Turning Waste Wool into a Circular Resource: A Review of Eco-Innovative Applications in Agriculture

Abstract

Agriculture significantly impacts the environment in terms of greenhouse gas emissions, soil nutrient depletion, water consumption, and pollution and waste produced by intensive farming. Wool has great potential and can be a valuable resource for agriculture due to its high nitrogen, carbon, and sulfur content and good water absorption and retention properties, benefiting soil carbon storage and fertility, as well as decreasing the risk of water contamination due to the slow decomposition and nitrogen release. This review aims to provide an overview of bio-based solutions that can benefit agroecosystems as a circular bioeconomy practice. Raw wool and wool hydrolysate are the most common applications, but also wool pellets, wool compost, and wool mats are interesting treatments for plant growing. Waste wool showed positive effects on soil fertility by primarily increasing nitrogen and sulfur content. Improved water retention capacity and microbial activity were also recorded in several studies. The use of wool as mulching is effective for weed control. Attention to the plant species tested aimed at identifying the most promising cultivations in terms of treatment efficiency, possibly lowering environmental impact on the agroecosystem. To eco-design and scale-up processes that strengthen the circular use of wool into widespread practices, further research should be encouraged in conjunction with environmental impact assessments and economic evaluations.

1. Introduction

The worldwide increase in agricultural and livestock production, especially from intensive farming systems, has resulted in a significant production of agricultural residues, namely, wastes co- and by-products [1]. Such an increase has placed strong pressure on the environment, with negative impacts on soil, air, and water resources [2]. All this adds up to the long-term consumption of agrochemicals [3], with harmful repercussions on the environment, which can be exacerbated by climate change [4]. IPCC [5] reports that, in 2019, approximately 22% [13 Gt CO2-e] of the total net anthropogenic greenhouse gas (GHG) emissions came from agriculture, forestry, and other land use (AFOLU). Livestock farming significantly contributes to global warming, accounting for more than half of total agricultural GHG emissions [6].

One strategy to counter these criticalities is to support actions that are built on the circular economy and bioeconomy, and that aim to exploit materials, such as by-products or residues, that would otherwise be managed as waste. In fact, the circular economy can contribute to design and develop appropriate strategies being addressed to reduce waste from production and distribution processes [7], thereby helping to reduce environmental impacts. Closing resource loops facilitates upcycling to restore or create new value from used materials [8], but it is also a promising strategy to optimize the environmental implications of agricultural activities while improving economic efficiency [9,10].

In 2021, worldwide, sheep accounted for 1.266 billion heads in the livestock value chain, showing an increasing trend over the last years, mainly due to the increasing demand for meat [11]. Although their contribution to the added value of livestock globally is relatively small, sheep farming systems play a significant socio-economic role both in developing countries, where subsistence economies prevail, and in industrialized or more developed countries that are more trade-oriented [12,13]. Undoubtedly, at global level, fleece is still one of the main reasons for raising sheep: it is estimated that, in the period 2024–2029, the global wool market is worth about USD 40 billion, 4/5 times the sheep meat market and 5/6 times the sheep milk market [14,15,16].

Considering the world sheep population and wool production, China, the country with the largest share of sheep, with about 194 million heads [12], represents the largest producer in terms of greasy wool weight [17], while India and Australia (over 70 million heads in 2022) are the second and third largest countries by sheep population, respectively [12].

The European Union (EU-27) also plays an important role with 59 million heads [18] and an annual wool production of approximately 89.000 tons. According to Eurostat [18], the largest producer is Spain (25%), followed by Romania (17%), Greece (13%), France (12%), Italy (11%), and Ireland (7%).

While the total sheep population has been increasing in recent decades, the demand for wool has been decreasing due to changing consumer habits, which have led to the replacement of wool products with synthetic fibers [19,20]. Moreover, wool is difficult to process and too expensive because it is traded in a volatile wool market [21].

The fall in prices [6] has led to a surplus of wool as a raw material that is unevenly distributed among countries. Australia, New Zealand, China, the United States, Iran, Argentina, Turkey, the United Kingdom, India, Sudan, and South Africa are the major producers of wool for the textile and fabric industry [22]. Conversely, most of the wool produced in the EU is generally considered to be of low quality when compared to the wool from major wool-producing countries [23]. Genes that follow Mendelian inheritance have been reported for many important traits (pigmentation, wool quality, and the keratin protein) in wool sheep [24]. The coarse characteristics of these wools, often derived from dual-purpose breeds (meat and milk production) or breeds intended for the dairy industry, do not allow European wool to meet the quality standards of the fashion market, thus contributing to the weakening of demand for wool. The latter, despite the high wool versatility that far extends beyond the traditional textile applications, is not supported by the technical textile sector (i.e., geotextile, green building components, composites), which is also subject to the weaknesses and uncertainties in the wool supply chain. This sort of situation poses a practical and economic burden for sheep farming, as the selling price of wool does not even cover the cost of shearing, which is a physiological need for sheep care [25]. If wool is not sold by sheep farms, its disposal can be logistically difficult to manage, and illegal storage, transport, and disposal practices (large-scale burial or burning) can be carried out, posing a threat to the environment [26]. Regardless of the wool variety breed and its quality, improper treatment can also occur during the manufacturing processes (carding, combing, spinning, weaving, etc.), in which nearly 10–15% of the wool residue/waste produced is often discarded or dumped on the ground [27]. Even if wool is properly disposed of in landfills, this represents an unsustainable and environmentally unfriendly practice [28].

Improper disposal of greasy wool can be harmful to the environment. In Europe, such practices are illegal, and the management of greasy raw wool and its application into the soil is ruled by the European Commission Regulation (EC) No. 142/2011 of 25 February 2011 [29] implementing Regulation (EC) No. 1069/2009. To our knowledge, there is no scientific literature regarding the impact of greasy raw wool on soil. However, raw wool contains lanolin (15%) and potential contaminants such as suint (4%), soluble contaminants such as potassium salts from perspiration and feces, and residues of pesticides used to protect sheep from ectoparasites [30]. Suint contains salts, ammonia, and urea, which, when present in large quantities, can damage the soil by altering pH, reducing fertility, and hindering plant growth. Malpractice discards valuable protein resources but also generates health hazards and environmental pollution [31]. Furthermore, some studies have demonstrated that wool can be used as a bio-indicator of sheep exposure to heavy metals [32], since contaminants from soil, air pollutants, food contaminants, plants, and animal droppings can be detected on wool fibers [33], as well as for biomonitoring trace element content in animals [34].

