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Review

Formiguer Fertilization: Historical Agricultural Biochar Use in Catalonia and Its Modern-Day Resource Implications

by
Nicolas Sesson Farré
1 and
Aaron Kinyu Hoshide
1,2,3,*
1
Born Global Foundation, 254 Commercial Street, Portland, ME 04101, USA
2
Sensei Economic Solutions, LLC, Rome, GA 30165, USA
3
AgriSciences, Instituto de Ciências Agrárias e Ambientais, Universidade Federal de Mato Grosso, Sinop 78555-267, MT, Brazil
*
Author to whom correspondence should be addressed.
Resources 2025, 14(8), 120; https://doi.org/10.3390/resources14080120
Submission received: 31 May 2025 / Revised: 21 July 2025 / Accepted: 22 July 2025 / Published: 28 July 2025
(This article belongs to the Special Issue Resource Extraction from Agricultural Products/Waste: 2nd Edition)
Browse Figures
Review Reports Versions Notes

Abstract

Biochar is an amendment that can enhance both soil fertility and sequester carbon. However, its historical applications continue to be underexplored. In this overview, we investigate the formiguer method of burning woody biomass to create agricultural biochar for use as fertilizer in Catalonia, Spain, within the context of historical biochar use. A literature review targeted searches of scholarly databases to compare the formiguer method to Amazonian terra preta and other traditional biochar use. We identified sources covering biochar properties, soil impacts, and historical agricultural practices within the Iberian Peninsula and briefly described the main methods or treatments used during this process. Past research demonstrates that the formiguer method, which involves pyrolytic combustion of biomass within soil mounds, improves microbial activity, increases soil phosphorus and potassium availability from soil structure, and leads to long-term carbon stabilization, even though it can result in short-term decreases in soil organic carbon and nitrogen losses. Despite being abandoned in Europe with the rise of chemical fertilizers, the use of formiguers exemplifies a decentralized approach to nutrient and agroecosystem management. The literature highlights the relevance that these traditional biochar practices can have in informing modern soil management and sustainable agricultural strategies. Understanding the formiguer can offer critical insights to optimize contemporary biochar applications and historical techniques into future sustainability frameworks.

1. Introduction

Biochar is a carbonaceous resource produced from pyrolysis of organic biomass under oxygen limitation [1]. Biochar has come under the spotlight for the immensely valuable role it plays in mitigating contemporary environmental issues, such as sequestration of carbon dioxide [2,3,4], managing solid waste as a potential feedstock, and remediation of toxic metals [3] and pollutants in soils [5], water [6,7], and wastewater [7], especially when the biochar is modified to enhance efficacy [3,5,6,7,8,9]. In addition to toxic metals, biochar can also treat other contaminants of emerging concern [10,11].
In this review, we contextualize the historical Catalonian formiguer method of agricultural biochar production, in which woody biomass was burned under soil cover to create lower temperature and oxygen conditions suitable for pyrolysis. With its multifunctional properties of increasing soil microbial biomass [12,13] and improving agricultural productivity and soil water holding capacity [14,15], biochar has the potential to contribute to a more sustainable agriculture [15]. Other potential agri-environmental benefits of biochar include creating higher value fertilizers from agricultural crop/livestock wastes [16], reducing nitrogen leaching [17], and thus, enhancing nitrogen cycling [18], pesticide adsorption [19], and soil water retention [20]. Biochar serves as a soil amendment option [11,21] among a diversity of natural/industrial soil amendments [22]. Biochar also supports composting and other industrial processes [21]. Such potential agricultural, agronomic, soil, and environmental benefits of biochar application have been thoroughly documented in recent meta-analyses (Table 1).
In addition to contributing to more efficient environmental remediation and sustainable agriculture, biochar can also contribute to current global efforts to de-carbonize industrial processes [48] (Table 1). The revived interest in biochar over the past two decades is closely linked with its role within the global quest for negative emission technologies. This more recent emphasis on industrial biochar production has become de-coupled from indigenous peoples’ production of biochar at a smaller scale within local communities [70].
Regular use of fire and charcoal production associated with human activities can be dated back to a period of time between 400,000 and 350,000 years ago [71]. Biochar has smaller particle size than charcoal [72]; like charcoal, biochar can be traced back through human history. The existence of biochar dates back at least 2000–2500 years to the Amazonian terra preta (i.e., dark earth) soils [70,73], which the Amazonian indigenous peoples amended with charcoal and organic matter [74]. These dark, fertile soils have persisted for centuries, offering a tangible historical example of biochar’s long-term benefits to soil fertility and carbon retention [74,75]. Interest in biochar began in the late 20th century, when it was realized that biochar had the potential to replicate these same advantages in other agricultural contexts [75]. Indeed, historical knowledge of terra preta forms the basic understanding of biochar in creating sustainable agricultural systems and, hence, acts as a yardstick for assessing its potential use in modern soil management practices [70].
The characteristics of biochar are predominantly defined by pyrolysis, the thermal decomposition of organic material under low or zero oxygen conditions. Temperature, feedstock type, and residence time have a great influence on the characteristics of the resulting biochar. For example, higher temperatures of pyrolysis, usually above 350 °C–400 °C, tend to yield carbon forms that are more resistant to microbial degradation, thus enhancing the capacity of biochar for carbon sequestration [76,77]. The feedstock and temperature utilized for the production of biochar defines its porosity, surface area, and nutrient content, and thus directly influences its functionality when applied to soil [77,78]. For instance, wood-derived biochar tends to have a higher surface area and more pronounced porosity when compared to biochar from agricultural residues [78].
Biochar also has the potential to sequester carbon in soils for long periods [79]. Biochar carbons are highly stable; indeed, it has a mean residence time of hundreds to thousands of years as a function of the pyrolysis conditions and the nature of the soil in which it is applied [80]. This is due to the aromatic carbon structures formed during pyrolysis that are resistant to microbial decomposition, which has the potential to annually sequester 1.3 Pg of carbon dioxide equivalent (CO2e; ~12% of 2010 global carbon emissions) over the next century [81]. Thus, biochar has been identified as a long-term carbon-negative technology [82]. Controlled burning of forest residues and waste can also help reduce the negative impacts of wildfires on food production and the environment [83]. Assuming no additional land is needed to just produce biochar, biochar can use less land globally at an estimated 40–260 million hectares or Mha, which is less than afforestation/reforestation (320 to 970 Mha) and bioenergy with carbon capture and storage (380 to 700 Mha) [84]. Production of biochar via pyrolysis can also produce bio-oil, which can be used as an alternative fuel source that can potentially further reduce carbon emissions [85].
Besides its carbon sequestration ability, biochar can bring about noticeable advantages in agriculture as well. Upon addition to soil, biochar improves soil structure, increases water retention [86], and provides enhanced nutrient availability to plants [87] (Table 1). Because it is porous, during the dry season in arid regions, biochar can retain water so that plants need less irrigation [87] (Table 1). Water retention capacity of soils could be increased by adding biochar, especially if the biochar undergoes water vapor activation [87]. In addition, nutrient leaching, such as that from nitrogen and phosphorus, which are both highly essential for plant growth, may decrease with biochar application, boosting soil fertility and reducing the environmental impact of fertilizer runoff [88] (Table 1).
The benefits of biochar to soil health do not only involve retention of water and nutrients. Biochar also enhances microbial activity in soils through the habitat that it provides for beneficial microbes, which are important for nutrient cycling and plant growth [89]. These microbial communities increase the efficiency of carbon use in soils, further enhancing biochar’s role in the promotion of soil organic carbon accumulation for long periods of time (Table 1). Biochar was found to decay at only 0.26% per year, suggesting persistence in the soil for up to 4000 years [90]. Therefore, biochar serves as a catalyst for sustainable agriculture by improving both soil health and crop productivity, particularly by increasing pH, soil water holding capacity, and crop nutrient availability [91].
The widespread application of biochar is not devoid of challenges. The increased variability in biochar quality due to production processes and feedstock type are likely one of the major reasons for inconsistent field performances [92]. While short-term studies suggest biochar can increase crop yields under drier conditions and in acidic, lower quality soils, a large knowledge gap still remains about the long-term impact of biochar amendment on crop yields, soil health, and carbon sequestration under various environmental conditions [93]. Determining the optimal application rate for particular soils and crops, together with understanding biochar’s long-term sustainability, requires further research on the use of biochar produced from different source materials [93,94].
Biochar is one of the promising tools needed to face the most daunting modern-day environmental and agricultural challenges. As such, it represents a dual-purpose tool for carbon sequestration and soil improvement for sustainable agriculture. However, for these potentials to be actually realized at a global scale, there are gaps in research and further development that need to be fulfilled. This literature review attempts to help connect the earliest documented global (e.g., terra preta, Indian jhum, Asian rice husk charcoal) and northern European (e.g., soil paring and burning) uses of biochar with biochar’s more recent popularization and modern application over the past twenty years. Our focal area of study is the historical production and agricultural use of biochar in Catalonia, Spain, from the 1800s to the 1960s. This formiguer method involved burning agricultural wastes (e.g., grape vineyard clippings) or biomass collected from the surrounding landscape under low-oxygen pyrolysis conditions to help fertilize crops.

