Soil Amendments: An Ecofriendly Approach for Soil Health Improvement and Sustainable Oilseed Production

Abstract

Oilseed crops are major part of human diet providing energy, used for cosmetics, health supplements and other purposes. Intensive agricultural practices, overexploitation of natural resource and climate change pattern have adverse impact on soil health, thus becoming serious concern for oilseed crop production and livelihood security of farmers. Maintenance of soil health with amendments can restore, revitalize and regain the soil quality for sustainable agriculture. Soil amendments, therefore have definite advantage by improving soil health and facilitating nutrient supply to oilseed crops. Soil organic amendments such as animal manure, compost, vermicompost, biosolids/sewage sludge, biochar etc. and inorganic amendments such as gypsum, zeolite, pyrite etc. are the most commonly available amendments which can be directly applied to soil after treatments. Direct and indirect effect of soil amendments on soil chemical, physical and biological properties significantly influences soil-plant-continuum, beneficial for soil health improvement, carbon sequestration and oilseed yield improvement. Soil organic amendments could substitute nearly 25–50% of synthetic fertilizers, enhance nutrient use efficiency and influencing oilseed yield response. Soil amendments may sustain or increase oilseed productivity at reduced production and environmental cost, thus, improve soil health and water use efficiency and its quality, and mitigating climate change impact.

1. Introduction

Soil degradation is the major obstacle for sustainability of crop production and human survival. With deteriorating climate change effects, of erratic rainfall patterns, sudden increase in rainfall intensity and temperature fluctuation around the world is a serious challenge for farmers, environmentalists and common man. Increasing pressure of human population, that has been estimated to rise up by 9.5 billion by 2050 would increased rastically food demand [1]. Food insecurity due to changing climate change vagaries and increasing population pressure is challenging scenario for researchers and policy makers. Furthermore, nutrients bioavailability, environmental factors as well as the biological soil health are other important criteria for improving crop yield per unit area for achieving the targeted goal of food security. Agriculture management strategies for improving crop growth and yield is usually achieved with genetic manipulation higher fertilizer doses, faulty irrigation practices, pesticides, weedicides etc. [2, 3]. Subsequently, few management techniques led soil health deterioration due to low soil organic matter, micro-nutrient or specific nutrient deficiency, biodiversity loss, persistence of chemicals in soil system etc. These factors are responsible for soil/land degradation which is a major problem for declining agricultural productivity [4]. Soil degradation adversely influence soil properties by productivity losses directly hampering human needs and capacity to perform various crucial service and functions, which are valued between US $1610 to US $19,420 ha⁻¹ y⁻¹ in organic farms and between US $1270 to US $14,570 ha⁻¹ y⁻¹ in conventional farming system [5]. Therefore, there is an urgent need to reverse soil degradation and improve soil properties to recover soil health in sustainable manner. Sustaining crop productivity and soil quality by site specific management strategies is essential, to conserve natural resources for the future generations.

Soil health improvement is pre-requisite for sustaining soil health and crop productivity. Agricultural management practices can reduce delivery of normal ecosystem services and functions, for instance by intensive use of chemical fertilizers and pesticides can increase nutrient losses, polluting water bodies (eutrophication) and aquatic organisms [6]. This could incur additional cost on water purification, decrease the esthetic and recreational values of water reservoirs. Thus, alternative solutions for reducing dependency on costlier chemical inputs in agricultural could be long term solution for maintaining and restoring soil fertility. Addition of organic and inorganic materials so as to improve soil properties so as to sustain crop production is a process of amending the sick soils. A healthy soil in arable system constitutes a good balance of organic and inorganic components. Such soil are usually characterized by higher biodiversity and lower concentration of inorganic and organic nutrients [7]. Rejuvenating soils by amendments with easily available products and environmentally safe is essential for improving soil health conditions. Soil amendments used as soil conditioners tom improve physical, chemical and biological properties so as to enhance crop productivity and livelihood security of farmers. The organic managed fields, with no application of chemical fertilizers and pesticides, are closer to natural soil than conventional fields, where soil fertility is maintained by consistent application of organic amendments. Generally, addition of organic amendments such as FYM, composts, animal manures etc. significantly increases SOC, on contrary to chemical fertilizers which have opposite effect [8, 9]. Biochar are carbon rich stable inert compounds produced from biomass pyrolysis that enhances soil carbon, positive impact on soil properties, crop yield and environment, are included in agroecosystem over a decade or two [10, 11]. Crop diversification by including oilseed crops (soybean, groundnut, mustard, sunflower etc.) in rotation or intercropping in conventional farming system are better options for withstanding climatic conditions, reducing irrigation and costlier agrochemicals, natural resource conservation, environmental safety and ensuring livelihood security of farmers [12].

Oilseed crops are the fourth most important category of agricultural produce globally next to cereals vegetables, fruits and nuts [13]. Oilseed crops contain triacylglycerol content making them important energy source containing bioactive compounds such as phytosterols, fat soluble vitamins, carbohydrate, phenols etc. [14]. Oilseed crops can be annual such as soybean, mustard, sunflower, groundnut, castor etc. and can be perennial such as coconut, oil palm, olive etc. Among oilseed crops, soybean and rapeseed/ mustard are the most globally produced oilseed crops [14]. Oilseed crops also known as vegetable oils is the most preferred crop as they are easily available, economically cheap and are cultivated in various agroclimatic regions around the world [15]. The bumper oilseed production during 1990 lead to ‘yellow revolution’ in India, however, could not sustain for longer period. Oilseed crops have the potential of improving productivity and profitability in various oilseed-based cropping systems. Unlike cereal crops, oilseed crops most grown under rainfed conditions are more tolerant to harsh weather conditions. However, few oilseed crops have higher market price, wider adoptability and provide optimum yield under stress conditions [12]. Poor nutrient management, excessive use of sulfur free fertilizers, micro-nutrient deficiency, salinity stress, moisture deficit, etc. are some major constrains in oilseed production. Soil nutrient mining and water depletion is the two most important constraints in oilseed production followed by pest and disease [16]. Soil management with soil amendments could be a helpful technique in improving oilseed yield and quality without deteriorating soil health. Application of chemical fertilizer or organic manure alone cannot sustain oilseed production. Therefore, judicious use of organics with inorganic fertilizers are essential to augment productivity, input use efficiency and soil health protection. Efficient management practices of oilseed crops includes higher production and processing oil with improved technology governs the economic health of the country. In India, oilseed crop is the second most category after cereal crops governing the agricultural economy, growing at 4.1% per annum during last three decades [17]. Most commonly grown oilseed crop includes soybean, rapeseed, canola, groundnut, sunflower, oilpalm, coconut, olive, cottonseed etc. Oilseed crops are known for their oil quality and quantity (yield) which determines the consumers choice and farmers profitability. Oil quality is characterized by its chemical components which includes, fatty acid, active compounds, microelements, vitamins and flavoring substances [18, 19]. Beside these components, oils are rich source of vitamins like A, D, E, K, in which vitamin E with antioxidant properties can remove free radicals in cells that can cause aging and cancer [14].

