover, routine soil tests may underestimate subsoil S in clay-rich profiles due to strong adsorption onto mineral surfaces, rendering some S non-extractable (Bankole et al., 2022; Tabatabai, 1982; Weil and Brady, 2016). Such conditions may explain the widespread low S reported in subsoils (Acquaye and Beringer, 1989a; Bankole et al., 2022). Soil texture and organic matter further mediate S retention by influencing leaching and adsorption: loam soils, for instance, maintained moderate to high S (21.6–32.0 mg/kg) throughout the upper 90 cm due to favourable nutrient-holding capacity (Uloro and Mengel, 1994). In contrast, coarse-textured, weakly developed soils readily lose SO₄²^-, while highly weathered tropical soils with abundant Fe and Al oxides can strongly retain it (Weil and Brady, 2016). These findings underscore the importance of depth-specific S assessments in developing tailored nutrient management strategies for diverse agroecosystems across SSA.

3.4 Yield response to sulphur-containing fertilisers

3.4.1 Status of sulphur fertiliser use in Sub-Saharan Africa

Crop sulphur (S) response studies have been conducted in Sub-Saharan Africa (SSA) targeting the four main groups of staple crops, Cereals: Barley, Maize, Sorghum, wheat and rice; Legumes: common beans, soybeans; Oilseeds: canola and sesame; and other crops like banana and cotton. The analysis revealed that cereals dominate the S management research, accounting for about 65% of study articles. Legumes represent about 25% of studies, while oilseeds are under-represented, with about 10% of study articles. Only 5% of articles focus on other crops, indicating a critical need for research on non-cereal crops.

The analysis of S fertilisation practices across crop categories reveals distinct patterns in material selection and application rates (Figure 4). Cereals predominantly received low to moderate S rates (0–30 kg S/ha), with gypsum (CaSO₄.2H₂O) and SA as the most frequently applied materials. Notably, 71% of cereal-focused applications used ≤20 kg S/ha, often in compound fertilisers (NPKS, SA), suggesting integration with macronutrient management. Legumes were treated with higher S rates (21–40 kg S/ha), primarily via potassium sulphate and NPS blends, reflecting legume high S demands. Oilseeds exhibited the widest rate range (11–50 kg S/ha), with sodium sulphate, gypsum and elemental-S used at maximal doses (41–50 kg S/ha).

Figure 4

Figure 4. Reported use of sulphur amendments and their corresponding rates across various researched crop categories in Sub-Saharan Africa. Note. S, Sulphur; Na-SO₄, Sodium sulphate; K-SO₄, Potassium sulphate; SA, Ammonium sulphate.

The predominance of cereal-focused S research in SSA reflects their socio-economic importance as staple crops, particularly maize and rice, which dominate smallholder diets and cropping systems (Agyin-Birikorang et al., 2022a; Amuri et al., 2023; Bekele et al., 2022; Sirikare et al., 2015). However, this emphasis often neglects legumes and oilseeds, despite their critical roles in nutrition and soil health. The observed variations reflect crop-specific S demands and agronomic management. Cereals frequently received S applications via gypsum, NPKS or SA, which align with cost-effective, multi-nutrient strategies in staple crop systems.

The prevalence of ≤20 kg S/ha rates aligns with cereals’ moderate S uptake (10–15 kg S/ha) from the soil for grain protein synthesis (Scherer, 2001). These rates may suffice in systems where deep-rooted cereals access subsoil S reserves, reducing dependency on surface applications (Kihara et al., 2017). The yield variations underscore the critical role of S in optimising cereal productivity. The high percentage yield increase in maize at 21–30 kg S/ha aligns with studies linking S to enhanced nitrogen use efficiency and chlorophyll synthesis in C4 plants (Marschner, 2012). However, the variability in responses at identical S rates suggests the variation in genetic potential of crop varieties and the environmental limitations, such as nutrient and drought stresses.