2. Materials and Methods

We conducted a literature review on biochar using GoogleScholar searches with the keywords “biochar” and “review”. Further GoogleScholar searches were added to find prior research articles on biochar attributes such as its carbon/soil remediation, water retention, and carbon sequestration using the keywords carboneras, terra preta, sequestration, microorganisms, Amazonian, anthropogenic, etc. Additional article searches for specific geographical regions of charcoal use and biochar production used keywords denoting the area of focus (e.g., “Iberian Peninsula,” “Spain,” “Catalunya,” etc.). Agricultural journal articles identified using keywords “biochar” and “meta-analysis” were summarized with average percent changes of biochar impacts weighted by observations used in each study.

3. Earliest Biochar Applications

Charcoal has been used in pottery and for other household uses in China over the past 7000 years [95], and biochar is a co-product of charcoal production. Biochar was used in traditional Chinese agricultural systems, which also relied on aerobically burning crop residues, applying dredged nutrient-rich canal sediment, and composting plant wastes, animal manure, and human manure [96]. The earliest known uses of biochar for soil improvement can be traced back to circa 500 BC–500 CE by pre-Columbian Amazonian civilizations who produced what is known today as terra preta or “black earth” [70,73] (Figure 1).
Terra preta soils are characterized by their distinct black color, which is different from the surrounding soil strata. Their color stems from the deliberate mixing of organic matter with charcoal by farmers [73]. Food waste and excrement/manure were the most likely human-derived nutrient inputs to the soil. Archaeologists believed that the widespread development of tropical soils close to these ancient homesteads was caused by deposits of domestic garbage that accumulated over time [97]. This led to an increase in the fertility of these soils compared to the naturally acidic and nutrient-poor soils of the Amazon, which played an instrumental role in facilitating the development of these Amazonian societies and peoples.
The discovery of terra preta in the 19th century drew scientific interest largely due to its anthropogenic origin [70]. This discovery substantiated the existence of advanced pre-Columbia civilizations in the Amazon, lending a degree of credibility to the accounts of both the conquistadors and explorers that had visited the Amazonian area in the 15th and 16th centuries. Early agricultural scientists began to investigate how indigenous people had managed to sustain high agricultural yields in such inhospitable environments [98]. These initial studies began in the early 20th century. This led to the discovery that biochar not only improves the phytochemical properties of soil, but its addition also has an impact on the biological properties of soil. Adding biochar leads to an increase in soil fertility by ameliorating existing soil structures, which is possible because biochar’s lattice structure allows it to contain more mineral complexes and pore space [75]. Furthermore, its porous structure enhances both nutrient cycling and retention, particularly nitrogen retention and immobilization. This decreases nutrient leaching from soil in which biochar acts as a soil aggregate [99]. However, interest remained sporadic until the late 20th century, when emphasis on sequestering carbon and reducing soil degradation launched research into biochar’s potential as a sustainable agricultural amendment [16,18,19,20].
In the late 20th century, biochar application was revisited as an alternative strategy for carbon sequestration to mitigate the effects of climate change. This was due to biochar’s potential to sequester carbon for centuries in a stable form [85]. Further research into its role has not only focused on improving soil health but also reducing greenhouse gas emissions. By the early 2000s, scientists began investigating further uses for biochar, including enhanced water retention, facilitation of nutrient cycling, and its role in reducing the need for chemical fertilizers. Recent studies and their theories have suggested that terra preta may have a natural origin, which suggests that these pre-Columbian populations that resided within the Amazon basin most likely intentionally enhanced and utilized the smaller patches of soil found in these regions with less fertile soils [98].
The Amazon’s naturally acidic and nutrient-poor soils, such as oxisols and ultisols, are highly weathered, have low cation exchange capacity, and are subject to rapid nutrient leaching [98] due to seasonal rainfall, regionally averaging between 1.5 and 3 m a year [100]. These conditions render the land unsuitable for sustainable agriculture. The Amazonian peoples utilized terra preta to offset this highly infertile land via their implementation of soil amendments.
Silva et al. (2021) presented a new theory for terra preta formation [98]. Previous theories have assumed that these Amazonian dark earths are exclusively anthropogenic in origin when the addition of biochar organic waste and other materials enhance fertility. However, Silva et al. (2021) propose that natural processes, particularly mid- to late-Holocene flood events driven by intensified monsoons, may have played a significant role in the formation of terra preta [98]. These floods likely led to deposits of nutrient rich sediments and micro-charcoals (i.e., fine particles of biomass produced by natural fires) onto flood plains. They propose that indigenous peoples discovered these deposits, settled in the area, and then their human activities (e.g., food production/procurement and waste disposal) led over time to the creation of highly fertile soils. This two-origin hypothesis suggests that Amazonian dark earths are the result of a complex interplay between fluvial processes and the intervention of individuals within these areas. This more nuanced understanding of the formation of terra preta is likely more plausible than the soil being created solely from human intervention; predictions indicate that this would have taken ~7500 years [98], which is three times longer than the oldest terra preta.
Although the notion of terra preta remained intimately linked to Amazonian societies, archaeology, and native customs, researchers have uncovered terra preta in other areas around the globe. Other terra pretas include those in South Africa (200 BCE to 1000) [74,101], West Africa [74,102,103], Egypt [104], Ecuador, Peru [74], in the Colla Valley in the Andes in southern Peru (within the past ~1500 years) [105], and in indigenous oven mounds in Australia (~500–1500 years before present) [106]. More recently, biochar has garnered global attention and has been incorporated into ecological modernization [70]. Understanding the complex and rich history of biochar and its relationship to terra preta is essential to comprehending how the idea has evolved over time and how global environmental discourses have impacted it.