2. Oilseed area and production

Globally among oilseed crops, largest production is recorded in soybean followed by canola/ rapeseed, sunflower, groundnut, cottonseed, oilpalm and copra. Some of the common oilseed crops, their oil content and uses are explained in Table 1. During the year 2021–2022, soybean production was 364 metric million tonnes. On consumption level, soybean oil is the most preferred, followed by canola and oilpalm [20]. Among vegetable oils, palmoil records highest annual yield and is continuously increasing in the recent years. South-Asian countries like Malaysia and Indonesia contributes to 85% of production. Soybean followed by rapeseed is the next important oilseed crop with higher yield [14]. Major producers of soybean includes China, USA, Argentina, Brazil etc. Countries like China, Canada, EU are major producers of mustard crop during 2017–2018. China and India are major producers of groundnut oil. Sunflower is a major oilseed crop in Ukraine, Russia and EU. South East Asian countries, Philippines are major producers of coconut oil. Approximately, 25 million hectares globally is under sunflower cultivation, accounting for almost 8% of world oilseed market [21]. Mediterranean countries Spain, Greece, Turkey accounts for 90% olive oil production worldwide [14]. India is the second largest consumer after China in vegetable oil consumption for food. Countries like Argentina, United States of America and Brazil are the major soybean exporters to India and China. However, in the recent year of 2021–2022, soybean oil imports in these countries are slowed down due to higher prices of soybean oil and shift towards other oilseed crops like rapeseed especially in India. The record harvest of rapeseed crop in India during 2021–2022, forecast the bumper production of 800,000 tons can reduce the import of oilpalm and sunflower [22].

Oilseed crop	Family	Oil percentage (%)	Type of oil	Uses
Soybean	Fabaceae	18–24	Vegetable oil	Protein source, Cooking oil, Flour, Pharmaceutical industry
Rapeseed	Brassicaceae	37.5–46.3	Vegetable oil, Diesel oil	Animal feed, Biodiesel
Peanut	Fabaceae	46–57	Cooking oil	Cooking, Cosmetics, Dyes, Textiles, peanut butter
Sunflower	Asteraceae	46–50	Seed oil	Cooking oil
Palmoil	Arecaceae	50–55	Vegetable oil	Cooking oil, Cosmetic industry; Detergent
Coconut	Arecaceae	65–74	Vegetable oil and biofuel	Cooking oil, Cosmetic and Food industry; Chocolate Beverages, Vinegar
Sesame	Pedaliaceae	43–61	Vegetable oil	Cooking oil
Cottonseed	Malvaceae	15–40	Seed oil	Detergents, Cosmetic industry; Insecticides
Safflower	Labiatae	About 40	Vegetable oil	Cooking oil
Olive	Oleaceae	31–56	Cooking oil	Cooking oil
Castor	Euphorbiaceae	30–50	Vegetable oil	Additive in food; Skin care products; Biodesel
Niger	Asteraceae	37–47	Vegetable oil	Cooking oil; As medicinal use in Asthma and reducing inflammation

Table 1.

Major oilseed crop and their uses.

Source: [14, 19].

According to an estimate by USDA [22] India consumes nearly 21.8 million tons of oilseed in 2021–2022. Among oilseed crops, highest vegetable oil for food use followed the order, oilpalm> soybean> rapeseed> sunflower> other oils during 2017–2018 to 2021–2022. India and China are major vegetable oil importers to meet their domestic demands. Globally, India being the fifth largest oilseed economy accounting for 7.4, 5.8, 6.1and 9.3% in oilseed, oil, oil meal and edible oil consumption respectively [23]. Global production of edible oils and major exporters and importers are given inTable 2.

Edible oil	Global production (million MT)	Major oilseed producing countries
Palm oil	73.49	Malaysia, Indonesia,
Soybean	56.97	USA, Argentina, Brazil, China
Rapeseed	27.96	European union, China, Canada
Sunflower	19.45	Ukraine, Russia, EU
Groundnut	5.57	China, India
Cottonseed	4.09	China, USA, India, Egypt, Uzbekistan
Coconut oil	4.09	Philippines, South-east Asian countries
Olive oil	2.04	Spain, Italy, Greece, Turkey

Table 2.

Oilseed producing countries and their production worldwide.

Source: [14, 24].

