Granulated organomineral fertilizers from by-products of the agricultural and forestry sectorABSTRACT
The present study evaluated the granular properties of organomineral fertilizers produced from charcoal fines (biochar) and natural phosphate with three distinct binders: cassava wastewater (CW), pyroligneous extract (PE), and bio-oil (BO). The objective was to ascertain the potential of these fertilizers to increase common bean production. To form the granules, a 3:1 ratio of biochar to natural phosphate was employed, along with five binder doses (50, 75, 100, 125, and 150 % of the biochar-phosphate mixture weight). The granules were evaluated based on their crushing strength, water absorption ratio (WAR), granule density, impact resistance index (IRI), salinity index (SI), and pH. The most optimal granules were selected for the experiment with the common bean, using a completely randomized design in a 3 × 4 + 4 factorial scheme with three replications. The treatments comprised three binders (CW, PE, and BO), four binder doses (75, 100, 125, and 150 %), and four controls: unfertilized soil (NS), natural phosphate (NP), biochar mixed with natural phosphate (B + NP), and biochar (B). Compared to the CW or PE granules, the BO granules exhibited superior crushing resistance, density, and IRI, accompanied by a reduced WAR. The PE granules exhibited a higher SI and a lower pH. The application of CW, PE, or BO granules resulted in a significant increase in shoot dry matter (SDM), with values of 98.31, 123.73, and 47.46 %, respectively, compared to the NS. Similarly, root dry matter (RDM) notably increased, with values of 331.03, 755.17 and 141.38 %, respectively. The physicochemical properties of organomineral fertilizers are influenced by the type of binder used, affecting common bean production. Further research is needed to ascertain the long-term effects of these slow-release fertilizers.
Keywords
biochar; bio-oil; cassava wastewater; pyroligneous extract; waste recovery
Introduction
One of humanity’s most significant challenges currently is the search for effective strategies to mitigate atmospheric greenhouse gas emissions. In response to this challenge, the concept of a circular economy has emerged as a prominent approach in the field of sustainable waste management, offering a potential pathway to reduce environmental impacts (Casau et al., 2022).
Pyrolysis is a thermochemical depolymerization process of organic compounds in an environment with a low concentration or no oxygen (Li et al., 2022). This process forms charcoal, known as biochar (B), for agronomic purposes. B is a porous material rich in recalcitrant carbon (Yang et al., 2022).
The production of charcoal at carbonization plants inevitably generates considerable quantities of waste, mainly attributable to handling the charcoal itself. These residues, designated as charcoal fines (B), are tiny fragments resulting from abrasion and mechanical shock, constituting approximately 20 % of the total charcoal produced. They are a significant source of economic loss and environmental liability (Ronix et al., 2021). Brazil is the world’s largest producer of charcoal, with an output exceeding 7.5 million tons in 2022 (FAO, 2022). This indicates potential losses through charcoal fines to reach over 1.5 million tons.
The economic viability of using B in agriculture remains a controversial topic, largely due to the considerable quantities that must be applied, the losses of material during handling, transportation, and application, and the inhalation risks faced by farmers (Mohammadi, 2021). One potential avenue for utilizing B agricultural settings is the production of granular organomineral fertilizers. The quality and stability of the granules are crucial properties, particularly in relation to binders (Riva et al., 2019). Nevertheless, some of these binders are costly and may increase production cost (Peng et al., 2021). One potential solution is the use of alternative binders, such as pyroligneous extract (PE) and bio-oil (BO), by-products of charcoal production, and cassava wastewater (CW), a waste product from the production of cassava starch.
The hypothesis proposed is that the physicochemical quality of the granules produced from organomineral fertilizers is altered by the type of binder used, including their ionic exchange capacity and nutrient bioavailability. This, in turn, is expected to increase shoot dry matter (SDM) and root dry matter (RDM) production in the common bean (Phaseolus vulgaris L. BRSMG Uai). The objective of this study was to assess the granular properties of organomineral fertilizers produced from B and natural phosphate (NP) with three binders (CW, PE, and BO), and their capacity to enhance common bean production.