4. Historical Use of Formiguers in the Iberian Peninsula

Throughout the centuries, the Iberian Peninsula has had a long-standing tradition of agricultural practices that reflect the ingenuity and adaptation of the local population to the region’s specific environmental conditions and demands [107,108,109]. The traditional formiguer method of fertilizer production was used where biochar was formed through controlled biomass burning and was used to close the nutrient gap that led to soil infertility [107,108,109]. It is known as the “anthill” method due to the mounds formed during the process of controlled burning, which resemble anthills [108,109,110] (Figure 1). However, the formiguer method is clearly distinguished from charcoal fuel production in that it is conducted exclusively for agricultural fertilization [108,109]. Organic biomass is collected from nearby crops or landscapes, and a reverse conical depression was likely dug below each anthill to improve pyrolysis efficiency (Figure 2). The municipality of Osor in the comarca of La Selva in Catalunya (i.e., Catalonia), Spain, also historically produced charcoal (i.e., carboner) [111] (Figure 3).
Historical records from Mediterranean agroecosystems, such as Les Oluges in Catalunya, have reinforced the significance of the formiguer method. The method has been shown to effectively recycle nutrients from the local environment [112] while also providing vital inputs of phosphorus and potassium. This contributes to nutrient cycling while simultaneously ensuring pest management and weed control [112]. Although nitrogen loss occurs during pyrolysis of the biomass, this is offset by the subsequent increases in organic matter mineralization from the soil cover, which enhances the availability of nutrients [112]. This technique represents an early, decentralized approach to nutrient management that predates modern day biochar techniques, as shown in Figure 1 [110].
In this section, we examine the origins of the formiguer and hormiguero techniques. We primarily refer to the practice as formiguer (single anthill) and formiguers (multiple anthills). Furthermore, we explore the agronomic impacts of the formiguer method. We also review potential environmental benefits of this practice.

4.1. Origins and Historical Use of the Formiguer Method

The formiguer/hormiguero method achieved widespread use throughout the Catalan autonomous communities (Valencia, Catalonia, Balearic Islands, etc.) where this was called formiguer, while in Spain they were called hormiguero [107,108,109]. This fertilization method was used until the mid-20th century, when the arrival of chemical fertilizers led to a decline in their use. Farmers created these anthill mounds by bundling firewood or other biomass and mounding it into a conical shape. This was then covered with soil in order to ensure that pyrolysis could occur. The low-oxygen combustion process was ignited using a plant-based wick. The pile would smoulder, which could last for several days; once the burning was completed, the resulting ash and soil was spread over the fields as a fertilizer [110].
Specific practices to create formiguers varied regionally due to local climates. In Sentmanat, the formiguer method was just one of many alternative techniques used to replace nitrogen lost through farming. A prior study found that, on the 1816 hectares of land which had been ploughed for agricultural use [112], all locally produced manure contained 12,164 kg of nitrogen, and that at least 50% of available nitrogen would be lost in the dung pile [112]. Therefore, biochar fertilization practices were needed to close this nutrient gap. Alternatively, in Mallorca, firewood was gathered between January and August. After this, mounds were carefully constructed with ventilation holes at both the bases and the tops of the mounds to ensure that combustion was controlled with minimal oxygen. This practice was labor intensive and deeply rooted in local agricultural traditions, encouraging the sustainable use of waste products and available resources [108,109].
Olarieta et al. (2011) [110] built a formiguer in an olive tree orchard. These researchers used dry woody material similar to what is shown in Figure 2, which contained small branches from trees such as the Aleppo or Jerusalem pine (Pinus halepensis) and the common olive (Olea europaea). They initially placed the material in a pile before converting the biomass with soil sods that were obtained from mineral soil deposits located within the vicinity. This was completed to replicate soil that would have been amended by farmers carrying out the formiguer method in the 19th century. They used a control of similar woody material which was burnt without soil cover to compare the ashes produced to those produced using the formiguer method.
Surveys of farmers in the Catalunya Priorat region have mentioned that the formiguer method was historically valued for its weed-killing ability as well as its potential to disinfect soil cover due to the high heat from pyrolysis of the soil when biomass is burnt in these mounds [114]. The disinfecting effects of the formiguer method depended on the temperature that the soil was heated. Due to the unpredictable nature of this along with the lack of standardization of the process, there is a large variation in these temperatures. Most soils would be sterilized at temperatures between 250 °C and 300 °C [115]. Past research by Serrasoles and Khanna (1995) demonstrated the potential of the formiguer method to act as a disinfectant and surrogate for autoclaving, where such biocidal treatment inhibits microbial activity and growth within the soil [114].
The use of formiguers has previously been thought to be solely motivated by using the ashes produced from burning biomass as a soil amendment. However, due to its labor-intensive production process coupled with the small yields of fertilizer produced, other soil amendments (e.g., animal manure) initially seem to be more appealing. However, there is likely another reason why farmers carried out this laborious process. Farmer interviews by Masip (2003) mentioned that the soil used to cover the formiguer mounds was as important as the biochar product formed during pyrolysis [116].
It can be assumed that the formiguer/hormiguero method played a different role depending on available biomass in the region and land area being fertilized. Due to how labor intensive the method was, it likely played a vital role in filling the remaining gaps within the nutrient balance of agriculture in the region [110]. This is because alternative methods of agricultural fertilization, such as human sewage/garbage and burying fresh biomass into the soil, as used in Sentmanat, Catalonia, Spain [109], were likely more readily available and required less of the limited manpower that was available before Spain’s industrialization. Furthermore, the efficiency of producing fertilizer using formiguers required substantial amounts of biomass. To fertilize one hectare, between 10 and 25 hectares of forest was needed in order to source the necessary amount of biomass [85] for fertilization of grain and vegetable production as well as for vineyards. The traditional formiguer/hormiguero method used dried branches and biomass from surrounding forests and scrub lands, which were then covered with soil during burning. This reduced the build-up of excess fuel in the landscape, which could otherwise contribute to wildfires [109].
The widespread adoption of chemical fertilizers and their increased use in the 1960s [117] led to decline of the use of formiguers [118]. In their study of the Les Oluges region in Catalunya, Díez et al. (2019) theorized that it was the availability of these mineral fertilizers, as a faster and more uniform solution for agricultural fertility management, which led to the capacity for greater nutrient use in local agroecosystems [112]. This led to the industrialization of agriculture; although it was a necessary innovation, this change contributed to the abandonment of the formiguer method, marking a loss of a traditional agricultural system that balanced agricultural productivity and ecological sustainability [112]. The resurgence of interest in biochar has resulted in comparisons to historical practices, like the use of formiguers.