Trace elements tend to accumulate in hair at concentrations at least tenfold higher than those found in blood serum and urine [34], and this may be related to the high capacity of lanoline to absorb contaminants [35,36]. Burying unwashed wool that may be contaminated can result in the accumulation of pollutants in soil with potentially dangerous consequences on leaching and groundwater pollution.

To limit the linear economic ‘take–make–use–dispose’ model [37], which is functional to increase profits by consuming primary resources and increasing demand for short-cycle products, sheep wool should be considered as a wasted resource [38].

Sheep wool fibers are composed of some essential nutrients such as carbon (C, 50%), nitrogen (N, 16–17%), and sulphur (S, 3–4%). These elements, along with various micronutrients like copper (Cu), manganese (Mn), iron (Fe), and zinc (Zn), are essential for plant nutrition [39]. In addition, wool fibers have good water absorbing and retention properties [40,41] and can reduce pH in highly alkaline soils [42]. Also, the slow release of nitrogen into the soil, due to the complex wool keratin structure [43], compared to the rapid release from mineral fertilizers that causes nitrate leachates [28], reduces water contamination, allowing soil fertility to be maintained over time, thus reducing the use of synthetic fertilizers [44,45]. These characteristics meet the need of the livestock systems to minimize GHG emissions [46], to increase soil C stocks, and to improve N use efficiency through more sustainable farming practices. The exploitation of keratinous wastes could contribute to tackling the high volume of biomass that can be harmful to the environment, instead providing resources, feedstock, and raw materials to produce keratin-based by-products for value-added applications [47].

The biomass produced from agroecosystems is the basis of our food system and future bioeconomy [48]. Sheep wool can be a protagonist of circular bioeconomy strategies that can be at the heart of agroecosystems. Agriculture and the environment are closely linked, so the use of sheep-wool-based materials, derived from livestock activities, can have a positive impact on agroecosystems and rural landscapes. As the application of circularity in agriculture could be a sustainable measure contributing to both the reduction of unexploited wool biomass and the potential to reduce the use of synthetic fertilizers, it is necessary to outline a set of information to support research on agroecosystems.

The present paper points to the general goal of understanding the interest of research in exploring innovative solutions for adding value to the sheep wool waste within circular bioeconomy approaches. Specifically, this paper aims to (i) provide a comprehensive analysis of the scientific literature related to the use of sheep wool in agriculture and (ii) examine the inherent challenges and potential opportunities for further studies in this sector.

2. Literature Search and Selection Process

An extensive literature screening was carried out to obtain a general overview of the literature regarding the use of sheep wool in agriculture. Research findings from 2000 to 2024 were taken into consideration during the review process. Web of Science^® (WoS) and Scopus^® platforms were used to search for peer-reviewed articles from indexed journals. Since there was a scarcity of scientific publications about sheep wool in agriculture, Google Scholar was also used to gather not indexed materials (referred to as “grey literature”) published in the same time period.

The collected grey literature was not analyzed, even though it can supplement knowledge from peer-reviewed publications for science-policy processes and applied research [49]. However, the quantity of grey literature findings was considered to track the dynamics of this research topic over time. Boolean search was performed on the search engines by combining “sheep wool” with the following words: agriculture, amendment, botany, container, crops, cultivation, fertilizer, geotextiles, horticulture, hydrolysis, hydroponic, landscaping, management, mats, mulch, pellets, plants, pots, soil improver, trees, waste.

The publications were selected based on their title and abstract content, as well as their compliance with the review’s objectives, which included, for instance, discussing the different types and purposes of applying sheep wool to the soil. Articles that contained some of the above-mentioned words about using wool for purposes other than soil improvement were discarded.

The collected records were first reported in a table by title, authors, year, key words, country, search platform, indexed or not indexed journal, type of publication (research article, abstract proceedings, review, book chapter), contents, and research gaps. The contents synthetically comprised the objective of the paper, the methods used, and the main results. The number of publications per continent was determined by considering the country of the institution affiliated with the first author.

Evidence of the properties, functions, and applications of sheep wool in agriculture was collected from the selected indexed publications and discussed, gathering them in the following nine domains: raw wool; wool hydrolysate; wool mats and substrates; wool pellets; scoured wool; wool management; wool compost. Two more domains came next: reviews and other, with the latter including a variety of subjects.

Moreover, papers focusing on plant cultivation tests were quantified.

The total number of publications was used for the following items: indexed and not indexed publications; indexed publications per domain; countries; plant cultivation tests; and related vegetal species. Percentages were calculated as regards the publications by continent. A schematic route of the selection process of the articles is reported (Figure 1).

Concerning the raw wool, it is referred to as “grease wool in a natural state before scouring” [50]. The adjective “coarse” is related to the characteristics of wool fibers in terms of micronage (31–40 μm) according to the classification of fibers by UNI 5423–64 rules [25].

To further understand the research addresses, a visual representation of the relevance attributed to the topic of waste wool was provided by means of a word cloud generated by using the online platform WordArt© (https://wordart.com/). The word cloud associates the size of the word characters with the frequency of the words used in the text, which are displayed in an appealing visual representation [51]. The word cloud was made by using the keywords of the selected indexed publications, which were uniformed into British language. In the case of derived words, the base noun was used (i.e., “keratine” and “keratinase” or “hydrolysis” and “hydrolysate”). The words that were cited only once were discarded.

3. Literature on Sheep Wool Application in Agriculture

A total of 136 records (indexed and not indexed) were found. The indexed publications were 90, comprising 72 research articles, 11 review papers, 6 book chapters, and 1 abstract proceeding. The remaining 46 records consisted of not indexed publications.

Regarding the geography of both indexed and not indexed publications, Europe was the most productive continent, followed by Asia, America, Oceania, and Africa (Figure 2). The leading countries in productivity were Romania (19), India (18), Italy and the USA (15), Poland (13), Germany (12), and Turkey (6), all accounting for 72% of the total records.

Since 2020, an increasing trend has been observed in the total number of publications (indexed and not indexed papers) per year (Figure 3). An average of 2.1 indexed publications per year (and 1 not indexed publication per year) during the period 2009–2016 and an average of 8.1 indexed publications per year (and 4.1 not indexed publications per year) during the period 2017–2024 were calculated. An exponential trend of article publishing with a fourfold increase over the last period was observed. In both periods, indexed publications have almost doubled grey literature production.