Materials and Methods
The experiment was conducted in two stages. During the initial stage, granular organomineral fertilizers were produced and a physical-chemical characterization was performed. Subsequently, the granules exhibiting the most favorable characteristics were selected for incubation in soil and for the cultivation of bean plants (Figure 1).
– Schematic representation of the experiment. BO = bio-oil; PE = pyroligneous extract; CW = cassava wastewater; NP = natural phosphate; WAR = water absorption rate; IRI = impact resistance index; SI = salinity index.
The B was obtained from the oven-furnace system of the Instituto de Ciências Agrárias of the Universidade Federal de Minas Gerais, located in the Minas Gerais state, Brazil (16°41’2.03” S, 43°50’19.28” W, altitude 646 m). The production of charcoal was achieved through the slow pyrolysis of wood derived from the Eucalyptus urograndis (Eucalyptus urophylla S.T. Blake × Eucalyptus grandis W. Hill ex Maiden clonal hybrid I144) species at 450 °C. The internal temperature of the ovens was monitored at 30-min intervals during the heating and drying stages and 8-h intervals during the cooling stage using an infrared sensor (Schettini et al., 2022).
The B was subjected to mechanical grounding and sieving using a 0.25 mm mesh to standardize the particles’ size and characterize their chemical and physical properties (Table 1). All analyses were conducted in quadruplicate. The pH and electrical conductivity (EC) were determined using a 1:20 ratio of B to deionized water after stirring for 1.5 h at 12.57 rad s–1 to ensure equilibrium between the solution and the B surfaces (Rajkovich et al., 2012). Bulk density was determined by filling a 10 mL graduated cylinder with the B, which was then shaken for 1 min, and subsequently weighed (Santos et al., 2019). The moisture, volatile material, ash and fixed carbon were determined by the methodology outlined in the American Society for Testing and Materials (ASTM) standard D1762-84 (ASTM, 2021). The organic carbon and nutrient content were determined in alignment with the approach described by Tedesco et al. (1995).
The CW, PE, and BO binders were employed in the initial process stage to produce the granules. The CW binder is a liquid residue resulting from the processing of cassava roots to produce starch. The PE and BO binders were derived from the production of charcoal using wood from E. urograndis in an oven-furnace system through the condensation of pyrolysis vapors. After six-month of standing, the layers were decanted and separated.
The granules were produced in a plate-type granulator. A 3:1 ratio of B to reactive NP (obtained from phosphorites containing 27 % total P2O5 and 8.1 % soluble P2O5 in 2 % citric acid) was employed to form the granules. The respective binders were uniformly sprayed onto the B and phosphate mixture and homogenized for approximately 40 min at 32.98 rad s–1.
The quantity of each binder was determined based on the weight of a 3:1 mixture of B and NP, corresponding to 50, 75, 100, 125, and 150 % of the weight of the mixture. Therefore, a dose corresponding to 50 % was prepared using 500 g of the mixture (comprising 375 g of B + 125 g of NP) and 250 mL of binder. For the doses corresponding to 75, 100, 125, and 150 % of the mixture (375 g of B + 125 g of NP), a respective volume of 375, 500, 625, and 750 mL of binder was utilized. The granules were subsequently subjected to drying in an oven maintained at a temperature of 65 ± 5 °C for approximately 72 h to attain constant weight (Santos et al., 2019). The granules were cooled to room temperature and stored in plastic containers.
The binders were characterized by their EC, pH, and total solids content (Table 2). Each analysis was conducted in triplicate. The EC and pH were directly measured in the CW and PE samples (liquid samples). For BO (liquid-pasty samples), a ratio of 1:5 was employed, whereby the BO was diluted in deionized water and the resulting aqueous extract was subjected to analysis (Tedesco et al., 1995). The total solids were determined in accordance with the ASTM method E1756-08 (ASTM, 2020). This entailed the placement of 50 mL of sample in porcelain crucibles within a water bath maintained at 65 °C for 16 h, followed by heating in an oven for a further 24 h at 103 °C. Thereafter, the crucibles were transferred to desiccators for cooling, and the weight of the solids was determined.