4.2. Agronomic Impacts of Formiguers

Soil fertility and soil organic carbon are important agronomic indicators for crop production. The formiguer method had a notable effect on the fertility of soil in which it was used [110]. The burning process increases levels of exchangeable potassium and labile phosphorus. The use of formiguers has a notable effect on the availability of nitrogen and nitrogen fixation properties of soil [110]. Olarieta et al. (2011) found that the burning process significantly changes nitrogen dynamics within the soil, such as decreasing mineral nitrogen, including both nitrate (NO3) and ammonia (NH4). In addition, biomass content was reduced from 7.0 mg.kg−1 to 1.4 mg.kg−1 within the inner layer of the formiguer [110].
While there were increases in ammonia nitrogen levels within the outer soil layer, this was not sufficient to compensate for the loss of nitrate. This contributed to an overall loss of short-term mineral nitrogen levels within the soil. The total nitrogen content within the soil remained at normal levels after using formiguers, which suggests that although this practice may lead to short-term nitrogen losses, it does not drastically impact nitrogen content in the soil in the long term [110,112]. While the formiguer/hormiguero method can initially lead to a decrease in nitrate, the burning of plant material also leads to nitrogen mineralization in the soil. Furthermore, the heat from the fire can also release nitrogen in the form of ammonium ions, which are available for plant uptake and can improve growth [110].
The use of formiguers also has an impact on potassium cycling and availability in the soil matrix [110]. Potassium is a vital nutrient for plant growth, serving as one of the three important macronutrients for plants which aids water regulation, enzyme activation, and photosynthesis [119]. Its presence in the soil is influenced by both the burning process and its interaction with other soil nutrients. After burning the biomass within the soil cover of the formiguer, Olarieta et al. (2011) found that exchangeable potassium within the soil increased by over 300% within the inner layer of the soil cover [110]. This increase in potassium availability within the soil matrix can be attributed to the high potassium content in the ashes and charcoal resulting from the combustion of the biomass. Comparative studies discovered that the areas treated with formiguers significantly outperformed nearby untreated soils.
The formiguer method had a positive effect on soil in the short term, resulting in a greater amount of soluble phosphorus [108]. This was also demonstrated by Olarieta et al. (2011), who found that mean values for labile phosphorus, the pool of phosphorus in the soil that is weakly bound to soil particles making it available for plant uptake, was 4–9 times higher in the inner layer of soil cover after it had been burnt using formiguers [110]. In a region of Catalunya called Les Oluges, Díez et al. (2019) found that formiguers played a key role in providing nutrient input into the soil. They found that they could provide 15% of the potassium and phosphorus extracted from the soil. However, much like Olarieta et al. found, there was a short-term net loss of nitrogen content in the soil, namely a 13% increase in extraction of this nutrient [112].
Regarding soil organic carbon (SOC), Olarieta et al. found that using formiguers led to a considerable decrease in SOC, with values in the inner layer of the formiguer reaching 60% of pre-burning levels and a general decrease of available SOC in the soil cover of 33% [110]. Despite this, the intentional practice of creating formiguers facilitates the incorporation of carbon-rich charcoal into the soil much like modern biochar, which can provide long-term benefits for soil structure, water retention capabilities, and soil health. The charcoal fragments added to the soil can act as a stable form of carbon that can remain in the soil for extended periods of time. This can potentially enhance soil fertility and microbial activity within the soil [108], much like the previously described impacts of biochar.

4.3. Agronomic and Environmental Benefits of Formiguers

In summary, the formiguer/hormiguero practice led to increases in soil fertility through several interconnected mechanisms, which can be separated into four main benefits: (1) nutrient release, (2) added organic carbon, (3) nitrogen mineralization, and (4) improved soil structure and microbial activity. The burning of plant material within the formiguer creates organic carbon, which encourages mineralization of nutrients such as phosphorus and nitrogen. Furthermore, plant ashes tend to be rich in exchangeable/labile potassium, which increases its availability within the soil [110]. The charcoal produced during the combustion process is rich in carbon. This charcoal is a form of organic carbon that can remain stable within the soil for long periods of time sequestering carbon [15].
In addition to agronomic benefits, formiguer/hormiguero fertilization increases biopores in the soil, which provide habitats for soil macroorganisms (e.g., earthworms). Furthermore, incorporating ashes, charcoal, and other organic materials from burnt plant material can enhance the physical structure of soil. This can increase soil porosity as well as water infiltration and aeration of soil, which is important for a healthy root system. In addition, the increased organic matter present in the soil supports beneficial microbial communities that are responsible for nutrient recycling, decomposition, and formation of soil aggregates and can improve overall soil fertility [108]. However, the extent of this impact can depend on the amount of formiguers and, thus, biomass used within the agricultural system, which depends on the surrounding native forest/habitat [109,112].