3. Soil amendments for soil health improvement

Sustainable crop production and improving soil quality is a major concern which need development of management strategies without negative effect on environment can lead to food security and natural resource conservation. In this approach, soil organic matter plays indispensable role which is directly related with soil ecosystem services and functions for long term oilseed productivity. Soil management for sustainable agriculture can be achieved by improving soil organic matter/ organic carbon of soil through organic amendments addition to soil at regular time intervals. Thus, soil organic matter will help to conserve or restore soil fertility to meet present and future food requirement, with acceptable impact on environment [25, 26]. Soil health is directly related to soil and crop productivity and is being recognized as a major component for mitigating climate change effect and food security. Soil health is closely associated with soil quality, in which the biological health of the organism in soil is critical for soil resiliency and ecosystem services [3]. Constant decline in soil health post green revolution in many countries are growing challenge for stakeholders to sustain oilseed productivity. Inorganic fertilizer application is effective to increase oilseed yield might be short term, but require long term additions. Increasing cost of chemical fertilizers are uneconomical for marginal farmers and creating environmental problems. Therefore, its essential to select locally available organic resources that are easily available, eco-friendly and at reasonable price for farmers application. Based on a recent report by Shukla [27], based on soil sample collected from various Indian states, indicated soils were deficient in sulfur, Zn, Fe, Cu, Mn and B by 41, 43,14.4, 6.1, 7.9 and 20.6% respectively. In soybean and mustard growing belt of North India, most soils are deficient of S an essential nutrient determining oil quality and productivity. Inadequate and imbalanced nutrient management in oilseed crops results in multi-nutrient deficiency in crops and arable soils. Therefore, integration of soil organic amendments for improving soil health by applying various organic sources includes, FYM, compost, vermicompost, biofertilizers, municipal solid waste, agro-industry waste etc. Addition of such organic amendments to soil will not only improve soil fertility, but would improve physical properties and enhance microbial activity and plant growth [28, 29]. Direct and indirect benefits of using soil amendments are illustrated in Figure 1. Soil amendments acts as soil conditioners by alleviating stress, improving soil properties and fertility, enhances ecosystem services and human health, showing minimum impact on environment.

Figure 1.

Potential benefits of soil amendments.

Stagnation in oilseed crop yield in many areas of the world has been attributed to suboptimal supply of nutrients, poor or no application of organic manures, negligible use of soil amendments, erratic rainfall pattern etc. Soils with low organic matter coupled with low native P, sulfur deficiency and micro-nutrient unavailability are some of the major constraint limiting oilseed yield. Best nutrient management practices, therefore, include integration of organic amendments with recommended chemical fertilizer doses to supply macro and micro nutrients to facilitate crop nutrient demand. Integrated nutrient management signifies the role of organic amendments in partially replacing inorganic fertilizers will not only supply essential macro and micro nutrients to crops but would regulate, water, air, temperature, nutrient transformation and biological activities [26]. Major challenges for maintaining sustainable crop production involves use of tools and techniques to enhance agricultural productivity ensuring food security, with minimum disturbance to environmental systems. Many ecological interventions have been addressed to improve the delivery of ecosystem services and functions, by reduce anthropogenic inputs in agriculture. With more emphasis on circular economy paradigm shift, decreasing the dependency on external inputs, reuse, recycling of the available resources at farm aimed at preserving and protecting soil ecosystem. Soil amendments could thus be effective strategy in gaining momentum towards natural or organic farming replacing costlier chemical inputs. Organic amendments such as manures, crop residues (CR), composts, green manures, biosolids, agri-processing industry wastes, industrial slurries etc. are usually applied to soils either as fertilizers or used for ameliorant and remediation purposes [30]. Biofertilizers commonly known as microbial consortia’s or micro-organisms are beneficial microbes used for improving soil health and productivity. They are also included under organic amendment category known as biofertilization techniques [31]. Soil health is defined as the capacity to perform various functions to support plants and organisms. Recovery, restoration and conservation of soil health is utmost important for survival of living beings. Reviving soil health with use of soil organic amendments is gaining much attention due to its cost-effective and eco-friendly approaches. The potential benefits of soil amendments on soil properties are shown in Figure 2.

Figure 2.

Soil amendments influence on physical-chemical and biological properties of soil.

4. Types of soil amendments

Soil amendments are generally added to soil for reclamation process which improve soil physical, chemical and biological properties. Soil amendments can be inorganic and organic resources. The most preferred soil amendments include natural minerals like gypsum, pyrite, lime; other amendments from biological origin such as animal manures, compost, vermicompost, farm yard manure etc. are organic in nature. Organic amendments generally used in arable lands are derived from agriculture, forestry sources and urban areas. In agriculture, animal manure is the most common source of amendments from animals, like cattle, buffalo, goat, poultry etc. Inefficient utilization of nutrients (viz., N, P, K, etc.) and rainwater by oilseed crops are mostly associated with low soil organic matter [32]. Therefore, addition of organic amendments stimulates microbial activity which releases organic acids and other metabolites that reduce nutrient sorption and enhance availability of nutrients (P, K). Addition of inorganic amendments such as gypsum, clays like bentonite, perlite etc., lime, sodium chloride, epsom salt, sulfur minerals etc. are allowed in soils reclamation with some restriction and are mostly based on site specific management practices. Thus, the two most common soil amendments used in agriculture are described below.

1. Organic amendments: Organic amendments are usually derived from live sources and natural sources of carbon for micro-organisms. Soil organic amendments are generally derived from variety of sources such as plant origin, animal manures, crop residue or waste parts, green manures, forest system, urban wastes, agro-processing wastes etc. Among these the most common organic amendments used in agriculture consists of animal manure which can be fresh, composted, liquid or solid fractions etc. from various animal species. Other organic resources include crop residues such as straw, grain husk, legumes, sometime mushroom spent wash. Forest origin amendments mostly includes, wood shavings, wood chips, sawdust, deink sludges, leaves etc. Industrial wastes such as coir-pith waste, sericulture waste is known to tremendous amount of nutrient content. Urban wastes mostly includes the organic fraction of municipal solid waste, sludge waste, industry waste are rich source of nutrients but are used with restriction as organic amendments in crop fields [33]. Interestingly, biochar is yet another and famous organic amendments gaining popularity in agriculture and other fields. Addition of microbial inoculants or biofertilizers are effective amendments for improving oilseed production by enhance biological N fixation in legume oilseed crops, uptake of nutrients, efficient utilization of water directly influencing yield. Treating legume oilseeds with rhizobia and P fertilizers improved soybean grain yield because P availability enhances N assimilation from atmosphere by root nodules [34].

2. Inorganic amendments: Inorganic amendments are either mined or man-made in nature. Inorganic amendments most commonly used as soil conditioners are lime and gypsum. Beside these, basic slag, sulfur, perlite, bentonite clays, Epsom salt, sodium chloride, etc. are other inorganic source of amendments. These minerals naturally occur in earth and can be used to improve soil physical chemical and biological properties especially by adjusting soil pH. Amendments such as gypsum are commonly used to rehabilitate saline-sodic soil supply S nutrient and alleviate salt stress. Gypsum is oldest soil amendment, important source of Ca and S commonly used for crops. Besides natural mine gypsum, flue gas desulfurization (FGD) gypsum is also used as soil amendment available as by-product from coal-fired power plant to remediate saline-sodic soils [35].