3.4.2 Crop yield response to sulphur fertilisation

S application demonstrated notable yield improvements across various crops, though the magnitude of response varied by crop and application rate (Figure 5). Maize exhibited the most significant increases, with percentage yield gains ranging from 20.1% to 125.4% under 0–10 kg S/ha, and reaching up to 260.9% at 21–30 kg S/ha (Bekele et al., 2022; Friesen, 1991; Getinet et al., 2022; Malley et al., 2013; Sirikare et al., 2015). Wheat responded strongly at 11–20 kg S/ha, with increases of 59.7–79.9%, while rice showed gains of 26.0–61.7% at the same rate and a peak of 214.4% at 31–40 kg S/ha (Assefa et al., 2020; Habtegebrial and Singh, 2009). Sorghum and millet demonstrated moderate improvements of 23.6–30.7% and 13.9–15.2%, respectively, at 0–10 kg S/ha (Friesen, 1991). In contrast, legumes and oilseeds showed more modest responses. Soybean increased by 25.4% at 21–30 kg S/ha (Getachew et al., 2017), and canola, sesame, and common beans displayed minimal responses (17.2%, 12.5%, and 1.9%, respectively) even at higher rates (31–50 kg S/ha) (Daudi et al., 2016; Ngezimana and Agenbag, 2014; Tadesse and Dechassa, 2017). These results highlight crop-specific differences in response to S fertilisation.

Figure 5

Figure 5. Yield increase (%) of various crops under Sulphur treatments across different S application rate categories in Sub-Saharan Africa. Error bars indicate standard errors of the mean yield increase.

Legumes required higher S rates (21–40 kg/ha), reflecting their demand for S-rich amino acids (cysteine, methionine) during nodulation and protein synthesis (Amir and Hacham, 2008; Haneklaus et al., 2007; Scherer, 2001). The preference for potassium sulphate and NPS blends suggests that targeted S supplementation alongside other nutrients such as potassium (K) and phosphorus (P), synergistically enhances root development and nitrogen fixation efficiency (Assefa et al., 2021; Marschner, 2012; Philp et al., 2021). Moreover, unlike acid-forming sources such as AS [(NH₄)₂SO₄] and elemental S, potassium sulphate (K₂SO₄) does not significantly lower soil pH, reducing the risk of S-induced acidification. Notably, S applications in legume were kept below 40 kg S/ha might be a deliberate strategy to additionally protect soil pH (Kılıç and Sönmez, 2024). Oilseeds’ extreme application rate (up to 50 kg S/ha) underscores their sensitivity to S availability for lipid and glucosinolate synthesis (Ahmad et al., 2001; Scherer, 2001).

Economic and edaphic factors may further explain material choices. Gypsum’s dual role in S and Ca delivery makes it cost-effective for smallholders, while compound fertilisers (NPKS) are more suited to commercial farms prioritising operational efficiency. Conversely, the minimal use of elemental S may be due to its slow oxidation in soils, which makes it less effective for short-season crops (Degryse et al., 2018). Similarly, the sodium build-up from repeated applications that threaten soil structure may explain the low adoption of Sodium sulphate as an S source.

3.5 Effect of soil S levels and application rates on crop yield response

The relationship between soil S concentration and yield response was evaluated across six crops (maize, rice, wheat, soybean, canola, and soybean) under varying S application rates (Figure 6). Yield responses were most reported at soil S concentrations below 2.0 mg/kg, where treatments resulted in yield increases exceeding 50% in many cases. For example, in soils with <1.5 mg S/kg, maize achieved yield increases up to 125% at 0–10 kg S/ha, while rice responded with yield increases as high as 62% at 11–20 kg S/ha. Linear regression lines fitted for each S application rate and crop combination further confirmed that yield responses diminished as soil S levels increased. In contrast, crops grown in soils with medium S levels (>6 mg S/kg) exhibited minimal response, with per cent yield increases ranging from 17% to 60% for canola and wheat, respectively.

Figure 6

Figure 6. Crop yield response to sulphur (S) by soil S levels and application rate in diverse soils in Sub-Saharan Africa.

These results indicate that yield benefits from S fertilisation are crop- and site-specific, with the greatest yield response achieved under low soil S conditions. Excessive application in soils with S levels above 6.0 mg/kg yielded no significant advantage in some crops, reinforcing the need for site-specific fertiliser recommendations based on soil test results (Amuri et al., 2023; Bekele et al., 2022).

