Organic Amendments: Enhancing Plant Tolerance to Salinity and Metal Stress for Improved Agricultural Productivity

Abstract

Salinity and metal stress are significant abiotic factors that negatively influence plant growth and development. These factors lead to diminished agricultural yields on a global scale. Organic amendments have emerged as a potential solution for mitigating the adverse effects of salinity and metal stress on plants. When plants experience these stresses, they produce reactive oxygen species, which can impair protein synthesis and damage cellular membranes. Organic amendments, including biochar, vermicompost, green manure, and farmyard manure, have been shown to facilitate soil nitrogen uptake, an essential component for protein synthesis, and enhance various plant processes such as metabolism, protein accumulation, and antioxidant activities. Researchers have observed that the application of organic amendments improves plant stress tolerance, plant growth, and yield. They achieve this by altering the plant’s ionic balance, enhancing the photosynthetic machinery, boosting antioxidant systems, and reducing oxidative damage. The potential of organic amendments to deal effectively with high salinity and metal concentrations in the soil is gaining increased attention and is becoming an increasingly popular practice in the field of agriculture. This review aims to provide insights into methods for treating soils contaminated with salinity and heavy metals by manipulating their bioavailability through the use of various soil amendments.

1. Introduction

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]. 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]. 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]. 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]. 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]. 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].

The objective of this study is to explore the impact of various OAs on mitigating the detrimental effects of salinity and metal stress in plants. This review will focus specifically on how the application of OAs can restore plant morphophysiological and biological parameters that have been compromised by salinity and metal stress.

2. Effects of Salinity and Metal Stress

2.1. Impact of Salinity on Soil Properties

Saline soils are characterized by high concentrations of dissolved salts such as sodium (Na⁺), calcium (Ca²⁺), potassium (K⁺), magnesium (Mg²⁺), chloride (Cl⁻), sulfate (SO₄²⁻), carbonate (CO₃²⁻), and bicarbonate (HCO₃⁻). These soils have an electrical conductivity (EC) greater than 4 dS m⁻¹ at 25 °C. Excessive Na⁺ content adversely affects soil quality by reducing soil permeability, structural stability, and bulk density [19] (Figure 1). It also decreases the soil’s water-holding capacity and rate of water infiltration. High salt concentrations inhibit the activity of nitrifying bacteria, leading to reduced nitrification, which is crucial for nitrogen (N₂) release in soil for plant growth [20]. Soil enzyme activities and respiration are also hindered by salinity. According to Sritongon et al. [21], a negative correlation exists between electrical conductivity and soil properties such as organic matter (OM), organic carbon (OC), and soil enzyme activities.

2.2. Impact of Metal Stress on Soil Properties

Soil is a natural source of heavy metals (HMs), originating from the weathering of metal-bearing rocks. Human activities (industrial activities, agricultural activities, and metal-containing wastes) have increased these concentrations, causing environmental damage [22,23]. When contamination reaches certain levels, HMs obstruct phytoremediation and reduce plant growth and production [24]. Heavy metals negatively impact soil quality, affecting the structure and abundance of soil microorganisms [25]. Heavy metals reduce enzymatic activities by decreasing urease and catalase activities, along with the reduced mineralization of soil organic matter (SOM), and affect their accumulation, resulting in hampered soil quality. However, Kumar et al. [26] argue that HMs enhance oxidative stress in plants, damage cell structures, and disrupt ionic homeostasis. The process of photosynthesis, which is essential for plant development and productivity, can be interfered with by HMs. For instance, photosystem II is inhibited by Hg, which lowers the amount of chlorophyll (chl) and decreases the effectiveness of light absorption and energy conversion. Similar to this, lead disrupts photosystem I’s electron transport chain, which hinders the production of ATP and NADPH needed for carbon absorption.

2.3. Impact of Salinity on Crop Productivity

Salt stress suppresses plant growth and productivity by affecting the availability of nutrients, which is regulated by rhizosphere microbial activity [27]. Salinity causes osmotic stress, reduced shoot growth, and stomatal closure owing to the accumulation of Na⁺ and Cl⁻ in leaves, where photosynthesis takes place [28,29]. It also accelerates the senescence of older leaves through chl degradation [30,31]. Higher Na⁺ accumulation in the intracellular space (cytosolic apartment) can inhibit enzyme activities and reduce water relations, photosystem II (PS II), and CO₂ assimilation in plants [31]. Salinity stress can reduce shoot biomass by 24–41% and grain yield by 7–30% in foxtail millet [32]. Excessive salinity impairs antioxidant activity, causes lipid peroxidation, denatures proteins and nucleic acids, and increases reactive oxygen species (ROS), causing cellular damage [33,34].

2.4. Impact of Metal Stress on Crops

Pedogenic and anthropogenic activities are the primary sources of soil metal pollution, which adversely affects plant growth and productivity. Metals like iron (Fe), cobalt (Co), copper (Cu), manganese (Mn), zinc (Zn), and molybdenum (Mo) serve as micronutrients at low concentrations but become toxic at higher levels, hampering plant development [35]. For instance, toxic levels of Cu hampered photosynthesis and plant growth. Heavy metal deposition disrupts various biochemical, physiological, and morphological processes in plants, thereby reducing agricultural yield [36]. In addition, inhibition of photosynthesis, chlorosis, low biomass accumulation, altered nutrient absorption, water balance, disturbing the redox balance, and causing oxidative stress are typical harmful impacts of Cd HM [37] (Figure 2).