An exponential growth in research on wool in agriculture from 2002 to 2024 was evident in the indexed papers (Figure 4). In particular, the most represented groups of papers are those related to wool hydrolysate, raw wool, wool mats and substrates, and scoured wool published from 2002 to 2005 and constantly growing until 2024. Other research lines, such as wool pellets and wool compost, appeared later and have not yet achieved many records, even though wool pellets tend to increase in the past three years as well as the “reviews” domain.

Figure 5 displays the most cited keywords related to the selected indexed articles. Considering the first ten cited words, “wool” was cited 56 times, followed by “waste” (27), “soil” (26), “sheep” (26) “hydrolysate” (16), “keratin” (15), “sustainable” (14), “fertility” (13), “organic” (11), and “plant” (10).

3.1. Domains of Sheep Wool Application

Table 1 shows the bibliographic records related to the domains of wool application: wool hydrolysate and raw wool comprise 16 articles, followed by wool mats and substrates (13), scoured wool (9), wool pellets (6), and wool management and wool compost (5).

Twenty-one additional records belong to the categories reviews (16) and other (5). All the reviews contained references to the use of wool in agriculture. Nine of them provided a general overview of the application of wool as residue or waste in different sectors such as building industry, pharmaceutics, etc., dedicating at least one paragraph to the use of wool in agriculture as fertilizer, amendment, or mulch. Five publications reviewed the different keratin extraction methods, also focusing on agricultural applications. Additionally, [52] concerned wool waste as a component for bio-based polymer composites, mentioning possible utilization as bio-plastic-based mulching, while [27] outlined the potential applications of wool waste in agriculture, providing a short reference list on related publications.

In the following paragraphs, the collected literature is illustrated and summarized by domain of application.

Table 1. Number of scientific publications and bibliographic references for different domains (types of application as raw and processed wool material, types of issues and of publication). The literature analysis was performed using the Web of Science© (WoS) and Scopus© platforms over the period of 2000–2024.

Domain	No. of Papers	References
Wool hydrolisate	16	Gousterova et al. [53]; Nustorova et al. [54]; Evangelou et al. [55]; Mokrejs et al. [56]; Gogos et al. [57]; Zoccola et al. [43]; Bhavsar et al. [23]; Holkar et al. [58]; Niculescu et al. [59]; Bhavsar et al. [60]; Akca et al. [61]; Karaca et al. [62]; Cristea et al. [63]; Metomo et al. [64,65]; Taskin [66]
Raw wool	16	Zheljazkov [67]; Zheljazkov et al. [68]; Mubarak et al. [40]; Zheljazkov et al. [45]; Butcaru et al. [69]; Kumar [70]; Butcaru et al. [71]; Choudhary et al. [72]; Malancu et al. [73]; Butcaru et al. [74]; Jungić et al. [75]; Lal et al. [76]; Ydyrys et al. [77]; Komorowska et al. [78]; Broda et al. [79]; Broda et al. [80]
Wool mats and substrates	13	Duppong et al. [81]; Böhme et al. [82]; Dannehl et al. [83]; Marczak et al. [84]; Liu et al. [85]; Marczak et al. [86]; Balawejder et al. [87]; Ferby et al. [88]; Herfort et al. [20]; Komorowska et al. [89]; Marczak et al. [90]; Gitea et al. [91]; Herfort et al. [92]
Scoured wool residues	9	Górecki and Górecki [44]; Butcaru and Stănică [93]; Abdallah et al. [41,94]; Broda and Gawlowski [95]; Palla et al. [96]; Gabryś and Fryczkowska [97]; Juhos et al. [98]; Li et al. [99]
Wool pellets	6	Böhme et al. [100]; Bradshaw and Hagen [101]; Haque and Naebe [102]; Lohr et al. [103]; Dal Prà et al. [104]; Kovács et al. [105]
Wool management	5	Stokes [106]; Ganci et al. [107]; Gillespie et al. [108]; Parlato et al. [1]; Midolo et al. [109]
Wool compost	5	Altieri and Esposito, [110]; D’Addabbo et al. [111]; Hustvedt et al. [42]; Malancu et al. [73,112]
Other	5	McNeil et al. [113]; Natali Murri et al. [114]; Chereji and Munteanu [115]; Løvbak Berg et al. [116]; Petek et al. [117]
Reviews	16	Johnson et al. [118]; Sundar et al. [119]; Rajabinejad et al. [19]; Sharma et al. [27]; Allafi et al. [120]; Marchelli et al. [121]; Perţa-Crișan et al. [122]; Petek and Marinšek Logar [28]; Chen et al. [31]; Ossai et al. [47]; Kulkarni et al. [52]; Sun et al. [123]; Kadam et al. [124,125]; Ozek [21]; Vijay and Narendhirakannan [126]

Table 1. Number of scientific publications and bibliographic references for different domains (types of application as raw and processed wool material, types of issues and of publication). The literature analysis was performed using the Web of Science© (WoS) and Scopus© platforms over the period of 2000–2024.

Domain	No. of Papers	References
Wool hydrolisate	16	Gousterova et al. [53]; Nustorova et al. [54]; Evangelou et al. [55]; Mokrejs et al. [56]; Gogos et al. [57]; Zoccola et al. [43]; Bhavsar et al. [23]; Holkar et al. [58]; Niculescu et al. [59]; Bhavsar et al. [60]; Akca et al. [61]; Karaca et al. [62]; Cristea et al. [63]; Metomo et al. [64,65]; Taskin [66]
Raw wool	16	Zheljazkov [67]; Zheljazkov et al. [68]; Mubarak et al. [40]; Zheljazkov et al. [45]; Butcaru et al. [69]; Kumar [70]; Butcaru et al. [71]; Choudhary et al. [72]; Malancu et al. [73]; Butcaru et al. [74]; Jungić et al. [75]; Lal et al. [76]; Ydyrys et al. [77]; Komorowska et al. [78]; Broda et al. [79]; Broda et al. [80]
Wool mats and substrates	13	Duppong et al. [81]; Böhme et al. [82]; Dannehl et al. [83]; Marczak et al. [84]; Liu et al. [85]; Marczak et al. [86]; Balawejder et al. [87]; Ferby et al. [88]; Herfort et al. [20]; Komorowska et al. [89]; Marczak et al. [90]; Gitea et al. [91]; Herfort et al. [92]
Scoured wool residues	9	Górecki and Górecki [44]; Butcaru and Stănică [93]; Abdallah et al. [41,94]; Broda and Gawlowski [95]; Palla et al. [96]; Gabryś and Fryczkowska [97]; Juhos et al. [98]; Li et al. [99]
Wool pellets	6	Böhme et al. [100]; Bradshaw and Hagen [101]; Haque and Naebe [102]; Lohr et al. [103]; Dal Prà et al. [104]; Kovács et al. [105]
Wool management	5	Stokes [106]; Ganci et al. [107]; Gillespie et al. [108]; Parlato et al. [1]; Midolo et al. [109]
Wool compost	5	Altieri and Esposito, [110]; D’Addabbo et al. [111]; Hustvedt et al. [42]; Malancu et al. [73,112]
Other	5	McNeil et al. [113]; Natali Murri et al. [114]; Chereji and Munteanu [115]; Løvbak Berg et al. [116]; Petek et al. [117]
Reviews	16	Johnson et al. [118]; Sundar et al. [119]; Rajabinejad et al. [19]; Sharma et al. [27]; Allafi et al. [120]; Marchelli et al. [121]; Perţa-Crișan et al. [122]; Petek and Marinšek Logar [28]; Chen et al. [31]; Ossai et al. [47]; Kulkarni et al. [52]; Sun et al. [123]; Kadam et al. [124,125]; Ozek [21]; Vijay and Narendhirakannan [126]

