Soil samples were collected in the 0-0.2 m layer of a native area (native vegetation of the Cerrado) located in the region of Campo das Vertentes, State of Minas Gerais, Brazil. According to the Brazilian Soil Classification System (Santos et al., 2018), soils were classified as typical dystrophic Red-Yellow Latosol (LVAd), and typical dystrophic Red Latosol (LVd), corresponding to Oxisol in the USDA Soil Taxonomy (Soil Survey Staff, 2014). The physical and chemical properties were determined in the air-dried fine earth (ADFE; < 2mm) ( Table 1 ).

Soil samples were dried, crushed, and passed through a 4-mm sieve. Then, the samples were homogenized and stored in plastic bags. A mixture of calcium (Ca) and magnesium (Mg) carbonates (p.a.) in a 3:1 molar ratio (Ca : Mg) was used to increase soil pH and increase base saturation to 60%, according to previous soil analyses (Alvarez and Ribeiro, 1999). The soil was incubated at room temperature for 60 days with soil moisture maintained at field capacity. Soil samples were homogenized once a week during the incubation period. At the end of the 60 days, the average pH values for LVAd and LVd were 6.15 ± 0.08 and 6.23 ± 0.06, respectively.

Soil pH in H₂O was measured using a 1:2.5 (v/v) soil:solution ratio (TEC-11 model from Tecnal). The content of soil organic matter (SOM) was determined by oxidation with a potassium dichromate solution in the presence of sulfuric acid (Carter and Gregorich, 2006).Calcium (Ca²⁺), magnesium (Mg²⁺), and aluminum (Al³⁺) were obtained with the 1 mol L^-1 KCl extractant (1:10 v/v soil: solution ratio), whereas available phosphorus (P), potassium (K), copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), and chromium (Cr) were obtained by Mehlich (1953) (0.05 mol L^-1 HCl + 0.0125 mol L^-1 H₂SO₄, in ratio 1:10 v/v soil: solution); sulfur (S) was extracted as sulfate by monocalcium phosphate with acetic acid by using 1:2.5 soil: solution ratio and activated charcoal (Hoeft et al., 1973). Exchangeable Al was determined by titration with NaOH (0.025 mol L^-1); P and S were determined colorimetrically using a UV/VIS spectrophotometer (B582 model from Micronal^®); K by flame emission photometry (DM-62 model from Digimed); and Ca, Mg, Zn, Fe, Mn, and Cu were determined by atomic absorption spectrometry (AAnalyst 400 model from PerkinElmer). Cr was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Blue model from Spectro). Attributes related to soil fertility (SB - sum of bases; CEC effective - effective cation exchange capacity; CEC at pH 7.0 - cation exchange capacity at pH 7,0; BS - base saturation index) were calculated according to Teixeira et al. (2017).

The experiments were carried out in two sequential steps: 1) incubation of fertilizers in soil samples for 30 days; and 2) growing wheat (Triticum aestivum L.) after incubating the fertilizers for 30 days. Two experiments were carried out, one in each soil (LVAd and LVd), with the same completely randomized experimental design, with five treatments and five replications. The treatments consisted of the composition of two types of OMF, which contained mineral fertilizers based on nitrogen (N), phosphorus, and potassium (NPK fertilizer), with and without the addition of S. The organic matrix of the OMF was BPIPT and organic compost commercial. The treatments were identified as OM-IPT (OMF based on BPIPT), OM-IPT+S (OMF based on BPIPT plus S), OM-CM (OMF based on organic compound), OM-CM+S (OMF based on organic compound plus S). In Table 2 describes the composition and nutrient content of each OMF.

All fertilizers were oven-dried (40°C) and crushed in a ball mill before their characterization and application to the soil. Then, they were passed in a 1-mm sieve to standardize the fertilizers’ granulometry and quartered in a stainless-steel sample splitter. The OMF were characterized according to the Manual of Official Analytical Methods for Fertilizers and Liming Materials (Brasil, Ministério da Agricultura, Pecuária e Abastecimento – MAPA, 2017) ( Table 2 ).