3. Types and General Role of Organic Amendments

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 3.

3.1. 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 [38], improves soil aeration and porosity [39,40], and enhances soil fertility [41,42,43,44]. Additionally, by promoting the rhizosphere’s biological environment with biochar, soil enzyme activity and microbial growth are enhanced [45]. It also assists in nutrient retention in soil micropores and supports easy plant nutrient assimilation [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 [47,48,49,50]. Biochar’s effects on degraded soil and crops are demonstrated in Figure 4.

3.2. 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 [51,52,53]. As compost alters soil properties [54], it elevates soil fertility for crop production. Moreover, it mitigates oxidative stress, boosts chl content and photosynthesis rates, and promotes crop growth [55,56]. Ahmed et al. [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 [58,59].

3.3. 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 [60] and harmful ions like Pb, Cd, nickel (Ni), and chromium (Cr) [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 [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 [63,64]. It also immobilizes soil HMs like Cd and diminishes their phytoavailability [65], subsequently increasing grain yields by supplying essential plant hormones [66]. The effects of VC on soil and crops are depicted in Figure 5.

3.4. Green Manure

Various green manuring crops are employed to enhance soil fertility [67] and reclaim soil salinity [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 6). 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 [69,70]. Sesbania and sunhemp demonstrate significant potential for reducing soil Na⁺ and ameliorating soil salinity. Choudhary et al. [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 [72]. Decomposed GM acts as a slow-release fertilizer, benefiting subsequent crops [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 [74]. Various GM crops, including mustard, phacelia, and borage, have been reported to boost soil respiration and diminish bioaccessible metal amounts, thereby reducing ecotoxicity [75]. Bruning et al. [76] hypothesize that legumes, despite their high salinity levels, can serve as GM due to their growth and atmospheric N fixation abilities.

3.5. 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 [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 [78]. Notably, duckweed can withstand pollutants such as ammonia and HMs, marking its potential as a purifier for agricultural and industrial wastewater [79]. However, some research indicates that DW efficacy in HM (Ni, Cd) pollutant removal diminishes under salt stress [80]. Contrarily, others have demonstrated DW’s capability to accumulate boron in environments with salinity under 100 mM, significantly improving osmotic stress resistance [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 [81]. Both Eichhornia crassipes and Lemna minor effectively remove HM ions, such as As, from water [82].

3.6. 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.

3.7. 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 [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 [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. [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.

3.8. 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 [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 [89]. Additionally, press mud is rich in hydroxyl ions, pivotal for metal adsorption and the diminishment of toxic metal bioavailability [88,90]. The manifold benefits of press mud on soil and crops, especially under salinity conditions, are illustrated in Figure 7.

3.9. Others

The lion’s share of humic compounds, notably humic acid, represent the most biologically vivacious components of soil and compost [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 [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 [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 [94]. Its inclusion diminished the accessible amount of Cd in tainted soils [95], consequently cutting down soil Cd concentration from root to shoot and enhancing rice growth and the soluble protein concentration of black gram [61,86].

4. Biochemical and Physiological Adaptations to Stresses in Crops and Soil through Different Organic Amendments

In recent decades, integrating organic resources into salt-affected soils has become a common practice. Before applying to the soil, OAs should be processed to mitigate potential risks to plants and achieve the desired transformation and stabilization of complex molecules into OM. Decomposed organic residues often result in more stable organic matter compared to fresh materials. These decomposed residues mineralize more slowly and benefit the soil over longer periods, especially in soils with low OM content [26]. The introduction of OAs improves the physicochemical and biological properties of soils, consequently enhancing plant growth.

4.1. Organic Amendments and Improved Soil Quality under Salinity

L’opez-Valdez et al. [96] found that OAs ameliorate soil properties by leaching Na⁺ and other salts and by reducing the exchangeable sodium percentage. Additionally, these amendments stimulate the biological and enzymatic activities in the soil, increasing the population of beneficial organisms and thereby enhancing soil fertility. In saline soils, both VC and compost additions have been shown to boost soil cation exchange capacity by 20–70% [57,84,97]. These amendments also enhance microbial biomass and soil respiration compared to unamended soils [55] and increase the salinity tolerance of rice, sugarcane, and fennel by increasing K⁺/Na⁺ and Ca²⁺/Na⁺ [66,84,98,99] (

Table 1. Application of compost and vermicomposting for soil and crop salinity reclamation.

