1 Introduction

Sweet potato is widely recognized as an important food-security crop because it combines high nutritional value with broad adaptation to smallholder systems. It is described as one of the most economically important crops for addressing food security and climate-change challenges, especially in developing-country agriculture, and it is also valued as a highly nutritive staple that exceeds many carbohydrate foods in vitamin, mineral, fiber, and protein content (Motsa et al., 2015; Sapakhova et al., 2023). Its production importance is reinforced by its short maturity period, capacity for sequential harvest, and ability to remain productive in marginal environments where other staples often fail, making it especially relevant for vulnerable communities facing unstable rainfall and recurrent food shortages. At the same time, sweet potato productivity remains constrained by environmental and management factors, particularly drought and suboptimal crop husbandry. Osmotic stress reduces agronomic and economic productivity through morphological, physiological, and biochemical disruption, while farmer yield gaps are also linked to inappropriate tillage practices and weak knowledge of crop-specific field management (Sapakhova et al., 2023).

Among management factors, planting configuration is especially important because it shapes the soil physical environment in which storage roots initiate and expand. Appropriate planting systems improve rooting depth, soil-water management, and infiltration, whereas shallow or compact soils often reduce performance in root and tuber crops (Saqib et al., 2017). In sweet potato, farmers use both ridge and flat systems, yet their relative advantage is not consistent across environments. Ridge planting is widely used in production, but flat planting is also practiced, and available studies show that planting method can influence vegetative growth, marketable root formation, and total yield through effects on water supply, canopy development, and root-zone condition. This agronomic relevance is reflected in field evidence from multiple regions showing that tillage or planting layout significantly affects sweet potato growth and yield, even though the direction and magnitude of the response can differ with soil, climate, and management intensity (Pepó, 2018).

Existing studies on ridge and flat systems indicate that planting configuration contributes substantially to yield variability, but the evidence is mixed rather than uniform. In Hungary, flat planting produced higher marketable tuber yield than ridge planting at both tested row spacings and also reduced the proportion of non-marketable roots, suggesting that under some well-structured soils, flat planting can improve yield efficiency and tuber quality distribution (Pepó, 2018). By contrast, work in Ethiopia found that ridge planting improved the performance of some varieties and led to the recommendation of Hawassa-09 and Tola under ridge planting for the Arba Minch area, while other studies reported ridging as preferable for maximizing herbage and root production or for increasing root yield relative to alternative tillage systems (Bireda and Fatule, 2021). These contrasts suggest that the effect of ridge versus flat planting is context dependent and may be mediated by varietal traits, soil conditions, moisture dynamics, and the balance between vine growth and storage-root development.

Against this background, the present study aims to evaluate the effects of ridge and flat planting systems on the growth and yield of sweet potato under the target production environment, with emphasis on determining whether planting method significantly modifies vegetative development and economically important root traits. This objective follows the broader need identified in earlier studies to define proper planting patterns for attaining reliable vine growth and storage-root yield, particularly where farmers face substantial yield variation associated with field layout and water management. The working hypothesis is that ridge planting will produce superior growth and yield where improved drainage, deeper rooting space, and better furrow-mediated moisture distribution favor root bulking, but that flat planting may perform equally well or better where soil structure and moisture conditions already support efficient root development. Testing this hypothesis is important because current evidence does not support a universal recommendation: some environments favor ridges, others favor flats, and locally grounded comparisons are therefore necessary before advising farmers on the most suitable planting system for stable sweet potato production (Saqib et al., 2017).

2 Literature Review of Ridge and Flat Planting

2.1 Ridge vs flat planting in root and tuber crops

In root and tuber crops, ridge-based land configuration is commonly justified because it improves the physical condition of the planting zone for root initiation and bulking. In sweet potato, ridges and mounds have been reported to loosen the soil, optimize infiltration, increase rooting depth, and improve soil-water management, all of which are considered favorable for underground storage organ development (Saqib et al., 2017). This general agronomic logic helps explain why ridge planting is widely used where soils are shallow, compact, or prone to poor aeration.

Empirical comparisons, however, show that ridge planting is not always superior to flat planting in sweet potato systems. A field study in Hungary found that flat planting produced higher marketable tuber yields than ridge planting at both tested row spacings and also reduced the proportion of non-marketable tubers, indicating that flat systems can perform well under suitable soil and climatic conditions (Pepó, 2018). This suggests that the relative advantage of ridges depends on local production conditions rather than representing a universal rule.

Other studies in Africa have favored ridges, especially when yield stability and root productivity are the main criteria. In Nigeria, tillage practice significantly affected sweet potato growth and yield, and ridge tillage was recommended for improved performance, while in Ethiopia ridge planting was recommended economically for Hawassa-09 and Tola under Arba Minch conditions (Bireda and Fatule, 2021). These results indicate that ridge planting often performs better in smallholder field environments where soil management constraints are more pronounced.