Chromium (Cr) contents were evaluated in the OMF according to the USEPA 3051A method (USEPA, 2007a). Five grams of each OMF were ground with an agate mortar and sieved through a 0.15 mm mesh. Then, 0.1 g aliquots plus 5 mL of concentrated HNO₃ were placed in Teflon^® vessels. The extract stood overnight at room temperature, and the digestion was performed the following day. The containers were sealed and heated in a Mars-5 microwave digestion oven (CEM Corp, Matthews, NC) to 175°C under controlled pressure conditions (0.76 MPa for 15 minutes). The sealed vessels were then removed from the oven and cooled to room temperature under a fume hood. Finally, the vessels were opened, and the resulting solution was filtered through Whatman #40 filter paper. The volume of the filtered extract was adjusted to 50 mL with ultrapure water. Each sample was digested in triplicate. Chromium contents were determined by ICP-OES (Blue model from Spectro) ( Table 2 ). Reference samples of a Natural Moroccan Phosphate Rock BCR^® 32 and a Trace Elements in Multi-Nutrient Fertilizer NIST 695, as well as a blank sample, were analyzed in triplicates. The analysis of the reference material for Cr had the following recoveries (%): 85 and 43 for BCR^® 32 and NIST 695, respectively.

The application of macronutrients (N, P, K, and S) was based on the fertilizer recommendation proposed by Malavolta (1980), with modifications to meet the objectives of this study, i.e., no micronutrients were added, which were already in the OMF source, and the doses of S changed with the treatments. Thus, the amount of fertilizers applied to the soil supplied 300 mg kg^-1 of N, 200 mg kg^-1 of P, 200 mg kg^-1 of K, and 50 mg kg^-1 of S. The sources of inorganic fertilizers (commercial sources) used to supply N, P and K were, respectively, urea, monoammonium phosphate (MAP) and potassium chloride (KCl). Micronutrients were not applied. The fertilizers were applied to the dry soil samples, which were stored in plastic bags, and stirred until complete homogenization. Then, 1 kg of the soil samples were placed in pots, weighed, and kept on benches in a greenhouse under controlled temperature conditions.

The first stage of the experiment was the incubation of the OMF in the soils for a period of 30 days. The purpose of this stage was to evaluate the interaction between the OMF and the soils, evaluating the levels of nutrients available after incubation and the effect on the soil microbiota. Soil samples were mixed and incubated with the dosage of each OMF. Daily irrigations maintained the soil moisture at field capacity. Eventual agitations ensured the reaction of the fertilizers with soil samples. After the incubation period, 200 g of soil were collected to determine the availability of Cr, Cu, lead (Pb), Zn, P, and K by the Mehlich-1 method (Mehlich, 1953). In addition, the following microbiological attributes were assessed: microbial biomass carbon (MBC), basal respiration (SBR), activity of β-glucosidase, arylsulfatase enzymes, and total soil enzymatic activity performed by hydrolysis of fluorescein diacetate (FDA).

The second stage of the experiment was to evaluate plant growth in the soil after OMF incubation. Ten wheat (Triticum aestivum L. cultivar TBIO Aton) seeds were sown in pots with 800 g of soil after the OMF incubation. Daily irrigations were carried out to maintain soil moisture at field capacity. After seven days, the seedlings were thinned to have only seven plants per pot. The experiment was conducted for 30 days under greenhouse conditions, with controlled temperature and air humidity. The plant material was oven-dried (60°C) until constant weight (after ± 72 h), and the dry mass of roots and shoots was weighed and recorded.

Statistical analysis was performed in the R programming language (R Development Core Team, 2020), version 4.0.3. Data normality and homoscedasticity were evaluated. Analysis of variance (p ≤ 0.05) was used to verify the significance of the treatments. Then, the treatments were compared using the Tukey HSD test with the emmeans v1.4 package (Length, 2020). Principal component analysis (PCA) was performed using the Vegan package v2.5-7 (Oksanen et al., 2016) to demonstrate the importance of variables in explaining nutrient contents and biomass increase of wheat plants.

The available contents of Cr, P, and K in soils after OMF incubation are shown in Figure 1 and Cu, Pb and Zn in Figure S1 . The application of OM-IPT in the LVAd and OM-IPT+S in the LVd caused the highest contents of Cr. However, the available contents of Cr in these soils were below 3 mg kg^-1 ( Figure 1A ). Contents of P and K increased in both soils following the application of OMF ( Figure 1 ). In the LVAd, K contents did not differ between the OMF tested. On the other hand, P contents were reduced in the treatments OM-CM+S, OM-IPT, and OM-IPT+S in the LVAd. In the LVd, OM-CM and OM-IPT+S increased the P and K contents.