The experimental design was completely randomized in a 3 × 5 factorial scheme, with five replications. The treatments comprised three binders (CW, PE, and BO) in five doses, corresponding to 50, 75, 100, 125, and 150 % of the weight of a 3:1 mixture of B and NP. The granules were characterized in terms of the following technological properties: crushing strength (consistency), water absorption ratio (WAR), granule density, impact resistance index (IRI), salinity index (SI), and pH. To determine the crushing strength, the granules were separated by size into fractions of 1, 2, and 4 mm.
The granules were subjected to a compression resistance test to determine their crushing strength, employing a methodology adapted from that described by Nunes et al. (2022). This was done with the objective of measuring the force required to fragment the granules. In order to conduct the test, the granules were separated into three distinct size categories: 1, 2, and 4 mm. A total of five granules were randomly selected from each size category for the resistance test, which employed the use of a portable digital penetrometer. The granules were positioned on a regular, flat surface and subjected to mechanical force, quantified in newtons (N), until they reached a state of fracture.
The WAR of the granules was determined using a method adapted from Santos et al. (2020). In a 20 mL bottle containing purified water, 1 g of granules was added. After 24 h at room temperature, the mixture was filtered, and the moisture content of the granules was determined. The granules were then dried in an oven until they reached a constant weight, after which the dry matter was determined. The WAR was calculated using Eq. (1):
The density of the granules was determined using a method adapted from Santos et al. (2019). First, a 10 mL graduated cylinder was filled with the respective granule samples after they were oven-dried to a constant weight. Then, the granules were weighed, and their density was calculated using Eq. (2):
The IRI of the granules was determined using a method adapted from Richards (1990). The granules were dropped from a height of 2 m onto a hard surface, and the number of fragments that resulted from the impact was recorded. The granules were then subjected to repeated impacts and releases until entirely fragmented. This procedure was repeated five times until all the granules had shattered. The IRI was then estimated using Eq. (3):
where Nd is the number of times the sample was dropped from a height of 2 m and Np is the number of fragments generated by the impacts.
The SI and pH of the granules were determined in accordance with the recommendations established by MAPA (2017). The SI represents the osmotic pressure exerted by a given material compared to sodium nitrate, with an estimated value derived from the EC. To determine the SI, 1 g of finely ground granules was solubilized and homogenized in 100 mL of deionized water. Following a 30-min period, the EC of the supernatant was determined using a conductivity meter. The procedure was then repeated using a 10 g L–1 solution of sodium nitrate. The SI was subsequently calculated from the relationship between the EC of the sample solution (ECS) and the EC of the nitrate solution (ECNaNO3), in accordance with Eq. (4):
To determine the pH value, 10 g of finely ground granules were added to 50 mL of a 0.01 mol L–1 calcium chloride solution. The mixture was agitated for 30 min, stirring at 10-min intervals. The pH of the solution was then determined using a pH meter.
In the second stage of the study, the treatments comprised granular organomineral fertilizers synthesized with doses of 75, 100, 125, and 150 % of the CW, PE, and BO binders. The additional treatments were natural or unfertilized soil (NS), natural phosphate (NP), a mixture of biochar and natural phosphate (B+NP), and biochar (B). The experimental design was a completely randomized 3 × 4 + 4 factorial scheme (three types of binder in four different doses for producing the granules and four additional treatments), with three replications, resulting in 48 experimental plots.
The quantity of granular organomineral fertilizer applied to the soil was calculated based on the dry matter produced by the mixture utilized to prepare the granules at the corresponding binder dose (B, NP, and binders) to achieve the level of P fertilization recommended by Malavolta (1980), namely 200 mg dm–3. This calculation was made with the understanding that the composition of NP is 27 % P2O5.