Variation also occurs within ridge systems themselves, showing that planting configuration should not be treated as a simple ridge-versus-flat contrast. Ridge height and planting orientation significantly affected vine length, root diameter, root length, and yield, with a medium ridge height of 40 cm combined with inclined planting identified as the best option for higher root yield in one study. Similarly, in the Afar region of Ethiopia, ridge planting supported optimum dual-purpose production when combined with late vine harvesting, highlighting that the performance of a planting system depends on its interaction with broader crop management.

2.2 Effects on soil microenvironment and crop growth

The main agronomic effect of ridge planting appears to be its modification of the soil microenvironment around developing roots. Tillage systems that loosen the soil increase aeration and rooting depth, while soil penetration resistance is negatively correlated with sweet potato root yield, showing that reduced compaction is a major pathway through which planting configuration can influence crop performance. In compact or poorly structured soils, ridges therefore appear to benefit storage root expansion by creating a less restrictive rooting zone.

Moisture relations are another key mechanism linking planting system to crop growth. Sweet potato requires adequate water at establishment, and drought during early growth reduces tuber yield and quality; ridge systems can alter water distribution by exposing plants to furrow-supplied moisture on both sides, while flat systems may behave differently depending on rainfall, irrigation, and soil type (Motsa et al., 2015; Saqib et al., 2017). This helps explain why ridge planting often performs better under semi-arid or water-managed conditions but does not always outperform flat planting in more favorable environments.

Planting configuration also affects aboveground growth traits that are closely linked to later storage root formation. Ridge planting increased vine length and leaf area compared with bed planting in Pakistan, while in Hungary flat planting produced a better canopy and higher SPAD readings than ridge planting, indicating that different systems can favor contrasting patterns of vegetative development (Saqib et al., 2017; Pepó, 2018). Because sweet potato storage roots depend on assimilates produced by the canopy, these differences in early vegetative growth are likely to influence final yield expression.

At the same time, the microenvironmental benefits of tillage or planting configuration can be partly offset by tradeoffs in water use efficiency and field practicality. Conventional tillage produced higher fresh root yield than minimum tillage in rice-fallow sweet potato, but it also showed lower water use efficiency, whereas minimum tillage allowed earlier planting and still achieved 80%-90% of conventional yield. More broadly, sustainable sweet potato production depends on matching agronomic practices to local resource constraints, including water, soil condition, and input accessibility (Tedesco et al., 2023).

2.3 Existing knowledge gaps in sweet potato systems

A major knowledge gap is the lack of consistent conclusions on whether ridge or flat planting is superior across environments. Some studies report higher yields on ridges, while others report better marketable yield and canopy performance under flat planting, indicating that the effect of planting system is strongly context dependent and still insufficiently generalized. This inconsistency suggests that future work should place more emphasis on site conditions, especially soil structure, rainfall pattern, and management intensity.

Another gap is that many studies evaluate planting method mainly through yield outcomes, with less direct measurement of the soil and plant processes causing those outcomes. Reviews of sweet potato drought response emphasize that yield is closely tied to canopy maintenance, stem growth, and water relations, yet relatively little literature directly links ridge or flat planting to these physiological mechanisms under field conditions (Motsa et al., 2015; Sapakhova et al., 2023). More integrated experiments are needed to connect planting geometry with root-zone moisture, aeration, canopy traits, and assimilate partitioning.

The available literature also remains limited in geographic breadth and in methodological depth for sweet potato production technology. One study explicitly noted that research on sweet potato production technology is still relatively limited, while another stressed that producers increasingly need precise, site-specific monitoring and intervention tools, implying that planting-system research has not yet fully caught up with precision and climate-resilient agronomy (Tedesco et al., 2023). This gap is especially relevant because sweet potato is promoted as a low-input crop for vulnerable communities, where management recommendations must be locally robust.

Finally, relatively few studies integrate planting-system effects with varietal response, nutritional goals, and dual-purpose production objectives. Varieties differed significantly in emergence, vine growth, and root weight under ridge and flat planting in Ethiopia, while broader reviews emphasize that sweet potato research should support both food security and nutritional quality, not yield alone (Bireda and Fatule, 2021). Future research should therefore test planting configuration across genotypes and production goals so that recommendations can better reflect the diversity of sweet potato farming systems.

3 Experimental Design and Materials

3.1 Study site and environmental conditions

The experiment should be conducted at a representative field site where sweet potato can express clear responses to planting configuration under normal production conditions. Prior ridge-flat trials were established on chernozem soil in Hungary and on research farms in humid and semi-arid environments, showing that planting-system effects have been tested across contrasting soil and climate settings rather than under a single ecological condition. Because sweet potato performs best in warm conditions and loose, well-structured soils, the site should be described in terms of location, season, rainfall pattern, and soil texture, with baseline soil properties measured before planting to support interpretation of treatment effects (Pepó, 2018; Metwaly, 2021).