5. Discussion

The formiguer/hormiguero practice is of particular interest because it represents a deliberate and intensive method of fertilizing an agricultural system. This demonstrates an intentional approach to nutrient management within the pre-industrial Spanish agroecosystem. Tello et al. (2012) found that the impact of the formiguer method within the region that they were studying, namely Setmanat, was crucial in filling the remaining gaps in the local agroecosystem’s nutrient balance due to the scarcity of pasture for livestock and subsequent access to manure [109]. This process has similarities to other historical biochar production for agricultural purposes, as distinguished from biochar created by wildfires or from exclusive focus on the production of charcoal.
The amount of effort required to produce formiguers becomes apparent when you take into account the amount of biomass per hectare required to produce agricultural biochar. Assuming 265 formiguers per hectare, within the range of 260–270 given by Olarieta et al., 2011, and a size for an individual formiguer of 0.5 m3 when used for soil remediation on a yearly basis [110], we would need 18,020 kg/ha of biomass for the formiguer process. This is a substantial amount of biomass to collect in a time when industrial machines were almost non-existent. Despite this, historical reconstructions in Les Oluges in the Catalonian region of Spain in 1860 showed that formiguers supplemented fertilization with manure for both small grains and vineyards [112].
However, while the formiguer method enhanced some aspects of soil fertility, it led to decreases in soil nitrogen [110]. When compared to modern day biochar, which retains carbon and stabilizes nitrogen within soil when used as a soil remediation tool [15], the formiguer method is a limited fertilization strategy [112]. Unlike the relatively uncontrolled combustion process inherent to the use of a formiguer when fertilizing, modern day biochar production involves precise temperature and oxygen regulation to ensure that the products of pyrolysis are in the most ideal proportion [15]. While using formiguers is a valuable historical approach to soil fertility, it fell out of practice by the 1960s due to its inability to compete with the much easier and time efficient method of using mineral fertilizer for soil remediation [118]. Furthermore, the ability of this historical biochar process to serve as a useful way of managing soil fertility is ultimately foreshadowed by the superior efficiency of contemporary biochar techniques [15].

5.1. Formiguer Method Compared to Similar Historical and Current Practices

The Spanish formiguer method of biochar production for agriculture falls within a historical framework of similar past and current biochar production ranging from Amazonian terra preta, jhum in India, rice-hull charcoal in Asia, soil paring-and-burning of grasslands and heathlands, and more recent modern biochar research (Table 2). We did not evaluate every single geographic region for historical use of agricultural biochar in this review. However, major cases for the intentional production of biochar through controlled burning via pyrolysis have been presented and summarized in this discussion.
The oldest physical evidence of biochar production in the anthropological record is Amazonian terra preta, which has been dated back to 475 BCE [70]. However, it is not clear if terra preta was intentionally produced for agricultural use, resulted from burning wastes and middens [120], or was a product of flood deposits [98]. While the jhum system in northeastern India is older, dating back to ~7000 BCE [121], it is part of a swidden (i.e., slash-and-burn) rather than a settled agricultural system, in contrast to other applications (Table 2). Like swidden, jhum typically clears smaller forested areas that are burned in high oxygen settings (i.e., slash-and-burn), cropped for a short period, re-established into forest, and then cleared/ burned again after 15–20 years [122]. In order to boost the fertility when cropping on shorter time scales (just a few years), jhum can involve slash-and-char rather than slash-and-burn [122].
Both Indian jhum and Spanish formiguers burned or burn biomass collected from area forests, natural habitats, or perennial crops (e.g., vineyards) in mounds covered with soil to maintain low-oxygen burning [110,123]. Forests and scrubland are easier to clear with axes and fire compared to how the preparation of grasslands and heathlands was accomplished prior to the invention and use of plows [124]. However, a comprehensive synthesis by McKey (2021) summarizes similar biomass burning conducted in Europe as early as the 10th century in grassland/heathland environments. In a more labor-intensive process, narrow soil blocks with grass/plant cover were dug (i.e., paring), dried, inverted, mounded or lined up, and then burned with lower oxygen availability. This process of “paring-and-burning” is still conducted in Sub-Saharan Africa (e.g., Congo, Ethiopia, and Zambia) [124]. Since rice hulls break down slowly, making them difficult to compost, Asian rice producer historically burned them [125,126] via slow pyrolysis, where oxygen was more limited, by adding fresh hulls to kilns [126]. This rice hull ash would be mixed with manure to fertilize crops [125,126].
Table 2. Agricultural biochar contrasts for Spanish formiguers and historical/modern applications.
Table 2. Agricultural biochar contrasts for Spanish formiguers and historical/modern applications.
Agricultural Biochar
Attribute		Amazon
Terra Preta		Spanish
Formiguers		Indian Jhum/Asian Rice Husk CharcoalSoil
Paring-and-
Burning		Modern
Biochar
Time period475 BCE to 1550s 1 [70,73]1800s to 1960s
[110]		~7000 BCE to
present [121]; 1000s of years [125,126]		As early as 900s in
Europe; present day
Africa [124] 		1980s to present
[127]
Agricultural:
  TypeSettledSettledSwidden/SettledSettledSettled
  EnvironmentTropical ForestTemperate Forest/ShrublandTemperate & Sub-tropical Forest
[122,125,126,127]		Grassland/Heathland [124]Various
  Location(s)
AmazonIberiaNortheast
India/Asia		Europe and
Sub-Sahara Africa [124]		Various
Pyrolysis:
Temperature RangeLower
[128]		250–300 °C when used for disinfectant/autoclaving [114,115]; low/high from lack of temperature controlHigher if slash-
and-burn;
Lower if slash-
and-char [121]/Lower [126]		Lower [124]Lower at 300–600 °C [76,77,127]
Low/No
Oxygen		Yes [128,129]Yes [108,109]Limited if slash-and-char, depends on quantity of soil cover/biomass [121] 2/Low [126]Variable depends on amount of soil cover [124]Yes [1]
Production SpeedUnknown and debated if dark earth entirely of anthropogenic origin [98]Slow smoldering process for each formiguer potentially taking several days [110]Depends on biomass quantity and burning intensity during December to February [121,130] 2
/Slow since smothered by more rice husks [126]		Slower since sod slower to turn over, dry out, and burn [124]Slow [127]
Intended Agriculture Use?Debated [98]Yes, clearly intended for fertilization [109,110,112]For short-term fertility [121,122] 2
/Yes [125,126]		Yes [124]Yes [16,18,19,20]
pH LevelAlkaline [120,127]Not evaluated [109,110,112]Slightly alkaline post-burning but can be variable
[121,123,130] 2
/Neutralizes [126]		Fire mineralizes
organic matter
lowering pH [124]		Biochars from plant residue alkaline, animal residue acidic [15]
Nutrient AvailabilityHigh [127]Increases labile potassium and phosphorus with short-term soil
nitrogen level
decrease [108,110,112]		Temporarily high from ashes [121,130] 2
/Increases [126]		Improved [124]Makes nutrients more available for plants [15], but can be variable [16,18]
Specific
Surface Area		High [127]Biopores result in greater soil total porosity [108]Improves initially
[121] 2/Similar to wood charcoal [126]		Not evaluated [124]High due to increased porosity due to carbon structure [77,78]
Water-Holding
Capacity 		High [127]Not evaluated [109,110,112]Improves initially
[121] 2/Improves [126]		Not evaluated [124]High due to porosity increases
[15,20,77,78]
1 Assume bulk of production stopped following indigenous population decline in wide-spread intensive settlements along the Amazon, which was initially documented in 1541 but not confirmed during subsequent Spanish explorations [131]. 2 Jhum slash-and-burn system typically completed at higher temperatures with no oxygen restriction but can involve pyrolysis [122].
McKey (2021) [124] classifies the Spanish formiguer method, which was used from the 1800s to 1960s [110], as a type of soil paring-and-burning. However, the formiguer method is more recent than soil paring-and-burning, which was practiced in Europe up to the late 1800s; however, it has also been argued that its European origins date back to the late Early Middle Ages during the 900s (Table 2) [124]. The drier Mediterranean environment meant that soil blocks did not need to be dried and, instead, could be added directly to burn piles in crop fields [110]. Therefore, we distinguish the formiguer method as its own category of agricultural biochar separate from soil paring and burning (Table 2).
Benefits of burning after paring included pest and weed control as well as consolidating clay particles into larger agglomerations for better soil tilth [124]. Since soil paring-and-burning was already being practiced in Europe prior to Spanish formiguers, it is reasonable to assume that the process of digging sod caused agriculturalists to be used to the labor-intensive nature of soil paring-and-burning. Collecting branches/shrubs and covering these with soil to build a formiguer is a logical extension of soil paring-and-burning in an Iberian environment with limited grass pasture, livestock, and manure [109]. Northern Europe also used “paring-and-composting,” where heathland profiles were dug, used as winter livestock bedding, and then manure/bedding were spread on adjacent farmland, creating Plaggen soils [124].