3.1.1. Wool Hydrolysate

The high stability of wool due to the cysteine, peptide, and hydrogen bonds between macromolecules makes the direct use of wool waste as fertilizer a disadvantage because soil enzymes cannot easily degrade the structure of wool fibres, thus limiting its application as a short-term fertilizer. In this regard, keratin, which has short molecular chains and low molecular weight, can be used in various fields as fertilizer, and it can be obtained through the hydrolysis of wool waste [43,58]. Different methods to produce hydrolysed wool have been tested, including modelling technological conditions for sheep wool hydrolysis [56]. For instance, hydrolysed wool obtained by acoustic cavitation assisted alkaline-hydrolysis-enhanced wheat seed germination by over 8.5% relative to the conventional method [58]. Properties of superheated wool hydrolysate in promoting N uptake and assimilation by maize and as a biostimulant for maize growth were shown by [64,65]. Evangelou [55] investigated hydrolysed wool to improve the extraction of Cu (850%) and Cd (30%) from tobacco soil compared to raw wool (68% and 5.5%, respectively). Alkaline hydrolysis using maximum K and minimum Na hydroxide to further increase the K content in soil was employed by Gousterova et al. [53]: the highest concentration of hydrolysate affected ryegrass growth (maximum biomass accumulation and height) at 160 days, while the total N content increased after 60 days. Nusturova et al. [55], testing the application of hydrolysed wool, showed the effects of reducing soil pH, increasing total C and N and C/N ratio in soil while the main effect on ryegrass cultivation was an increase in germination rate, plant height, and mineral N fixing and ammonifying bacteria. Hydrolysed wool was found to act as a Zn and Fe fertilizer in wheat plants, increasing grain yield two fold and protein content in grains 1.5-fold compared to mineral fertilizer [57]. Bhavsar et al. [23] applied superheated water hydrolysis to obtain wool hydrolysate. Germination tests showed a remarkably high germination index and no phytotoxic effect. The hydrolysate content in N, amino acids, micronutrients, and the neutral pH supported its application as a biostimulant for soil microbial activity through fertirrigation and as a plant growth promoter. The same method has been shown to increase biodegradability in soil when mixed in biocomposites [60]. Keratin obtained by hydrolysis was used to develop biocomposites, with controlled biodegradability, which, when used as a substrate for growing rape, can increase biomass up to 38% [59]. Cristea et al. [63], conducting a germination study on pepper and tomato seeds based on protein-based gels obtained from protein hydrolysates of tanned leather of bovine and sheep wool, found a significant stimulatory effect on both plants. Interestingly, hydrolysed wool has been mixed with biochar and then pelletized to obtain balanced C/N fertilizer [121].

3.1.2. Raw Wool

The use of raw wool waste added to soil as short wool fibres or as a geotextile has positive advantages in terms of soil fertility and water retention. Kumar et al. [70] reported that geotextile application initially serves as a moisture reservoir to support plant growth and, during the decomposition period, it acts as a natural fertilizer source. A 20% higher yield compared to conventional mineral fertilizers was found in a study on winter wheat in Poland [79], where nitrate increased significantly with increasing soil wool concentration. Even much higher yields (70%) were found in marigold [68]. The presence of wool also stimulates enzyme activities in the soil [76], while it has no significant effect on mycorrhizae [68]. However, some negative consequences of the use of raw wool can occur through nitrate leaching after a single application of wool waste in field production systems and an increase in Na [67], as well as a possible decrease in soil pH [45]. When used as a mulch, raw wool has been shown to be effective in controlling weeds, while also providing additional N and C sources, thus improving plant growth and yield [74,75] and increasing the population of heterotrophic aerobic bacteria [69]. Raw wool could also be a valid alternative as fertilizer to reduce GHG emissions. In a study conducted on green bean, Komorowska et al. [78] showed that the use of wool waste is justified from an environmental point of view since part of the C contained in the wool undergoes a humification process, which is very important for sequestrating C in the soil.

3.1.3. Wool Mats and Substrates

Wool mats as biotextiles have been developed for mulching and vegetation substrates. Degradable nonwoven wool mulches deriving from the wool waste industry were found to be effective in weed suppression and showed superior thermal insulation properties compared to degradable plastic mulches [85], as well as positive effects on plant growth and yield [81]. On the other hand, a drawback was that soil moisture retention capacity was lower than that of biodegradable plastic film mulches [85]. Herfort et al. [20] demonstrated that vegetation mats made from coconut mixed with at least 50% of sheep wool were effective in the pre-cultivation of perennials such as Achillea clypeolata Sm, Aster dumosus Nees, and Salvia nemorosa L. due to the high nutrient supply assured by wool component. Marczak et al. [84] presented a new type of water-absorbent geocomposite (BioWAG) consisting of a wool biotextile enveloping a superabsorbent polymer, which absorbs water and its solutions, and an internal skeleton wooden structure, which provides space for free water absorption. This mat, when applied under a depth of 15–30 cm of soil resulted in a 400% increase in N content due to the addition of wool and, consequently, the grass mixture, also grew 40–430% higher [86], while the dry weight of the roots increased by 130–200% [90]. In hydroponic cultivations, wool panels have been tested as an environmentally friendly alternative to rock wool slabs. However, in greenhouse experiments on cucumbers and tomatoes, the pore volume of sheep wool was significantly higher than that of other substrates such as peat moss, resulting in a low water capacity [82,83]. Results on plant growth rates and yields are controversial, being lower in the case of tomatoes [83] but higher in the case of cucumbers [82] when compared to another environmentally sound substrate such as peat slab. Reduced yields were also found in lettuce, probably due to the important level of salinity and water deficit in the wool substrate [88]. Komorowska et al. [89] pointed out the importance of recycling wool waste for reducing the GHG emissions and water consumption, although hydroponic cucumber cultivation yield decreased by 10% compared to mineral wool substrate. Sheep wool fibres in the vegetation mats provided fertilizing effects on perennial pre-cultivation without any need for additional fertilization [92], thus preserving the environment from the overuse of synthetic fertilizers. Coco substrates enriched with wool influenced the content in polyphenols in raspberries [87], as shown also by the synergistic effect of soil conditioner and sheep’s wool on plum trees, resulting in high-quality fruit production [91].