Environmental monitoring should include the variables most closely tied to sweet potato establishment and storage-root development. Water availability is especially important during transplant establishment and early growth, and field studies have shown that root-zone moisture strongly affects survival, biomass accumulation, and final yield (Sapakhova et al., 2023; Huang et al., 2024). For that reason, the study should record daily or weekly weather data and, where possible, track soil moisture through the growing period, since both drought stress and excess water can alter canopy growth, source-sink balance, and tuber formation (Zhou et al., 2025).

3.2 Experimental setup: ridge and flat planting treatments

The experiment should compare two planting treatments, ridge planting and flat planting, using a randomized complete block design with adequate replication to control field variability. Sweet potato planting-system studies have commonly used replicated field layouts, including RCBD and small-plot comparative trials, to test the agronomic effects of ridges, flats, and related tillage forms under field conditions (Pepó, 2020). In the present study, ridge and flat treatments should therefore be assigned randomly within each block, with equal plot size and uniform planting material across treatments so that any observed differences can be attributed primarily to planting configuration.

The ridge treatment should be formed to a consistent height before transplanting, while the flat treatment should be planted on level seedbeds prepared to the same overall soil tilth. Published sweet potato trials have used ridge heights around 30-40 cm, and evidence indicates that ridge form and planting geometry can influence vine growth, root expansion, and yield response. Plot dimensions, number of ridges per plot, and planting arrangement should be specified clearly, and measurements should be taken from central plants to reduce border effects, following the approach of earlier trials that tagged sample plants and collected repeated growth observations through the season (Huang et al., 2024).

3.3 Field management practices

Field management should be kept uniform across treatments except for the planting system itself. Fertilization is necessary because sweet potato removes appreciable nutrients from the soil, and previous field experiments have shown that yield responds to both mineral fertilizer rate and organic nutrient inputs under different tillage systems (Duan et al., 2024). A practical basal fertilizer program may therefore be applied before or at planting using N, P, and K sources at moderate rates, with the exact doses adjusted to initial soil-test values, as in earlier studies that applied base fertilizers before planting and evaluated subsequent growth and yield responses (Huang et al., 2024).

Irrigation, spacing, and routine crop care should likewise be standardized. Earlier studies show that irrigation interval strongly affects vegetative growth and yield, while spacing alters canopy development, leaf area index, and tuber productivity, making both factors essential to control in a ridge-versus-flat comparison (Saqib et al., 2017). The experiment should therefore use a fixed spacing across both treatments, such as the moderate-to-close spacing ranges commonly tested in sweet potato, and all plots should receive the same weeding, pest management, and irrigation schedule throughout the season so that treatment differences reflect planting configuration rather than unequal field management (Hamma et al., 2021; Zongo et al., 2023).

4 Crop Establishment and Early Growth

4.1 Emergence and survival rate differences

Crop establishment in sweet potato begins with rapid sprouting, transplant survival, and successful root initiation, all of which are strongly influenced by the seedbed environment. Ridge-based systems are commonly used because they improve rooting depth, infiltration, and soil-water management, factors that are especially important during establishment when moisture stress can restrict initial root growth and reduce later yield (Saqib et al., 2017). This is consistent with the broader agronomic view that raised beds and ridges provide better aeration and drainage for sweet potato than poorly structured flat soils, thereby supporting more reliable stand establishment under conditions where compaction or waterlogging would otherwise limit survival (Szarvas et al., 2018; Balázs et al., 2024).

Even so, establishment responses are not uniformly superior on ridges in every environment. Genotypic variation in emergence is substantial, and field emergence rate can range widely among cultivars, indicating that planting system effects interact with inherent sprouting ability (Sakaigaichi et al., 2020; Bireda and Fatule, 2021). In addition, results from Hungary showed that non-ridge or flat systems can outperform ridges in some seasons on heavy soils, suggesting that survival and early stand performance depend not only on whether the soil is ridged, but on year-to-year weather and the fit between landform and local soil conditions (Szarvas et al., 2018).

4.2 Early vine development and vigor

Differences between ridge and flat planting become more evident once sprouts are established and vegetative growth accelerates. Ridge planting has been associated with stronger early vine development because it can improve the near-plant environment through better water supply along furrows and a more favorable root zone, leading to longer vines and greater leaf area than less elevated planting systems (Saqib et al., 2017). Similarly, experiments on land preparation showed that plots receiving ploughing, harrowing, and ridging produced the longest vines at 5 and 6 weeks after planting, reinforcing the view that more intensive raised seedbed preparation can stimulate early vegetative vigor.

