Sustainable Soil Amendment: Effect of Reusing Saturated Dolomitic Calcareous Amendment (DCAS) on Chemical Properties of Two Types of Agricultural Soils

Abstract

Replacing the linear process based on production, consumption, and disposal gives rise to the circular economy, in which materials are reincorporated into a new production process to create new amendments, following the model of sustainable agriculture. Through the circular economy approach, the aim is to add value to the waste generated during the adsorption process by recovering and reusing it as sustainable soil amendments. The present study analyzes the effects of saturated dolomitic calcareous amendment (DCAS) on the chemical properties of sandy-textured and clayey-textured agricultural soils. For this purpose, the dolomitic calcareous amendment, saturated with nutrients from hydroponic effluent through an adsorption process, was reused, and its effects on the chemical properties of agricultural soils were evaluated during incubation periods of 30, 60, and 90 days and compared with other amendments. A completely randomized experimental design was used, applying 4 treatments with 5 replications, totaling 20 experimental units for each soil type (sandy and clayey): T1 (control), T2 (dolomitic calcareous amendment in natura—DCAN), T3 (saturated dolomitic calcareous amendment—DCAS), and T4 (granulated dolomitic calcareous amendment—DCAG). The chemical properties evaluated were: pH in water, exchangeable aluminum, exchangeable calcium and magnesium, and available phosphorus. An interaction test between treatments and incubation periods was performed for each soil type and analyzed through analysis of variance, with means compared using Tukey’s test (p < 0.05) in InfoStat software, version 2020I. Through statistical analysis, it was confirmed that there was both interaction and a time effect for the variables pH, exchangeable aluminum, and available phosphorus in both sandy and clayey soils. Furthermore, the results showed that the saturated dolomitic calcareous amendment—DCAS (T3)—had good compatibility with both soil types, highlighting its ability to improve soil chemical properties by increasing pH, and available phosphorus levels, as well as completely reducing exchangeable aluminum concentration. This indicates that the saturated dolomitic calcareous amendment (DCAS) derived from the adsorption of nutrients from hydroponic effluent, can be effectively used to amend soil chemical properties, thereby promoting more efficient and environmentally sustainable agriculture.

1. Introduction

Hydroponics is revolutionizing agricultural production techniques worldwide due to its minimal ecological footprint, improved pest control, and high production yield. However, the hydroponic system requires large amounts of water and chemical fertilizers to optimize crop production. Consequently, this type of agriculture produces large quantities of point-source pollution highly concentrated in nitrate (200–300 mg NO₃-N L⁻¹) and phosphate (30–100 mg PO₄-P L⁻¹) [1,2]. At the same time, hydroponic effluents are discharged into surface and groundwater environments, leading to eutrophication and the consequent degradation of ecosystems [3].

Technologies for treating hydroponic effluents, such as ultrafiltration and reverse osmosis, are efficient but entail high operational and maintenance costs [4]. Among the emerging technologies for treating contaminated water, adsorption processes hold great potential [5]. Adsorption is a process by which a substance concentrates on the surface of another solid or liquid phase, making it a surface phenomenon; in addition, it is considered relatively simple, efficient, environmentally friendly, and economically viable [6,7,8]. However, only certain solids exhibit sufficient specificity and adsorption capacity to be useful as adsorbents, since solids often have the specific ability to adsorb large amounts of certain substances [9]. Boeykens et al. [10] demonstrated that dolomitic calcareous amendment (DCA) is an efficient adsorbent for phosphate from aqueous solutions containing other contaminants. However, once the adsorbent becomes saturated with the contaminant, a residue is generated that could cause a new environmental problem if not properly managed [11].