Plant Species	Salinity Level	Types and Doses	Effect of Amendments		Reference
			On Soil	On Crop
Rice (Oryza sativa)	Salinity, 2.9 dS m−1	VC with rice husk ash (1000 kg Rai−1)	Increased exchangeable K+, Ca2+, and Mg2+ in soil, decreased EC.	Increased plant growth and grain yield.	[99]
Lettuce (Lactuca sativa)	Salinity, 8.32 dS m−1	VC 50% and eggshell 12.5%	Reduced soil salinity	Increased germination, growth of seedlings, and yield.	[100]
Rice–wheat	Salinity, 5.02 dS m−1	Hyacinth compost, 10 and 15 t ha−1 with gypsum, 50%	Increased water-holding capacity, soil aggregation, and soil CEC.	Rice and wheat yield increased.	[57]
Sugarcane (S. officinarum L.)	Salinity, 4.12 dS m−1	VC, 10, 20 t ha−1 and N fertilizer, 50, 75, and 100 kg ha−1	Reduced soil EC and Na+/K+ ratio and reduced salinity mitigation.	Increased sugarcane growth and production.	[101]
Bean (Phaseolus vulgaris L.)	NaCl (20, 40, 60, and 80 mM)	VC and sand mixture (0:100; 10:90; 25:75; 50:50, and 75:25)	Not observed	Increased photosynthetic rate, concentration of K+ and Ca2+ in leaves, and growth of bean improved.	[102]
Potato (Solanum tuberosum L.)	NaCl (15, 20, and 25 mM)	VC, 300, 580, and 860 g plant−1	Not observed	Increased plant height and stem diameter; potato production.	[103]
Tomato (S. lycopersicum L.)	NaCl (150 mM)	VC extract, 6 mL L−1	Increased nutrient availability and reduced soil salinity.	Increased accumulation of proline, total sugars leaf water content, reduced osmotic stress.	[104]
Mustard–pearl millet cropping system	Salinity, 7.2 ds m−1	Rice straw compost 3 to 5 t ha−1	Reduced soil salinity and increased microbial activity	Increased plant growth, grain, and straw yield.	[105]
Rice (O. sativa)	EC (7.5 dS m−1)	Compost (15 t ha−1)	Improved soil nutrient availability and ameliorated salinity.	Increased essential micronutrients in rice grain, increased crop yield.	[106]
Tomato (S. lycopersicon)	NaCl (40 and 80 mM)	Compost (25 t ha−1)	Increased macro- and micronutrients and reduced salinity	Increased accumulation of osmoprotectants, such as soluble sugars and amino acids. Increased crop yield.	[56]
Fennel (Foeniculum vulgare)	NaCl (40, 80, and 120 mM)	VC extract (10%)	Increased Ca2+ content, alleviated salinity stress of plants.	Increased root Ca2+ content, reduced Na content, enhanced germination and growth of fennel.	[107]
Maize (Zea mays)	Salinity (10.6 dS m−1)	VC (72 g pot−1) + cow dung (33 g pot−1)	Soil physical and chemical properties improved	Increased germination percentage, plant height, root length, and crop yield.	[54]
Tomato (S. lycopersicon)	50 (100 mM of NaCl)	Compost (55 g kg−1)	-	Increased enzymatic activities, reduced oxidative stress, promoted plant growth and productivity.	[108]

). They also improve the availability of soil P, which had previously been tightly bound due to soil pH. This results in effective mitigation of salt stress in plants, like tomatoes [56].

The combined application of compost with BC in saline soil provides both micro- and macronutrients to the rhizosphere, boosting soluble sugars and amino acid levels. This enhances nutrient (N, P, and K) assimilation in the soil [46,51]. Incorporating VC with eggshells and rice husk improves nutrient availability, particularly Ca²⁺ and K⁺, by fostering a more diverse soil microbial community in rice and wheat crops under salinity [98,99].

Using legume green manuring, especially Sesbania, raises SOC, improves nutrient availability, and enhances the soil’s physical, chemical, and biological characteristics. It also bolsters the yield of crops like rice and wheat [109,110,111,112] (

Table 2. Application of green manuring crops and duckweed for salinity reclamation of crops and soil.

GM Crops as OA	Crop	Salinity	Effect of Amendments		Reference
			On Soil	On Crops
Green manure + FYM (1:1 w/w) at 12.5 kg m−2	Oryza sativa	1–2% salt	Soil fertility improved and alleviated the problem of salinity.	Increased total chl, photosynthesis abilities, crop growth, and grain yield.	[109]
Sesbania + Compost+ FYM, 5% volume	O. sativa and T. aestivum	Total soluble salts = 25.3 mg L−1	Increased soil fertility.	Increased rice–wheat production in saline-affected area.	[110]
(Sesbania + Gypsum), 12.5 to 20 Mg ha−1	Rice–Wheat (T. aestivum)	Salinity (2.7–4.5 dS m−1)	Alleviated soil salinity	Increased crop growth and grain (rice and wheat) yield.	[111]
Water hyacinth (E. crassipes) and Duckweed (Lemna minor)	Industrial wastewater	45 mM NaCl	Decrease in pH, EC, oxidation redox potential (ORP), and salinity.	-	[112]

). Sunnhemp green manuring has shown promise in saline soils by improving soil fertility and nutrient status [72]. Once decomposed, GM crops release organic acids, fostering a favorable environment for soil microbes. This promotes the release of various nutrients, thus improving soil quality.