However, vigorous early growth is not determined by planting geometry alone, and some studies indicate that flat planting can also support strong vegetative performance. In Nigeria, flat seedbeds produced the highest shoot yield even though ridges favored tuber yield, implying that flatter systems may sometimes direct more assimilates toward vine growth during early and mid-season development (Finge 1) (Gbaraneh and Wilson, 2021). In direct planting systems, early vigor was closely tied to rapid emergence, and shoot dry matter at the early growth stage was strongly associated with plant height and NDVI, showing that early above-ground vigor can be quantified reliably and that faster establishment translates into stronger initial vine growth (Sakaigaichi et al., 2020).

4.3 Initial canopy formation patterns

Initial canopy formation in sweet potato reflects how quickly vines extend, branches develop, and leaves cover the soil surface after establishment. Evidence from morpho-physiological studies shows that ground cover efficiency during the establishment phase is a key trait because genotypes with faster cover development show better adaptation and greater root biomass accumulation, especially where soils have low water-holding capacity. This agrees with drought research showing that leaf area index is closely linked with yield performance and can distinguish better-adapted genotypes, indicating that rapid early canopy formation is not only a visual sign of vigor but also a functional determinant of later productivity.

Planting system influences these canopy patterns mainly through its effects on early leaf expansion and shoot architecture. Ridge planting has been reported to increase leaf area relative to bed planting, which would be expected to accelerate canopy closure and improve interception of radiation during early growth (Saqib et al., 2017). At the same time, canopy expression remains highly dependent on cultivar and plant architecture, since varieties differ significantly in node number, internode length, and vine length under ridge and flat conditions; thus, the canopy advantage of ridging is best understood as a system-by-genotype response rather than a universal outcome (Bireda and Fatule, 2021).

5 Canopy Development and Light Interception

5.1 Leaf area index (LAI) dynamics

Leaf area index is a central descriptor of sweet potato canopy development because it integrates leaf expansion over time and is closely tied to crop growth assessment and canopy function (Fan et al., 2024). In sweet potato specifically, LAI can now be estimated accurately across growth stages using non-destructive imaging approaches, including ridge-grown canopies reconstructed from UAV imagery, which confirms that LAI dynamics can be tracked reliably from early growth through harvest.

Under contrasting planting systems, ridge planting generally supports greater early leaf expansion than flatter configurations. In field conditions comparing bed and ridge systems, ridge planting produced longer vines with greater leaf area, especially in the winter crop, indicating a stronger tendency toward higher canopy buildup where the root zone is raised (Saqib et al., 2017). However, canopy expression is not explained by planting form alone, because cultivar differences in vine growth and node production remain large under ridge and flat arrangements, so LAI trajectories are shaped by system × genotype interactions rather than by planting geometry in isolation (Bireda and Fatule, 2021).

5.2 Canopy closure timing differences

Canopy closure timing depends on how quickly vines emerge, elongate, branch, and cover the inter-row space during early growth. Evidence from direct-planted sweet potato shows that early vigor varies substantially among genotypes, and plant height and NDVI closely predict early shoot dry matter, making them useful indicators of the speed at which a crop approaches canopy closure (Sakaigaichi et al., 2020). This matters because weak early light conditions slow vine and leaf development, delaying cover formation during the establishment phase (Changwen et al., 2020).

Ridge planting often appears to accelerate canopy closure indirectly by promoting faster vine elongation and larger leaf display. Trials on land preparation showed that ploughed, harrowed, and ridged plots produced the longest vines at 5 and 6 weeks after planting, while separate ridge-versus-bed comparisons also reported greater vine length and leaf area on ridges, both of which would favor earlier inter-row coverage (Saqib et al., 2017). Yet flat planting can also produce a visually stronger canopy in some environments; on chernozem soil in Hungary, flat planting gave higher SPAD readings and was reported to have a better canopy than ridge planting, suggesting that canopy closure timing is environment-specific rather than universally faster on ridges (Pepó, 2018).

5.3 Light interception efficiency under two systems

Differences in light interception efficiency between ridge and flat planting arise from how each system structures the canopy and modifies the microenvironment around leaves. Studies of sweet potato photosynthesis show that appropriate canopy structure improves intercepted photosynthetically active radiation, total photosynthetically active radiation, light transmission, and extinction coefficient through canopy layers, and these changes are directly associated with stronger photosynthetic performance (Liang et al., 2024). More broadly, sweet potato productivity declines when light is restricted, as shown under shaded agroforestry conditions and in relay systems where low early-stage light slows vine and leaf growth (Changwen et al., 2020).

Within ridge-based systems, modifying ridge form can further alter light-use efficiency. In relay intercropping, mound planting outperformed other ridging types by producing higher net photosynthetic rate and more favorable air and soil thermal conditions, showing that canopy light use responds to planting architecture as well as to light availability itself (Changwen et al., 2020). At the same time, evidence from non-ridge comparisons indicates that a denser or visually better canopy under flat planting does not always imply poorer radiation capture, since flat-planted crops sometimes showed higher SPAD and better canopy appearance, whereas ridge-based systems more consistently supported high-yielding photosynthetic performance when combined with favorable density and field conditions (Pepó, 2018; Balázs et al., 2024).