3.6 The environmental implications and the need for the management of sulphur containing amendments

The use of S-containing amendments and fertilisers such as Sub-Saharan Africa (SA) and elemental S is critical for improving soil fertility and crop yields. However, these amendments may have environmental side effects that must be carefully managed to prevent undesirable consequences. Among primary concerns associated with S amendments is soil and water acidification (Gerson and Hinckley, 2023). When elemental S is applied to the soil, it undergoes microbial oxidation to form sulfuric acid (Tabatabai, 1982), which can lower soil pH (Marschner, 2012; Mengel and Kirkby, 2001), potentially harming crops not adapted to acidic conditions and disrupting soil microbial communities (Gerson and Hinckley, 2023). In Tanzania, soil acidification is a substantial concern, particularly in cashew-growing regions where S is used to control powdery mildew disease (Dondeyne et al., 2001). This practice leads to reduced crop yields and soil fertility, especially in sandy soils with low buffering capacity (Dondeyne et al., 2001; Majule and Omollo, 2009). Acidification can lead to nutrient imbalances, affecting plant health and growth (Gerson and Hinckley, 2023; Mengel and Kirkby, 2001; Weil and Brady, 2016). According to Gilliam et al. (2020), extreme soil acidity can raise the solubility of heavy metals such as aluminium to a toxic level, and render the availability of phosphorus, further hampering plant development. Runoff from acidic soils can carry S material into water bodies, leading to the acidification of water ecosystems (Munodawafa, 2007). This can be observed in poorly managed soils where water erosion can easily carry away soil to nearby water bodies.

Studies have primarily examined the effects of nitrogen fertilisation on greenhouse gases (GHG) emissions like CO₂, CH₄, and N₂O (Mosongo et al., 2022; Pelster et al., 2017). However, the particular influence of S fertilisation on GHG emissions in SSA has been less explored. In other areas, S fertilisation is recognised for its ability to regulate methane (CH₄) emissions from wetland environments like flooded rice fields (Gauci et al., 2004). However, using these materials can lead to the release of S gases like SO₂ upon oxidation, which play a significant role in climate change (Ward, 2009). The use of elemental S increases the solubility and availability of heavy metals such as iron (Fe) and cadmium (Cd), aiding plants in absorbing other metals like zinc (Zn), manganese (Mn), lead (Pb), and cadmium in maize (Safaa et al., 2013). This is attributed to the decrease in soil pH, which facilitates the solubility of these metals in the soil. This situation threatens food safety and human health, as toxic metals can accumulate in crops and infiltrate the food chain.

In the long run, applying elemental S and SA regularly without effective management can increase soil and environmental susceptibility to degradation, thereby reducing soil productivity. The use of these amendments should be tailored to agro-ecological zones and crops, based on the results of soil analysis, with application right fertiliser source, at the right rate, the right time, and in the right place (4R fertiliser stewardship) (Nalivata et al., 2017). S amendments should be sourced appropriately, ensuring high-quality sources free from impurities, and applied at the correct rates, timings, and placements in line with soil nutrient recommendations. Integrated soil fertility management, combining S with other nutrients and organic matter, can enhance long-term environmental and soil health by promoting soil biodiversity and structure, improving nutrient retention, and ultimately boosting agricultural production.

4 Conclusion and recommendation

This systematic review highlights the growing recognition of sulphur as a critical nutrient in Sub-Saharan Africa (SSA) crop production. The increasing number of researches, particularly from countries like Ethiopia, Nigeria and Tanzania, reflects rising S deficiencies in agricultural soils. S status across soil profiles shows clear vertical stratification, shaped by soil type, leaching, and texture, with generally higher concentrations in surface layers and highly variable subsoil. Studies should explicitly report the S extraction methods to improve data comparability and enhance the development of regionally relevant recommendations. Addressing infrastructural limitations through investment in laboratory facilities and technical training is crucial to support the adoption of new and advanced technologies for accurate and routine S analysis across SSA. Moreover, integrating S into broader nutrient management frameworks is an essential step toward sustainable agricultural intensification in SSA.

Consistent evidence indicates that S application significantly enhances crop yields, especially in cereals such as maize and rice, by improving nitrogen use efficiency and promoting photosynthetic activity. However, the varied responses among crop types underscore the need for crop-specific and site-specific S management strategies. The widespread use of S-containing amendments, including elemental S and SA, raises concerns about soil acidification, heavy metal mobilisation, and potential environmental degradation. To balance productivity and sustainability, S management must follow 4R nutrient stewardship (right source, rate, time, and place) and be integrated into broader soil fertility frameworks. Site-specific strategies that combine S with organic inputs and other nutrients are essential to enhance soil health, crop productivity, and environmental safety across SSA.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Author contributions

MM: Validation, Conceptualization, Writing – review & editing, Writing – original draft, Formal Analysis. NA: Supervision, Writing – review & editing. RW: Writing – review & editing, Supervision.

Funding

The author(s) declare that financial support was received for the research and/or publication of this article. This study was funded by the Food Shot Global Ground Breaker Prize.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declare that no Generative AI was used in the creation of this manuscript.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fagro.2025.1656622/full#supplementary-material

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