The MBC increased significantly after the application of OM-CM, OM-CM+S, and OM-IPT+S in the LVAd ( Figures 2A, D ). The SBR had a varying behavior in the soils, and the effect of OMF on the metabolic capacity of the microbiota was unclear ( Figures 2B, E ). The qCO₂ was higher in the control treatment ( Figures 2C, F ), indicating that the application of OMF enhanced the metabolic efficiency of the microbial community. In the LVAd, the FDA increased significantly with the application of all OMF compared with the control ( Figure 3A ). As for the LVd, only the OM-IPT and OM-IPT+S treatments increased the FDA activity ( Figure 3D ). The β-glucosidase activity increased significantly with the OM-IPT+S treatment in the LVAd ( Figure 3B ), and after the application of OM-IPT in the LVd ( Figure 3E ). The arylsulfatase activity significantly decreased after the application of OMF in the LVd relatively to the control ( Figures 3C, F ).

The elemental composition of the soil determined via Mehlich-1 (plant-available fraction) via pXRF is shown in Table S1 . The contents of total Cr (pXRF) and the relationship between total Cr and the available Cr fraction are found in Figure 4 . The LVd has the highest contents of Fe₂O₃ and the lowest contents of SiO₂ ( Table S1 ), and the highest levels of total Cr ( Figure 4D ), as well as other elements (Ni, Cu, Zn, and As) ( Table S1 ).

In the LVd, the average Cr content in the control treatment is 1472 mg kg^-1. However, the available Cr fraction corresponds to less than 1% of the total Cr ( Figure 4E ), regardless of the treatment. As for the LVAd, the highest Cr available fraction occurred after the application of OM-IPT, but it did not exceed 5% of the total Cr ( Figure 4B ). In the OM-IPT+S treatment, the available Cr fraction is approximately 2% lower than in the OM-IPT treatment. This available fraction (5%) is relative to a total Cr content of 63 mg kg^-1 in the OM-IPT treatment and to a total Cr content of 54 mg kg^-1 in the control treatment.

The Cr contents in wheat SDM were higher in the control treatments of LVAd (6.6 mg kg^-1) and LVd (5.4 mg kg^-1) ( Figures 5A, D ). The lowest Cr content in wheat SDM was found in the OM-IPT treatment (2.4 mg kg^-1) in the LVd ( Table 3 ).

The application of OMF increased Al contents in wheat SDM. The highest Al levels were found in the OM-IPT+S treatment in the LVAd (350 mg kg^-1) and LVd (151 mg kg^-1) ( Table 3 ).

The application of OMF increased wheat SDM ( Figures 5B, E ). In the LVAd, higher SDM values were observed after the application of OMF, with an average of 1.7 g pot^-1. However, within OMF treatments, no differences in SDM were observed ( Figure 5B ). In the LVd, the SDM values in the OMF treatments did not exceed 0.5 g pot^-1 ( Figure 5E ). In this soil, OM-IPT increased SDM relatively to the control. The OM-IPT treatment caused the highest RDM in the LVAd, and a high RDM in the LVd, which did not differ from the control.

In wheat grown in the LVAd, P and K contents in SDM increased significantly in all treatments compared with the control. However, no difference was observed among OMF treatments. The Ca and Mg contents did not differ from the control. The S content in SDM was the highest for OM-CM+S (3.4 g kg^-1) and the lowest for the control (1.8 g kg^-1; Table 3 ). Overall, the application of OMF in the LVAd increased the contents of micronutrients ( Table 3 ). Specifically, Mn had an average content of 36 mg kg^-1 in the control, and an average content of 225 mg kg^-1 for the OMF treatments in the LVAd.

For the LVd, the OM-IPT treatment increased the P content in SDM (2.5 g kg^-1). The OM-IPT and OM-IPT+S treatments had the highest K and S contents. Lastly, Ca and Mg contents decreased after the application of OMF in the LVd ( Table 3 ), except for Ca content in the OM-IPT+S treatment, which did not differ from the control. Overall, the contents of micronutrients in SDM increased after the OMF addition. However, probably due to the buffering effect of the higher clay content of this soil, the increases in Mn and Zn contents were less pronounced when compared with LVAd.