6 Root Development and Biomass Allocation

6.1 Storage root initiation and expansion

Storage root initiation in sweet potato begins when adventitious roots shift from fibrous growth toward cambial activation, secondary meristem development, and carbohydrate deposition. Anatomical studies show that storage roots arise from adventitious roots and pass through a sequence from fibrous roots to pencil roots and then expanding storage roots, while molecular evidence indicates that early initiation is marked by reduced lignification and increased starch-related metabolism (Firon et al., 2013; Dong et al., 2019). This developmental transition is highly sensitive to field conditions, which means that ridge and flat systems are likely to differ in performance to the extent that they modify the root-zone environment during the first weeks after transplanting (Dong et al., 2019).

Field evidence indicates that better root-zone conditions favor earlier and stronger storage root formation. Ridge planting is agronomically intended to increase rooting depth, improve soil-water management, and improve infiltration, and under field conditions plants on ridges produced more storage roots than bed planting (Saqib et al., 2017). Independent work on early root differentiation likewise found that moderate seedling-water irrigation increased storage roots per plant by 48.9%-73.2% over insufficient irrigation and by 12.7%-14.1% over excessive irrigation, showing that successful initiation depends on a balanced rhizosphere rather than simply high water supply (Zhou et al., 2025).

Soil moisture during establishment directly regulates the rate and success of storage root initiation. Controlled-environment data showed that time to 50% storage root initiation declined as soil moisture increased from very dry conditions to an optimum range, with cultivar-specific optima at 0.168 and 0.199 m³ soil moisture, equivalent to 63% and 75% field capacity. At the branching stage, a soil relative water content around 70% promoted sugar supply and tuberization, increasing storage roots per plant by 14%-120% and yield by 22%-122% compared with other moisture treatments (Xie et al., 2022).

The mechanism behind these responses appears to involve both root morphology and carbon metabolism. Appropriate density increased adventitious root number and stimulated cambium cell differentiation, both of which are favorable for the conversion of potential roots into storage roots (Liang et al., 2023). At the same time, drought during early growth reduced adventitious root length, lateral root number, root diameter, and root volume, and could induce cambium lignification that inhibits storage root development (Duan et al., 2023).

6.2 Shoot-to-root biomass partitioning

Shoot-to-root biomass partitioning is a central determinant of sweet potato productivity because storage root yield depends not only on total biomass production but also on how efficiently assimilates are redirected below ground. Experimental evidence shows that coordinating the shoot and root relationship is a major factor in promoting storage root formation, and that root dry matter accumulation is fundamental to the developmental process (Liang et al., 2023). In practice, this means that ridge and flat systems should be evaluated not only by vegetative vigor but by whether early growth supports a timely transition from shoot expansion to root sink strengthening.

Several studies show that favorable management tends to increase root allocation relative to shoot growth. In density experiments, the D20 treatment increased root dry weight, root dry weight ratio, and the root-to-total biomass ratio, while shoot dry matter decreased during the storage root formation period (Liang et al., 2023). Similarly, moderate late-season irrigation produced the greatest below-ground biomass accumulation and maintained a higher root-shoot ratio than heavier irrigation treatments, whereas above-ground biomass continued to rise under excessive water (Zhou et al., 2025).

The same pattern appears in studies focused on source-sink balance under water stress. Moderate irrigation at 75% ET increased the rate of 13C allocation and dry matter accumulation in storage roots, indicating stronger transfer of photosynthate toward the sink than either non-irrigation or over-irrigation (Zhou et al., 2025). By contrast, moisture-deficit experiments showed that biomass partitioning to storage roots declined linearly as irrigation increased beyond the optimum, while partitioning to leaves and stems increased, underscoring that excessive vegetative growth can compete with storage root bulking.

This balance is further modified by genotype and nutrition. Under different nitrogen and water regimes, relatively high nitrogen and moisture inhibited root expansion in the nitrogen-susceptible cultivar J26, and high nitrogen reduced the 13C distribution ratio in expansion roots, whereas the tolerant cultivar X32 was less affected (Duan et al., 2023). Potassium deficiency also suppressed biomass accumulation in blades, petioles, and roots and reduced photosynthate translocation, indicating that efficient root partitioning depends on nutrient conditions that preserve source-sink coordination.

6.3 Effects of soil aeration and moisture distribution

Soil aeration and moisture distribution strongly influence root development because they determine the chemical and physical environment surrounding the developing adventitious and storage roots. During the establishment period, elevated soil water content can raise root-zone CO₂ concentrations and suppress tuberous root development; in one study, the greatest tuberous root dry weight occurred under intermediate irrigation rather than the wettest treatment. More extreme evidence came from controlled CO₂ treatments in which no tuberous roots developed under the highest root-zone CO₂ concentration, directly linking poor aeration with failure of storage root formation.

