Salinity and metal stress are pressing concerns that pose significant threats to soil microbial communities, soil fertility, food security, biodiversity, and the sustainability of agriculture ^[1][2][1,2]. Climate change and global warming contribute to rising sea levels, which, in turn, result in new areas becoming saline and barren each year. In addition, human activities such as the use of sewage water for irrigation, industrial operations, mining, and the overuse of pesticides further contribute to the toxicity of soils [3]. These anthropogenic and geogenic actions are responsible for the accumulation of salts and toxic metals like arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg), posing risks to both plants and the environment ^[4][5][4,5]. Although some metals are harmless or even beneficial at low concentrations, they become toxic as their levels increase [1].

Organic amendments (OAs), such as Sesbania rostrata biomass, vermicompost (VC), compost, biochar (BC), and poultry and farmyard manures (FYMs), provide an alternative approach to alleviate these abiotic stresses ^[6][7][8][6,7,8]. These high-nitrogen organic amendments contribute to the improvement of soil quality and promote plant growth by reducing the bioaccumulation and translocation of metal stress under salinity [9]. Additionally, they have the added benefit of enhancing soil health by increasing nutrient availability, reducing the uptake of harmful metals, and strengthening antioxidant defenses in plants. According to multiple studies, OAs significantly contribute to the improvement of soil health, increase nutrient uptake, enhance the stability of cellular membranes in plants, and also decrease the bioavailability of pollutants [10]. This results in increased biomass production in soils affected by salinity and metal contamination. Organic amendments act by increasing the content of soil organic matter, which, in turn, stimulates the activity of soil microorganisms ^[11][12][11,12]. These microorganisms convert nutrients into forms that are readily available for plant uptake. Moreover, OAs have a positive impact on the physical and chemical properties of the soil, thereby enhancing soil health, crop yield, and quality ^{[13][14][15][16]}[13,14,15,16]. Furthermore, phytoextraction of metals by using different plants like Brassica rapa, Cannabinus sativa, Helianthus annuus and Zea mays is also found effective to remove metal pollution from soil [17]. Given these benefits, it is crucial for researchers and agriculturalists to adopt a more comprehensive approach to improve soil fertility and bolster plant defenses against abiotic stresses. Several studies have already validated that the application of OAs is an environmentally sound, economically viable, and agronomically effective technique [18].

Organic amendments positively amend degraded soil structures and enhance soil productivity and quality. Originating from both plants and animals, their types are illustrated in Figure 1.

2. Biochar

Biochar, a carbon © rich byproduct of biomass pyrolysis, contains various amounts of C, hydrogen (H), sulfur (S), oxygen (O₂), N, and minerals. Although almost 70% of its composition is C, the rest depends on the feedstock used to make it. It has recently been recognized for its beneficial economic and environmental impacts on soil and crop productivity. Biochar amends pH, increases CEC, sequesters C, enhances P availability ^[19][38], improves soil aeration and porosity ^[20][21][39,40], and enhances soil fertility ^{[22][23][24][25]}[41,42,43,44]. Additionally, by promoting the rhizosphere’s biological environment with biochar, soil enzyme activity and microbial growth are enhanced ^[26][45]. It also assists in nutrient retention in soil micropores and supports easy plant nutrient assimilation ^[27][46]. Salinity is mitigated by replacing Na⁺ from exchangeable soil sites, reducing Na⁺ adsorption ratios, and alleviating oxidative stress from NaCl. Researchers have also found that the presence of oxides, hydroxides, and carbonates in BC improves soil productivity. Furthermore, biochar’s strong adsorption capacity, particularly in bamboo charcoal, makes it an ideal nutrient preserver and stabilizer for HMs, notably Pb and Cd in polluted soils ^{[28][29][30][31]}[47,48,49,50]. Biochar’s effects on degraded soil and crops are demonstrated in Figure 2.