Furthermore, BC possesses a high adsorption capacity, aiding in the retention of negative ions and enhancing cation exchange capacity by introducing Ca²⁺ into the soil solution [98] (

Table 3. Application of biochar for soil and crop salinity reclamation.

Plant Species	Salinity Level	Type and Dose of Amendment	Effect of Amendments		Reference
			On Soil	On Crops
Rice (Oryza sativa)	50 mM and 75 mM of NaCl	BC + Trico compost + Phospogypsum	Increased soil N, P, K+/Na+, Ca2+/Mg2+, enhanced SO42−, NO3−, Mn4+, Fe content in rice rhizosphere, and reduced CH4 emission.	Increased plant height, shoot biomass, and crop yield.	[98]
	Salinity, 1 and 3 dS m−1	Rice straw BC (0.3%)	Reduced Na+ and Cl− contents of soil and improved physiochemical properties of soil.	Granum lamellae in mesophyll cells’ structure is improved, improved rice productivity.	[117]
	Salinity (2, 4, 6, and 8 dS m−1)	BC (2 kg m−2)	Increased soil moisture content and physicochemical properties	Increased chl content, relative water content, stomatal conductance, reduced proline content, increased plant growth and productivity.	[118]
Wheat (Triticum aestivum L.)	NaCl, 3000 ppm	Soybean straw BC (5%) (w/v) + selenium (0.15%)	Not observed	Increased biomass assimilation, mineral uptake, chl synthesis, photosynthesis rate. Reduced EL, improved salinity tolerance.	[119]
	Saline water irrigation (10 dS m−1)	Wheat straw BC (10 and 20 t ha−1)	Reduced soil bulk density, increased permeability and nutrient status of soil	Improved growth, photosynthesis and reduced aging of leaves.	[120]
	150 mM NaCl	BC (5%) and jasmonic acid (5 μM)	Reduced accumulation of Na+	Reduced oxidative stress and boosted antioxidant activity.	[121]
	EC = 7.17 dS m−1	BC (2% w/w) + Lysin (1.0 and 2.0 mM)	Reduced soil salinity and increased nutrient availability.	Increased chl a, chl b, total chl, and carotenoid, photosynthesis, reduced MDA, H2O2, and EC, increased growth, biomass, and grain yield.	[122]
Maize (Zea mays)	100 mM NaCl	Wheat straw BC + Arbuscular mycorrhizal fungi	Soil nutrient status improved, and mitigated salinity.	Improved photosynthetic performance, reduced oxidative damage, enhancement maize production.	[123]
	EC = 0.01955 dS cm−1	Mixture of cotton straw, peanut shell, and sawdust (90:5:5, w/w/w) 30, 50 and 75 t ha−1	Increased soil bulk density, soil pore space, macroaggregates, CEC, total carbon, N, P, K and decreased exchangeable Na+ and decreased salinity.	Reduced oxidative stress. improved palmitoleic acid, oleic acid, and linolenic acid contents, increased crop yield.	[124]
Soyabean (Glycine max)	Salinity (5 and 10 dSm−1)	BC (50 and 100 g kg−1 soil)	Enhanced nutrient availability and lower Na+ content.	Improved leaf chl (a, b, c) content	[113]
Mungbean (Vigna radiata L.)	Salinity, 5 and 10 dS m−1	BC (50 and 100 g kg−1)	SOM status improved and salinity stress mitigated.	Improved xylem structure, decreased ABA and ACC, increased root/shoot ratio, total root area, and plant growth.	[125]
Sorghum (Sorghum bicolor L.)	Salinity, 0.8, 4.1, and 7.7 dS m−1	BC, 2.5, 5, and 10% (w/w)	Decreased soil degradation and reduced salinity.	Increased plant height, DM RWC, crop yield, and mineral availability, decreased osmotic stress.	[114]
Quinoa (Chenopodium quinoa L.)	Saline water irrigation (400 mM)	BC, 5% (w/w)	Increased soil water content, nutrient availability and reduced Na+	Increased plant height, shoot biomass, and grain yield, increased leaf photosynthetic rate and stomatal conductance, maintained ionic balance.	[126]
	Salinity (20 dS m−1)	BC (1%) w/w and Endophytic bacteria	Reduced soil salinity and increased nutrient availability	Reduced antioxidant activities, increased plant growth, grain yield and grain nutrient content.	[127]
	Salinity, 11.5 dS m−1	Cotton shell BC 1 and 2% (w/w)	Reduced soil salinity.	Reduced Na+ induced phytotoxicity and increased yield. Increased plant growth, water contents, stomatal conductance, and chl contents.	[128]
Potato (S. tuberosum)	25 and 50 mM of NaCl	BC, 5% (w/w)	Reduced soil salinity.	Increased photosynthetic rate, stomatal conductance, relative water content, increased shoot biomass, root length, and tuber yield.	[129]
Tomato (Solanum lycopersicum)	Salinity level 0.3% and 0.6% of soil dry weight salts	BC, 1% of soil dry weight	Not observed	Increased total soluble solids and vitamin C in tomato.	[130]
	Salinity (1, 3 dS m−1)	BC (2, 4, 8%)	Released mineral ion K+, Ca2+, and Mg2+ in soil solution, increased organic matter.	Increased vegetative growth and production.	[131]
Cabbage (Brassica oleracea var. Capitata)	150 mM NaCl	BC doses (weighed at the rate of 2.5%, and 5% by soil weight)	Reduced salinity stress.	Reduced oxidative stress, ABA content, Na+ content and increased growth of cabbage seedling.	[132]
Jute (Corchorus capsularis)	50, 100, and 150 mM NaCl	BC (2.0 g kg−1 soil) + Chitosan (100 mg L−1)	-	Improved enzymatic and non-enzymatic antioxidant systems, enhanced glyoxalase enzyme activities, increased Na+/K+ ratio, reduced oxidative stress, plant growth improved.	[133]