Currently, the concept of the circular economy is promoted worldwide as an innovative approach. This term represents a paradigm shift that emphasizes the revalorization of by-products, allowing them to be recycled at the end of their useful life and reused to manufacture other inputs [12]. According to Borrello et al. [13], the circular economy model applies recycled materials to food production to reduce the use of external resources. In Brazil, an innovative strategy for correcting acidic soils was recently proposed, consisting of producing a soil conditioner made from bauxite residue (used in alumina production) and residual palm oil biomass as raw materials [14]. Likewise, Kizito et al. [15] used nutrient-enriched biochar (from anaerobic digestion of urban effluents) as a soil amendment during maize growth, obtaining positive effects on micronutrients, macronutrients, soil organic matter, and biomass yield; they also indicated that nutrient-enriched biochar could partially replace chemical fertilizers and promote organic agriculture through the circular economy concept. Other soil amendments derived from by-products include mushroom residues and fly ash. Mushroom residue compost is a substrate material from mushroom cultivation that contains rich organic matter, nitrogen, phosphorus, potassium, calcium, magnesium, and micronutrients. It has been proposed as a slow-release fertilizer for plants, improving soil fertility, structure, moisture retention, and microbial activity [16]. Fly ash, a by-product of power plants, has been shown to enhance key soil properties, including neutralization of acidic soil pH, water infiltration, root penetration, and soil aeration, all of which are essential for healthy plant growth [17]. In addition, sewage sludge, and to a lesser extent certain types of industrial sludge, have been proposed as alternatives to conventional organic amendments, with restrictions for specific crops or soil conditions [18].

In tropical regions, oxisols and ultisols predominate (ferralsols and acrisols, according to the FAO soil classification system), which are acidic soils dominated by minerals rich in silica, iron, and aluminum [19,20]. These soils are highly weathered and have low natural fertility due to various pedogenetic processes such as ferralitization and argilluviation, which cause base loss and the concentration of iron and aluminum oxides [21]. Aluminum (Al) is a non-essential metal that can be toxic to plants, particularly in acidic soils, as it inhibits root growth [22]. In contrast, calcium (Ca), potassium (K), magnesium (Mg), and phosphorus (P) are essential nutrients that support vital plant functions such as photosynthesis, cell structure, cell division, and the absorption of water and nutrients. In general, soil acidity reduces the availability of essential nutrients [23]. Rodrighero et al. [24] indicate that calcareous amendment is the most widely used soil acidity corrector in agriculture.

In this context, an alternative for maintaining the fertility of agricultural soils and reducing the impact on natural resource exploitation is the use of residues (by-products) as amendments and sources of soil nutrients. Moreover, since 2017, the European Parliament has promoted the use of recycled materials for fertilizer production as part of the circular economy. It is estimated that around 30% of mineral fertilizers can be replaced with residues [25].

Based on these premises, and given that the recovery and reuse of saturated dolomitic calcareous amendment with nutrients adsorbed from hydroponic effluent have already been evaluated as an agricultural soil improver and for its impact on the agronomic performance of beans [26], the present study presents the effects of saturated dolomitic calcareous amendment (DCAS) on the chemical properties of sandy- and clay-textured agricultural soils during incubation periods of 30, 60, and 90 days. It is hypothesized that the use of DCAS, saturated with nutrients from hydroponic effluents, will significantly improve soil chemical properties (pH, Al, Ca, Mg, and available P) compared to other forms of calcareous-based amendments.

2. Materials and Methods

2.1. DCAS Used as a Sustainable Soil Amendment

The dolomitic calcareous amendment (DCA) was sieved using a U.S. standard testing sieve (Thermo Fisher Scientific, 168 Third Avenue, Waltham, MA, USA), ASTM-11 specification, mesh sizes 50 and 270 [27]. Figure 1 details the nutrient saturation (concentration) process of hydroponic effluent in the DCA.

Hydroponic effluent samples were collected from a lettuce production system. The samples were analyzed according to standard methods for water and effluent analysis [28]. Phosphorus was determined using the 4500-P-E Ascorbic Acid Method. Meanwhile, the other nutrients were determined using the Atomic Absorption Spectrophotometric Method, 3111 B–Direct Air-Acetylene Flame Method. pH was measured using the Electrometric Method, 4500-H⁺B.