3. Compost

Compost is rich in OM and essential plant nutrients like N, P, and K, fulfilling deficiencies found in saline-affected soils. It also decreases the sodium absorption ratio by increasing Ca²⁺ in the soil solution. Furthermore, compost enhances SOM by binding soil particles into aggregates, thus improving soil air circulation and infiltration, increasing the available micronutrients, and promoting plant and microbial growth ^[32][33][34][51,52,53]. As compost alters soil properties ^[35][54], it elevates soil fertility for crop production. Moreover, it mitigates oxidative stress, boosts chl content and photosynthesis rates, and promotes crop growth ^[36][37][55,56]. Ahmed et al. ^[38][57] advocate for using affordable water hyacinth compost to amend degraded saline-sodic soils and improve crop yields. Composting livestock dung can quickly transform it into a biofertilizer, eliminating harmful chemicals, HMs, pathogens, and antibiotics ^[39][40][58,59].

4. Vermicompost

Produced by using earthworms to convert organic waste into nutrient-rich compost, VC has various plant nutrients. It acts as a biosorbent, reducing the negative impacts of salinity ^[41][60] and harmful ions like Pb, Cd, nickel (Ni), and chromium (Cr) ^[42][61]. In the composting process, earthworms elevate the mineralization and humification rates in soil, increasing soil pore space, water infiltration rates, and water retention, which increase microbial populations and organic C content and promote growth, yield, and fruit quality ^[43][62]. Researchers have identified that VC has more nutrients than regular compost, enhancing soil fertility in multiple ways. It bolsters SOM and exchangeable minerals like K⁺, Ca²⁺, and Mg²⁺ in soil, reducing EC. Additionally, VC improves plant physiological factors, reducing harmful effects like oxidative stress and enhancing plant growth ^[44][45][63,64]. It also immobilizes soil HMs like Cd and diminishes their phytoavailability ^[46][65], subsequently increasing grain yields by supplying essential plant hormones ^[47][66]. The effects of VC on soil and crops are depicted in Figure 3.

5. Green Manure

Various green manuring crops are employed to enhance soil fertility ^[48][67] and reclaim soil salinity ^[11][49][11,68]. Sesbania, a leguminous plant, is effectively utilized as green manure (GM). It mitigates soil salinity by drawing out excess salt and harnessing it through its biomass, simultaneously improving soil structure and nutrient availability (Figure 4). This leads to optimized crop growth. Decomposed GM crops elevate soil CO₂ concentration, aiding CaCO₃ dissolution and hastening the removal of exchangeable Na⁺ ions from saline soils ^[50][51][69,70]. Sesbania and sunhemp demonstrate significant potential for reducing soil Na⁺ and ameliorating soil salinity. Choudhary et al. ^[52][71] found that incorporating GM decreases soil pH in saline-sodic soils due to its acidifying effect, which, in turn, boosts the available soil and plant minerals. Organic materials not only ameliorate conditions but also augment the physical attributes of the soil, nutrient availability, and the SOM status in degraded soils. Sesbania, given its ample biomass and nodulation, is a widely preferred OA. It enriches the soil with N, P, K, and OAs, enhancing the C:N ratio, Ca²⁺ status, and salinity mitigation ^[53][72]. Decomposed GM acts as a slow-release fertilizer, benefiting subsequent crops ^[54][73]. Shirale et al. [11] posited GM as a potential gypsum substitute, attributing to its incremental salinity reclamation capabilities and bolstering of biological N fixation and C sequestration. Mustard species, utilized as GM, improve soil fertility due to their rhizosphere activity and phytoremediation potential ^[55][74]. Various GM crops, including mustard, phacelia, and borage, have been reported to boost soil respiration and diminish bioaccessible metal amounts, thereby reducing ecotoxicity ^[56][75]. Bruning et al. ^[57][76] hypothesize that legumes, despite their high salinity levels, can serve as GM due to their growth and atmospheric N fixation abilities.