5.2. Formiguers Versus Biochar from Wildfires and Charcoal Production

While all of the biochar systems listed in Table 1 were and continue to be used for their ability to enhance soil fertility, these agricultural systems can be distinguished from biochar produced by wildfires and remaining from historical production of charcoal. Kamarudin et al. (2022) discuss the role of fire in creating naturally occurring biochar in the Great Plains of the USA [73]. However, wildfire charcoal is formed under higher temperatures, with more oxygen, and with shorter heating times, resulting in lower carbon sequestration potential compared to modern day, slow-pyrolysis biochars [132]. Another historic source of biochar production was charcoal production in hearths, in which the resulting charcoal was used for producing iron [133].
Although they have similar production processes, biochar production for agricultural applications (e.g., formiguers) is typically its own process and is not a co-product of charcoal production [110]. Biochar can store carbon for hundreds to thousands of years [134]. Ancient Roman charcoal rich deposits dating back to the Bronze Age have also been documented to last a long time with favourable carbon sequestration potential [135,136]. Historic charcoal production sites in Germany and Belgium have been shown to improve carbon storage capacity of soils [137,138]. Charcoal kiln sites in southern Italy demonstrate soil microbes more favourable for plant growth [139]. It is not guaranteed that soils at historic charcoal production sites were concurrently used for agriculture. For example, southern Belgium earth-mound kiln charcoal production in the forest peaked in the late 1700s, with limited cases of kiln soils used for agriculture, but not until the late 1800s [140].
Use of charcoal has been historically documented to be integrated into different agricultural systems. Plaggen soils typically use a “paring-and-composting” approach [124]. However, Davidson et al. (2006) documented incomplete softwood combustion and charcoal inputs in a Plaggen soil in Scotland between the 1600s and mid-1800s, where town waste was burned [141]. Furthermore, in a greenhouse study, Lehmann et al. (2002) demonstrated the benefits of shifting from “slash-and-burn” (i.e., swidden) in the Amazon to a system of “slash-and-char,” where charcoal produced from secondary forest is used as a soil amendment for rice (Oryza sativa L.) and ice-cream bean (Inga edulis) [142].

5.3. Applying Formiguers to Biochar as a Modern-Day Resource

5.3.1. Formiguers for Developing Nations

In comparison to modern biochar production, the formiguer method is a highly laborious process that requires minimal machinery and external inputs [110]. The production of modern biochar requires precise temperature controls, chemical analysis, and the use of modern technologies to optimize the quality and yield of the product [143,144]. While an effective method, this requires a substantial financial investment, technical expertise, and industrial infrastructure. The formiguer method operates based on a different pretense and can be completed using simple farm equipment available to most farmers [110]. The only tools and resources needed for producing a formiguer are: (1) digging tool to form the furnace structure (Figure 2B), (2) implements to cut down and transport biomass, and (3) available biomass for charring (Figure 2A). By providing an alternative solution for communities with limited capital or technical capabilities but access to organic matter/waste, using formiguers can be a viable alternative for communities because it relies on simple earthen mounds for pyrolysis (Figure 1).
The formiguer method can be environmentally sustainable [145], promoting self-sufficient agriculture in localities that lack access to chemical fertilizers or sufficient capital for expensive/technical machinery. The use of formiguers can provide accessible solutions for rural agriculture producers looking to enhance soil fertility and food security while mitigating adverse impacts of industrial fertilizer use in developing nations. It provides rural communities with the opportunity to produce, cultivate, and fertilize the land using a modular process that requires little pre-existing infrastructure and capital investment (Figure 1). Therefore, it is well suited to the rural economic realities in many developing regions. Unlike modern biochar production, which requires complex supply chains and technical expertise [9], the formiguer method can be constructed using readily available materials, such as clay (Figure 1). In addition to this, the flexibility of the process allows farmers to adapt their production scale depending on the availability of biomass and according to their fertilization needs [109,110,112]. This causes it to be more easily implemented and applicable to the local population than other methods.
The process can help foster an adaptable approach to soil improvement that can work with pre-existing soil remediation practices. In regions where traditional slash-and-burn methods continue to be practiced, such as those observed in India and parts of Congo, adopting formiguers can offer a sustainable and transformative alternative. For example, if we look at the practice in India called the jhum system of crop burning, we are faced with a potential example in which the formiguer method can help speed up the process of slash-and-burn. The conventional cycle of clearing and burning forests typically lasts from 25 to 30 years before fields are rotated [122]. By transitioning from the slash-and-burn to the slash-and-char approach, communities can decrease the period of time of long rotations in swidden production cycles while also reducing nutrient loss.
This, in turn, leads to increased conservation of the carbon formed from the biomass, which ensures increased fertility of soil and nutrient availability, mitigating long-term soil degradation. Similarly, this process to improve agricultural fertility could prove both useful and transferrable to nations such as Haiti. Formiguers could be an alternative to smaller, mobile biochar kilns [146]. This assumes household fuel use in Haiti is diversified or made more efficient to produce enough biochar/charcoal while minimizing land degradation from overuse of wood coppicing currently used for charcoal production [147]. As documented by historic charcoal sites in Europe, there could be potential agricultural benefits of such residual biochar [135,136,137,138,139] in developing nations, including Haiti.
Soils at traditional charcoal sites tend to show improvement in soil microbes known to enhance plant growth [139], which can lead to increased soil fertility due to unintentional incorporation of carbon from residuals into the soil. However, given that these additions are incidental because they are not the primary objective of the process, they can lead to less-than-optimal conditions for strategic soil improvement. By intentionally applying the formiguer method in such environments, there is potential to transfer existing practices into a sustainable and structured agricultural system harnessing biochar’s soil-amending properties while also addressing environmental and economic barriers to development.
Biochar can have both agricultural and sustainability benefits for both plant [148] and animal [149] production, in addition to enhancing soil ecology [148,150], increasing carbon sequestration [150], and meeting United Nations’ Sustainable Development Goals [151]. However, these benefits tend to be more apparent in marginal tropical soils, found in many developing nations, rather than more fertile temperate soils [152,153]. Therefore, more research is needed to support biochar adoption in developing nations with poor soils using agricultural wastes versus wood [154].