4.1 Commonly used soil amendments in agriculture

Utilization of organic amendments as soil conditioner/ fertilizers though is old, but increasingly become popular worldwide not only to reduce chemicals but are a sustainable, eco-friendly approach. Soil amendments such as manures, compost, mineral/organic fertilizers, biochar, biosolids, microbes etc. generally improve plant growth and soil health [8, 36]. Reutilization of organic waste is also socially acceptable approach which could be better substitute to landfills, incineration and directly contribute to SDG goal 3 (Good health and wellbeing) and 6 (Clean water). Moreover, to combat soil degradation, a circular economy concept is a sustainable approach which had recently gained global attention. Organic farming could rejuvenate, and restore soil health, thus improving soil productivity by altering physical, chemical and biological properties. In this chapter, therefore, more emphasis is given to the role and importance of organic amendments on oilseed crops. Farmers are therefore, encouraged to integrate organic amendments in their farming system which can reduce chemical input cost and improve soil quality for long term cropping. Some of the commonly used organic amendments/ manure used in agriculture are described as follows.

1. Animal manures: Animal or livestock manures are the easily available organic amendments and used by man since decades in agriculture. Major limitation with use of manure is due to their large volume, bulky nature, high moisture, and transportation cost. The quality of manures in terms of nutrients and other components depends upon the feeding stock, animal type, quantity of dung or litter produced, their management practices etc. Poultry manure are considered rich source of nutrients compared to animal manure. Cattle manure on dry weight basis is characterized by 0.56–4.24% of ammonia N, 1.18–2.9% of organic N, 42 ± 6% of Carbon and 11.7 ± 5 C:N ratio [37]. Integration of animal manure with chemical fertilizers increases soil organic carbon, thus maintaining soil fertility for longer time.

2. Composting: Composting is the most common sustainable technique of oxidative disintegration of biomass with micro-organisms. Composting provides good quality end products which are cost effective, environmentally safe and enhances crop production. Animal manure composting is an effective method of nutrient enriched stable compounds, decreased mass and water content, supressed weed seeds and pathogens etc. The quality of compost obtained depend upon various factors such as the feedstock, aerobic/ anaerobic decomposition, heavy metal content etc. Therefore, to improve efficiency of composting addition of various additives such as zeolites, biochar, clay minerals, phosphate sources, gypsum, micro-organisms etc. are used for crop production.

3. Vermicompost: Composting techniques in which earthworms are used for degrading and accelerating biodegradable substances is vermicomposting. The most commonly used species of earthworm are Eisenia fetida, and Lumbricusrubellus are the most efficient species. Compared to normal composting, vermicomposting is faster due to worm activity producing compost rich in nutrients and plant growth regulators.

4. Crop residues: Most of the developing countries generate more than 1000 million tonnes of crop residues, which is either burnt in-situ or part of it utilized as animal feed, or other purposes. These CR if recycled properly in fields can significantly contribute to organic carbon and essential nutrients. CR recycling along with animal manure and fertilizers are economical and environment friendly approaches to improve soil health and decrease dependency on chemical fertilizers. Plant stover/ straw is a major source of humus, and lingocellulosic waste generated, traditionally use as soil amendments and also raw material for composting process.

5. Biofertilizers: Biofertilizers are considered green technology and eco-friendly alternative strategies to fertilizers which can improve plant nutrition and soil health under oilseed-based cropping system. Use of microbial inoculants or consortia of micro-organism that promote plant growth by enhancing uptake and bioavailability of mineral nutrition. Combine application of microbial inoculants are known to give better results compared to single microbial species. Biofertilizers include plant growth promoting microbes by direct and indirect process such as biological nitrogen fixation, phytosiderophores, chelates, sugar/ root exudates, hydrolytic enzymes, hormones etc. which increase bioavailability of nutrients to crops. Chelates released by crop roots complexes with micronutrients such as Zn, Fe improving their solubility and enrich crops with micronutrient content. Plant growth promoting organisms such as Azotobacter, Azospirillium, Rhizobium, Pseudomonas, Trichoderma, Flavobacterium, Streptomyces etc. are important biofertilizers utilized for enhancing crop growth and yield. Seed biopriming, increasing N and P availability, with biofertilizers will have positive influence on yield and yield components. Compared to chemical fertilizers, biofertilizers supply essential plant nutrients, reduces leaching losses and improve nutrient use efficiency. Few biofertilizers recommended for oilseed crops include: (i) Rhizobioum for soybean and groundnut; (ii) Azotobacter for sunflower, safflower, mustard and sesame; (iii) Arbascular mycorrhizae for soybean and sunflower; and (iv) Bacillus for K and Zn solubizers for all crops [38]. Soils amended with microbial consortium thus, enhances crop growth and yield, enhance nutrient uptake thereby reducing the negative impacts of chemical fertilizers.

6. Biochar: Biochar is carbonaceous material produced by heating different biomass at various temperature range under limited oxygen supply for stipulated time. Biochar strongly adsorbs ammonia, phosphate, potassium through functional group, reduce N emission, leaching losses, niche to various micro-organisms. Biochar use as slow fertilizer release components to improve nutrient use efficiency and reduce losses. Biochar is one of most appreciated soil amendments for its potential carbon sequestration capacity, nutrient supply, water holding capacity, aeration, enhanced microbial activity etc. [10, 39] resulting in overall soil health improvement. Compared to biochar alone, combined application of biochar with fertilizer or organic manures/ compost proved better nutrient use efficiency, improved soil properties resulting in better crop yields [36].

7. Municipal solid waste (MSW): Sewage sludge from waste water treatment plants are applied to soil as organic amendments. Approximately 50–70% of biosolids produced is applied to land in these countries. Biosolids or treated sewage sludge are easily available and cheap source of nutrients such as N, P, K, micronutrients and organic matter. Thus biosolid application can improve soil quality by increasing soil fertility and could be safe disposal of treated sludge. However, biosolids contains sometimes heavy metals, or other pollutant and pathogens. Therefore, potential risk associated with application of such amendments should be carefully analyzed before direct application to soils to prevent contamination of soil and water.