6. Duckweed and Water Hyacinth

Over recent decades, phytotechnologies, which utilize plants for pollutant removal, have gained prominence. Both terrestrial and aquatic plants possess remarkable metal-sorption capabilities ^[58][77]. Duckweed (DW, Lemna), an aquatic member of the Lemnaceae family, is enriched with trace minerals, K, and P, and vital sources of vitamins A and B, proteins, fats, amino acids, and starch. Infusing soil with duckweed biomass increases the uptake of nutrients like N, K, Ca, Mg, Fe, and Zn, subsequently boosting crop production. Duckweed extracts have been employed as biostimulants for olive plant growth ^[59][78]. Notably, duckweed can withstand pollutants such as ammonia and HMs, marking its potential as a purifier for agricultural and industrial wastewater ^[60][79]. However, some research indicates that DW efficacy in HM (Ni, Cd) pollutant removal diminishes under salt stress ^[61][80]. Contrarily, others have demonstrated DW’s capability to accumulate boron in environments with salinity under 100 mM, significantly improving osmotic stress resistance ^[47][66]. Water hyacinth, a rapidly proliferating aquatic plant, owes its growth to nutrient content. Activated C derived from water hyacinth has applications in salinity reduction through mineral absorption ^[62][81]. Both Eichhornia crassipes and Lemna minor effectively remove HM ions, such as As, from water ^[63][82].

7. Poultry Manure

Poultry manure serves as an organic material for enhancing soil fertility because it is rich in both macro- and micronutrients. Organic N-rich poultry manure (PM) is commonly utilized to amend and enhance fertility in saline soil. As found by numerous researchers, such as Leithy et al. [13], PM ameliorates the physical, chemical, and biological properties of soils and mitigates the toxic impacts of salinity across various plant species. Additionally, PM has been shown to decrease certain trace metal concentrations in soil.

8. Farmyard Manure

Farmyard manure (FYM) is a composted blend of cow dung, cow urine, litter, and other dairy byproducts. It is a reservoir of nutrients, including N, P, and trace elements, all of which enhance soil fertility and soil quality, along with the stable humic substance ^[64][83]. As an integral source of soil C, it bolsters the activities of soil flora and fauna and effectively reduces EC and pH in saline-sodic soils. Singh and Agrawal ^[65][84] emphasize that FYM is invaluable for elevating soil fertility and diminishing soil metal contamination. Its solo use or in conjunction with N, P, and K (inorganic fertilizers) can mitigate the phytoavailability of HMs in the soil. This results in maintaining plant vitality and bolstering growth and yield, especially at contaminated agricultural sites. Chicken and cow manures, when added to polluted soil, drastically cut down the phytoavailability of Cd while amplifying the growth and yield of sweet basil [9]. Rani et al. ^[66][85] underscored that FYM, in combination with cow dung and pig manure, can alleviate soil metal stress and markedly reduce Ni by forming resilient metal complexes with organic manure. Among the modifications to reduce chromium toxicity, FYM has been the most effective.

9. Press Mud

Press mud, a byproduct of the sugar industry, is esteemed for augmenting SOM, cultivating a conducive environment for microbial communities, and, ultimately, boosting soil fertility and crop yield ^[67][68][69][86,87,88]. Beyond being a vital nutrient source, press mud also magnifies plant nutrient uptake through roots, fortifies membrane integrity, and enhances osmoprotectant processes ^[70][89]. Additionally, press mud is rich in hydroxyl ions, pivotal for metal adsorption and the diminishment of toxic metal bioavailability ^[69][71][88,90]. The manifold benefits of press mud on soil and crops, especially under salinity conditions, are illustrated in Figure 5.

10. Others

The lion’s share of humic compounds, notably humic acid, represent the most biologically vivacious components of soil and compost ^[72][91]. Incorporating humic substances leads to an elevation in soil pH, cation exchange capacity, and OC content, released P, controlled N loss, reduced metal mobility, and improved crop growth ^[73][92]. Sewage sludge is embraced as an OA due to its ample concentrations of N, P, and K. Typically, urban sludge is benign relative to its industrial counterpart. Steel slag, an industrial residue rich in Ca, Si, Fe, and P ^[74][93], holds promise for remediating HM pollution. Historically, steel slag, along with BC application, significantly improved growth performances, reduced the oxidative stresses of okra, and mitigated the adverse effects of As stress ^[75][94]. Its inclusion diminished the accessible amount of Cd in tainted soils ^[76][95], consequently cutting down soil Cd concentration from root to shoot and enhancing rice growth and the soluble protein concentration of black gram ^[42][67][61,86].