1 Introduction

Eggplant (Solanum melongena L.) is an important vegetable crop whose productivity depends strongly on soil fertility, nutrient supply, and the maintenance of favorable physical and biological soil conditions. At the same time, broader evidence from sustainable agriculture shows that soil degradation has become a major constraint on food production and environmental quality, especially where intensive cultivation depletes soil organic matter, weakens structure, and reduces the capacity of soil to retain water and nutrients (Singh et al., 2024). This issue is especially relevant to eggplant because the crop has relatively high nutrient demand, is often grown repeatedly in protected or intensively managed systems, and responds strongly to fertilization strategy and soil condition (Mahamad et al., 2022; Duri et al., 2025). In many production regions, continuous cultivation, heavy dependence on synthetic fertilizers, and limited return of organic matter have contributed to declining fertility, lower biological activity, and reduced long-term soil resilience, making soil restoration a central concern in eggplant agronomy. As a result, the study of soil-improving practices is not only relevant to maintaining yield, but also to protecting the ecological foundation on which sustained vegetable production depends.

The significance of soil degradation in eggplant production is evident from studies of continuous monocropping, acidification, contamination, and other forms of soil stress that directly reduce plant performance and yield. Under plastic tunnel systems, continuous cropping conditions degrade soil chemical and biological properties, alter microbial communities, and increase soil-borne disease pressure, thereby threatening the sustainability of eggplant production over time. Experimental work in greenhouse eggplant has also shown that soil acidification lowers soil organic matter, reduces total nitrogen, phosphorus, and potassium, increases exchangeable aluminum and electrical conductivity, and weakens nutrient uptake by the crop as pH declines. Other studies indicate that degradation can also arise through salinity, drought-linked soil deterioration, and toxic contamination, all of which diminish biomass and fruit production. Together, these findings show that soil degradation in eggplant systems is multidimensional: it is expressed through declining fertility, disrupted microbial balance, impaired nutrient acquisition, and ultimately lower crop productivity and stability.

Against this background, organic amendments are increasingly recognized as a practical strategy for sustainable soil management because they act simultaneously on soil physical, chemical, and biological properties. Across diverse agricultural systems, amendments such as compost, manure, vermicompost, and biochar increase soil organic matter, improve aggregation and structure, enhance water retention, and support nutrient availability, although their effects vary with soil type, climate, and application rate (Matisic et al., 2024). More specifically in eggplant, amendment-based approaches have improved growth, yield, and soil function under both normal and stressed conditions: vermicompost and biochar increased vegetative growth, total yield, and water-use efficiency under deficit irrigation, while organic nutrient management with farmyard manure or similar inputs has repeatedly enhanced soil organic carbon and available N, P, and K alongside yield (Ebrahimi et al., 2021; Nisar et al., 2025). Organic amendments also help stimulate beneficial microbial activity and nutrient cycling, and in some monocropping systems they suppress pathogens while promoting more favorable rhizosphere communities. Their value therefore lies not only in nutrient supply, but in rebuilding the soil processes that support resilient crop production over the long term.

In this context, the objective of the present review is to synthesize current evidence on the soil improvement effects of organic amendments in eggplant production, with emphasis on their role in restoring degraded soils, sustaining fertility, and supporting crop productivity under intensive cultivation. The review considers both direct agronomic outcomes, such as growth, yield, and fruit quality, and underlying soil responses, including changes in organic carbon, nutrient availability, aggregation, enzymatic activity, and microbial community composition (Bonanomi et al., 2020). It also recognizes that amendment performance is not uniform: some benefits emerge most clearly when organic inputs are integrated with complementary practices such as crop rotation, microbial inoculation, or balanced mineral fertilization, and some trade-offs remain possible depending on the amendment source, rate, and production environment. Accordingly, the scope of this review is to evaluate the evidence for major organic amendment types used in eggplant systems, identify the main mechanisms through which they improve soil and crop performance, and highlight research gaps related to long-term field validation, amendment combinations, and site-specific recommendations for sustainable eggplant production.

2 Types and Characteristics of Organic Amendments Used in Eggplant Production

2.1 Farmyard manure and compost-based amendments

Farmyard manure and compost are among the most established organic amendments used in eggplant production because they supply nutrients while also improving the soil matrix that supports water and root function. Animal manures provide major nutrients such as nitrogen and phosphorus and are valued for improving soil structure, aeration, and moisture retention, while compost-based inputs are broadly recognized as tools for rebuilding soil organic matter and restoring degraded soils (Matisic et al., 2024; Singh et al., 2024). In eggplant systems, these materials are used either alone or in combination with mineral fertilizers, and their value lies not only in immediate fertility effects but also in their contribution to longer-term soil quality maintenance. Evidence from eggplant production shows that farmyard manure often performs strongly when integrated with balanced fertilization programs. In a four-year nutrient management study, combining the recommended fertilizer dose with FYM increased fruit yield by 47% over mineral fertilizer alone and produced the largest gain in soil organic carbon stocks and available N, P, and K, indicating a clear soil-building effect alongside productivity benefits (Nisar et al., 2025). Earlier field work similarly found that eggplant grown with organic manures showed more vigorous vegetative growth and higher total fruit yield, with farmyard manure outperforming compost and Tithonia among the tested organic sources.

Compost-based amendments are more heterogeneous than FYM because their characteristics depend on feedstock and processing, but they are increasingly important in sustainable eggplant production. Broad review evidence describes compost, vermicompost, biochar, pomace, and manure as amendments with distinct but complementary effects on soil physical and chemical properties, which means compost performance should be matched to crop needs and site conditions rather than treated as uniform (Matisic et al., 2024). This variability is practically relevant in eggplant because municipal composts, poultry-litter co-composts, and vermicompost differ in nutrient concentration, salinity risk, and stability of organic matter. Field studies in eggplant confirm that compost can substantially improve soil fertility indicators, especially when combined with other amendments or rational fertilizer inputs. Wheat-straw compost increased cation exchange capacity by 115% relative to the control in one field study, although biochar was superior for many other soil-health traits, while another brinjal experiment reported that integrating STCR-based NPK with 12.5 t/ha vermicompost increased organic carbon and available N, P, and K compared with the control (Figure 1) (Mohan et al., 2023; Kuzhalarasi et al., 2024). These results suggest that compost-based materials are best viewed as multifunctional amendments whose agronomic value depends on amendment chemistry, application rate, and whether they are used alone or in integrated nutrient management.