The FDA activity increased after wheat cultivation in the LVAd, with the highest value after the application of OM-IPT ( Figure 6A ). On the other hand, FDA activity in the LVd was unaffected by OMF application ( Figure 6C ). β-glucosidase activity varied in both soils ( Figures 6B, D ). In the LVAd, all OMF treatments had higher β-glucosidase activity than the control ( Figure 6B ). As for the LVd, the application of OM-IPT ( Figure 6D ) increased the β-glucosidase activity compared with the control.

The pH values in CaCl₂ ( Figures 4C, F ) and in H₂O ( Figure S2 ) at the end of cultivation decreased after the application of OMF in the soils.

In the LVAd, the application of OM-CM+S and OM-IPT+S caused the highest acidification ( Figures 4C, F ). As for the LVd, the application of OMF reduced the pH compared with the control treatment, but no differences were observed among OMF treatments.

The PCA shows the clusters between the OMF applied in both soils ( Figure 7 ). Variables related to plant productivity, SDM, and soil microbial activities were in the same group, suggesting that OMF applications led to increased SDM production and improved the microbiological quality of both soils ( Figure 7 ). In the LVAd, the treatments had a similar response, with close clustering, influenced by the increased contents of elements in SDM and soil microbiological activity ( Figure 7A ). In the LVd, the group of OM-IPT was directly related to SDM, nutrient content in SDM, and β-glucosidase activity. Such pattern suggests that this product positively influenced SDM and plant nutrition in this soil ( Figure 7B ).

The contents of Cr, P, and K in the soil were modified after the incubation of OMF in both soils ( Figure 1 ). Although the responses were diverse concerning the effects of treatments in the different soils ( Figure 1 ), it was possible to note that this greater availability of elements - plant nutrients or not - was beneficial for the development of wheat and for the biological activity of the soil ( Figures 2 ; 3 ; 5 ).

The total Cr contents varied within the soils ( Figure 4 ), with a much higher total Cr content in the LVd (1472 mg kg^-1) than in the LVAd (54 mg kg^-1). Such increased Cr levels in the LVd are likely a result of the wreathing of the parent mafic rocks, which can contain up to 3400 mg kg^-1 of Cr (Kabata-Pendias and Mukherjee, 2007). Nonetheless, the Cr available fraction in the LVd was lower than 0.2%. Even at such low levels, the Cr available fraction increased after the OMF application in the LVd, thus evidencing the potential of these fertilizers to increase Cr availability in this soil. The LVAd had lower levels of Cr, clay, and Fe₂O₃, suggesting a lower buffering capacity and possibly greater responses to management practices. Indeed, the application of OM-IPT and OM-IPT+S in this soil increased the Cr available fraction, whereas the S-containing treatments (OM-CM+S and OM- IPT+S) caused soil acidification compared with the control ( Figure 4 ).

Adding organic and inorganic amendments can increase the sorption of Cr (III) in soils. These products release hydroxyl ions and increase the negative charges in the soil surface layers, thus favoring the retention of Cr (III) and decreasing Cr (VI) sorption (Choppala et al., 2018). The increase in Cr (VI) sorption is possible by adding elemental S (S⁰), as the S⁰ oxidation causes soil acidification and thus results in the opposite effect of liming on Cr (VI) and Cr (III) species (Choppala et al., 2018). Besides, the sorption of Cr in the soil also relates to the type of SOM and clay (Covelo et al., 2007). Chromium - Cr (III) - can form inner-sphere complexes with SOM, with varying solubilities, or remain in the insoluble humin fraction of SOM (Ratke et al., 2019). In the present study, the addition of OMF likely increased the SOM levels in the soils, contributing to higher sorption of Cr compared with the control. Thus, Cr was less available to plant uptake, which explains the overall lower Cr contents in SDM after the OMF application in both soils, particularly the LVAd ( Figures 5A, D ). Organic matter additions can also stimulate microbial activity and the biotic reduction of Cr (VI) to Cr (III) in soils. Accordingly, organic sources (e.g., animal manure, compost, biosolids, or plant biomass) are commonly used as a recovery strategy in Cr-contaminated sites (Shahid et al., 2017; Choppala et al., 2018).