3.1.4. Wool Pellets

Dal Prà et al. [104] showed that pelletizing can be a useful method of applying sheep wool as a soil fertilizer, as it can reduce the microbiological load of greasy wool, thus avoiding the costly industrial wool scouring phase, in compliance with the “Do No Significant Harm” (DNSH) certification [127] (Art. 17 of Regulation (EU) 2020/852). The high total N and total organic carbon contents as well as the dynamics of water retention provided by sheep wool pellets [104] suggest the possibility of using this product as a fertilizer and soil amendment. The capacity of sheep wool pellets to positively affect plants yields (20% and 30% higher in Iceberg lettuce and Ricinous whole meal, respectively; 20–30% higher in tomato; 65% higher in Poinsettia) was found by [100], even though the authors found a negative effect on kohlrabi cultivation (less 50%). Sheep wool pellets were studied by Lohr et al. [103] to characterize the N release. Wool pellets showed a longer lag phase than pellets made from other fertilizers, possibly due to the large amount of wool wax (mainly lanolin) present in raw wool. Kovács et al. [105] showed that the combination of microbial inoculants and wool pellets increased NO³⁻ uptake efficiency, addressing a major environmental concern associated with conventional nitrogen fertilizers. The effect of sheep wool pellets on soil moisture retention in pot tests was found to be less effective than sheep wool powder, and according to Hacque and Naebe [87], this may be due to the larger surface of wool powder in contact with the soil.

3.1.5. Scoured Wool

Residues derived from wool scouring, that is, the first stage of processing, were tested as soil amendment and fertilizers. Two types of residues were used. One was obtained by mechanical beating of scoured wool and consisted of wool fibres and vegetal residues. The other residue resulted from applying sulphuric acid (“carbonization”) to the scoured and beaten wool to remove all cellulosic impurities. Such materials, rich in N as well as of S, K, P, Mg, and Ca, can be used as a balanced and nutrient-rich organic fertilizer. Moreover, they can improve soil infiltration capacity and reduce surface runoff because they can slightly modify the soil, affecting the size and continuity of the soil pores, thereby increasing water movement [41]. However, due to the increased water percolation, they are not suitable for efficient rainwater conservation [94]. Conversely, high amounts of Na can affect soil salinity [41]. The residues can be used as fertilizers, especially for crops that require little fertilizer such as sunflower [94], although in the case of carbonized materials, the presence of sulphuric acid residues had a negative effect on seed germination [41] as well as bacterial community composition at concentrations of 2% or higher [96]. Scoured wool residues were tested by Gabryś and Fryczkowska [97], who highlighted the role of scoured wool in preventing water evaporation and facilitating water access for plants. When applied as mulching, wool was shown to effectively control weeds, providing, at the same time, an additional source of N and C, thus improving plant growth and yield [93]. According to Juhos et al. [98], the insulation effect of wool mulching resulting in lower temperatures can have effects on the balanced biological activity in the soil. Increase in crop yields (30%) influenced by wool resulted in solanaceous species cultivation, such as tomato and pepper [44]. Broda and Gawlowski [95] highlighted the function of wool as a fertilizer, slowly releasing N, thus assuring higher and healthier grass cover.

3.1.6. Wool Compost

Wool composting has been developed and tested in some field experiments. Wool waste has often been mixed with carbon-rich residues such as olive mill waste and straw to produce a balanced fertilizer and amendment that can be used as an alternative to commercial fertilizers [110,111]. Wool compost in purity also showed good performance as a fertilizer, especially in stimulating plant growth due to the presence of Fe, Cu, and Zn [112]. In addition, it can be used as a biostimulant for soil fauna. In this regard, D’Addabbo et al. [111] suggested that the release of nutrients as well as an improved drainage and water retention in the amended soils would promote microorganisms’ growth, which in turn would contribute to improved plant growth. Moreover, wool compost proved to be effective in pest control. In a trial on nematode-infested tomatoes, N content and pH were very effective in suppressing nematodes, reducing eggs and juveniles by 77% and 73%, respectively [111]. Mixed compost with olive mill waste, wool, and straw showed lower phytotoxicity due to reduced polyphenol content [110].

3.1.7. Wool Management

The use of wool waste in agriculture also involves a reflection on the management issues related to its collection and storage before processing and use. To this extent, Midolo et al. [109], Parlato et al. [1], and Ganci et al. [107] conducted several studies in Sicily (Italy) for planning the location of collection centres aimed at developing a logistic and supply sheep wool waste model, thus improving profitability and minimizing social and environmental impacts. Although not specifically focused on the agricultural use of wool, they highlighted the importance of GIS-based approaches that rely on data such as sheep census and farm locations to address location–allocation issues for wool waste [109]. Furthermore, questionnaire surveys of sheep farmers demonstrated the interest in transferring wool to collection centres to find ways to commercialize wool [107]. Opportunities from a circular bioeconomy perspective were also considered in a study conducted in Ireland by [108]. This research found that the establishment of a national facility for the hydrolysis of wool waste, centrally located between the areas of highest livestock density and those of major cereal production, would cover 4.78% of the entire Irish cereal crop demand in terms of fertilizer production. The following scheme (Figure 6) summarizes the different domains of wool, as raw and processed material, along with its different applications in agriculture.