8. Organic wastes from agro-industries: Large quantities of organic waste are generated from vegetable markets, agro-based industries, food-based industries, sugar industries etc. Sugarcane bagasse, pressmud, paddy husk, oil waste, jute waste, groundnut shell, cotton stover, tea, fruit pulp waste etc. are different sources of waste generated during processing. Some agro-waste are composted and used as soil amendments showed significant improvement in crop yield. Sugar industry waste such as pressmud is used along with gypsum to alleviate sodicity problems in semi-arid regions [40]. Major constrain with the usage of such organic resources are the transportation costs and environment problems such as presence of heavy metals, pathogens etc.

9. Other organic amendments: Other amendments such as coirpith, seri-waste, tank silt etc. are available as location specific materials for agriculture usage. Coir-pith waste is generated as by-product of coir industries that decomposes slowly due to complex lingo-cellulose complex and are known to reduce soil erosion and enhance water holding capacity [41]. Sericulture waste on otherhand is silkworm litter enrich with nitrogen (280–300 kg), phosphorus (90–100 kg) and potassium (150–200 kg) and more effective than conventional FYM manure [41]. Hence such organic waste can be recycled back to soil to enhance its health.

10. Inorganic amendments: In oilseed crops gypsum, pyrite etc. are the common soil amendments used due to high S demand, which influence crop yield by 10–48% in irrigated and 25–124% under dryland conditions [42, 43]. Gypsum improves soil conditions with better root expansion, exploring more volume of soil, reducing sub-surface Al toxicity, better nutrient and water utilization etc. with positive yield response [44]. Zeolite play important role in improving physical property of soil, increase cation exchange capacity, and reduce N leaching [45]. Zeolite is thus considered as an efficient soil amendment in retaining water and essential nutrients in crop root zone [46].

Soil amendments interacts and modifies soil chemical, physical and biological properties for optimizing oilseed production. However, considerable variability in climate, crop type, management practices, and types of organic amendments significantly impact crop yields. Agronomic evaluation of soil amendments in organic matter supply, plant nutrient availability and soil property modifications determine the potential of amendments in cropping systems. Different sources of soil amendments that can be used for improving oilseed production are explained in Table 3.

Amendments	Availability	Uses	Advantages	Disadvantages
Animal manure	Higher availability; Sustainable supply	Fuel, fertilizer, organic matter (OM) source	Multiple uses; versatile soil amendments	Bulky nature; large volume; loading and transportation problems;
Compost / Vermicompost	Location specific; Availability varies; Competing users	OM source; organic fertilizer for crops	substitute to chemical fertilizers; stable product	Low availability; price fluctuations; low N availability compared to manures
Green manures and crop residues	Location and site-specific	OM and nutrient source	Addition of biomass, organic matter, Nitrogen and other nutrients to soil	Availability of GM seeds; CR could impact tillage operations; High C:N ratio of CR can immobilize nutrients
Biosolids	Surplus availability in urban areas; continuous supply	Organic matter source; essential nutrient source	Multiple uses as amendments; cost effective	Heavy metal and harmful pathogen hazard; High nutrient build up in some location causing pollution
Industrial waste	Site-specific	OM and nutrient source	Cost effective, multiple uses as amendments	Contaminants/ pathogen; transportation cost
Digestate	Highly location specific; limited availability	OM and nutrient source		Variable quality; not fully characterized; odor problem; pose pathogen problem
Biochar	Not easily available	OM source; soil amendment; increasing soil pH	Conversion of biodegradable waste into stable carbon product; improves carbon sequestration; used as liming material; good filtration method for waste water and polluted soils	Initial investment; availability of raw material; complex mechanism
Inorganic amendments
Gypsum	Locally available; industrial by-product	Calcium and sulfur source; versatile soil amendment for salt affected soils;	Easily available, economically feasible; good source of S for crops	Source of gypsum might vary sometimes, by-product such as phospho-gypsum could be loaded with contaminants; radioactive compounds
Basic slag	Location specific; Materials are usually free	Acts as Ca source for acidic soils	Acts as sorbent	Cause NH3 volatilization
Zeolite	Location specific; Naturally available	Improves soil properties such as cation exchange capacity, water holding capacity and high adsorption capacity	Regulate nutrient availability and enhance fertilizer use efficiency; acts as soil amendment; acts as heavy metal trap; slow release of herbicide; water and gas adsorption	Very costly; Not easily available; Research studies are less

Table 3.

Types of soil amendments for oilseed production.

5. Soil properties as influenced by soil amendments

1. Chemical properties: Combined use of organic amendments such as FYM, compost, vermicompost, green manures etc. with chemical fertilizers are essentially required to improve soil health. Addition of soil organic amendments result in carbon accumulation and enhance SOC content in long run for sustaining any cropping system. Organic amendments such as FYM, composts, CR can sustain SOC at higher levels in soil as compared to mineral fertilizers. Soil organic amendments effect pH and cation exchange capacity, which indirectly influence essential nutrient bioavailability thereby direct impact on soil fertility [47]. Composition and maturity of soil amendments significantly influence soil reaction. Some amendments have high Ca or basic cations which results in ‘liming effect’ of soil thus increasing soil pH [48]. Thus liming improves soil microbial activity and diversity in acid soils. On contrary some organic amendments reduce soil pH which is attributed to either humic acids released during carbon pool degradation and /or by nitrification of ammonium in the amendment [49]. Blanchet et al. [50] reported that addition of FYM and CR amendments with reduced mineral fertilizers improved SOC by 6.2 and 2.4% respectively over only fertilizers plots after 50 years of cropping. Application of these organic amendment improved C/N ratio, available P and K content. In soybean crop, addition of 5 t ha⁻¹ FYM with RDF increased grain yield by 13% over the control [51]. Improvement in crop yield with organic amendments application could be attributed to increased N supply directly and indirectly improving soil environment for plant growth. Organic amendments such as FYM, animal manures etc. contain varying amount of N, P, K and other nutrients. Among nutrients, N amount and availability is associated with the type of organic amendments added to soil which undergoes mineralization to provide available N to crops. Organic manures or compost supply N, but slow mineralization by micro-organism limits its availability in short-term. However, with continuous application of FYM, compost etc., builds up N and residual effect on N availability is usually visible after 4–5 years [25]. Depending upon the origin and type of organic resources, variable effect is observed on soil pH. Long term application of organic amendments was found to decrease [52] and increase [53] soil pH in many studies. Chemical properties like cation exchange capacity improves with addition of FYM, biosolids etc. Beneficial effects of FYM and phosphocompost with recommended fertilizer doses in soybean crop on significant improvement on SOC, N, P, K and S availability in vertisol has been reported by many researchers [54]. Organic amendments regulate P and K supply by (i)chelating with Al, Fe, Ca ions thus decreasing precipitation of P and enhancing P availability; and (ii) reduce K fixation, solubilization and release slowly K by organic-clay interactions. Application of biosolid-municipal waste compost once in 4 years improves P supply in soils [55]. Beside enhancing nutrient availability, soil amendments viz., organic manures/ compost improve rain water use efficiency of oilseed crops by increasing moisture retention capacity of soil [56].