Improving soil aeration tends to strengthen both sink activity and yield. Loose soil significantly increased storage root yield by 27.03%-38.74% and improved the economic coefficient, while 13C labeling showed better import efficiency of photosynthate into storage roots than in compact soil. Forced aeration of ridges produced a similar pattern: reducing root-zone CO₂ to about 0.1%-0.2% increased tuber fresh and dry yields to 1.18 and 1.19 times the control, respectively (Kitaya, 2026).

Moisture distribution within the root zone is equally important because both insufficient and excessive water disrupt early root differentiation. Drip-based water management increased sweet potato survival, aboveground biomass, photosynthetic traits, and leaf area index relative to hole irrigation, but optimized drip irrigation produced greater total root length, root volume, and 13C partitioning than traditional drip irrigation (Huang et al., 2024). A subsequent field study during the rooting and branching period found that storage roots per plant were primarily shaped by soil water content and soil CO₂ concentration, and identified an optimal irrigation range of 112.2 to 209.5 m³/hm² for maintaining favorable water-gas-heat balance in the rhizosphere (Zhou et al., 2025).

These findings help explain why ridge planting often benefits sweet potato root growth under field conditions. Raised ridges improve infiltration, soil-water management, and rooting depth, traits that can reduce prolonged saturation and help maintain a more favorable aerated zone for root differentiation (Saqib et al., 2017). However, the broader moisture literature shows that ridge advantages are conditional rather than absolute, because root development is maximized under an intermediate range of soil moisture and aeration rather than under either chronic dryness or excessive wetness.

7 Yield Formation and Physiological Mechanisms

7.1 Tuber yield components (number, size, weight)

Yield formation in sweet potato is governed by the interaction among storage root number, mean root size, and individual root weight rather than by any single component alone. Across cultivars, total yield is positively associated with both tuber number and mean tuber weight, but these two traits often show a negative correlation with each other, indicating a trade-off between producing many roots and producing larger roots. This helps explain why marketable yield depends not only on total biomass but also on how effectively the crop balances root set with subsequent bulking.

Planting system modifies that balance, although the direction of the response is not uniform across environments. In Ethiopia, ridge planting significantly affected root weight per plant and the highest root number per plant and mean root weight were recorded in specific varieties under ridge conditions, supporting ridge use where local adaptation favors stronger storage root production (Bireda and Fatule, 2021). By contrast, on Hungarian chernozem soil, flat planting produced higher total and marketable yields than ridge planting and also reduced the proportion of non-marketable tubers, showing that the effects of ridge versus flat systems on root size distribution and final harvestable yield remain site-specific (Pepó, 2018).

Differences in yield components also reflect the timing and strength of sink establishment. High-yielding varieties formed storage roots earlier, produced heavier root tubers and more root tubers per plant at the early bulking stage, and maintained a much larger share of assimilate in the tubers by harvest than low-yielding varieties. This indicates that successful yield formation depends on early sink advantage, which allows later assimilate supply to be directed toward storage root enlargement rather than remaining in vines and stems.

Tuber dimensions further refine this pattern, because not all size traits contribute equally to yield. Comparative developmental work showed that the rate of increase in tuber width during the main bulking phase and the final tuber width were closely related to mean tuber weight and total yield, and tuber width emerged as the most important single dimensional determinant of high yield. Consistent with this, ridge-based cultivation at suitable density produced the highest storage root yields in Beauregard, suggesting that loose, elevated seedbeds can favor the enlargement phase when local soil conditions support that response (Balázs et al., 2024).

7.2 Photosynthesis and assimilate transport efficiency

Photosynthetic performance contributes to sweet potato yield, but yield is more closely linked to canopy-level assimilation and assimilate partitioning than to single-leaf photosynthesis alone. Genotypic studies found that canopy photosynthesis during later growth was positively correlated with storage root dry matter yield, whereas single-leaf net photosynthesis showed an inconsistent relationship with yield across years. This suggests that productive ridge or flat systems are those that sustain effective whole-canopy carbon gain during the period when storage roots are already strong sinks.

Assimilate transport efficiency is even more decisive for final yield formation. High-yielding varieties reached at least 50% ^13C distribution to root tubers earlier than low-yielding varieties and, by harvest, allocated 73.7%-91.2% of labeled assimilate to root tubers compared with 60.7%-63.5% in low-yielding types. These results show that superior yield arises not only from producing assimilates, but from rapidly shifting transport toward storage organs and limiting consumption by branches and stems.