2.2 Biochar and carbon-based organic amendments

Biochar and related carbon-based amendments are distinguished by their porous structure, relative stability, and capacity to modify soil hydraulic and structural behavior. Review evidence shows that biochar generally reduces bulk density by 3%-31%, increases porosity by 14%-64%, and increases available water by 4%-130%, although the magnitude of response depends on soil texture, biochar rate, and particle size. These properties make biochar particularly attractive in eggplant systems exposed to drought, salinity, or degraded soil structure, where improvements in water retention and aeration can translate quickly into crop response. Eggplant studies support that functional expectation. Under deficit irrigation, biochar and vermicompost improved soil physicochemical properties including pH, cation exchange capacity, nutrient retention, and water retention, and the amendments increased vegetative growth, yield, and water-use efficiency (Ebrahimi et al., 2021). In a separate field comparison, wheat-straw biochar was the most beneficial single amendment for most measured soil-health parameters, producing a 20% decrease in bulk density, a 30% increase in water-holding capacity, and large gains in soil organic carbon, total nitrogen, and available phosphorus relative to the control (Mohan et al., 2023).

Carbon-based amendments also appear especially effective when combined with compost or with reduced-input irrigation and fertilization strategies. Co-composted poultry-litter biochar improved bulk density, hydraulic conductivity, useful pores, water-holding pores, available water, and soil biota under water stress, while also increasing fruit yield and irrigation water-use efficiency in eggplant grown on salt-affected soil (El-Mageed et al., 2021). In greenhouse Mollisols, adding biochar together with 20% reductions in water and fertilizer increased surface-soil organic carbon and aggregate stability, with mean weight diameter rising by 32.6% and 30.6% relative to the comparison treatments (Xu et al., 2023). A key characteristic of biochar is that its effects are often context dependent rather than universally positive. Review synthesis indicates that sandy soils tend to respond more strongly than clayey soils and that long-term field studies are still needed to define the persistence of biochar effects on soil physical properties. This caveat is consistent with eggplant evidence showing that biochar performs best when embedded in broader management systems that coordinate irrigation, fertilization, and sometimes compost co-application rather than when it is treated as a stand-alone input.

2.3 Green manure and crop residue incorporation

Green manure and crop residue incorporation represent biologically active amendment strategies that differ from FYM and biochar because they rely on fresh or partially decomposed plant biomass returned to the soil. In vegetable systems, green manure can improve water relations and pore organization, while residue retention helps accumulate soil carbon and sustain microbial processes over time (Ansari et al., 2022). Their main agronomic value lies in converting seasonal biomass into soil organic inputs that improve aggregation, nutrient cycling, and resilience against physical degradation. Comparative work on organic vegetable soils shows that green manure can be especially effective for available water content, whereas farmyard manure may have stronger effects on total porosity and bulk-density reduction. In a two-year study, green manure produced the highest available water content, while FYM produced the highest volumetric water content and porosity and the lowest bulk density, indicating that different organic materials improve different dimensions of soil physical quality (Mujdeci et al., 2020). This distinction is useful for eggplant production, where water supply and root-zone aeration are both critical, but not always limited by the same soil constraint.

Longer-term evidence also shows that the quality of the incorporated biomass matters. After five years, Sesbania green manuring increased total and fractionated soil organic carbon pools, improved water-stable macroaggregates, and raised aggregate ratios relative to non-green-manured soils, while residue retention further increased carbon pools and microbial biomass carbon (Ansari et al., 2022). In eggplant-specific regenerative systems, living mulches improved soil structure and the water resistance of soil aggregates, although these gains could come with some yield reduction, indicating that physically beneficial cover-based practices may require careful management to avoid excessive crop competition (Adamczewska-Sowińska et al., 2022).

3 Effects of Organic Amendments on Soil Physical Properties

3.1 Improvement of soil structure and aggregation

Organic amendments generally improve soil structure by increasing aggregate formation, stabilizing macroaggregates, and lowering bulk density, although the size of the response depends on amendment type, soil properties, and management conditions (Matisic et al., 2024; Shihao et al., 2024). A global meta-analysis found that organic amendment increased the proportion of >2 mm and 0.25-2 mm aggregates by 32% and 12%, and increased aggregate stability measured as MWD and GMD by 17% and 22%, respectively (Shihao et al., 2024). Mechanistically, aggregation improves because amendments add organic carbon, stimulate microbial activity, and in some cases alter hydrophobic behavior that reduces aggregate turnover (Sarker et al., 2022). Eggplant-focused evidence supports the same pattern under protected cultivation. In greenhouse Mollisols, biochar combined with 20% reductions in water and fertilizer increased the proportion of >2 mm aggregates and raised MWD by 32.6% and 30.6% relative to the comparison treatments in the 0-20 cm layer (Xu et al., 2023). More broadly, long-term review evidence shows that repeated organic inputs increase physical fertility mainly through improved aggregate stability and reduced bulk density, indicating that structural gains are usually cumulative rather than transient.

The structural response also differs across amendment materials. Short-term incorporation of sheep-manure compost improved aggregate stability and increased mean weight diameter, whereas corn stover with decomposing agents reduced stability compared with compost treatments, showing that not all organic inputs strengthen aggregation equally (Dong et al., 2021). A three-dimensional field study similarly found that all tested organic materials reduced bulk density and increased water-stable macroaggregates, but the straw plus organic fertilizer mixture produced the clearest improvement in pore connectivity and overall microstructure. There are also important rate effects and potential trade-offs. In facility agriculture, biochar and vermicompost reduced soil bulk density and increased porosity, but the highest biochar rate was not favorable for aggregate-structure parameters, whereas the V3 vermicompost treatment gave the best aggregate stability response. Review evidence likewise concludes that amendment effects vary with texture, application rate, and cropping system, so structural improvement in eggplant soils is best framed as context dependent, not universal (Sarker et al., 2022; Matisic et al., 2024).