2. Physical properties: Soil amendments, both organic and inorganic alter soil physical properties supporting both chemical and biological parameters in improving soil health. Organic amendments, in particular improves soil structure, better soil aggregation, enhance water holding capacity, maintains optimum soil temperature, reduces bulk density (BD), increases porosity favorable for extensive root growth and development to extract plant nutrients. Moreover, organic amendments stimulate microbial activity, secretion of exopolysaccharides improve soil structure, other compounds like hyphae enhances soil aggregation and stability. Addition of FYM with fertilizer dose, decreased BD significantly compared to only fertilizer plots [57]. This decrease in BD is due to higher pore space, organic carbon, better aggregation, root growth, and micropores in soil amendment treated fields. Application of soil amendments/ conditioners significantly decreases soil bulk density and a decrease in bulk density would be expected when soil is mixed with less dense organic material. Addition of municipal solid waste compost improved water stable aggregate and macro pore distribution, leading to higher hydraulic conductivity of clay loam and loamy sand soils. Microbial respiration improvement with addition of MSW compost directly improved soil structural and hydrological properties [58]. In addition, soil rich in organic matter are less prone to erosion processes than soils with low organic matter those that are predominate in arid and semi-arid areas, the reason being organic matter improves soil structure and tilth as well as it cements individual soil particles into larger aggregates, which leads to less runoff and erosion. The amendments can increase the stability of surface soil aggregation, which has ability to resist movement by wind or water and soil pores created by aggregation promotes water infiltration, thereby reducing runoff and transport of soil particles with the water, indicating lower soil erosion. Use of agricultural gypsum is one of the alternative to reduce soil density, on reaction in soil solution it acts as a binder by the cations such as calcium and sulfur, which acts to neutralize soils [43]. Crop residues amendments acts as surface mulch, decreasing evaporation and enhancing water retention in soil. When incorporated, CR acts as carbon source enrich soil organic matter, improves soil aggregation, improves water infiltration thereby reducing soil erosion. Addition of 3 t ha⁻¹ mustard CR with gypsum amendment reduced runoff and soil loss by 26 and 29% respectively in soybean crop [59]. CR retention on soil surface benefits (i) protecting bare soil surface against raindrop slashing; (ii) enhancing aggregate stability; and (iii) prevents compaction of surface soils due to rain drop effect [60]. In another study by Soinne et al. [61] reported biochar addition improved aggregation of clay textured soils reducing negative impact of tillage on soil aggregates.

3. Biological properties: Soil microbes are crucial components for sustaining soil health as they have affirmative role in enhancing nutrients availability by nutrient cycling through the processes of mineralization and immobilization [62]. Addition of organic amendments improve biological properties by stimulating microbial growth, energy, nutrients, indirectly promoting crop growth and development. These amendments bring biodiversity leading to functional changes promoting plant growth and disease suppression. Furthermore, enhancing structural and functions biodiversity reinforce soil ecosystem, which could become tolerant to natural and anthropogenic stress and turmoil [63]. The presence of active, abundant and diverse microbial community in the soil is an important indicator of improved soil health and agronomic production. Moreover, the activity and abundance of diverse soil microbes govern the stability and function of agro-ecosystems. The activity of soil enzymes is a measure of soil microbial health and is sensitive to various factors including climate, temperature, soil moisture, soil temperature, edaphic properties, crop management practices, etc. [64]. However, the chemical degradation (salinization and alkalization) of soils hinders the biological activities viz. mineralization, enzyme activities, microbial biomass carbon and nitrogen, etc. and lowers the crop productivity. Hence, the application of organic amendments i.e. green manures, crop residues, FYM, compost, vermi-compost, etc. is one of the most suitable management options for degraded soils [64]. The potential benefits of organic amendments in stimulating microbial activities by providing energy and principal nutrients are well recognized [47]. The increased supply of nutrients and growth substrates enhance the numbers of ecological niches and promotes ecological interactions between the microorganisms which may affects the microbial diversity and their abundance [65]. Thus, the increased functional diversity of microbes promotes plant growth, strengthen the soil ecosystem stability and promotes resilience against anthropogenic stresses [63]. The positive role of organic amendments on soil microbial activity, their diversity and composition has been reported in several studies. Organic amendments reduce the soil pH and ESP of the calcareous soil by producing organic acids and increasing the availability of Ca²⁺ which exchange Na⁺ of clay particles and create favorable soil environment for microbial multiplication and activity [66]. The application of sesbania green manure increased the soil microbial biomass carbon (SMBC) by 90%, microbial respiration by 104% and DHA by 265% as compare to control along with the increased C sequestration. Similarly, the application of gypsum improved the SMBC by 29.5% over control [66]. The improved biological activities attributed to increased supply of energy from carbonaceous materials, sugar, amino acids and organic acids produced from the decomposition of organic matter and decay of plant roots. Further, the reduced pH and ESP of soil improved the microbial C into the soil [67]. The black soils are very susceptible to degradation and can be improved by applying gypsum together with organic amendments which improve the microbial activity. It has been recommended that application of organic amendments (green manures) is suitable alternative to costly gypsum for reclamation of sodic black soils, enhancing crop productivity and developing a sustainable agro-ecosystem [66]. The quality and quantity of the organic amendments also influence the diversity and activity of microbial community. Meena et al. [68] reported that application of municipal solid waste compost and rice straw compost at a rate of 16 and 14 t ha⁻¹, respectively significantly improved the dehydrogenase, phosphatase and urease activities than control. The combined use of gypsum and bio-organic amendments (compost and crop residues) provides essential nutrients to crops and improves soil biological properties besides improving physical and chemical properties and productivity of saline soils [69]. The structural and functional diversity of soil microbes and bacterial richness and evenness were improved by application of organic amendments to soil [70]. Thus, the organic amendments have been reported to sustain soil health and crop production by improving soil biological properties. Although, the adoption of soil conditioners/amendments should be based on specific crop production standards for developing sustainable agro-ecosystem [71].