3.2. Plant Species Studied

Of the 90 indexed papers, 55 (61% of the total records) concerned tests on plants (Table 2). The most represented group was horticultural productions with 28 papers, followed by field crops (15), ornamental plants (10), grass (9), medicinal plants and fruit trees/shrubs (3), and aromatic plants (2). Among horticultural crops, tomato scored the highest number of articles (7), being represented in all domains except scoured wool and wool management. Lettuce was studied in five papers in different experiments involving raw wool, wool pellets, wool mats and substrates, and wool compost, while pepper, with a similar number of articles, was only tested on raw wool and wool hydrolysate. Grasses were studied as single species (ryegrass or red clover) and in mixtures. Ornamental plants were represented by a relatively large number of articles but with only one type of wool product applied to each species. A similar trend was observed in the crop group with a lower number of papers and species studied. Fruit trees/shrubs were the least represented category with only three trials: olive trees, plum trees, and raspberries. Considering the most represented plants among horticultural productions, tomato yields increased by 30% following the application of scoured wool as compared to commercial fertilizer standard dose [44]. Wool pellets had similar effects in one study [100], but another study did not find significant differences between fertilization with wool pellet and standard fertilizer [101]. Wool-based compost was effective in reducing nematode density in the soil [111]. Conversely, the use of slabs composed of hemp and sheep wool is not suitable as growing substrate for hydroponic production. Wool decomposition is too fast, thus causing plant instability and yield reduction [83]. Similarly, wool slabs are not effective in hydroponic cultivation of lettuce as compared to other substrates such as coco peat [88]. Wool pellets, instead, provided 20% yield increase in lettuce compared to commercial fertilizer [100]. Even raw wool applied as mulching showed interesting results in terms of yields, thus offering a valuable alternative to polyethylene mulches [75]. Among crops, raw wool application to maize led to an average 25% increase in dry matter and a threefold increase in soil moisture content compared to the control group [40]. Even scoured wool positively influences crop yields with no need for further mineral N-fertilization [94]. Superheated wool hydrolysate proved to be an effective biostimulant on maize, especially for the growth of the foliar biomass [65]. One experiment with hydrolyzed wool on wheat highlighted that grain yield increased twofold as compared to mineral fertilizer application [57]. Raw wool is also effective in increasing the growth of wheat, particularly in terms of dry matter [79].

Table 2. Experimental trials conducted on different plant species according to application type (P = wool pellet; R = raw wool; M = wool management; H = wool hydrolysate; MS = wool mat and substrates; S = scoured wool; C = wool compost). Application types are marked with X. The number of publications and corresponding references for each plant species is also indicated.

Plant Species	Type of Application							No. of Papers	References
	P	R	M	H	MS	S	C
Horticultural
Tomato	X	X		X	X		X	7	Altieri and Esposito [110]; Górecki and Górecki [44]; D’Addabbo et al. [111]; Böhme et al. [100]; Dannehl et al. [83]; Bradshaw and Hagen [101]; Cristea et al. [63]
Lettuce	X	X			X		X	5	Altieri and Esposito [110]; Böhme et al. [100]; Jungić et al. [75]; Ferby et al. [88]; Kovács et al. [105]
Bean		X						2	Ydyrys et al. [77]; Komorowska et al. [78]
Cucumber					X			2	Böhme et al. [82]; Komorowska et al. [89]
Pepper		X		X				4	Górecki and Górecki [44]; Juhos et al. [98]; Karaca et al. [62]; Cristea et al. [63]
Eggplant		X						1	Górecki and Górecki [44]
Guar gum		X						1	Kumar [70]
Kohlrabi	X							1	Böhme et al. [100]
Pea		X						1	Kumar [70]
Spinach	X							1	Bradshaw and Hagen [101]
Swiss chard		X						1	Zheljazkov et al. [45]
Cabbage		X						1	Choudhary et al. [72]
Strawberry		X						1	Broda et al. [80]
Field crops
Barley		X						1	Lal et al. [76]
Lupine				X				1	Marchelli et al. [121]
Maize		X		X		X		4	Mubarak et al. [40]; Abdallah et al. [94]; Metomo et al. [64,65]
Rapeseed				X		X		2	Stokes [106]; Niculescu et al. [59]
Sorghum		X					X	1	Malancu et al. [112]
Sugar beet				X				2	Akca et al. [61]; Taskin [66]
Tobacco				X				1	Evangelou et al. [55]
Wheat seeds		X		X				3	Gogos et al. [57]; Holkar et al. [58]; Broda et al. [79]
Fruit tree/shrubs
Olive tree						X		1	Palla et al. [96]
Prunus domestica L.				X				1	Gitea et al. [91]
Raspberries					X			1	Balawejder et al. [87]
Grass
Ryegrass		X		X		X		4	Gousterova et al. [53]; Nustorova et al. [54]; Abdallah et al. [94]; Broda et al. [79]
Mixed grass pasture					X			3	Marczak et al. [84]; Marczak et al. [86]; Marczak et al. [90]
Red clover						X			Li et al. [99]
Weed germination					X			1	Liu et al. [85]
Other
Ornamental	X	X			X	X		10	Böhme et al. [100]; Butcaru et al. [69]; Butcaru and Stănică [93]; Abdallah et al. [94]; Butcaru et al. [71]; Butcaru et al. [74]; Gabryś and Fryczkowska [96]; Haque and Naebe [101]; Herfort et al. [20]; Herfort [91]
Aromatic		X						2	Zheljazkov [67]; Zheljazkov et al. [45]
Medicinal		X			X			3	Duppong et al. [81]; Zheljazkov et al. [68]; Herfort [92]

Table 2. Experimental trials conducted on different plant species according to application type (P = wool pellet; R = raw wool; M = wool management; H = wool hydrolysate; MS = wool mat and substrates; S = scoured wool; C = wool compost). Application types are marked with X. The number of publications and corresponding references for each plant species is also indicated.