3.2 Regulation of soil water retention and hydraulic properties

Organic amendments generally improve soil water retention by modifying pore-size distribution and increasing the share of water-holding pores, but hydraulic responses are more variable than structural responses (Matisic et al., 2024). In a 33-year Vertisol experiment, wheat straw and animal manures significantly increased water retention in the wet range through improved structure and higher SOC, but they did not significantly increase plant-available water and had little benefit for hygroscopic water retention at high suction. This indicates that amendments most consistently improve capillary-water storage rather than every component of the soil water-retention curve. Short-term studies show that these gains can appear quickly. Composting and vermicomposting increased soil macroporosity, produced a more complex pore system, and improved water-holding capacity and plant water-use efficiency even at low application rates and short incubation times (Rivier et al., 2022). In sandy-loam soils, compost, vermicompost, and biochar all improved water-holding capacity, macroaggregate formation, and bulk density at low rates, although biochar was the only amendment with a clear dose-response relationship for PAWC, aeration capacity, and relative field capacity.

Eggplant studies show that these hydraulic changes have direct production value under irrigation stress. In salt-affected soil, co-composted poultry-litter biochar improved hydraulic conductivity, useful pores, water-holding pores, available water, and fine capillary pores, while increasing fruit yield and irrigation water-use efficiency under deficit irrigation (El-Mageed et al., 2021). Under open-field deficit irrigation, biochar and vermicompost also increased eggplant growth, yield, and water-use efficiency, supporting the view that improved soil water relations are a key pathway through which amendments buffer drought stress (Ebrahimi et al., 2021). Farmyard and green manure treatments further show that different amendments optimize different hydraulic properties. In an organic vegetable field, FYM produced the highest volumetric water content, total porosity, and lowest bulk density, whereas green manure gave the highest available water capacity (Mujdeci et al., 2020). Biochar-specific review evidence also shows that available water commonly increases by 4-130%, while saturated hydraulic conductivity tends to decrease in coarse soils and increase in fine soils, reinforcing that hydraulic outcomes depend strongly on texture.

3.3 Effects on soil temperature and aeration conditions

Organic amendments influence soil temperature mainly through their effects on porosity, water content, and thermal buffering, although direct temperature evidence is less developed than evidence for aggregation and water retention. Review synthesis indicates that biochar can moderate soil thermal properties, while broader analyses of organic amendments emphasize that they alter multiple physical properties together rather than in isolation. In practical terms, this means changes in soil heat status in eggplant systems are likely to emerge from amendment-driven shifts in moisture retention, bulk density, and pore distribution rather than from a single thermal mechanism. Aeration responses are better documented and consistently positive when amendments enlarge and connect pore networks. In semiarid soil, organic amendment increased air-filled porosity, pore radius, throat length, and coordination number, and the resulting higher gas diffusivity and air permeability indicated improved soil aeration (Dong et al., 2021). A separate three-dimensional pore study found that all organic materials reduced bulk density and increased field capacity, while the straw plus organic fertilizer mixture markedly increased medium and large porosity and improved the connected-to-isolated pore ratio by 4.7-fold.

Biochar appears especially effective where drainage and oxygen supply are constrained. In waterlogged soil, biochar increased total porosity by 54.28% and connected porosity by 119.75%, and both biochar and straw-plus-organic-fertilizer improved connectivity indices that are relevant to air and water movement. In clay soil, a large one-time application of organic amendments increased macroporosity by 20%-27% even five years later, a change associated with better aeration and faster infiltration during wet periods. Evidence from vegetable systems suggests that these aeration effects are agronomically meaningful for eggplant because they improve the root-zone environment under both normal and stressed conditions. Farmyard manure and green manure increase total porosity while decreasing bulk density, and greater aggregation increases both micro- and macropore numbers, which improves the air-water balance needed for root activity (Mujdeci et al., 2020).

4 Effects of Organic Amendments on Soil Chemical Properties

4.1 Enhancement of soil organic matter and carbon sequestration

Organic amendments consistently increase soil organic matter and strengthen the carbon-holding function of cultivated soils, which is one of their clearest chemical benefits in vegetable systems. Recent review evidence identifies compost, vermicompost, biochar, pomace, and manure as major inputs that increase soil organic matter and support carbon sequestration, while a mechanism-based synthesis shows that nutrient-rich organic amendments mainly build organic matter through added carbon inputs and short- to medium-term increases in soil organic carbon stocks (Matisic et al., 2024; Oyebiyi et al., 2026). For eggplant production, this matters because repeated cultivation and intensive fertilization tend to deplete soil carbon, whereas amendment-based management can rebuild the organic fraction that underpins long-term soil fertility. Longer-term vegetable studies support that carbon gains are durable rather than merely transient. In a seven-year field sequence that included eggplant, compost-based fertilization increased SOC by 8.1 and 5.7 Mg/ha under two amendment strategies, and SOC concentrations of 1.2%-1.4% maintained yields equal to or above mineral fertilization (Morra et al., 2021). In a separate three-year vegetable system under semiarid conditions, organically managed plots had higher organic carbon than the mineral-fertilized control, and the authors concluded that amendment use together with cereal-legume rotation enhanced the soil’s function as a carbon sink (Sánchez-Navarro et al., 2023).

The form of amendment also influences how carbon is stabilized in soil. Biochar emerged as particularly effective for carbon sequestration across contrasting soil textures in greenhouse soils, while green manure and straw stimulated carbon-transformation enzymes and promoted carbon sequestration capacity through aggregate stabilization and changes in the fungal community (Li et al., 2025). This indicates that some amendments act mainly by direct carbon addition, whereas others work more strongly through biological processing and structural protection of carbon within aggregates. Not all carbon gains are equally efficient, and the best strategy depends on dose and amendment chemistry. In the seven-year vegetable trial, annual inputs of 4-5 Mg C/ha produced a positive soil-C stabilization efficiency of 15%, suggesting that moderate compost inputs can be more efficient than heavier applications (Morra et al., 2021). At the same time, a recent synthesis emphasized that no single amendment is universally superior and that long-term sequestration outcomes remain context dependent, especially for carbon-rich materials such as biochar and humic substances (Oyebiyi et al., 2026).