Additional treatments with B and NP, whether alone or in combination, were applied in a conventional (non-granular) form. For the combined application of B and NP, a B to NP ratio of 3:1 was utilized, which is the same ratio employed in the production of granular fertilizers.
The soil utilized in the experiment was collected from the 0 to 20 cm layer of an Oxisol from a native region of the Cerrado and subjected to sieving through a 2 mm mesh. The chemical properties of the soil were determined in accordance with the methodology proposed by Teixeira et al. (2017): pH (H2O) = 5.80; P (resin method) = 1.30 mg dm–3; available P (Mehlich 1) = 0.95 mg dm–3; K = 13.71 mg dm–3; S = 1.87 mg dm–3; Ca = 0.31 cmolc dm–3; Mg = 0.05 cmolc dm–3; Al = 0.05 cmolc dm–3; base saturation (V %) = 38 %; cation exchange capacity at pH 7.00 = 1.19 cmolc dm–3, and soil organic carbon = 2.00 g kg–1.
The soil and treatments were homogenized and placed in 500 cm3 pots. Four seeds of the common bean were then sown per pot. Ten days after sowing, at the vegetative stage V1, the plants were thinned out to leave one plant per pot, which was then allowed to develop for 50 days, corresponding to the end of stage V4 and the start of flowering.
Four distinct top dressings were applied at the specific growth stage of the bean plants: at 23 (early V3), 28 (mid V3), 36 (late V3), and 41 (mid V4) days after sowing, utilizing a concentration of 50 mg dm–3 of 20-00-20 NPK fertilizer. Soil acidity was corrected with calcium and magnesium carbonate (Ca:Mg ratio of 4:1) in each treatment to increase the soil V % to 70 %. The soil moisture was maintained at a level close to field capacity throughout the experimental period.
In the initial phase of the experiment, the data were evaluated for normality of variances using the Shapiro-Wilk test and subjected to analysis of variance (ANOVA) using the F-test. When a significant difference was identified, the binders were compared using Tukey’s test (p < 0.05), with regression analysis employed for the doses. The data were transformed into arcsine (√x) to analyse crushing strength. The Kruskal-Wallis non-parametric test was utilized for IRI, and multiple comparisons were conducted using the Dunn’s test (p < 0.05).
At the conclusion of the second phase of the experiment (phenological stage V4), the SDM, the concentration of macro- and micronutrient, the accumulated amount, and the RDM were quantified. The data were subjected to ANOVA, and when significant, regression analysis was conducted for the variables under study as a function of the binder dose. Regardless of dose, the mean values for each binder were compared using the Tukey’s test (p < 0.05). The additional treatments were compared with the other treatments using the Dunnett’s test (p < 0.05). The R software, version 4.2.2, was utilized.
Results
The crushing strength increased linearly with the amounts of binder present in the mixture of B + NP used to produce the organomineral fertilizer (Figure 2A), irrespective of the specific type of binder or granule size employed.
– Physicochemical properties demonstrated by the granules with charcoal fines (biochar) as a function of different doses of cassava wastewater (CW), pyroligneous extract (PE) and bio-oil (BO). *significant at 5 %. WAR = water absorption rate; SI = salinity index; pH (CaCl2) = hydrogen potential in 0.01 mol L–1 CaCl2.
The larger the size of the granules in each of the binders (CW, PE, and BO), the greater the resistance to breakage. This is evidenced by the greater crushing strength required to fragment the granules (Table 3). For particles of sizes 1, 2, or 4 mm, the force required to fracture the BO granules was 6.0, 6.7, and 7.2 times greater, respectively, than that needed to fracture the CW granules (Table 3). With regard to the binders, the granules with BO exhibited the greatest resistance to breakage, while those with CW demonstrated the least (Figure 2A).