Boeykens et al. [10] conducted dosage trials to optimize the amount of adsorbent mass for the removal of phosphate and nitrate ions from a synthetic solution. For this purpose, different masses of adsorbent were contacted in batch mode with 50 mL of solution containing phosphate, nitrate, or a mixture of both. The systems were agitated at 200 rpm for 24 h at 25 ± 2 °C. From these trials, it was concluded that the appropriate adsorbent dose is 3.000 ± 0.002 g of DCA; at this dose, 74% of phosphorus was removed. Based on this, in the present study, 3.00 g of DCA was mixed with 50.0 mL of hydroponic effluent (in triplicate) for 24 h under continuous agitation (160 rpm, 25 ± 2 °C) to allow sufficient nutrient adsorption and saturation of the DCA.

Table 1. Initial and final concentrations and percentage of nutrient removal [26].

Nutrient	P	Ca	Mg	Na	Fe	Zn	Mn
Initial Concentrations (mg/L)	2.67	100.50	30.00	4.52	4.24	0.48	0.05
Final Concentrations (mg/L)	0.68	80.65	30.00	4.52	4.24	0.42	0.02
Removal (%)	74.53	19.75	-	-	-	12.50	60.00
Standard Deviation	0.05	1.80	-	-	-	1.08	3.31

shows the means and standard deviations of the initial and final nutrient concentrations after contact with DCA, as well as the percentage of nutrient removal achieved. The initial and final pH of the sample were 8.01 and 8.22, respectively. Under these conditions, DCA retained phosphorus (74.53%), manganese (60.00%), calcium (19.75%), and zinc (12.50%). The phosphorus removal result reproduces that obtained by the authors mentioned above.

The following are the chemical characteristics of the amendments in their natural state and saturated with nutrients from hydroponic effluent:

DCA: CaO: 37.8%; MgO: 12.04%; PRNT*: 86.71%

DCAS: CaO: 38.74%; MgO: 12.33%; PRNT*: 90.64% [29].

* Total Relative Neutralizing Power (PRNT).

2.2. Soil Sampling and Analysis

An analysis was carried out on two types of soils to determine the availability of nutrients adsorbed by the dolomitic calcareous amendment for the purpose of amending them. Agricultural soils were collected from a depth of 0–20.0 cm in two locations in Paraguay. The sandy-textured soil sample (81.60% sand, 5.36% silt, and 13.04% clay) was taken from the District of Yhú, Paraguay: Latitude 25°08′39.4″ S and Longitude 55°51′49.4″ W; and the clay-textured soil sample (37.44% sand, 26.64% silt, and 35.92% clay) was taken from the District of Minga Guazú, Paraguay: Latitude 25°29′23.7″ S and Longitude 54°51′23.0″ W. The Minga Guazú soil belongs to the Oxisol order, while the Yhú soil corresponds to Argisol [30].

The samples were dried, ground, and sieved through 2 mm mesh, and then subjected to the respective physicochemical analyses to obtain baseline data prior to applying treatments. The following properties were analyzed: texture, cation exchange capacity (CEC), pH, available phosphorus, and exchangeable cations (Ca, K, Mg, and Al), following the methodology of the Brazilian Agricultural Research Corporation–EMBRAPA [31]: pH in H₂O; exchangeable Al, Ca, and Mg extracted with 1M KCl; exchangeable K and available phosphorus extracted with Mehlich 1 solution; cations quantified by Atomic Absorption Spectrophotometry, using a Thermo Scientific iCE 3000 Series AAS; CEC was calculated as the sum of the different exchangeable basic and acidic cations present in the soil. The pH meter used was the TEC-2 Tecnal (Brazil). Standards for calibrating the atomic absorption spectrophotometer were prepared from 1000 ppm standard solutions.