4.2 Improvement of soil nutrient availability and nutrient cycling

Organic amendments improve soil nutrient availability by increasing nutrient pools and by stimulating the microbial processes that release, transform, and retain nutrients. A global meta-analysis found that, relative to mineral-only fertilization, organic amendments increased soil organic carbon by 38%, total nitrogen by 20%, microbial biomass carbon by 51%, and enzyme activities for C, N, and P acquisition by 39%, 22%, and 48%, respectively. This broad pattern is consistent with review evidence that organic amendments increase nutrient availability across soils, although the magnitude of the response depends on amendment type, application rate, and cropping system (Matisic et al., 2024). Field evidence from vegetable systems shows that these benefits extend to major and micronutrients. Over three years in a vegetable rotation, organic amendments increased total nitrogen, phosphorus, potassium, magnesium, and several micronutrients relative to conventional fertilization, improving the soil’s function for food production (Sánchez-Navarro et al., 2023). In saline-alkali soils, meta-analysis likewise showed strong nutrient gains under amendment use, with increases of 50.0% in SOC, 55.1% in total N, 25.6% in total P, 31.3% in available N, 58.9% in available P, and 41.3% in available K (Li et al., 2025).

A key mechanism is the stimulation of microbial nutrient cycling rather than simple nutrient addition alone. In greenhouse vegetable production, long-term partial substitution of chemical fertilizer with manure or straw increased microbial taxonomic and functional diversity and enhanced genes linked to P solubilization and mineralization, including gcd, ppx, and phoD, which were positively associated with labile P and Olsen P (Zhang et al., 2023). Another greenhouse study showed that amendment effects on nutrient pools depended strongly on amendment chemistry and soil texture, with biochar, rice husk, and sheep manure shifting microbial resource allocation and enzyme activities involved in C, N, and P acquisition (Li et al., 2025). These improvements are often strongest when organic inputs are integrated with mineral fertilizers rather than used in isolation. A 33-year field experiment found that co-application of straw or manure with inorganic fertilizer increased the soil quality index by 161.9% and 285.7%, respectively, and manure plus fertilizer also increased nutrient content and microbial biomass linked to yield formation. Long-term meta-analysis similarly concluded that OA + inorganic fertilizer can provide additional yield and resilience benefits over inorganic fertilizer alone, but amendment type and rate must be chosen carefully to maximize nutrient-use efficiency and avoid undesirable side effects.

4.3 Regulation of soil ph and mitigation of soil salinity

Organic amendments also regulate soil pH and help mitigate salinity, especially in degraded or salt-affected soils. A global meta-analysis of saline-alkali land showed that amendment input reduced soil electrical conductivity by 23.6%, pH by 2.3%, and salt content by 20.9%, while increasing cation exchange capacity by 22.7% (Li et al., 2025). Review evidence further indicates that organic amendments improve saline soils by adding organic matter, increasing water retention and microbial activity, and encouraging salt leaching through improved pore structure. The mechanisms of salinity alleviation are both chemical and biological. Organic matter can accelerate the leaching of sodium and other salts, reduce exchangeable sodium percentage, and improve the movement of water through soil, which together lower salinity stress. Bio-organic amendments also release organic acids during decomposition, helping solubilize salts and carbonates, while reducing Na uptake and improving essential nutrient uptake by crops in saline environments.

Experimental studies confirm these effects at the field scale. In saline-sodic soil, lignite humic acid significantly lowered soil pH by 1.36 units and EC by 0.2 mS/cm, while increasing soil organic matter and the availability of potassium and phosphorus (Guo et al., 2022). In another saline-soil study, organic amendments produced desalinization rates of 11.66% to 37.17% in the 0-40 cm layer, and humic-acid-based treatments increased soil nutrient content alongside improved maize performance. The pH response, however, is not always in the same direction across all production systems. In an organic vegetable field in Maine, compost increased soil pH, organic matter, and nutrient contents relative to fertilizer alone, showing that amendments can also correct acidity rather than only reduce alkalinity. This variability matches broader evidence that amendment effects on chemical properties depend on soil texture, amendment composition, and application rate, so pH regulation in eggplant soils should be interpreted as site specific rather than universal (Matisic et al., 2024).

5 Effects of Organic Amendments on Soil Biological Functions

5.1 Stimulation of soil microbial diversity and activity

Organic amendments consistently increase microbial biomass, functional activity, and catabolic diversity relative to mineral-only fertilization. In long-term rotations, combined organic and inorganic fertilization increased microbial biomass C and N by 17.65%-40.86% and 18.63%-50.76%, respectively, while also increasing substrate-induced respiration and microbial catabolic diversity (Song et al., 2022). Global meta-analysis likewise showed that organic amendments increased microbial diversity components and shifted community structure, and these biological gains were positively associated with microbial functionality and crop yields. The diversity response is not uniform across microbial groups, which is important when interpreting eggplant soils under different amendment regimes. A global analysis found that bacterial Shannon and Chao1 diversity increased significantly, whereas fungal diversity showed no significant overall response, and the strongest taxonomic increases were in copiotrophic groups favored by nutrient enrichment (Cui et al., 2023). In greenhouse vegetable production, partial substitution with manure or straw also increased microbial taxonomic and functional diversity, showing that the same pattern holds in intensive protected systems similar to commercial eggplant cultivation (Zhang et al., 2023).

Microbial activity appears to respond not only to the presence of organic matter, but also to amendment quality and system context. In greenhouse vegetable soils, microbial communities were more responsive to compost than to cover crops alone, and the highest microbial biomass occurred where residue-based cover and compost were combined. In saline-sodic soils, different amendments also increased microbial richness and diversity while shifting community structure, indicating that amendment-driven biological recovery can occur even in chemically stressed soils (Guo et al., 2022). Mechanistically, these responses are linked to greater carbon supply and altered microbial resource allocation. Organic amendments provide diversified carbon substrates that can rapidly shift microbial population structure and sustain those changes for months or years. Short-term studies further suggest that changes in microbial function are associated more strongly with bacterial community composition than fungal composition, with taxa such as Luteimonas and Gemmatimona enriched under organic amendment.