The WAR of the granules decreased linearly with the increase of the binder dose (Figure 2B). At the lowest dose (50 % binder), no distinction was observed between the CW and PE binders, which exhibited WAR values of 71.37 and 69.21 %, respectively (Figure 2B). As the binder dose increased, the WAR values for the PE granules were consistently lower than those for the CW granules (Figure 2B). Conversely, at all doses, the WAR values for granules with BO were lower than those with CW or PE (Figure 2B). Regardless of the binder dose, the WAR values for granules with BO, PE, or CW, were 39.43, 54.08, and 67.32 %, respectively (Table 4).
The type of binder and respective doses influenced granule density (Figure 2C), which increased linearly for granules with PE and BO. For CW, the maximum density (5.35 g cm–3) was obtained at a dose of 140 % (Figure 2C). In general, regardless of the dose, granules with BO exhibited a higher density than those with PE or CW (Table 4), with the granules with CW displaying the lowest values for density (Table 4).
The highest IRI values were observed in granules with BO (409.52 %), which differed from those with CW and PE, regardless of the binder dose (Table 4). Similarly, granules with PE (241.53 %) exhibited higher IRI values than those with CW (32.86 %) (Table 4).
No differences were observed for SI between the doses of BO and CW (Figure 2D). Conversely, a linear increase in SI was noted with an increasing dose of PE (Figure 2D). The highest mean values were obtained with PE, followed by BO and CW (Table 4).
The pH values of the granules with CW, adjusted to a quadratic model, are shown as a function of the binder dose in Figure 2E. The pH of granules with PE or BO decreased linearly with the respective dose (Figure 2E). In contrast, the highest pH was obtained with CW, regardless of the dose, as demonstrated in Table 4. Furthermore, there was no discernible difference in the average pH of granules with PE or BO (Table 4).
The application of B, whether individually or in combination with NP, irrespective of granulation, resulted in a notable increase in the SDM and RDM of the bean plants when compared to the NS. The increases were as follows: B + NP = 72.88 and 182.76 %; B = 72.88 and 148.28 %; CW = 98.31 and 331.03 %; PE = 123.73 and 755.17 %; and BO = 47.46 and 141.38 %, respectively (Table 5).
The application of granular fertilizer containing CW did not result in significant differences in SDM compared to the conventional application of B, whether used alone or in combination with NP, while PE resulted in significantly higher SDM values (Table 5). Conversely, bean plants treated with granular organomineral fertilizer containing CW or PE exhibited a marked increase in RDM, reaching 78.57 and 254.29 %, respectively, compared to the treatment containing BO, regardless of the binder dose (Table 5).
The binder dose did not affect SDM production in soils fertilized with granular organomineral fertilizer (Figure 3A). In contrast, for RDM, the treatments containing PE or BO granules exhibited a linear reduction, while those containing CW granules demonstrated a linear increase (Figure 3B).
– Regression equations adjusted for the dry matter of shoots (SDM) and roots (RDM) of bean plants observed at phenological stage V4, following the application of granular organomineral fertilizer with different binder doses. * and **significant at 5 and 1 %, respectively. BO = bio-oil; PE = pyroligneous extract; CW = cassava wastewater.
The data revealed a no statistically significant difference among the treatments concerning the macro- and micronutrient content of the bean plants. The results indicated that the average nutrient concentrations were as follows: N = 54.21 g kg–1; P = 1.25 g kg–1; K = 35.20 g kg–1; Ca = 19.31 g kg–1; Mg = 5.93 g kg–1; S = 7.86 g kg–1; B = 44.00 mg kg–1; Zn = 14.00 mg kg–1; Fe = 180.00 mg kg–1; Mn = 65.00 mg kg–1, and Cu = 3.32 mg kg–1. However, regarding to the quantity of nutrients accumulated in the SDM of the bean plants, the highest values were observed in the treatment with PE, while the lowest values were evident in NS and NP (Table 5).