2.3. Experimental Design

A completely randomized experimental design was used, with 4 treatments and 5 replications, totaling 20 experimental units for each soil type. The treatments applied for each soil type are detailed in

Table 2. Applied treatments [26].

Treatment
T1 (Control)	Soil without DCA.
T2 (DCAN)	Soil with DCA in natura.
T3 (DCAS)	Soil with saturated DCA.
T4 (DCAG)	Soil with granulated DCA.

.

The experimental units were installed at the Faculty of Agricultural Engineering of the Universidad Nacional del Este (National University of the East) (Paraguay): Latitude 25°49′59.8″ S and Longitude 54°77′15.8″ W, under 50% white shade cloth.

Granulated DCA was included for comparison in terms of efficiency among the types of amendments. It was sourced from BM Brasilminas.

The applied amendment doses were calculated based on the results of the soil analysis, conducted in a preliminary evaluation prior to the experiment, taking into account textural class differences. Additionally, it was considered that a 20 cm-thick soil layer of one hectare weighs approximately 2,000,000 kg [32]. Using this information, the amount of amendment for 2000 g of soil was calculated through the base saturation method, applying Equation (1) [33]:

$$NC\left(t/ha\right)={\displaystyle \frac{CEC(V_2-V_1)}{100}}$$

(1)

where NC is the calcareous requirement in kg/ha; V₂ is the target base saturation percentage for common bean cultivation (70%), V₁ is the current soil base saturation percentage, and CEC is the cation exchange capacity in cmol_c/dm³ (as provided in the analysis report).

The resulting rates for each soil type were: 2747 kg/ha (2.75 g/pot) for sandy soil and 2122 kg/ha (2.12 g/pot) for clay soil. These quantities were weighed and mixed with 2000 g of soil, and then placed in pots (polyethylene bags 25.0 cm × 20.0 cm) properly labeled for a period of 90 days. Distilled water (due to the presence of calcium in tap water) was added to each treatment to maintain soil moisture and facilitate the calcareous reaction. This condition was maintained throughout the experimental phase. The experimental units were set up on 5 November 2022.

After applying the treatments with the different amendments, 25 g of soil samples were taken from the experimental units during each evaluation/incubation period (30, 60, and 90 days). The samples were extracted using a soil sampler consisting of a PVC tube (6 cm diameter, 30 cm length) with a 15 cm vertical opening to facilitate soil removal and transfer to a resealable plastic bag for transport to the laboratory. Once in the laboratory, the samples were dried, ground, and sieved for subsequent analysis, which included: pH in water, exchangeable aluminum, calcium, magnesium, and available phosphorus. Soil sample collection dates were 5 December 2022; 4 January 2023; and 3 February 2023, corresponding to 30, 60, and 90 days of incubation, respectively.

An interaction test between treatments and study periods was carried out for each soil type, analyzed by analysis of variance, and the means compared using Tukey’s test (p < 0.05) in InfoStat software, version 2020I. Data normality was verified using the Shapiro–Wilk test.

3. Results and Discussion

3.1. Soil Analysis

Table 3. Results of chemical analyses for the two soil types [29].

	pH H2O	Al	Ca	Mg	K	CEC	P
		cmolc/dm3					mg/dm3
Sandy Soil	4.55	0.70	0.24	0.10	0.05	4.49	3.11
Clay Soil	5.00	0.30	3.20	2.34	0.64	11.85	4.77

presents the results of the chemical analyses performed on the two soil types.

The sandy-textured soil exhibited a very high acidic reaction when measured in water (pH H₂O: 4.55) and an aluminum concentration of 0.70 cmol_c/dm³ (toxic to crops). The clay-textured soil showed a high acidic reaction when measured in water (pH H₂O: 5.00) and an aluminum concentration of 0.30 cmol_c/dm³ (slightly toxic to crops) [34].

3.2. Application of Treatments to Soil

3.2.1. Effect on the pH

Figure 2 and Figure 3 show the results obtained for pH and the interaction between treatments and study periods in sandy and clay soils, respectively.