6. Oilseed yield and quality as influenced by soil amendments

Oilseed crop yield and oil quality is influenced by both biotic and abiotic factors. Oil quality is governed by three major compounds namely, fatty acids, triglycerol and bioactive compounds which depend upon crop variety, weather conditions and post-harvest methods [72]. Soil health deterioration with less to no use of organic manures, chemical fertilizers, intensive cultivation, sulfur free fertilizers etc. are major constrains in enhancing oilseed productivity. Sulfur is an essential macro-nutrient that play important role in protein synthesis, regulates N assimilation inn crops, coenzyme-A, biotin, vitamins etc. and determines oil quality. Oilseed crops are heavy S feeder crops and can remove 10 to 25 kg per year depending upon soil, crop and environmental conditions [73]. Incorporation of soil amendments with recommended fertilizer doses to oilseed crops will have beneficial effect on improving soil physic-chemical and biological properties, improving soil health directly influencing productivity and oil quality (Figure 3). Potential yield gap can be minimized by best soil management practices to enhance nutrient availability and uptake by crops. Integrated nutrient management of incorporating fertilizers and organic resources are effective strategy to enhance oilseed productivity. Organic amendments such as compost, green manures etc. acts as slow release organic fertilizers to meet crop demand. Thus, enhancing nutrient uptake by decreasing loss via leaching, runoff, volatilization and increasing nutrient use efficiency.

Figure 3.

Soil dynamics and crop performances influenced by soil amendments.

Soil amendment as nutrient source and soil conditioner can alleviate salinity problems, micro and macro nutrient stress, enrich biodiversity thereby enhancing nutrient supplying capacity of soil. Positive effect of soil amendments such as organic manures, zeolites etc. improved crop yield under water deficit conditions in soybean [74] and sunflower [46]. Soil amendments such as organic manures enhances rain water use efficiency of oilseed crops. According to Adeli et al. [75] organic amendments could increase crop yield by 42% over only mineral fertilizer application. However, crop yield response varies depending upon crop species, soil amendments, edaphic factors and climatic parameters. Ulzen [56] reported that soils treated with rhizobia, P fertilizer and fertisol (manure) showed higher water retention in soil and improved water use efficiency for increasing soybean yield. Organic amendments supply micronutrients, stimulate soil microbial activity and continuously provide nutrients to improve crop yields. Thus substituting chemical fertilizers with organic amendments can be economically feasible and environment friendly approach for sustainable production. Substituting nearly 25–50% of synthetic fertilizers with organic manures/ compost provide better crop yield response and enhance fertilizer use efficiency in oilseed crops. This is due to modification in soil properties such as soil pH, soil fertility, resilience of soil aggregates, soil microbial biomass which enhances ecosystem services and functions, influencing carbon and nitrogen use efficiency. Thus combined application of different types of organic amendments could improve crop yield for long term without deteriorating soil environment. Several studies have shown significant impact of soil amendments on oilseed yield and quality is described in Table 4.

Oilseed crop	Amendment/treatment	Improvement in yield	Improvement in crop quality	Reference
Soybean	Gypsum, cow manure	Soybean grain yield increased by 44 and 31% with cow manure and gypsum ameliorant in saline soils	Gypsum and cow manure increased leaf chlorophyll content	[76]
Soybean	Poultry litter, Gypsum	Soybean grain yield improved by 42% over control	—	[75]
Soybean	Cattle manure, biofertilizers	Grain yield improved by 132% over uninoculated treatments	—	[77]
Soybean	Rhizobia, organic manure, P fertilizer	Combined application of rhizobia+ P fertilizer + organic manure improved73–93% increase in yield over control.		[56]
Soybean	FYM, Vermicompost, Poultry manure	Higher grain and monetary returns with integration of organic amendments with fertilizers		[78]
Safflower	FYM, Subabul leaves	Maximum grain, straw, petal yield was observed in integrated application of 50% through inorganic and 50% through organic resources.		[79]
Safflower	vermicompost + vermiwash spray + Azotobacter + cow dung urine	improvement in dry matter (3154 kg/ha) and seed yield (1230 kg/ha) of safflower		[80]
Safflower	Gypsum, FYM, Gypsum+FYM	Improved safflower yield in combined application of FYM and gypsum in alkali soils		[81]
Safflower	Vermicompost, Azospirillum, PSB	Safflower yield improved by 19% in vermicompost + biofertilizer treatments Oil content and oil yield improved with organic amendment inclusion with fertilizers.		[82]
Safflower	Municipal waste compost, vermicompost, sheep and cow manure	Improved seed yield with sheep manure (1.67 g per plant) and municipal compost (1.51 g per plant) was recorded. Higher micro-nutrient uptake by plants was obserd with municipal compost plots.		[83]
Mustard	Sheep manure and humic compounds	Mustard yield improved by 140–190% over control in combined application of sheep manure and humic substances		[84]
Rapeseed	Biochar with urea	Mustard grain yield increased by 16.6% over only urea treatment	Improved NUE 58.8% over urea only	[85]
Mustard	Composted cattle manure (CCM)	Significant yield improvement observed in CCM+ 75 kg N per ha treatment over urea alone	—	[83]
Mustard	10 t FYM + 10 t sewage sludge/ha with fertilizers	Best treatment with 1.73 t/ha yield	Oil content was 37%	[86]
Sunflower	Composted cowdung and gypsum, sole gypsum, sole gypsum	Improved crop yield by 82% with combined amendment application over control		[87]
Sunflower	Zeolite, organic manure, integrated treatment with different irrigation regimes	FYM (50% or 100%) with zeolite (5 t ha^−1) under normal irrigation condition improved biological yield by two times than control	Increased linoleic and olic acid content in organic fertilizer treatment	[46]
Sunflower	FYM, Vermicompost, Azotobacter +PSB	Seed yield improved by 68.1 and 107.5% over control in VC/ FYM+ Azotobacter+PSB + 50% NPK		[2]
Sesame	Compost, chicken manure, FYM with or without reduced N fertilizer	Seed yield improved by 15.9 and 9.7% in chickenmanure and FYM plots respectively over RDF	Oil and protein content improved by 15.1 and 25% in FYM with reduce N application dose	[88]
Groundnut	Biofertilizers (Rhizobium and phosphate solubilizing microbes), FYM	pod and haulm yields of groundnut increased by 40.19 and 35.96 per cent, respectively over no manuring in manuring plots with FYM + Rhizobium + PSM		[89]
Castor	FYM, Sewage sludge, Gypsum	Application of FYM and gypsum improved castor seed yield by 4–7% over sewage sludge treatments		[90]
Niger	FYM, Azotobacter	Improved better growth and increased yield by 63% in 75% RDF + Azotobacter + PSB or 50% RDF + 5 t FYM/ha treatments		[91]