Plant Species	Type of Application							No. of Papers	References
	P	R	M	H	MS	S	C
Horticultural
Tomato	X	X		X	X		X	7	Altieri and Esposito [110]; Górecki and Górecki [44]; D’Addabbo et al. [111]; Böhme et al. [100]; Dannehl et al. [83]; Bradshaw and Hagen [101]; Cristea et al. [63]
Lettuce	X	X			X		X	5	Altieri and Esposito [110]; Böhme et al. [100]; Jungić et al. [75]; Ferby et al. [88]; Kovács et al. [105]
Bean		X						2	Ydyrys et al. [77]; Komorowska et al. [78]
Cucumber					X			2	Böhme et al. [82]; Komorowska et al. [89]
Pepper		X		X				4	Górecki and Górecki [44]; Juhos et al. [98]; Karaca et al. [62]; Cristea et al. [63]
Eggplant		X						1	Górecki and Górecki [44]
Guar gum		X						1	Kumar [70]
Kohlrabi	X							1	Böhme et al. [100]
Pea		X						1	Kumar [70]
Spinach	X							1	Bradshaw and Hagen [101]
Swiss chard		X						1	Zheljazkov et al. [45]
Cabbage		X						1	Choudhary et al. [72]
Strawberry		X						1	Broda et al. [80]
Field crops
Barley		X						1	Lal et al. [76]
Lupine				X				1	Marchelli et al. [121]
Maize		X		X		X		4	Mubarak et al. [40]; Abdallah et al. [94]; Metomo et al. [64,65]
Rapeseed				X		X		2	Stokes [106]; Niculescu et al. [59]
Sorghum		X					X	1	Malancu et al. [112]
Sugar beet				X				2	Akca et al. [61]; Taskin [66]
Tobacco				X				1	Evangelou et al. [55]
Wheat seeds		X		X				3	Gogos et al. [57]; Holkar et al. [58]; Broda et al. [79]
Fruit tree/shrubs
Olive tree						X		1	Palla et al. [96]
Prunus domestica L.				X				1	Gitea et al. [91]
Raspberries					X			1	Balawejder et al. [87]
Grass
Ryegrass		X		X		X		4	Gousterova et al. [53]; Nustorova et al. [54]; Abdallah et al. [94]; Broda et al. [79]
Mixed grass pasture					X			3	Marczak et al. [84]; Marczak et al. [86]; Marczak et al. [90]
Red clover						X			Li et al. [99]
Weed germination					X			1	Liu et al. [85]
Other
Ornamental	X	X			X	X		10	Böhme et al. [100]; Butcaru et al. [69]; Butcaru and Stănică [93]; Abdallah et al. [94]; Butcaru et al. [71]; Butcaru et al. [74]; Gabryś and Fryczkowska [96]; Haque and Naebe [101]; Herfort et al. [20]; Herfort [91]
Aromatic		X						2	Zheljazkov [67]; Zheljazkov et al. [45]
Medicinal		X			X			3	Duppong et al. [81]; Zheljazkov et al. [68]; Herfort [92]

4. Evidence from the Selected Papers

Analysis of the collected literature shows an increase, from 2000 until present, in scientific production, especially indexed articles, related to the application of wool in agriculture. As shown in Figure 4, this increase is exponential and indicates not only the growing interest in this topic but also the need to find bio-based solutions that promote circular and sustainable agricultural practices. The relevance of the main research addresses on the topic can be intuitively grasped through the visual representation provided by the frequencies of the key words related to the indexed articles. The results highlight that Europe is the continent where most of the scientific literature is produced, suggesting that European countries mostly need to find alternatives to the use of their wool. As we have already mentioned, European wool can be defined, generally, as coarse wools with unfit quality standards for the garment industry, showing the highest market demand. Although other sectors such as furnishing and technical textiles (i.e., geotextiles or insulated panels) may require coarse wools, the scarcity (or in many areas the absence) of industrial scouring facilities seems to have created a bottleneck in many European wool textile supply chains, making wool production unsustainable both economically and environmentally. Wool quality depends on sheep breeds, but it may be influenced also by its management. In fact, if wool, after the shearing phase, is not selected according to the different breeds, it becomes commercially undervalued. This issue can be further complicated by the fact that local sheep breeds are often typical of remote and mountainous areas, where wool logistics are more complex. In fact, many areas of the Mediterranean and Alpine regions and of central Europe have been and are currently participating in European projects to exploit local sheep breeds and wools, e.g., Med-Laine [128] and Alp Textiles [129]. The collection of scientific literature on the application of sheep wool in agriculture has provided a wide range of domains that, by promoting bioeconomy and circularity, can effectively contribute to explore alternative uses of this neglected resource while improving the sheep systems sustainability. Looking at the number of publications per plant cultivation, the research interest seems to be more oriented towards horticultural productions. While crops, grass, and medicinal/ornamental/aromatic plants are the subject of a similar number of studies, research on fruit production is less represented. Concerning the domains, most publications focus on research on wool hydrolysate and raw wool, while research on wool compost, scoured wool, and wool pellets is less represented. A systematic economic analysis on the application of sheep wool in agriculture is currently unavailable. As Zoccola et al. [43] pointed out, in Europe, wool management and processing costs are economically unsustainable, whereas hydrolyzed wool as a fertilizer is economically viable. For raw wool, it can be assumed that the research interest is driven by the need to find a direct way to apply the wool without post-shearing treatments, thus avoiding management costs. Additionally, raw wool, compared to other products such as wool hydrolisate or wool pellets, can be more versatile since it can be applied as a fertilizer but also for mulching [69,71,74,75,93]. However, this type of application should consider the impact of wool components, other than the fibre, such as suint containing grease, droppings, vegetal material, and chemicals, which can be harmful to the environment and affect agronomic performances, as it represents a less standardized material subjected to several variables. Wool hydrolysis is, instead, a process that allows the use of raw wool without scouring, while reducing the microbial load. Additionally, keratin hydrolysate can provide a means of easier and more uniform application on the soil. Considering the fertilizing effects of wool hydrolysate and raw wool, both products showed comparable benefits in terms of plant growth as well as differences in the dynamics of nutrient releasing into the soil. However, more tests are needed on specific plants and in different farming systems in order to select the most suitable option. Pelletization represents an interesting solution to overcome the problems of raw wool about the presence of microbial load and to obtain a wool-based product with a relatively low impact transformation process. Although pelletization can also ensure a reduction of the microbial load of wool, thus avoiding industrial cleaning, the literature on wool pellet production is scarce. A survey conducted within the Bresov Horizon2020 project [130] showed that only 2% of the farmers surveyed used wool pellets as an alternative fertilizer. Despite that, another study by Heward et al. [131] reported the increasing popularity in the UK of bio-based fertilizers such as chicken manure pellets, sheep wool pellets, and digestate in agriculture linked to the growth of the horticultural market due to strong consumer demand. Further research is expected to be conducted on all the types of application of wool in agriculture, particularly in less represented domains. However, it should also be noted that both wool pellets and scoured wool show a growing trend. Moreover, the increased number of reviews in the period 2019–2024 could indicate the need to frame knowledge development and create guidelines for policy and practice [132], thus confirming the general increasing attention of research on these topics. The properties of wool also need to be analysed more in depth in order to evaluate the net beneficial effects on the environment. Most studies have focused on the fertilizing properties of sheep wool in relation to plant growth and yield. In this regard, further research is needed on the dynamics of N and S release from wool, especially to better synchronize plant demand [68] and nutrient release, optimize application rates [73], and thereby reduce groundwater eutrophication. Concerning amendments, Li et al. [99] suggested research on combinations of multiple amendments. Also, soil physical parameters need to be better investigated because pH, soil water retention, porosity, and bulk density have been proved to be affected by wool. This will provide a better understanding of how to develop more efficient water-saving strategies using sheep wool, which has been shown to be a water retention tool in fruit tree cultivation under drought conditions [91]. Alternative applications, such as bio-stimulant effects of wool on soil fauna [66] and pest control [110], also deserve more investigation to reduce the use of agrochemicals and potentially reduce pollution. Moreover, exploring protein sources, such as keratin, as alternatives to conventional and plant-based sources could help preserve agricultural land and biodiversity [64]. Also, the combination of mulching and fertilization effects on slope greening [95] is a promising topic for developing environmental-friendly solutions that can replace fossil-based materials. Despite the potential of wool products in agriculture, the research in this field remains fragmented, as demonstrated by this review. The comparison between the different sheep wool products in terms of effects at soil nutrients level is difficult to provide. The reasons behind this can be the different methodologies used to obtain the final products, as in the case of the different hydrolysis methodologies used to extract keratin from wool. Similar considerations can be made for the other main wool products analysed in this review (i.e., pellets, compost, and wool mats and substrates). In addition, besides the production process, the final composition of the wool products can be influenced by the conditions of the greasy wool, which, in turn, depend on the type of sheep breeding (extensive or intensive) and the wool management and logistics (e.g., shearing phase, wool storage and transportation). Moreover, the dynamics of nutrients at the soil level can depend on the interaction with the cultivated plant and, in this respect, the number of studies per plant species/wool product available from the collected scientific literature is still insufficient. Finally, the management of wool waste is an important issue to find environmentally sound solutions that avoid rebound effects. In this regard, life cycle thinking approaches such as LCA (Life Cycle Assessment) and circular economy evaluations are currently lacking in the scientific debate on sheep wool use in agriculture. LCA studies and circularity assessments can provide effective data and information to evaluate the most appropriate application in different contexts, thereby integrating information on crop yields, nutrient cycle, and effects on soil in a life cycle perspective. On the other hand, a comprehensive economic feasibility assessment (production costs and willingness of farmers, agricultural advisors, and consumers to purchase bio-fertilisers) is essential to design large-scale development of products such as wool pellets, wool hydrolysate, or wool compost.