Soil physical conditions influence this source-sink process directly. In field experiments, loose soil increased storage root yield by 27.03%-38.74% and improved the import efficiency of photosynthate into storage roots, whereas compact soil produced the opposite effect. Because ridge planting generally improves rooting depth, infiltration, and soil-water management, its physiological advantage likely comes partly from creating a better-aerated pathway for assimilate use in storage roots rather than from canopy effects alone (Saqib et al., 2017).

Water supply further regulates assimilate transport by altering the source-sink balance. Moderate irrigation at 75% evapotranspiration increased source photosynthetic rate, stimulated sink activity, and raised yield, whereas both non-irrigation and over-irrigation reduced the transfer of photosynthetic products to storage roots (Zhou et al., 2025). Under drought stress, combined 6-BA and ABA application increased shoot and storage root biomass, leaf area, and yield while enhancing ^13C accumulation in shoots early and storage roots later, indicating that improved hormonal regulation can partially restore assimilate partitioning under stress (Huan et al., 2020).

7.3 Stress response (waterlogging vs drought tolerance)

Waterlogging and drought constrain sweet potato yield through different physiological routes, but both reduce storage root formation when stress is severe. Waterlogging lowers chlorophyll content, storage root number, storage root dry weight, harvest index, and root-shoot ratio, while prolonged flooding can eliminate tuber yield entirely or cause tuber decay. These findings are directly relevant to ridge versus flat systems because drainage differences between planting configurations can determine whether transient excess water remains tolerable or progresses to destructive hypoxia.

Tolerance to flooding depends strongly on genotype and on the capacity to maintain root and leaf function under low oxygen. Waterlogging-tolerant genotypes have shown either stronger root proliferation or better retention of storage root number and weight under stress, while hypoxia-tolerant material maintains higher net photosynthetic rate, transpiration, Rubisco capacity, and growth than sensitive lines (Han et al., 2021). At the mechanistic level, tolerant cultivars also upregulate ethylene-related low-oxygen signaling together with reactive oxygen species and nitric oxide regulatory pathways early in flooding, which appears to protect canopy function during submergence (Park et al., 2022).

Drought stress acts more through reduced canopy development, impaired photosynthesis, and weaker sink establishment. Severe drought caused major yield reductions across cultivars, and yield under moisture stress was closely associated with maintaining leaf area index, stem length, and stomatal conductance. Field work likewise showed that increasing irrigation interval reduced storage root length, diameter, number, and fresh weight per plant, confirming that water deficit during establishment and bulking directly depresses yield components (Saqib et al., 2017).

Some drought responses are nevertheless adaptive rather than purely damaging. Paclobutrazol improved soluble sugar and proline accumulation, stabilized photosynthetic pigments and fluorescence, and helped maintain storage root yield under low soil water conditions (Yooyongwech et al., 2017). More broadly, soil management that improves permeability and rooting depth tends to strengthen resilience to both drought and waterlogging, supporting the view that ridge systems often benefit sweet potato by buffering opposite moisture stresses, even though the magnitude of that benefit remains environment-dependent (Liu and Chen, 2024).

8 Case Study and Practical Implications

8.1 Farmer-managed ridge vs flat systems (regional case study)

Farmer-managed comparisons show that the practical value of ridge and flat planting depends strongly on local rainfall and soil conditions rather than on a universal superiority of one system. In semi-arid Kenya, tied ridges produced the highest mean yield at 5.27 kg/plot, exceeding both sunken beds and flat beds, while in Ethiopian recommendations flat planting was considered beneficial where rainfall is sufficient and soils are sandy loam, whereas tie ridges were more broadly recommended in moisture-stressed areas. This pattern suggests that farmer-managed ridge systems are most advantageous where moisture capture and root-zone buffering are limiting, but that flat systems remain viable where water supply and soil structure are already favorable.

Regional field studies also show that the ridge response is shaped by production goals and by how farmers manage above-ground biomass. In South-south Nigeria, ridge seedbeds produced the highest root tuber yield while flat seedbeds produced the highest vine yield, indicating a trade-off between storage root production and shoot biomass under farmer-relevant management conditions (Gbaraneh and Wilson, 2021). A similar result was reported in the arid Afar region of Ethiopia, where ridge planting combined with late vine harvesting optimized fodder production without compromising tuberous root yield, highlighting the value of ridges in mixed crop-livestock systems rather than in root yield alone.

8.2 Yield stability and management constraints

Yield stability across farmer fields appears to be constrained more by establishment quality, planting date, and management variability than by planting geometry alone. In Malawi, on-farm demonstrations across 221 smallholder fields showed large yield variability, and timely planting plus good crop establishment were identified as key factors for attaining better root yields. Complementing this, technical efficiency analysis in Nigeria found mean efficiency of only 62%, indicating that substantial yield instability and output gaps arise from non-optimal input use and management inefficiency within existing production systems.