5.2 Regulation of soil enzyme activities and nutrient transformation

Organic amendments regulate soil enzyme activities by stimulating the biochemical machinery responsible for decomposition and nutrient release. A global meta-analysis found that organic amendment increased enzyme activities linked to nitrogen and phosphorus decomposition, although effects on carbon-decomposition enzymes were not significant overall (Cui et al., 2023). In long-term field rotations, organic amendment also increased the potential activity of extracellular carbon-cycle hydrolases and reinforced the link between microbial biomass and functional diversity (Song et al., 2022).

Vegetable-system evidence shows that enzyme activity often tracks yield-relevant microbial changes. After five years of organic substitution in greenhouse production, microbial biomass carbon explained 89.5% of total enzyme activity variation, and phosphomonoesterase, N-acetyl-glucosaminidase, and urease were positively related to vegetable yields. Compost and manure also increased soil enzyme activities in organic production systems, although manure tended to raise nitrification, nitrite oxidation, and denitrification potential more strongly than compost, indicating a greater potential for nitrogen loss as well as faster N turnover.

At the gene level, organic amendments promote nutrient transformation by increasing the abundance of microbial functions tied to phosphorus and nitrogen cycling. In a 10-year greenhouse vegetable experiment, organic substitution increased taxonomic and functional diversity and raised the abundance of gcd, ppx, and phoD, genes associated with phosphorus solubilization and mineralization (Zhang et al., 2023). The same study showed that these shifts increased microbial P solubilization, mineralization, and immobilization capacity, improving P availability while reducing environmental losses (Zhang et al., 2023).

Nitrogen-cycling responses show a similar pattern but with strong dependence on amendment type, dose, and soil properties. Meta-analysis of 124 datasets found positive responses of nifH, amoA, nirS, nirK, and nosZ under organic amendment, with especially strong effects for manure, 10-20 t/ha application rates, and alkaline soils (Yang et al., 2024). Controlled greenhouse studies likewise found that amendment chemistry shaped microbial nutrient limitation and enzyme expression, with biochar and rice husk promoting C-, N-, and P-acquisition enzymes in clayey soils and sheep manure or biochar driving different enzyme responses in sandy soils (Li et al., 2025).

5.3 Suppression of soil-borne diseases and improvement of soil health

Organic amendments can improve soil health partly by enhancing natural suppressiveness against soil-borne pathogens, but the effect is not universal. Reviews consistently report that amended soils often show greater suppressiveness, especially against soil-borne diseases, because amendment-driven changes in microbial biomass, diversity, and hydrolytic activity strengthen antagonistic functions in the rhizosphere. However, suppression varies with amendment type, crop, pathogen, and environment, so disease control should be treated as context dependent rather than guaranteed. The biological basis of suppressiveness is increasingly clear. Disease-suppressive soils are typically characterized by high microbial diversity and abundant beneficial bacteria and fungi that inhibit pathogens through antimicrobial production, competition for resources, and induction of plant defenses such as induced systemic resistance (Priyadarshini et al., 2025). Organic amendments help establish these communities by supplying substrates for beneficial microbiota and by shifting soil conditions toward networks that outcompete pathogens (Priyadarshini et al., 2025).

Compost-based materials appear especially important in this context, although their performance is still variable. Disease-suppressive compost can introduce beneficial microbiota into conducive soils and increase suppression against soil-borne pathogens by reshaping soil microbial communities. Across a much larger evidence base, compost showed disease suppression in more than half of reported cases, whereas crop residues produced more variable effects and some amendments were suppressive only for specific pathogens (Priyadarshini et al., 2025). Pathogen specificity is one of the main limitations for practical use. Large-scale synthesis showed that disease suppression often changed during organic matter decomposition and that the same amendment could suppress one pathogen while being ineffective or even conducive for another. Enzymatic and microbiological indicators, including FDA activity, substrate respiration, microbial biomass, fluorescent pseudomonads, and Trichoderma populations, were more informative predictors of suppressiveness than chemical parameters alone.

Long-term soil health management therefore matters as much as amendment addition itself. Repeated organic amendment can strengthen suppressiveness over time, but its durability depends on how well soil management preserves the beneficial microbiome and avoids practices that disrupt it (De Corato, 2023). In particular, fumigation or other diversity-reducing practices can counteract amendment benefits by simplifying the microbiome and abolishing suppression that organic inputs had induced.

6 Influence of Organic Amendments on Eggplant Growth, Yield and Quality

6.1 Effects on plant growth and root development

Organic amendments improve vegetative growth by enhancing the soil conditions needed for eggplant rooting and canopy development. Deep, well-drained soils with high organic matter are described as essential for root development in eggplant, and organic fertilization improved plant height, branch number, stem diameter, and chlorophyll relative to control treatments in field experiments (Duri et al., 2025). In a two-year field study, 90% RDN plus Panchagavya and Jeevamrut significantly improved plant height, biomass, and root mass density, indicating that organic bioformulations can stimulate both aboveground and belowground growth (Rathore et al., 2022).

The response differs among amendment materials, with some manure- and vermicompost-based treatments showing especially strong effects. Chicken dung significantly increased plant height, leaf number, leaf area, fresh weight, dry weight, and flower number compared with vermicompost and biochar in one greenhouse study, while farmyard manure gave the best overall plant growth and fruit-size parameters among FYM, leonardite, and vermicompost in a two-year trial (Mahamad et al., 2022; Başay et al., 2025). Other organic sources also improved structural growth traits, as solid organic fertilizers significantly affected plant height, leaf number, stem diameter, and primary branches, and combined onion-skin liquid fertilizer with coconut-dregs compost increased plant height and advanced reproductive development.

Root responses are especially important where eggplant growth is constrained by stress or pathogens. Under deficit irrigation, vermicompost and biochar increased vegetative growth and yield, likely through higher soil organic matter and better water and nutrient retention, and they also increased leaf N, P, K, Fe, and Mn concentrations (Ebrahimi et al., 2021). In nematode-infested systems, increasing manure dose increased shoot and root length and shoot weight, with poultry manure producing the greatest total plant length, while some seed-powder and manure amendments also improved root and shoot weights before Meloidogyne javanica inoculation.