5. Research Gaps and Future Directions

The present review provides interesting findings about the use of sheep wool for agricultural purposes. However, despite the promising perspectives of a more sustainable agriculture, there are certain research spaces that can be further structured, and additional investigations can be recommended, including the potential concerns of consumers and the general public regarding the use of sheep wool as a source of nutrients for crops.

5.1. Raw Wool

The dynamics of nutrient release need to be better synchronized with plant demand [45], and the optimal rates of wool application to different crops [112] need to be assessed, thus maximizing crop yields and avoiding possible eutrophication of underground water. In addition, the possible presence of both biological and chemical contaminants should be explored in depth.

5.2. Wool Hydrolysate

Further research on the impact of prolonged wool hydrolysate application on the physical, chemical, and biological properties of soil [55] is necessary. Keratin extraction methods, such as acoustic cavitation, provide encouraging results in experimental trials, but tests on larger-scale studies should be conducted to verify their environmental and economic sustainability [58].

5.3. Wool Pellets

Similar gaps can be related to wool pellets, as regards to the influence on soil properties, especially changes in pH and EC values [100], as well as the environmental and economic implications of this production that are linked to greasy wool management.

5.4. Greasy Wool Management

The few studies conducted in this field [1,107,108,109] highlighted the importance of creating wool collecting centers. Nonetheless, the uneven distribution of wool production within countries and across different regions requires contextualized studies to assess the economic and environmental sustainability of wool logistics, including possible transportation from low density wool producing areas to collection and transformation centers. This calls upon the need for the calculation of LCA of greasy wool.

5.5. LCA

To our knowledge, there are no studies on LCA calculated on wool products for agricultural applications. Some examples of LCA studies on sheep wool were found, such as the evaluation of the impact of sheep wool production in Australia by Wiedemann et al. [133]. Other studies focused on the worsted and woolen processing phases of wool production [134], while others investigated the value chain based on the recycling of pre- and post-consumer discarded textile [135,136]. Many studies, instead, analyzed the impact of sheep milk/meat production systems [137,138]. LCA studies should address the environmental implications of greasy wool processing, hereby including scouring, hydrolyzation, pelletization, and composting, in order to quantify the reduction of GHG emission by using wool-based fertilizing products instead of synthetic fertilizers.

6. Conclusions

The large amount of sheep wool globally produced represents a neglected resource for which environmentally sound solutions are needed in order to turn potential waste into valuable products. Sheep wool is a product that incorporates (i) relevant environmental values as a renewable resource; (ii) economic values provided by its production costs (shearing process and breeding system); and (iii) cultural values provided by the rural areas to which the wool productions belong, which are returned in the form of landscape and innovation. In addition to being a commodity, sheep wool provides several ecosystem services thanks to this combination of values and inherent qualities as those generated by its use in agriculture. The present paper has provided an in-depth review of the possible and multifunctional applications of wool in agriculture. This review highlighted that sheep wool is rich in high N, C, and S content and shows good water absorption and retention properties. Moreover, acting as a slow N-releasing fertilizer, it demonstrates itself as a viable bio-based solution. The exponential trend of academic literature production on sheep wool in agriculture is a positive signal of the interest of agronomic sciences on the potential of the wool as a biomaterial, even though some knowledge gaps and controversial issues still need to be filled and clarified. Research is also needed to better assess the environmental impacts that can be avoided by using wool in agriculture in terms of nutrients and soil properties as well as plant growth. In this context, LCA and circular economy studies as well as management issues enquiries should be encouraged to support the overall sustainability of the widespread use of wool in agriculture as bio-based solutions for agroecosystems.