Regarding the interaction between treatments and study periods over time, there was both an interaction and effect on pH in sandy soil, given the p-value obtained from the statistical analysis (<0.0001). When testing for significance, treatments T1, T3, and T4 did not show significant differences; however, considering treatments T2 and T3 at 90 days, a significant difference was observed compared to the other treatments, except for T3 at 60 days.

Figure 2 highlights the results obtained for pH in the sandy soil after treatment with T2 (DCAN) and T3 (DCAS), as this parameter increased over the 90-day period. According to the classification scale, the pH values observed at 30 and 60 days ranged from very acidic to low and moderate acidity, respectively, at 90 days [34].

In another study using DCAN, it was reported that applying 2500 kg/ha of DCAN increased sandy soil pH from 4.66 to 5.51 after 60 days of incubation [35]. Notably, in the present study, after 60 days of incubation with DCAS, pH reached 5.63 with a dose of 2747 kg/ha.

In T4 (DCAG), pH decreased slightly after 90 days. Although this reduction was not statistically significant, it is worth noting that this amendment did not increase soil pH due to its particle size, as it did not react within the timeframe of the trial. This was evident at the end of the incubation period, where product granules were still visible in the soil. This same result was reported by Díaz-Poveda & Sadeghian [36], who evaluated the effectiveness of commonly used amendments in Colombian coffee cultivation for correcting soil acidity. Regarding the two granular amendments used, it was demonstrated that, despite their adequate chemical purity, they did not increase soil pH due to their particle size, as they did not react within the time frame established for the trial (45 days); this was evidenced at the end of the incubation test, when granules of the products were observed in the soil. Studies by Viadé et al. [37] and Deus et al. [38], which evaluated the effectiveness of amendment particle size, concluded that finer fractions react more quickly and raise soil pH in less time; however, the residual effect lasts for a shorter period compared to larger particles. Therefore, the efficiency of amendments depends on particle size, as reaction rate is directly related to the particle’s contact surface area with the soil [32].

With regard to the clay soil, it was also confirmed that there was an interaction and effect on soil pH (p-value: 0.0001). However, when testing the significance of the results, treatments T1 and T4 showed no significant differences. In contrast, treatment T3 (at 30 days) presented a significant difference compared with the other treatments, except for T2 (30 days), which was similar to T3. This can be observed in Figure 3.

Although the statistical analysis did not show a significant difference between T1 and T4, Figure 3 reveals a positive trend considering the increase in pH for these treatments. The pH results for clay soils fell within the classification range of very high to high acidity [31]. In another study, results indicated that applying 2500 kg/ha of DCAN increased the clay soil pH from 4.28 to 4.79 after 60 days of incubation [35]. In this context, the present study highlights the result obtained with DCAS, which reached a pH value of 4.93 after 60 days of incubation with a dosage of 2122 kg/ha.

DCAS (T3) exhibited good affinity with both soil types evaluated, demonstrating greater effectiveness in reducing acidity in sandy soil, which may be attributed to the higher PRNT in DCAS, as a higher PRNT indicates a more reactive amendment.

3.2.2. Effect on Aluminum (Al)

Figure 4 and Figure 5 show the results obtained for aluminum and the interaction between treatments and study periods in sandy and clay soils, respectively.

According to the statistical analysis and the interaction between treatments and study periods, there was a confirmed interaction and effect on aluminum levels in sandy soil (p-value: <0.0001). When testing for significance, treatments T2 and T3 showed significant differences compared with T1 and T4. There were also differences between the results of T1 and T4 at 30 days compared with the values obtained at 60 and 90 days. In Figure 4, the results obtained in sandy soil regarding aluminum content after treatment with DCAN (T2) and DCAS (T3) stand out, since Al concentrations decreased to 0.06 cmol_c/dm³ after 30 days. At 60 days, concentrations decreased further in line with the increase in pH, becoming undetectable analytically, and remained at this level until the end of the incubation period. This may have been caused by the rise in pH, which in turn allowed a notable increase in CEC, thereby fixing Al in clay particles and preventing its displacement into the soil solution [39], or by the precipitation of Al³⁺ as Al(OH)₃ [40]. The Al concentrations obtained in T2 and T3 are classified as non-toxic to crops [34].