Integrated amendments often outperform single inputs because they improve nutrient access and root function simultaneously. Vermichar plus Trichoderma with recommended NPK consistently produced the best plant height, branch number, fruit number, and fruit weight, and the study attributed part of that response to improved nutrient absorption and root development. In organic soilless culture, arbuscular mycorrhizal fungi did not significantly improve growth or yield, but fish- and plant-based organic fertilizers performed similarly to inorganic fertilizers, suggesting that nutrient availability rather than inoculation alone can be the dominant growth driver in some protected systems (Shaik and Singh, 2022).

6.2 Effects on eggplant yield formation and nutrient utilization efficiency

Organic amendments generally support eggplant yield formation, but the strongest yield responses often occur when they are integrated with mineral nutrients or biostimulants rather than used as direct substitutes. Digestate promoted earlier fruiting in a Mediterranean greenhouse, although mineral fertilization still produced higher total yield, while long-term synthesis across crops shows that organic amendments alone often improve yield relative to no fertilization but not consistently relative to full mineral fertilization. That pattern indicates that organic inputs are effective for sustaining production, but their yield advantage depends on background fertility, amendment type, and management context.

Where integrated strategies are tested directly, yield gains are clearer. The combination of vermichar, Trichoderma, and recommended NPK increased fruit production by 13.93% over the control and produced the highest marketable fruit number and fruit weight per plant, while poultry manure combined with NPK gave the highest leaf and fruit yields in gboma eggplant. Similarly, high-rate vermicompost plus repeated vermitea drenches produced the highest fruit number, fruit size, and shoot nutrient uptake, with N, P, and K uptake in shoots exceeding the chemical-fertilizer treatment (Goud and Amal, 2020).

Amendments also improve nutrient utilization efficiency by increasing plant nutrient acquisition under both normal and water-limited conditions. Biochar and vermicompost raised leaf concentrations of N, P, K, Fe, and Mn and improved water-use efficiency under deficit irrigation, allowing acceptable yield even at 50% of plant water requirement (Ebrahimi et al., 2021). In organic fertilization systems, vigorous yield formation appears closely tied to better nutrient movement and uptake, as vermichar-enhanced treatments were interpreted to improve nutrient absorption by roots and support development of stems, leaves, and fruits.

Not all organic strategies deliver equal total yield, and trade-offs should be stated explicitly. In greenhouse eggplant, some organic fertilizers produced growth and yield close to inorganic fertilization in soilless production, but top-performing inorganic fertilizer still gave 12-15% higher total yield per plant than the best organic liquid fertilizer (Shaik and Singh, 2022). Likewise, organic amendments can raise pest pressure while improving growth and yield, since cow dung and poultry droppings increased plant height, leaf number, and yield but also increased whitefly and aphid abundance in field-grown eggplant.

6.3 Effects on fruit quality and nutritional characteristics

Organic amendments often improve fruit nutritional quality, especially antioxidant-related traits, although the effect is not fully consistent across studies. Organic fertilization increased ascorbic acid and hydrophilic antioxidant activity in one greenhouse study, and microbial biostimulants further improved carotenoid content and antioxidant activity (Duri et al., 2025). In another trial, high-rate vermicompost plus vermitea produced the highest fruit N, P, and K concentrations and the highest anthocyanin content, indicating that amendment-rich systems can improve both mineral and phytochemical quality (Goud and Amal, 2020).

Several studies also report better mineral and phenolic composition under organic management. Organically produced eggplants had higher K, Ca, Mg, and total phenolics than conventionally grown fruits in one year, and they still maintained higher K, Mg, and Cu in the second year. In manure-amended soil, fruits from vermicompost plus biochar had the greatest total phenols, while fruits from biochar-amended soil had the highest vitamin C, showing that different amendment types can shift quality traits in different directions.

Fruit size and market quality also respond to amendment choice. The 90% RDN plus Panchagavya and Jeevamrut treatment significantly improved fruit length, diameter, weight, and TSS in a two-year field experiment, while FYM, leonardite, and vermicompost all altered fruit height and diameter, with FYM giving the best overall fruit dimensions (Rathore et al., 2022). Organic seed-oriented production further showed that organic fertilizers significantly increased the nutrient-element contents of eggplant seeds compared with the control, with FYM maximizing seed K and vermicompost increasing Fe, Zn, and Mn (Başay et al., 2025).

Quality effects, however, are not uniformly favorable across all compounds or processing conditions. One study found no consistent differences in polyphenols, flavonoids, or antioxidant capacity between organic and conventional eggplant, and total monomeric anthocyanin was threefold higher in conventionally produced fruit. Overall, the evidence indicates that organic amendments usually enhance eggplant growth, support efficient yield formation, and often improve nutritional quality, but the magnitude and direction of quality responses depend on the amendment used, the production system, and the specific trait being measured.

7 Case Study: Application of Organic Amendments in Eggplant Production Systems

7.1 Case study background and experimental design

Case studies of organic amendment use in eggplant production have been conducted under both open-field and greenhouse conditions, with most experiments using replicated comparative designs to test amendment type, rate, and management context. A representative open-field study used a split-split plot randomized complete block design with three replications to test deficit irrigation, vermicompost, and two biochar types, while a greenhouse study in the Mediterranean compared compost and digestate in combination with plant- and microbe-based biostimulants while tracking yield and fruit quality traits (Ebrahimi et al., 2021; Duri et al., 2025). These designs reflect a shift from simple amendment-versus-control comparisons toward more system-based experiments that evaluate how organic inputs interact with irrigation, fertilization, and biological stimulants in commercial eggplant production.

Other case studies broadened this framework by testing integrated nutrient packages and contrasting multiple organic sources within the same trial. A two-year field experiment evaluated ten treatments based on reduced recommended nitrogen doses combined with vermicompost, poultry manure, Panchagavya, and Jeevamrut, whereas another two-season brinjal study used a randomized block design with eight treatments including FYM, vermicompost, Beejamrit, Jeevamrit, and their combinations (Rathore et al., 2022; Suraj et al., 2025). Longer-duration designs were also used to capture cumulative soil effects, including a four-year eggplant study comparing synthetic fertilizer alone with synthetic fertilizer plus FYM and a seven-year vegetable sequence that included eggplant under repeated biowaste compost strategies (Morra et al., 2021; Nisar et al., 2025).