Discussion
The linear increase in crushing strength with the different binder doses can be attributed to the proportional increase in the solid fraction that remains following the evaporation of water from the binders. It is possible that the solid fraction of the binders may have occupied the pores of the Bs and NP particles through polymerization (Riva et al., 2019). In this context, the binding mechanisms between the B and the different binders may have played a role in stabilizing the granules. These mechanisms are influenced by the various polar functional groups, including hydroxyl (OH), carboxyl (COOH), and/or carbonyl (CO) groups, present in the structure of Bs and binders. This hypothesis posits that these functional groups can facilitate an electrostatic attraction between particles, such as Van der Waals forces and hydrogen (Anukam et al., 2021).
In this study, a high crushing strength was required to fragment the granules containing the BO binder, which may be related to the presence of various aromatic and heterocyclic compounds of high molecular weight originating from the polymerization and condensation reactions of low-molecular-weight components (Frainetti and Klinghoffer, 2023). These compounds are derived from the thermal decomposition of lignin, hemicellulose, and cellulose from the biomass (eucalyptus wood), which renders the BO highly viscous and adhesive (Zhang et al., 2020).
The greater resistance to breakage exhibited by the granules with PE, in comparison to those with CW, can be attributed to the viscosity and composition of the binder. Despite this inferiority in relation to BO, the PE binder can contain chemical compounds, including aldehydes, phenols, pyrolytic lignin, and aromatic and aliphatic groups, which promote robust interactions between the solid particles (Asafu-Adjaye et al., 2022).
The high compressive crushing strength of the granules, irrespective of the binder utilized, may also be associated with the degree of cohesion between the binder, the B particles, and the NP particles. The binder plays a significant role in the mechanical strength and durability of the granules, influencing their consistency. It facilitates the formation of solid bridges between the particles (Peng et al., 2021). Moreover, the drying temperature of the granules (65 ± 5 °C for 72 h) may have facilitated thermal interactions between the solid fraction of the binder and the other components of the organomineral fertilizer after the evaporation and volatilization of the liquid and volatile fractions of the binders (Butler et al., 2023).
In terms of size, the larger granules demonstrated greater resistance to crushing, potentially due to the distributed force required to break the granules across a larger surface area. These findings align with those obtained by Nunes et al. (2022), who observed similar results with granules ranging in diameter from 3.35 to 4 mm, composed of B, organic polymers, and MgO, necessitating a breaking force of approximately 88.26 N.
The WAR decreased as the binder dose increased. This phenomenon can be attributed to the porous structures of the B being filled by the solid fraction of the binders, thereby reducing the available spaces for water absorption. Furthermore, at higher doses, binders have the potential to form a more cohesive matrix around the B particles, which helps to further reduce the water absorption capacity of the granules (Sarker et al., 2023).
The discrepancy the WAR values of the three binders can be attributed to both their hydrophobicity and their capacity to fill the porous spaces of the B structure, particularly the mesopores and micropores measuring less than 100 µm in size. The presence of hydrophobic compounds, including aliphatic and aromatic hydrocarbons, in the PE and particularly the BO binders may be associated with the moderate temperatures (< 500 °C) utilized to produce the B, as proposed by Gondim et al. (2018).
The same hypothesis previously used to explain the crushing strength of the granules can also be employed to elucidate the underlying mechanisms of the WAR. The higher water absorption of granules with PE compared to those with BO can be attributed to the presence of hydrophilic components and oxygenated functional groups (Cheng et al., 2022). Similarly, the elevated level of water absorption exhibited by the granules with CW in comparison to those with PE and BO can be attributed to the presence of starch. Due to its hydrophilic nature, even with prior gelatinization of the polysaccharide, less resistant and brittle granules may be formed because of the swelling caused by water absorption and breaking intermolecular bonds (Butler et al., 2023).
Concerning density, the lower values observed in granules with the CW binder may be attributed to the weak bonds between the particles, which influence on the cohesion forces between them and compaction, as previously discussed. The hydrophilic nature of starch may affect the density, as the irreversible swelling of the granules may result from the changes in this polysaccharide when the intermolecular hydrogen bonds are broken. The drying temperature of the granules may have prevented or hindered gelatinization and, subsequently, on cooling, recrystallization of the amylose and amylopectin chains, the main constituents of starch (Channab et al., 2023).