). This replaces and releases essential mineral nutrients to the soil and boosts its OM content [113,114]. Biochar applications also mitigate soil compaction, encourage evaporation, and reduce salinization, making it a potent remedy for reducing soil ESP [114]. The addition of cow dung, PM, also reduced soil ESP and EC and notably increased soil Ca²⁺ status and improved soil salinity [115,116] (

Table 4. Salinity reclamation through PM, press mud, sewage sludge, cow dung, and FYM.

Plant Species	Stress Level	OA Doses and Application Method	Effect of Amendments		Reference
			On Soil	On Crops
Rice (Oryza sativa)	Salinity, 6.4 dS m−1	FYM (5 to10 t ha−1) + PM (4 to 8 t ha−1 + proline	Reduced soil salinity.	Increased nutrient uptake, plant height, panicle length, grain yield, and straw yield of rice, decreased K+/Na+ in both grain and straw.	[134]
Wheat-Maize	Salinity, 5.4 dS m−1	PM + FYM + GM (12 t ha−1)	Increased CEC, total N, soil carbon, reduced soil EC, pH, and SAR, improved soil structure.	Increased crop growth, biomass, and grain yield.	[135]
Rice–wheat	Salinity, 3.6 dS m−1	Sugarcane press mud (10 t ha−1)	Reductions in soil pH, ESP, reduced soil salinity.	Enhanced leaf water potential, membrane, reduced membrane injury stability, Na+/K+ accumulation, increased photosynthetic efficiency, plant growth, and yield.	[136]
Wheat (Triticum aestivum)	Salinity, 11.72 dS m−1	Sugarcane press mud 10–15 g kg−1	Increased SOM, improved microbial activity, enhanced nutrient availability, reduced soil salinity.	Increased nutrient availability in rhizosphere, fertile tiller, plant biomass production, and plant growth, grain yield.	[137]
Wheat (T. aestivum)	Salinity (6, 12 dS m−1)	Sugarcane press mud (3, 6, and 9%)	Improved soil properties, increased Ca2+ and K+ in soil, leaching of Na+, improved salt induced toxic effect.	Increased chl content (a, b, and total chl), soluble sugar, proteins, free amino acids, leaf water content, proline, K+, and activity of antioxidant enzymes; APX), CAT, and POD, rice growth and yield reduced EL, H2O2, MDA.	[138]
Pepper (Capsicum annuum)	Salinity (6 dS m−1)	PM (10% and 30%) with exogenous gibberellins (0, 250 mg L−1)	Decreased EC and osmotic stress in soil solution, increased nutrient availability.	Increased photo synthetic rates, stomatal conductance, total chl, total biomass, leaf N, P, and K, reduced proline and Na content, increased fruit set.	[139]
Potato (S. tuberosum)	Salinity, 0.9 to 5.9 dS m−1	PM (20, 30, 40, 50, 60 mt ha−1)	Decreased nutrient losses and soil salinity.	Increased K, N in leaves and roots, growth, yield, and nutritional status of tuber.	[140]
CherryTomato (Lycopersicon esculentum)	Salinity, 0.44 mS cm−1	Poultry manure (0. 25, 31, 38, and 44%)	Improved soil properties, soil available nutrients and reduced salinity.	Increased plant height, root length, fresh and dry weight, number of flowers and shoot K concentration.	[116]
Saline soil	Salinity, 4 dS m−1	Cow dung (2%)	Improved soil aggregation, Ca2+, reduced ESP and EC, soil pH, Na+	Not observed	[115]

). Again, press mud application in soil improved soil OM and nutrient availability and reduced salinity.

In conclusion, organic additions can significantly improve the mineral nutrient status and growth of plants in saline soils, mainly through the reduced translocation of harmful salts [116].

4.2. Organic Amendments Improve Crop Growth under Salinity

Salinity impedes nutrient availability due to osmotic stress. Organic amendments directly contribute by adding C, promoting microbial cells that counteract osmotic stress through osmoprotectants under various stresses [141,142] (Figure 8). A high Na⁺ buildup inhibits N absorption, thereby reducing protein levels in grains [143]. Organic amendments, on the other hand, enhance SOM content. They provide a substrate for decomposing organisms, resulting in better nutrient assimilation and, consequently, higher grain yields in rice and lettuce [100,101,102,104,106]. The negative effects of salinity were mitigated, and rice root and shoot growth was amplified by the FYM + PM treatment [89].