The amendment materials tested in these case studies were chemically and functionally diverse, which is important for interpreting response variability. Some experiments focused on carbon-rich materials such as date-palm or pistachio biochar incorporated into the top 40 cm of soil, whereas others evaluated nutrient-rich inputs such as vermicompost, farmyard manure, poultry manure, digestate, or municipal solid waste compost under either sole or integrated application (Ebrahimi et al., 2021; Kuzhalarasi et al., 2024). Organic mulching studies extended the concept of amendment beyond incorporated materials by comparing straw mulch, compost mulch, and plastic mulches for their effects on soil physicochemical and microbial properties under conservation-oriented greenhouse production (Amami et al., 2025).

Across studies, measured outcomes typically included soil chemical or biological indicators together with plant growth, yield, quality, and economic traits, allowing relatively complete assessment of amendment performance. For example, greenhouse mulch trials explicitly evaluated soil physicochemical properties, microbial communities, and eggplant growth and yield, while a vermicompost-versus-NPK field study tracked both root and shoot traits and cultivation economics (Figure 2) (Kalika-Singh et al., 2021; Amami et al., 2025). This breadth of measurement is a major strength of the case-study literature because it links soil improvement directly to agronomic performance rather than treating soil and crop responses as separate outcomes.

7.2 Soil improvement responses after organic amendment application

The clearest soil response across case studies is improved soil organic carbon and nutrient status, especially where amendments are applied repeatedly or combined with mineral fertilizers. In a four-year eggplant study, the 100% RDF + FYM treatment produced the greatest increase in soil organic carbon stocks and the highest availability of N, P, and K, while in a split-plot brinjal experiment 100% STCR NPK + 12.5 t/ha vermicompost increased organic carbon and available N, P, and K relative to the control (Kuzhalarasi et al., 2024; Nisar et al., 2025). These results indicate that integrated amendment strategies not only feed the crop during the current season but also build the soil resource base that supports subsequent production.

Case studies also show that the form of amendment shapes the type of soil improvement observed. In an organic greenhouse mulch experiment, compost mulch significantly increased soil organic matter, available phosphorus, potassium, and total nitrogen, whereas straw mulch promoted the greatest microbial abundance, suggesting a trade-off between short-term nutrient enrichment and stronger stimulation of soil biota (Amami et al., 2025). A seven-year vegetable sequence that included eggplant similarly showed that compost-based fertilization increased SOC by 8.1 and 5.7 Mg/ha under two amendment strategies, while maintaining nitrate concentrations in the monitored 0-30 cm layer below thresholds associated with unnecessary additional fertilization (Morra et al., 2021).

Amendment effects on soil conditions were especially important in stressed or low-fertility environments. Under deficit irrigation, biochar and vermicompost improved soil conditions associated with better water and nutrient retention, helping sustain acceptable eggplant yield even at 50% plant water requirement, while a four-year field experiment in sandy alkaline farmland showed that organic amendments sharply increased SOC, total N, available K, and available P in the 0-20 cm layer (Ebrahimi et al., 2021). Although the latter study was not eggplant-specific, it strengthens the case-study interpretation that organic inputs are particularly effective where coarse texture or water limitation constrain nutrient retention.

Biological and broader soil-health responses also improved under repeated organic input. A two-year mesocosm study found that organic amendments promoted beneficial microbiota, improved soil fertility, and increased crop yield, whereas synthetic fertilizer plus fumigation reduced microbiota diversity and functionality while increasing acidification and salinity (Bonanomi et al., 2020). In eggplant specifically, recent brinjal field evidence reported that organic supplements improved soil health and increased plant resistance to biotic stressors, reinforcing the view that amendment benefits extend beyond chemistry into the biological resilience of the production system (Suraj et al., 2025).

7.3 Effects on eggplant growth and production performance

Most case studies report better eggplant growth and yield with organic amendments, but the strongest responses usually come from integrated treatments rather than single organic inputs used alone. In the four-year eggplant nutrient-management study, 100% RDF + FYM increased fruit yield by 47% over 100% RDF alone, while in the two-year brinjal trial vermicompost at 5 t/ha combined with Beejamrit and Jeevamrit gave the highest yield, biomass, and N, P, and K uptake (Nisar et al., 2025; Suraj et al., 2025). This pattern suggests that organic inputs work best when they improve nutrient supply and soil condition simultaneously rather than acting only as nutrient substitutes.

Several case studies also show that amendment choice affects which component of production improves most. Under open-field deficit irrigation, the combined application of vermicompost and pistachio biochar produced the highest total yield at full irrigation and the highest water-use efficiency at 50% PWR, whereas in a Mediterranean greenhouse digestate promoted earlier fruitification even though mineral fertilizer still gave the highest total yield (Ebrahimi et al., 2021; Duri et al., 2025). These contrasting responses show that some amendments are especially useful for stabilizing production under stress or advancing early harvest, even when they do not always maximize absolute seasonal yield.

Growth responses were equally consistent across a range of amendment systems. The treatment combining 90% RDN with Panchagavya and Jeevamrut significantly improved plant height, biomass, root mass density, fruit size traits, and yield over two years, while 100% STCR NPK plus 12.5 t/ha vermicompost significantly increased plant height, branch number, fruit number, fruit weight, yield, and nutrient uptake compared with the control (Rathore et al., 2022; Kuzhalarasi et al., 2024). Even where vermicompost did not significantly outperform chemical fertilizer for all aboveground traits, it still improved root and shoot length and was considered a cost-effective soil conditioner with favorable implications for plant growth (Kalika-Singh et al., 2021).

Some case studies also captured fruit-quality and economic benefits that strengthen the practical case for amendment use. Vermicompost plus biochar increased total phenols in eggplant fruit and biochar-based treatments produced the highest vitamin C concentrations in one field study, while microbial biostimulants used alongside compost or digestate improved carotenoid content and antioxidant activity in greenhouse fruit (Duri et al., 2025). Economic responses were also favorable in several trials, including higher benefit-cost ratio under STCR NPK plus vermicompost and maximum net return under vermicompost combined with Beejamrit and Jeevamrit (Kuzhalarasi et al., 2024; Suraj et al., 2025).