The OMF produced from BPIPT increased the content of available Cr in the soils ( Figure 1 ). However, such an increase did not seem to affect the development of wheat plants ( Figure 5 ). Indeed, the OMF produced with BPIPT provided a high level of nutrients for the plants, comparable to the other fertilizer sources ( Table 3 ). For plants grown in the LVAd, the highest SDM yields occurred after the application of OMF produced from BPIPT. Tannery by-products-based fertilizers have shown many benefits to soil attributes and plant production. They have increased C and N contents in their composition (Ciavatta et al., 2012; Majee et al., 2021) and have a gradual N release (Lima et al., 2010), which optimizes the use of N fertilizers by the crops. Such enhanced N use efficiency can result in increased yields (Castilhos et al., 2002; Coelho et al., 2015) and lower soil acidity (Ferreira et al., 2003; Teixeira et al., 2006). As such, using the BPIPT to produce OMF favors a circular economy (Chojnacka et al., 2021), as the by-product is incorporated into a different production sector, that is, from the tannery industry to agriculture.

The application of OMF in soils reduced the Cr uptake by wheat plants ( Figure 5 ). Plants can capture both Cr (III) and Cr (VI). Cr (III) uptake by plants is a passive mechanism and does not require energy. As for Cr (VI), its uptake by plants is an active process (generally via phosphate or sulfate transport), due to the similarity of CrO₄ ^2- with these anions. The presence of sulfate in soils can even inhibit Cr (VI) uptake by plants (Shahid et al., 2017). The Cr available contents in soils (total Cr divided by Cr determined via Mehlich-1 extractor) below 3 mg kg^-1 ( Figure 1 ) were not toxic to wheat plants in both soils. The Cr contents in SDM ranged from 6.6 to 2.4 mg kg^-1, with the highest level in the LVAd control treatment ( Table 3 ). Toxicity symptoms were not observed, even at the highest Cr contents in SDM. Toxic effects of Cr for most plants occur at foliar contents higher than 18.0 mg kg^-1 (Losi et al., 1994). Coelho et al. (2015) and Oliveira et al. (2008), after using the tannery by-product as an N source, also did not report toxic levels of Cr in wheat and elephant grass, respectively. Wheat germination can be affected by Cr contents ≥ 4,000 mg kg^-1 and ≥ 2,000 mg kg^-1 for oats and sorghum that received the application of tannery by-products (López-Luna et al., 2009).

Liming was performed to simulate the process of soil acidity correction in tropical soils and to maintain the appropriate pH range for the cultivation of plants. It is known that the use of fertilizers with elemental sulfur (S°) may cause soil acidification, from the increasing content of hydrogen ions (H⁺) formed during its microbiological oxidation (Lisowska et al., 2022). Soil acidification with the presence of S was observed in the LVAd, which has lower buffer capacity, yet this did not cause a higher availability of Cr.

The OMF application promoted increases in MBC and in the biological and enzymatic activities of these soils ( Figures 2 , 3 ), except for arylsulfatase in the LVd ( Figure 3F ), thus evidencing the beneficial effects of these fertilizers. This effect was probably due to the increased availability of nutrients and energy for the microbial community in this soil. The tannery by-products are rich in proteins and lipids, which represent a promising source of nutrients, they can stimulate the decomposing and mineralizing activities of the heterotrophic community of the soil, providing nutrients to plants and microorganisms (Majee et al., 2021). What makes the reuse of this waste an eco-friendly activity, is because in addition to disseminating the concept of circular economy in the industry (Velenturf et al., 2019), it is also a source of multi-element fertilizers for plants and microorganisms.

The Cr contents present in the OMF do not seem to have an impact on the microbial activities in the soil ( Figures 2 , 6 ). In fact, the application of OMF led to higher wheat yields and increases in enzymatic activities in the soil. However, the activity of the enzyme arylsulfatase decreased after the application of OMF in the LVd ( Figure 3F ), which may be related to a higher S availability after applying the products. This enzyme acts in some stages of the mineralization of organic S, and reductions in arylsulfatase activity may occur if inorganic S is present in these fertilizers. Oliveira-Longatti et al. (2017) also observed increases in microbial biomass and the activities of β-glucosidase and urease enzymes after the application of tannery by-products in two tropical soils. The authors demonstrated that the trace levels of Cr in the by-product were not harmful to the biological activity in the soil.