According to Ahmed et al. [57], incorporating FYM into the soil boosted its OM content and decreased SAR. This replaced Na⁺ in the soil with Ca²⁺, promoting growth, increasing biomass output, and reducing the negative impacts of salt stress on crop growth. The combined application of FYM and PM augmented maize growth [7], while combining FYM with GM crops increased essential soil nutrients, thus improving soil quality and crop yield [135].

Biochar application in saline soil creates a conducive environment for rice seedlings by augmenting nutrient availability and enhancing soil physical properties [117] while also maintaining leaf water potential [125]. Additionally, BC has been shown to boost the shoot biomass of mung bean [125], quinoa [126], and tuber yield of potatoes, as well as elevate total soluble solids and vitamin C in tomato plants in saline soil [3,130]. Again, VC addition in soil increased the growth of sugarcane, bean, potato, fennel, and tomato [101,102,103,104,107].

4.3. Organic Amendments Enhance Relative Water Content, Photosynthetic Pigments, and Reduce EL under Salinity

Salinity markedly decreases crop photosynthesis by increasing the activity of the enzyme chlorophyllase, which breaks down chl. Moreover, chl synthesis in plants is suppressed due to the accumulation of MDA and hydrogen peroxide (H₂O₂), resulting in diminished Mg uptake under salinity stress. However, the use of OAs reduces the accumulation of MDA and H₂O₂ in plants, thereby increasing Mg uptake and, consequently, chl levels [127,143].

Organic amendments enhance SOM content, fostering a favorable soil environment that notably increases relative water content and reduces EL in plants, bolstering their growth [106,136,144]. Press mud, for instance, plays a crucial role in minimizing EL by reducing reactive oxygen species production [89]. Furthermore, a combined application of FYM and PM resulted in increased concentrations of chl a and b, total chl, and carotenoids, higher relative water content, and decreased EL compared to controls.

The introduction of BC to soil has been observed to amplify photosynthetic rates, enhance chl synthesis, and reduce EL in crops like wheat, maize, tomato, and soybean [113,118,122,123,124,125,126,127,128,129,130,131]. Alharbi and Alaklabi [121] have pointed out that wheat benefits from the combined use of BC and jasmonic acid in terms of growth, photosynthesis, and salt stress tolerance. As per Cha-um and Kirdmanee [109], applying GM in conjunction with FYM to RD6 rice grown in saline soil leads to an increase in chl a concentration, overall chl pigments, and photosynthetic capabilities.

4.4. Organic Amendments Improve Antioxidant Activities

Salt stress triggers an excessive generation of ROS, which negatively impacts proteins, lipids, and carbohydrates [145]. Organic amendments help manage these issues by maintaining reduced Na⁺ concentrations and lowering MDA and H₂O₂ accumulations. Applying VC, either through foliar or edaphic methods, bolsters the activity of antioxidant enzymes, namely SOD, POD, and CAT [108]. This leads to a decrease in EL and oxidative stress and benefits maize seedling growth [146]. Furthermore, applications of FYM + PM under salinity stress notably enhance antioxidant activities, elevating CAT and APX levels by 59.9% and 68.8%, respectively. This also boosts grain protein and Fe and Zn contents in rice [89]. Separate studies showed that PM can decrease CAT activity while augmenting nutrient availability in saline soils [139]. Lastly, BC has proven beneficial for plant growth under salinity. It achieves this by diminishing oxidative stress, moderating phytohormone production, enhancing stomatal attributes, and promoting seed germination. It also bolsters microbial activities, which, in turn, boosts maize growth [124].

4.5. Organic Amendments Maintain Ionic Homeostasis under Salinity

Research indicates a pronounced accumulation of Na⁺ and a reduction in K⁺ around plant roots under salinity stress. This elevates the osmotic pressure in the soil solution. However, using OA facilitates a higher K⁺ buildup and curtails Na⁺ accumulation in rice plants. This notably decreases the Na⁺/K⁺ ratio; Ca²⁺ plays a vital role in enhancing membrane integrity [147]. The application of press mud has been found beneficial in improving Ca²⁺ accumulation, reducing Na⁺ accumulation, and mitigating salinity stress [89]. Vermicompost and regular compost applications aid in nutrient assimilation and ion balance [108]. This offers relief to plants from the severe damage inflicted by salinity. In BC-treated soils, there is a notable improvement in the K⁺/Na⁺ ratio, a reduction in Na⁺ and Cl⁻ contents, and an augmentation of abscisic acid and plant nutrient contents, as observed in cabbage seedlings [132].