The density of granules density is correlated with their crushing strength. Therefore, the hypothesis posits that the incorporation of BO and PE into the constituents of the granules facilitates their penetration into the B pores, thereby creating a homogeneous and compact structure. Furthermore, the presence of these elements strengthens the bonds between the particles, establishing solid bridges that enhance the granules’ mechanical properties (Sarker et al., 2023). However, compared to PE, BO contains compounds of high molecular weight that are insoluble in water and demonstrate high resistance to volatilization, contributing to an increase in granule density (Lachos-Perez et al., 2023).
In this regard, both BO and PE serve as binders, facilitating the formation of denser granules that are more homogeneous in shape and size and exhibit greater resistance to breakage during transport and storage (Sarker et al., 2023). These attributes contribute to a reduction in the rate of water diffusion within the granule, resulting in a fertilizer with a slower release of nutrients (Cheng et al., 2022). With regard to the application of the granules, a more uniform distribution is achieved when they are placed directly in the planting furrow, thereby reducing the loss due to wind drift when they are broadcast.
The linear increase in the dose of PE suggests a proportional addition of water-soluble salts by the binder, as evidenced by the elevated EC levels (Table 2). The PE may contain more than 200 water-soluble chemical compounds, predominantly derived from the pyrolysis of cellulose and hemicellulose. These can potentially influence the EC, SI, and pH of the granules. Conversely, water-insoluble compounds in BO may account for the comparatively lower SI of granules with this binder (Lachos-Perez et al., 2023).
Using fertilizers with a high SI can have detrimental effects on plants, particularly when applied in large quantities and near the plant roots. In this regard, the high capacity of Bs to adsorb ions, which is attributed to the presence of surface electric charges resulting from the ionization of functional groups, can serve to reduce the SI of binders, such as PE (Nunes et al., 2022).
The elevated pH values observed in granules with CW can be attributed to the reduced buffering capacity and less acidic nature of the binder (Table 2), which can be attributed to the presence of basic compounds such as Ca(OH)2 (Cheng et al., 2022). Moreover, the alkaline pH of these granules, irrespective of the dose, is influenced by interaction with NP, which is also alkaline in nature. This hypothesis is supported by the findings of Mikolaitienė and Šlinkšienė (2022), who observed a pH close to neutral in fertilizers based on urea, microalgae, and potato starch.
As the binder dose decreased, granules with BO and PE showed a linear reduction in pH. This behavior is associated with the higher acidity of the binders (Table 2), which, due to the complexation of porous structures and basic cationic groups, exceeds the alkalinity of the B (Shetty and Prakash, 2020).
The acidity of the BO and PE components is attributable to both the raw material and the pyrolysis process. During pyrolysis, the cellulose, hemicellulose, and lignin present in wood undergo degradation, resulting in the production of a range of organic acids, including acetic acid, butanoic acid, formic acid, and propanoic acid, among others (Cheng et al., 2022; Zhang et al., 2020).
The present research demonstrated that the application of B, whether alone or in combination with NP, significantly influenced the production of SDM and RDM in bean plants, regardless of the granulation of organomineral fertilizers. This improvement in productivity can be attributed to the increased availability of nutrients for the plants and the liming effect of B with a high ash content (Table 1). This effect is particularly important in acidic tropical soils, as it reduces P fixation, increasing its availability for crops (Santos Júnior et al., 2024).
Furthermore, B exhibits a high reactive potential due to the surface charges. These charges predominantly originate from functional heterocyclic oxygen groups, including the carboxylate, phenolate, and oxonic groups. This attribute enables the B to compete with P for adsorption sites on minerals in weathered soils. Consequently, there is a reduction in nutrient precipitation reactions with Al and Fe oxides (Nunes et al., 2022).