5.3.2. Industrial Upscaling of Formiguers

Future technological improvements could commercialize laboratory production [155,156] and flame-curtain kilns [157] to varying scales [158]. However, industrial production of biochar is currently limited, with individual entities producing ≤10,000 metric tons per facility per year [159]. Alternatively, simpler historical processes like Iberian formiguers can be upscaled using excavators and other larger scale equipment (Figure 4). However, the agricultural returns for the enterprises using such biochar would have to be sufficient to cover the higher capital investment and costs of equipment (e.g., excavators). Additionally, such upscaling assumes that agricultural waste residues (e.g., from perennial plant pruning) are available for use instead of having to harvest woody biomass from the surrounding landscape, which may have adverse environmental impacts. Use of annual crop residues for agricultural biochar should also not be competitive with other uses (e.g., animal feed and bedding).
In the early 2010s, the use of biochar exploded within research on sustainability and the environment, with an annual growth rate of 13.6% for biochar research between 2013 and 2023 [107]. Furthermore, since 2020, research into biochar production and its utilization has significantly diversified. The majority of recent studies have concentrated on its applications and economic feasibility, with the most research emerging from China and India [107]. However, if biochar produced from agricultural wastes [10,11] is to be recognized as a viable solution for multi-faceted environmental applications [21], including de-carbonization in Spain [160] and elsewhere, it must become more globally adaptable and accessible, particularly for developing nations with limited access to capital and technology. Formiguers can also be applied industrially in developed nations in regions with highly weathered soils [161] and perennial commodity crops (e.g., pecans), in which woody biomass from pruning or culling can serve as a potential feed stock for this historical resource similar to how grape orchards served this purpose historically in Catalonia, Spain. This includes producing joint outputs of charcoal and biochar, as has historically been the case in Catalonia [162]. Future biochar research on formiguers can add to the global data set on agricultural biochar use recently consolidated by Li et al. (2024) [163].
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Figure 4. Production of formiguers at more industrial scale using excavator. Picture was created by second co-author using free images from https://pngtree.com [164].
Figure 4. Production of formiguers at more industrial scale using excavator. Picture was created by second co-author using free images from https://pngtree.com [164].
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6. Conclusions

Our literature review’s findings reveal the multifunctional role of biochar within the context of the Catalonian formiguer method that was historically utilized in the Iberian Peninsula from the 1800s to the 1960s. We highlight potential lessons for contemporary agricultural soil fertilization practices producing biochar as a resource. This review reveals how producing formiguers resulted in biochar-rich soils that contributed to soil fertility, water retention, and long-term carbon sequestration. By combusting both agricultural crop wastes and biomass collected from the surrounding landscape under pyrolytic conditions, farmers were able to recycle nutrients, closing the nutrient gap while also improving soil structure for crops. Evidence from the literature positions the formiguer method, specifically within Catalonia, as a unique example of historical agricultural soil improvement that draws on the principles of classical écoubage (i.e., paring and burning) with the potential to contribute to modern-day terra preta analogs. These practices could be integrated into current agricultural systems to support regenerative farming while enabling the regeneration of agroecosystems that have been affected by excessive use of conventional fertilizers, as is currently the case in the Mar Menor lagoon in southeastern Spain. Furthermore, this review highlights the underutilized value of rural ecological knowledge, which can foster solutions to improve agricultural resource use. Future researchers may be able to add to this body of knowledge by conducting greater in situ analysis (e.g., pH impacts on soil fertility, toxic metal remediation potential) of former sites of formiguers over a greater period of time than has currently been conducted. Biochar produced using the formiguer method still needs to be directly compared to the composition of modern biochar. In addition to this, it would be helpful to quantify long-term carbon stock changes and shifts in microbiological dynamics caused by this practice, while also investigating policy mechanisms that can encourage the integration of formiguers into local and industrial agricultural settings around the world, particularly in regions and countries whose biochar use has been understudied in the literature.

Author Contributions

Conceptualization, A.K.H. and N.S.F.; methodology, A.K.H. and N.S.F.; formal analysis, A.K.H. and N.S.F.; investigation, A.K.H. and N.S.F.; resources, A.K.H.; data curation, A.K.H. and N.S.F.; writing—original draft preparation, A.K.H. and N.S.F.; writing—review and editing, A.K.H. and N.S.F.; visualization, A.K.H. and N.S.F.; supervision, A.K.H.; project administration, A.K.H.; funding acquisition, A.K.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by The Born Global Foundation.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data used for this review is available upon request by contacting the authors.

Acknowledgments

We would like to thank Kimberly Samaha and others at Born Global Foundation for administrative support. Marina Moura Morales supported our review of agricultural biochar meta-analyses. We would also like to thank the three anonymous reviewers for their constructive feedback and edits which have substantially increased the quality of this work.