With the addition of OMF, it was possible to observe that there were increases in the availability of resources, carbon, and nutrients for the microbiota, which favored the increase in MBC and microbial activities, even in a short experimental period ( Figures 2E , 6 ). Microbial ecophysiological characteristics, such as MBC and SBR, are known to control the renewal, accumulation, and losses of C in soils (Schimel, 2013; Liang et al., 2017). MBC stabilizes C in soils through the formation of microbial biomass. Such function is crucial for efficient nutrient cycling, given that microorganisms are the primary agents in the decomposition process (Bradford et al., 2013; Liang et al., 2017; Majee et al., 2021). At the same time, microorganisms rely on the availability of organic matter as a source of energy and nutrients, particularly N, to build their biomass and release and activate their enzymes (Moorhead et al., 2013; Sokol and Bradford, 2019).

The metabolic quotient (qCO₂) was more efficient than SBR in showing a decrease in the metabolic stress of the microbial community after the addition of OMF in both soils ( Figures 2C, F ). Soil microbiota was more efficient in storing C in the biomass in treatments that received OMF, considering the reduced qCO₂ in the OMF treatments ( Figures 2C, F ). Such reduced qCO₂ is a positive aspect of these materials on the soil microbiota. The qCO₂ is the ratio of basal respiration rates to microbial biomass and represents a simplified indicator of energy demands for cellular maintenance and microbial C use (Anderson and Domsch, 1993; Joergensen and Wichern, 2018). As a result, higher qCO₂ rates can potentially enhance C losses from soil microorganisms, thus decreasing C accumulation in soils and ecosystem functioning (Schimel, 2013; Malik et al., 2020).

The activities of β-glucosidase and FDA were evaluated before and after wheat cultivation ( Figures 3 , 6 ). These enzymes were very sensitive in evaluating soil microbiological activity in the incubation stage and the plant development after the OMF application. Both soils presented a trend for increasing β-glucosidase and FDA activities after the OMF application and higher wheat yields ( Figures 3 , 6 ). Thus, our results corroborate the use of enzymatic activities of soil microbiota as a sensitive indicator of management changes in the soil (Wahsha et al., 2017; Anghinoni et al., 2021). Besides, we evidenced the positive relationship between enzymatic activities and crop yield (Lopes et al., 2013; Mendes et al., 2019), and demonstrated that tannery by-products containing trace levels of Cr were not detrimental to the microbiological activity in the soil (Oliveira-Longatti et al., 2017).

The total Cr contents determined by pXRF in the LVAd (average value of 54 mg kg^-1) are below the prevention value for Cr in soils in Brazil (75 mg kg^-1; Brasil, Conselho Nacional do Meio Ambiente – CONAMA, 2009). The application of OM-IPT+S led to a total Cr content of 106.8 mg kg^-1, higher than the other treatments, but below the intervention value for agricultural use (150 mg kg^-1). The LVd had a much higher total Cr content (1472 mg kg^-1). Such increased value reflects the composition of the parent material and strengthens the premise that threshold values must be determined with respect to different assumptions and modeling. Some countries or regions emphasize that threshold values must vary according to soil types (clay content), extractors, pH values and soil depth (Carlon, 2007; Vácha et al., 2016). Most regulations have threshold values for Cr (VI) in soils, due to its deleterious effects to human and environmental health.

The Cr contents in the OMF were 187.2, 94.1, 10,061.2 and 10,307.7 mg kg^-1 for OM-CM, OM-CM+S, OM-IPT, and OM-IPT+S, or 0.02, 0.01, 1.01 and 1.03%, respectively. Most threshold values for fertilizing products do not consider total Cr contents, which complicates our attempt to establish safe levels for the use of tannery by-products-based fertilizers. The threshold value for organic fertilizers and soil conditioners in Brazil is 2 mg kg^-1 of Cr (VI) per dry matter and 200 mg kg^-1 of total Cr for inorganic fertilizers containing macronutrients (Brasil, Ministério da Agricultura, Pecuária e Abastecimento – MAPA, 2006). In Europe, organic or organo-mineral fertilizer can have up to 2 mg kg^-1 Cr (VI) per dry matter (European Commission, 2019). Future studies of Cr in soils following the OMF application should include direct speciation analyses (e.g., synchrotron-based spectroscopies) to investigate the chemical forms of Cr in soils, tannery by-products, and organo-mineral fertilizers. This knowledge would demonstrate the suitability of these products within the current regulations and evidence the impacts of long-term OMF applications on Cr (VI) levels in soils.