The notable contribution of granular organomineral fertilizer to the production of SDM and RDM in bean plants can be attributed to the CW, PE, and BO binders, which enrich the soil with a variety of bioactive compounds and nutrients. The binders facilitate the release of a range of compounds, including aldehydes, ketones, alcohols, organic acids, esters, and essential minerals. This process stimulates root development and microbial activity in the soil. The granular form of these fertilizers allows for a controlled and continuous release of nutrients, thereby enhancing availability and the efficiency with which the plants absorb them (Cheng et al., 2022).
As previously discussed, the combination of B with the CW, PE, and BO binders results in the formation of surface functional groups, particularly oxygenated groups (OH, COOH, and CO), which are responsible for enhancing the polarity of the organomineral fertilizer and facilitating the adsorption of nutrient ions through various binding mechanisms, including as surface complexation, ion exchange, coprecipitation, electrostatic attraction, and physicochemical adsorption (El-Naggar et al., 2022).
The diminished dry matter production of plants cultivated in soils treated with granular fertilizers comprising BO, in comparison to those treated with granules containing PE and CW, can be attributed to the compact structure of the BO granules. This density creates a barrier that limits the diffusion of water from the soil and the dissolution of mineral elements from the granules, resulting in a slower release of nutrients and, consequently, a reduction in the initial crop yield (Cheng et al., 2022). The hydrophobic nature of the granular BO fertilizer also contributes to this effect (Zhang et al., 2020).
In contrast, the granules comprising the CW and PE binders contain a considerable quantity of water-soluble compounds, which can exert a more rapid influence on the physicochemical properties of the soil. This has the effect of improving the soil pH, water retention capacity, nutrient availability, and soil microbial community. Furthermore, the greater WAR of these binders may have increased water absorption in the rhizosphere, promoting the dissolution and availability of nutrients. This process contributes to better root development, mineralization, diffusion and nutrient assimilation, especially P from NP, increasing crop productivity (Santos et al., 2020).
The linear reduction in RDM as a function of the dose of the BO and PE binders can be explained by the proportional reduction in the water absorption capacity of the granules due to their reduced porous structure, resulting in less nutrient dissolution. In contrast, granular fertilizers containing CW had the opposite effect since starch, due to its hydrophilic properties, favors gelatinization and, consequently, better water absorption in the rhizosphere.
The effects of concentration and dilution (Jarrell and Beverly, 1981), associated with lower and higher dry matter production, respectively, may have contributed to the lack of any significant difference between treatments regarding the nutrient content of the bean shoots. The brief cultivation period may have constrained nutrients’ bioavailability due to the organomineral fertilizers’ slow-release nature, which retains nutrients within their structure. This may have initially favored the bio-stimulant effect of the binders. The greater amount of nutrients accumulated by the plants in the PE treatment is associated with the increase in dry matter production obtained with this treatment (Table 5).
Therefore, the technological properties of the organic fertilizer granules containing B and NP were modified by the diverse binders. The granules with BO exhibited the highest values for crushing strength, density, and IRI; CW demonstrated the highest values for WAR and pH, while PE exhibited the highest values for SI. The granular fertilizers, regardless of the binder utilized, were observed to increase SDM and RDM production in the bean plants. However, the results suggest that the biostimulant effect of the binders was more pronounced during the short cultivation period due to the slow-release nature of the fertilizer. Consequently, further research with longer cultivation periods is necessary to investigate more thoroughly the soil-plant-environment interactions with organomineral fertilizers containing distinct binders.
Acknowledgments
The authors would like to thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the scientific productivity fellowship granted to the last author. This research was funded by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq/Brazil), the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES/Brazil) and the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG/Brazil).
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Data availability statement
The data supporting the findings of this study are available in the paper. Should any raw data files be needed in another format they are available from the corresponding author upon request.
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Declaration of use of AI Technologies
No generative artificial intelligence (AI) was used in this study.
Edited by
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Edited by:
Ricardo Enrique Bartosik
The data supporting the findings of this study are available in the paper. Should any raw data files be needed in another format they are available from the corresponding author upon request.