8 Future Perspectives and Conclusions

Future optimization of organic amendment management in eggplant production should move away from uniform recommendations and toward site-specific strategies based on soil texture, climate, amendment chemistry, and cropping context. Broad reviews consistently show that amendment effects vary with soil conditions, application rate, and management duration, while mechanism-based synthesis indicates that no single amendment is universally superior because composts, manures, biochars, and humic materials act through different biogeochemical pathways. For eggplant systems, this means that choosing among vermicompost, digestate, biochar, or manure should be guided by the dominant constraint to productivity, whether low organic matter, poor water retention, nutrient imbalance, salinity, or biological degradation. Optimization also requires better definition of rate, timing, and combination effects under long-term field conditions. Long-term meta-analysis shows that OA + inorganic fertilizer often delivers more stable performance than organic inputs alone, but amendment type and rate must be selected carefully to maximize nutrient-use efficiency and avoid environmental losses, while recent fertilizer-by-amendment experiments indicate that moderate nitrogen with mushroom residue or biochar can outperform higher-input regimes for soil fertility and resource-use efficiency. This supports a future research agenda in eggplant centered on optimization rather than simple substitution, especially under intensive vegetable production where overapplication can increase salinity, phosphorus accumulation, or inconsistent crop response.

Organic amendment management should also be optimized at the level of material production and quality control. Recent reviews emphasize the need to improve composting processes, optimize conversion of organic wastes into higher-quality fertilizers, and develop fortified products with more predictable nutrient composition, while another review highlights persistent challenges related to quality variation, scalability, overapplication, and regulation. In practical terms, eggplant production systems will benefit from amendment products that are more uniform in maturity, nutrient density, and contaminant safety than many farm-derived materials currently available. A second management priority is to refine amendment placement and repeated-use strategies rather than focusing only on single applications. Emerging evidence suggests that innovations such as deep banding of compost or manure can improve nutrient availability and reduce runoff, while repeated applications of organic amendments can promote beneficial microbiota, improve soil fertility, and sustain higher yields over time under intensive production. For eggplant, long-term trials should therefore compare surface incorporation, localized placement, mulch-based delivery, and repeated low-dose applications to identify management schedules that build soil function while maintaining economic feasibility.

The next step for eggplant systems is to integrate organic amendments with other sustainable agriculture technologies rather than treating them as stand-alone inputs. Reviews published in 2023-2025 emphasize the synergistic potential of combining traditional amendment practices with digital agriculture, precision farming, conservation tillage, crop diversification, and living ground cover, showing that integrated systems are more likely than isolated practices to rebuild soil function and strengthen resilience. This framing is especially relevant for eggplant, which is commonly grown in high-input open-field and greenhouse systems where nutrient losses, heat stress, and declining soil organic matter can interact. Precision management is one of the clearest future directions. Current reviews identify a need for predictive models and precision in-field analysis of organic fertilizers, and broader assessments of agronomic management also conclude that organic matter management works best when coordinated with irrigation, nutrient management, rotations, and mulching rather than optimized in isolation. In eggplant production, this suggests practical value in coupling amendment use with sensor-based irrigation scheduling, soil testing, variable-rate fertilization, and decision tools that match amendment type and dose to plant demand and seasonal risk.

Integration with biological technologies is also promising. In greenhouse eggplant, compost and digestate combined with microbial or plant-based biostimulants improved early production and fruit nutraceutical quality, and field evidence shows that vermichar plus Trichoderma with reduced mineral fertilizer outperformed other treatments for plant growth and yield. These findings support a shift toward multi-component biological management in which amendments serve as both nutrient sources and carriers of a more favorable rhizosphere environment. Organic amendments can also be integrated with stress-mitigation technologies tailored to degraded or water-limited environments. In eggplant, biochar and vermicompost improved growth and water-use efficiency under deficit irrigation, while in sodic soils the conjunction of gypsum, crop residue, or FYM with mineral fertilization improved soil quality, nutrient recovery, and economic return more effectively than simpler treatments. This indicates that future sustainable eggplant production will likely depend on integrated packages that combine amendment use with irrigation management, salinity reclamation tools, protected cultivation, and pest-management practices adapted to local constraints.Overall, the literature supports a clear conclusion: organic amendments are effective tools for improving soil quality in eggplant production, but their benefits are most reliable when embedded in systems-based management rather than applied as universal replacements for mineral fertilizers. Reviews across diverse environments show that amendments improve soil structure, nutrient availability, microbial functioning, and climate resilience, while long-term evidence indicates that they also increase the resilience of agronomic systems over time. For eggplant, the strongest practical implication is that amendment programs should be designed around local soil limitations and integrated with complementary practices that reduce disturbance and maintain biological activity. Future research priorities should therefore emphasize long-term, eggplant-specific comparisons among amendment sources, rates, and combinations under both open-field and greenhouse systems. Current reviews explicitly call for more work on interactions among soil management, amendments, and time, while recent eggplant research notes that synergistic effects of organic fertilization and biostimulation remain only partly explored in this crop. Priority topics include nutrient-release kinetics, salinity and pH risks, amendment effects on soil-borne disease suppression, and the persistence of soil carbon and microbial changes over successive eggplant cycles.

Research also needs to expand beyond agronomy alone to include economic, social, and adoption dimensions. Recent reviews argue that comprehensive socioeconomic analysis, farmer-oriented evaluation, and region-specific validation are necessary to support adoption, especially where costs, labor demand, market access, or limited input availability constrain the use of organic amendments. This is particularly important for eggplant, which is often cultivated by smallholders and in peri-urban vegetable systems where profitability and labor efficiency strongly shape management decisions. Finally, future studies should better connect amendment research with environmental safeguards and practical deployment technologies. Meta-analysis shows that combining organic materials with chemical fertilizer often gives the best balance between crop yield and soil quality, but the joint use of amendments with other farmland management practices still needs more research, while region-specific organic vegetable trials confirm that optimized combinations of compost, biochar, and auxiliary management can improve both soil properties and crop outcomes. In conclusion, the future of organic amendments in eggplant production lies in precise, integrated, and locally adapted management that improves soil function while sustaining yield, quality, and long-term agroecosystem resilience.

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