Table 4.

Oilseed yield response as influenced by soil amendments.

Beside the beneficial effect of organic amendments, they also have some detrimental effect on soil ecosystem and human health. The major drawback with application of amendments such as biosolids, sewage sludge, etc. is poor segregation of waste at source that could have hazardous effect on soil environment. Application of immature compost, untreated materials often lead to agronomic and environmental problems. Organic amendments of various types can contain pathogen, pollutants, heavy metals, emerging contaminants such as antibiotic resistance genes, endocrine disruptors, microplastics etc. [33, 92]. Moreover, if unstable organic amendments are applied to land could result in ammonia volatilization, deplete oxygen, release some toxic compounds and immobilize nutrients hindering crop uptake. Indiscriminate and overuse of some amendments might result in environmental pollution by high release of N and P into ground water, rivers etc., eutrophication problems, immobilization of nutrients, greenhouse gas emission, acidification, salinization etc. [93]. Therefore, safe handling of soil amendments especially from municipal solid waste, sewage sludge, industrial waste etc. is essential for maintaining environmental and human health.

7. Future research gaps

Many studies on soil amendments have highlighted the positive impact of soil amendments viz., manure, compost, biofertilizers, biosolids etc. on crop yield. Gypsum application on oilseed crops has enhanced oilseed yield and quality. However, studies on nutrient release pattern, carbon sequestration and build-up of contaminants in soil with amendments application, though are slow process, but could be evaluated in oilseed cropping sequence. Influence of biosolids, animal manures, industrial waste etc. effect on oil quality can be tested to determine the impact of soil amendments. Biochar amendment is added to many field crops, but its application on oilseed crop is less studied. Studies on impact of biochar amendment on oilseed crop will improve understanding on carbon sequestration and oilseed quality. Exploring the possibility of converting woody CR like cotton, castor, pigeonpea stalks etc. into biochar amendments for soil carbon management. Biofertilizers or microbial inoculants for improving composting process of CR and its impact on oilseed crops is lacking. Most of the studies in oilseed are associated with rhizobial strains in soybean and groundnut crops. Limited studies are reported on the use of phosphate solubilizing microbes, sulfur oxidizing organisms, micro-nutrient activating microbes etc. in oilseed crop production of semi-arid regions. Studies on the effect of soil amendments on oilseed crops such as safflower, seasame are lacking. More research is needed to enhance nutrient content in composts and accelerate process of composting using suitable microbial consortia. More research is needed to transfer the advanced scientific knowledge for composting, enriching with micro-nutrients, use of different strains of microbes, reducing nutrient loss with additives should be promoted. Sound approaches to educate and create awareness among farmers about the benefits of various soil amendments in improving soil health, rather than supplying nutrients.

8. Conclusion

Soil amendments could bring paradigm shift in oilseed production, by including as suitable strategies for maximizing crop productivity and optimize environmental sustainability. Poor soil fertility, salinity, sodicity, low soil organic matter, reduced bioavailability of sulfur and micronutrients etc. are some major constrain in oilseed production. Therefore, inclusion of soil amendments in management practices will improve soil productivity and crop yield. Organic manures, compost, biosolids, biochar, biofertilizers etc. are few organic amendments. Inorganic amendments such as gypsum, pyrite, lime, zeolite etc. are few inorganic amendments which are used for enhancing oilseed production. Soil organic amendments will supply both nutrients and organic matter providing more opportunities to enhance soil physical, chemical and biological properties. Inclusion of amendments such as carbon stable amendments like biochar, can improve soil properties and act as carbon reserve for longer time. Biofertilizers are the potential organic amendments that would improve nutrient cycling, enhance soil biodiversity, producing siderophores, organic acids, chelateswould enhance essential nutrient accumulation in crops. The positive impact of soil amendments depends upon their composition, types of materials, soil type, utilization, weather conditions, cropping system etc. However, soil amendments are valuable asset that meets the present circular economy paradigm approach. Comprehensive analysis of organic and inorganic soil amendments should be performed prior to application in oilseed crops, to determine the potential and limitations with respect to soil health and crop quality. Locally available soil amendments such as organic manures, compost, green manure, CR, biofertilizers etc. should be utilized for soil and crop productivity. Use of organic amendments can be a low input system which could help in achieving sustainability of agroecosystem. Soil amendments have the potential positive impact on various ecosystem services depending upon their composition, stability, origin, application rate and utilization, soil type, cropping sequence, weather parameters etc. Hence, comprehensive characterization or soil amendments in relation to different agroecosystem must be carried out before soil application, so as to identify the potentials and limitation of give soil amendments on soil health and crop quality.