4.6. Organic Amendments Increased the Yield under Salinity

Organic amendments boost crop yields by an average of 27% compared to mineral fertilization [148]. Biochar, due to its high adsorption potential, can mitigate EL, even in situations with high salt concentrations. This is achieved by decreasing Na⁺ absorption [113,118,120,122,123,124,125,126,127,128,129,130,131]. This results in increased plant height, total biomass, and overall productivity of sorghum, rice, and wheat [98,114,117,118,119,120]. According to She et al. [131], BC, through its adsorption of Na⁺ ions, releases other essential minerals like K, Ca, and Mg into the soil solution. This helps reduce salt stress and augment tomato production. Hafez et al. [118] observed that BC treatments markedly reduced Cd and Na⁺ uptake in plants, subsequently enhancing the growth and photosynthesis of rice. Moreover, a combination of BC and selenium–chitosan nanoparticles have shown promise in protecting wheat plants from salt damage and increasing plant growth and production by restoring nutrient balance, ionic homeostasis, and C assimilation [119].

Naveed et al. [127] found that the combined application of BC with endophytic bacteria significantly boosted the grain yield and grain quality of quinoa. On the other hand, when used on its own, BC improved Na⁺-induced oxidative stress and increased quinoa’s grain yield [128]. Using FYM alongside PM application noticeably improved the yield traits of the rice crop by 9.83% and 15.58% [89,134]. Moreover, PM application was found to enhance wheat production and P absorption [135]. The highest concentration of vital macronutrients in rice grain, wheat, and pearl grain was consistently associated with compost treatment [105,106]. Olive yield, when considering pomological character, also saw an uptick with compost application [56,149]. Savy et al. [56] reported that applying compost effectively countered salt stress in tomato plants, favoring metabolite accumulation compared to mineral fertilization. In addition, the introduction of Sesbania GM and sunhemp notably elevated cotton production in salt-affected soils [72]. Ghafoor et al. [111] opined that Sesbania green manuring yielded better results for wheat compared to rice under saline conditions. Sesbania GM notably enhanced the dry matter in aerial parts and the grain yield of wheat in saline soils [20]. The combined application of PM with BC lowered salinity and increased the total dry matter and yield of pulse crops [89]. In contrast, applying only PM boosted rice and tuber growth and yield in semi-arid regions [134,140]. Furthermore, the application of gibberellic acid (GA) combined with PM invigorated growth and increased pepper’s resilience against salinity stress [139]. An emerging and promising aquatic extract, DW, functions as a biostimulant. This extract has been found to augment leaf chl content, enhance the essential nutrient composition of olive plants [78], and decrease salinity in industrial wastewater [112]. Another substance, water hyacinth, is showing potential for the desalinization of seawater [81].

4.7. Alleviating Metal Stress through OA

Organic amendments contain humic acid, which binds metal particles, including Cd, Cr, Cu, and Pb, rendering them immobile [9]. According to Ho et al. [150], carboxyl and oxygen groups are responsible for lessening HMs through the ion exchange process. Consequently, these HMs transition from being highly accessible to being less bioavailable through organic OA (Figure 9). These amendments, such as compost, BC, cow and pig dung, and FYM, reduce the mobility and uptake of HMs in soils. This has resulted in significant reductions in metals like Pb, Co, and Cr, which subsequently benefitted the growth of cabbage, bean, wheat, Spinacia oleracea, Brassica, and lettuce [79,86,141,151,152,153,154] (

Table 5. Metal mobility on soil through different organic amendments.

Crops	Heavy Metal	Organic Amendment Used	Effect on Soil and Crops	Reference
Rice (Oryza sativa)	Cd (5 mg kg−1)	Steel slag (3 gm kg−1)	Increased soil pH, Si, Ca concentration I roots, decreased Cd content, improved crop growth and grain yield.	[61]
Black gram (Vigna mungo)	Cd contaminated soil (10 and 20 mg kg−1)	Cow manure (5%), sugarcane press mud (5%), and (cow dung + PM)	Improved photosynthetic pigments, leaf water status, reduced hydrogen peroxide production, EL, MDA accumulation, and increased accumulation of soluble protein and free amino acids	[86]
	Cd (0.2 mg kg−1) and Cr (2.75 mg kg−1)	BC, 1.5% (w/w)	Reduced soil Cr, Cd concentration, increased available carbon, microbial activity, plant growth.	[151]
Maize (Zea may)	Cd (2.5, 5) mg kg−1	Compost with BC (0.50, 0.75, and 1.00%)	Improvement in soil organic matter, plant height, root length, number of leaves, leaf fresh and dry weight, plant fresh and dry weight, chlorophyll a, b, and total, and carotenoids.	[53]
Rice (O. sativa) and Wheat (T. aestivum)	Pb (54.39 mg kg−1) Cd (0.83 mg kg−1)	BC and green stabilizing agent Poultry manure (34% and 25%)	Reduced Pb concentration accumulation in wheat and rice roots, shoots, and leaves. Increased biomass and yield.	[152]
Cacao (Theobroma cacao L.) beans	Cd (5 mg kg−1)	Compost + zeolite (0.5, or 2%)	Increased soil pH and reduced soil Cd concentration.	[65]
Brassica chinensis	Cd (1 mg kg−1)	Cow dung + cow dung derived Biochar (3.0 and 6.0% w/w)	Decreased cd availability, increased trace elements and biomass production	[141]
Mustard (B. juncea)	Cd, Cu, and Pb (5, 160, and 1000 mg kg−1)	Wood BC (1%)	Reduced toxicity of metals and increased nutrient availability.	[158]
	Ni (50 mg kg−1 and 100 mg kg−1)	BC with muscle cell (1 g 250 mL−1 volumetric glass)	Reduced Ni bioavailability, increased plant biomass, chl content.	[49]
	Cd (1 mg kg−1), Pb (74.4 mg ha−1)	Rice husk BC (0.5, 1, and 2% w/w)	Reduced phytoability of metals.	[159]
Wheat and Maize T. aestivum Z. mays	Cd (5, 20, 50 mg kg−1 soil)	Compost and Biogas slurry, 15 t ha−1	Total dry biomass increased; Cd concentration reduced.	[3]
Pakchoi (Brassica chinensis L.)	Cd (50 mg kg−1 soil)	Poultry manure compost (120 g kg−1)	Increased soil pH, reduced Cd concentration in soil, favored antioxidant capacity dissolved OM.	[156]
Duckweed Extract	Cr (1.2 μg L−1) Ni (0.9 μg L−1), and Co (0.5 μg L−1) concentrations	Lemna gibba and L. minor	Cr, Co, and Ni concentration reduced.	[160]