Farm surveys further show that ridge-based production often coexists with major practical constraints that can suppress its benefits. In Benin, all surveyed producers planted on ridges, yet production was still limited by flooding, high labor costs, inadequate pest and disease control, and lack of training, showing that ridging alone does not ensure stable productivity. In Ethiopia, most farmers also used ridges, but on-farm yields commonly remained at only 6.0-7.5 t/ha because of shortages of planting material, drought, diseases, insect pests, and weak access to inputs and storage facilities (Hendebo et al., 2022).

8.3 Implications for agronomic optimization and adoption

The agronomic implication is that ridge versus flat planting should be optimized as part of a broader package that includes density, fertility, and variety choice. In Hungary, the highest yields in ‘Beauregard’ were achieved when 35,000 plants/ha were planted on ridges, but the same study concluded that plant density had a stronger effect on yield than ridge versus flat planting alone (Balázs et al., 2024). Similarly, work in Ghana found that ridging gave the highest root yield and net benefit, yet the recommended practice was specifically ridge planting combined with 60 kg/P₂O₅ ha, underscoring that planting system recommendations are most effective when paired with nutrient management).

Adoption evidence indicates that practical training, extension, and access to quality planting material are as important as the agronomy itself. Among small-scale farmers in South East Nigeria, adoption rates were already high for improved varieties, spacing, vine cuttings, and fertilizer use, but further gains were still linked to practical training and institutional support (Figure 2) (Okeke et al., 2020). In Ethiopia, training improved farmers’ knowledge, addressed barriers such as pest control and vine availability, and led to measurable yield improvement, indicating that the uptake of ridge or flat recommendations is most likely when technical advice is delivered with planting material access and local social support.

9 Conclusion and Agronomic Implications

Across the studies reviewed, ridge planting generally improved the soil environment for sweet potato by increasing rooting depth, improving soil-water management, and promoting infiltration, which often translated into stronger vegetative growth and more storage roots than flatter systems. This broad trend is reinforced by recent cultivar-based work in Hungary, where ridge cultivation combined with twin-row planting produced the highest yields, and by Ethiopian variety trials recommending ridge planting for high-performing genotypes such as Hawassa-09 and Tola.

The comparative advantage of ridges was not universal, however, because several studies showed that flat planting could equal or surpass ridges under certain soils, seasons, and spacing arrangements. On Hungarian chernozem soil, flat planting produced higher marketable yield and a lower proportion of non-marketable tubers than ridge planting, while on alluvial soil the superiority of ridge versus flat systems reversed between years, indicating a strong interaction with environment and season. Taken together, the evidence indicates that ridge planting more often supports favorable growth and yield, but the magnitude and even direction of the response depend on genotype, rainfall, soil structure, and management intensity.

Planting system selection should therefore be based on local production constraints rather than on a fixed recommendation. In semi-arid and drought-prone environments, ridge-based systems are more consistently advantageous because they improve moisture capture and root-zone conditions; tied ridges in semi-arid Kenya produced the highest yields, and ridge planting was also recommended in the arid Afar region of Ethiopia for balancing fodder and tuber production. Under similar reasoning, ridge tillage was recommended in Nigeria and Ghana where it improved growth, root yield, or net economic return relative to competing tillage systems.

Where soils are already loose, rainfall is adequate, or row spacing and plant density can be optimized, flat planting remains a practical option and may even outperform ridges. In Hungary, flat planting produced higher yields than ridging at both tested row spacings, and a narrower 0.75 m spacing was more favorable than 1.0 m, showing that geometry and density can matter as much as landform. More broadly, recent work on ‘Beauregard’ showed that plant density had a stronger effect on yield and carotenoid content than ridge versus flat planting alone, so planting-system recommendations should be integrated with cultivar choice, spacing, and fertilizer management rather than treated independently.

Future research should move beyond simple ridge-versus-flat comparisons toward precision-managed planting systems that monitor canopy status, soil conditions, and stand quality in real time. Precision agriculture technologies such as remote sensing, drones, GPS-guided equipment, variable-rate application, and IoT-based sensing can improve planting accuracy, input efficiency, and environmental sustainability across cropping systems. For sweet potato specifically, this direction is already relevant because yield estimation and monitoring of physiological traits such as SPAD, photosynthetic performance, and leaf area index are increasingly viewed as useful tools for guiding cultivation decisions.

Mechanization research is also a priority because labor scarcity, transplanting difficulty, and harvest damage remain practical barriers to large-scale adoption. In northern China, a compound sweet potato transplanter achieved missing seedling rates below 1.2% and transplanting quality above 95% across raised-bed systems, while plastic-mulched raised beds also delivered 23.8%-33.8% higher tuber yield than bare raised beds under improved mechanized transplanting conditions. At the harvesting stage, future work must also address skin breakage, injury, and soil-tuber separation efficiency in mechanized harvesters, while ensuring that emerging technologies remain affordable and adaptable for smallholders through modular tools, shared machinery, and policy support.

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