Conflicts of Interest

Author Aaron Kinyu Hoshide was employed by the company Sensei Economic Solutions, LLC. The remaining author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Formiguer production by layering of biomass and soil for biochar production for agriculture [110,112,113]. Hand drawings by the lead author.
Figure 1. Formiguer production by layering of biomass and soil for biochar production for agriculture [110,112,113]. Hand drawings by the lead author.
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Figure 2. (A) Biomass collected by the lead author from the forests near Vila-Rodona which historically would have been used to produce formiguers and (B) reverse conical excavation to ensure more efficient pyrolysis dug by lead author demonstrating the base of a formiguer before piling collected biomass, sticks, and soil cover in Tarragona Province, Catalunya, Spain. Pictures taken by the lead author.
Figure 2. (A) Biomass collected by the lead author from the forests near Vila-Rodona which historically would have been used to produce formiguers and (B) reverse conical excavation to ensure more efficient pyrolysis dug by lead author demonstrating the base of a formiguer before piling collected biomass, sticks, and soil cover in Tarragona Province, Catalunya, Spain. Pictures taken by the lead author.
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Figure 3. Charcoal kiln site historically used for both charcoal and biochar production located in the forest near the town of L’Arboçar Aiguamurcía, Tarragona Province, Catalunya, Spain—coordinates 41.37390° N, 1.43270° E. Picture views are from (A) interior looking outward through entrance, (B) exterior looking at entrance, (C,D) interior looking upward where original roof would have been before it collapsed. The charcoal production process involved heating wood via pyrolysis, cooling the kiln to prevent charcoal from igniting upon being exposed to the air after carbonization, followed by charcoal collection. Pictures taken by the lead author.
Figure 3. Charcoal kiln site historically used for both charcoal and biochar production located in the forest near the town of L’Arboçar Aiguamurcía, Tarragona Province, Catalunya, Spain—coordinates 41.37390° N, 1.43270° E. Picture views are from (A) interior looking outward through entrance, (B) exterior looking at entrance, (C,D) interior looking upward where original roof would have been before it collapsed. The charcoal production process involved heating wood via pyrolysis, cooling the kiln to prevent charcoal from igniting upon being exposed to the air after carbonization, followed by charcoal collection. Pictures taken by the lead author.
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Table 1. Impacts of agricultural biochar on crop, plant, soil, and environment from recent meta-analyses.
Table 1. Impacts of agricultural biochar on crop, plant, soil, and environment from recent meta-analyses.
SectorAgroecosystem
Metric		Percent
Change		Type/
Measure		Number of
Studies		Observations/
Contrasts		Years
of Studies		References
AgricultureCrop Production 116.08%Crop yield1511,0131980 to 2024[23,24,25,26,27,28,29,30,31,32,33,34,35,36,37]
Crop/Plant16.42%Crop productivity419151990 to 2024[34,38,39,40]
 Productivity−6.95%Lodging (grain)113022011 to 2023[33]
 Tomato Quality5.53%Total soluble solids/Vit. C14582013 to 2024[37]
AgronomySoil Nitrogen 116.99%Total soil nitrogen (N)22781990 to 2024[38,40]
−9.86%Soil inorganic N318631990 to 2020[38,41,42]
18.7%Soil microbial biomass N1331990 to 2020[38]
15.7%Biological N fixation1191990 to 2020[38]
Soil Phosphorus (P)54.8%Total soil available P210532000 to 2019[42,43]
48%Soil microbial biomass P1742000 to 2017[42]
460%P availability11081980 to 2016[44]
Plant Biomass16.6%Plant biomass227002000 to 2023[32,33]
Photosynthesis23%Plant photosynthetic rate19652012 to 2020[45]
Plant Nutrient18.8%Plant N uptake210311990 to 2020[38,46]
 Uptake/Fixation12.04%Rice N use efficiency111002011 to 2021[36]
63%Biological N fixation1252003 to 2017[46]
55%Plant P uptake15162011 to 2019[43]
SoilAcid /Alkaline 1,222.9%Soil pH23072012 to 2024[28,40]
Carbon (C) 1,239.04%Soil organic carbon834492012 to 2024[26,28,29,30,40,47,48,49]
10.26%Dissolved organic carbon12762012 to 2022[29]
37.08%Easily oxidized carbon11382012 to 2022[29]
139.98%Particulate organic carbon1402012 to 2022[29]
C:N Ratio 223.81%Carbon: Nitrogen ratio13942012 to 2022[29]
Cation Exchange 220%Cation exchange capacity1732012 to 2021[28]
Conductivity 2−7.4%Soil electrical conductivity1180Up until 2024[40]
Enzyme Activity22.7%Four enzymes 313972009 to 2019[50]
Immobilization1750%Chromium and nickel118,7022006 to 2019[51]
Microbes 1,418.9%Soil microbial biomass C34632012 to 2024[29,47,50]
Quality16.9%Aggregates and stability312052010 to 2024[52,53,54]
−12.25%Soil bulk density44712010 to 2024[28,40,52,55]
15.4%Soil mean weight diameter15542011 to 2024[54]
41.2%Soil porosity21082012 to 2021[28,52]
Water Retention
and Transmission		26.13%Available H2O holding
capacity		37661990 to 2020[52,55,56]
25.99%Field capacity26501990 to 2020[55,56]
25.2%Saturated hydraulic
conductivity		124Up until 2018[52]
47%H2O content held at
wilting point		11762010 to 2019[55]
14.3%Permanent wilting point13331990 to 2020[56]
EnvironmentGreenhouse
Gas (GHG) 		12.14%Soil carbon dioxide
emissions		411252005 to 2020[57,58,59,60]
 Emissions 519%NH3 volatilization1992003 to 2017[46]
1.3%Soil methane emissions36112002 to 2023[31,59,60]
−14.2%Nitrous oxide emissions828151980 to 2024[31,38,46,57,58,59,61,62,63]
−29%Yield-scaled GHG
emissions		1812011 to 2018[64]
−24.43%Global warming potential210592006 to 2022[26,30]
Nutrient Leaching−20.4%Soil nitrogen leaching21791990 to 2020[38,46]
Soil Loss−22.9%Soil erosion and runoff25422002 to 2023[65,66]
1 Biderman and Harpole (2013) confirmed that this increased (n = 371) for studies from 1980 to 2012 [67]. 2 Confirmed by Sun et al. 2022 as improved after evaluating 138 research articles [68]. 3 Alkaline phosphate (n = 77), dehydrogenase (n = 108), nitrogen acquisition (n = 121), and urease (n = 91 [50]. 4 Deshoux et al. (2023) analyzed 181 studies from 2010 to 2022 for soil microbial biomass and diversity [69]. 5 Jeffrey et al. (2016) showed that methane emissions decreased in rice paddies (42 studies, n = 189) from 2009 to 2014 [23].
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Farré, N.S.; Hoshide, A.K. Formiguer Fertilization: Historical Agricultural Biochar Use in Catalonia and Its Modern-Day Resource Implications. Resources 2025, 14, 120. https://doi.org/10.3390/resources14080120

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Farré NS, Hoshide AK. Formiguer Fertilization: Historical Agricultural Biochar Use in Catalonia and Its Modern-Day Resource Implications. Resources. 2025; 14(8):120. https://doi.org/10.3390/resources14080120

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Farré, Nicolas Sesson, and Aaron Kinyu Hoshide. 2025. "Formiguer Fertilization: Historical Agricultural Biochar Use in Catalonia and Its Modern-Day Resource Implications" Resources 14, no. 8: 120. https://doi.org/10.3390/resources14080120

APA Style

Farré, N. S., & Hoshide, A. K. (2025). Formiguer Fertilization: Historical Agricultural Biochar Use in Catalonia and Its Modern-Day Resource Implications. Resources, 14(8), 120. https://doi.org/10.3390/resources14080120

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