). Compost application not only elevated soil pH but also bound metal particles with SOM and caused them to co-precipitate with soil P [155]. Additionally, compost application cut down Cd accumulation by 97.8% in wheat grain and up to 50% in Brassica fields [155,156] and promoted the growth and quality of Brassica napus, simultaneously reducing the stress ability of MDA and antioxidant enzymes to HMs [157]. Ahmed et al. [3] found that compost and biogas slurry could neutralize and stabilize Cd, thus mitigating its adverse effects on the growth and dry biomass production of wheat and maize. Furthermore, amended compost applications have been observed to bolster the growth of rye grass by removing metals like Cd and Pb from its stem [154].

Mining soils laden with a plethora of HMs adversely impact radish growth and yield. However, organic fertilizers, including VC and C, by forming stable metal complexes, not only enhance radish growth but also minimize risks to human health [48]. Biochar applications form stable metal complexes, drastically cutting down Cd concentrations in various crops such as rice, wheat, maize, and Brassica [58,140,159,161].

Biochar has been found to augment plant biomass production and diminish the concentration of HMs in plant tissues. Additionally, it reduces copper uptake in soils polluted by Cu mines [162]. The use of soybean and rice straw BC significantly decreased Cd content in various parts of the rice plant, including the roots, shoots, husks, and grains [163]. The combined application of BC and compost considerably enhanced SOM as well as the content of leaf chl a and b and carotenoids [153]. Conversely, pairing BC with chicken manure boosted the height, biomass, and enzyme activity (SOD, POD, and CAT) of maize plants while reducing MDA content [153]. Sewage sludge, due to its OM, acted as a metal chelator, lowered metal concentrations in the Sorghum bicolor crop, and significantly promoted crop growth and soil quality [164].

Cow manure has proven effective in cutting down on metal concentrations, elevating soil pH, and boosting soil nutrients [116]. As per the researcher, FYM substantially curtails Cd and Pb contents in both the shoots and roots of amaranth cultivated in sandy soil. Green manure crops also reduced Cd content in the aerial parts and Cu concentrations in the roots of Indian mustard [165].

Certain aquatic plants have shown a significant ability to decrease metal concentrations [160]. Water hyacinth (Eichhornia crassipes), DW (Lemna minor, Spirodela intermedia), and water lettuce (Pistia stratoites) are used for the phytoremidation of HMs (As, Ca, Pb, and Hg) from wastewater through their extensive root systems [166]. Again, industrial hemp (Cannabis sativa L.) can accumulate soil metal and metaloid in its shoot through the root system and form a stable complex, thus reducing soil metal pollution [167]. Due to their influence on adsorption, complexation, reduction, and volatilization processes, these OAs can serve as a means to reduce the bioavailability of metal(loid)s in polluted soils and sediments [158]. The ways in which OAs improve metal-contaminated soils are illustrated in Figure 9.

5. Conclusions

The significance of OAs in reducing the bioavailability of HMs and alleviating salinity stress in soils, along with their impact on soil quality, is highlighted in this review. Salinity and HM contamination are primary abiotic stresses that diminish crop productivity and can render soil infertile for extended periods. Such conditions adversely affect the overall GDP. Organic amendments are rich in OM, which is readily soluble and available for soil microbial activity. Increased microbial activity results in the production of more carbon dioxide, which can displace sodium ions from the soil solution, thus aiding in saline soil reclamation and diminishing metal stress. The application of VC, BC, and FYM has been observed to mitigate the detrimental effects of salinity and metal concentrations. This review reveals that VC, BC, and FYM boost antioxidant enzyme activities, stabilize ionic balance, alleviate osmotic and oxidative stresses, and modulate gene expression, collectively supporting enhanced plant growth and productivity. In summary, the incorporation of the aforementioned OAs is a promising strategy to bolster soil fertility and productivity for field crops cultivated in salt-affected terrains. Agriculturists and farmers in regions with salt-impacted soils can adopt this method to increase crop yield and decrease crop failure due to soil salinity and metal contamination.