In the T1 (control) and T4 (DCAG) samples, Al concentrations showed a slight increase, indicating that DCAG requires more time (over 90 days) to react with or dissolve in the soil, thus behaving similarly to the control. The Al concentrations observed in T1 and T4 are classified as toxic to crops [34].

Regarding the clayey soil, it was confirmed that over time there was an interaction and effect of the amendments on soil aluminum levels. When testing the significance of the results, treatments T2 and T3 showed significant differences compared with T1 and T4. Differences were also observed between the values obtained for T1 and T4 at 30 days and those recorded at 60 and 90 days.

Figure 5 highlights the complete neutralization of Al in T2 (DCAN) and T3 (DCAS) at 30 days, with this concentration remaining stable after 90 days. Likewise, a progressive decrease in the concentration of this element in T1 (control) and T4 (DCAG) is observed starting at 60 days. The Al concentrations obtained in T2 and T3 are classified as non-toxic for crops; however, those observed in T1 and T4 are classified as slightly toxic [34]. The reduction in aluminum content is attributed to the increase in soil pH. This effect is evident from the negative relationships between the two variables in both soil types, leading to the precipitation of Al³⁺ as Al(OH)₃ [40].

3.2.3. Effect on Calcium (Ca)

Figure 6 and Figure 7 show the results obtained for calcium and the interaction between treatments and study periods in sandy and clay soils, respectively.

From the statistical analysis and the interaction between treatments and study periods, it was confirmed that there was no interaction or effect on Ca in sandy soil (p-value: 0.3498). However, when testing the significance of the results, treatments T2 and T3 showed significant differences compared to those obtained for T1 and T4, as shown in Figure 6.

In Figure 6, the results in sandy soil after treatment with T2 (DCAN) stand out, without significantly differing from T3 (DCAS). The Ca concentrations obtained in these treatments are classified as low [31]. T4 (DCAG) behaved similarly to the control, indicating that more time is required for it to react in the soil.

Regarding the results on the interaction between treatments and study periods in clay soil, there was no interaction or effect on soil Ca (p-value: 0.5534). When testing the significance of the results, those obtained for T3 at 30 and 90 days showed significant differences compared with T1 and T4, except for T2.

Figure 7 highlights the Ca concentration obtained in T3 (DCAS) at 30 days. In addition to increasing soil pH and neutralizing exchangeable aluminum, the amendments provide calcium and magnesium according to their composition [36]. In this regard, the high Ca concentration observed could be attributed to the calcium content in the amendment, as the CaO percentage increased in DCAS by adsorbing the calcium present in the hydroponic effluent.

In T2 (DCAN), Ca concentration remained constant throughout the incubation period. The Ca levels obtained in T3 and T2 at the end of the incubation period are classified as high and medium, respectively [31,34].

In general, the highest calcium levels were observed after 30 days of study, highlighting the rapid reaction of the cation with the soil type used. This is attributed to the texture, which provides a larger reactive surface compared to sandy soil, thereby allowing greater dissolution of the applied amendment. In this regard, Melo et al. [41] found that in clay soils, higher retention of cations K⁺, Na⁺, Ca²⁺, and Mg²⁺ was observed, therefore being influenced by soil texture. Furthermore, according to other authors, the low mobility of Ca in the soil profile, due to its retention on the negative charges of clays and organic matter, could favor the levels of available Ca after amendment application [42].

3.2.4. Effect on Magnesium (Mg)

Figure 8 and Figure 9 show the results obtained for magnesium and the interaction between treatments and study periods in sandy and clay soils, respectively. From the statistical analysis and the interaction between treatments and study periods, it is confirmed that there was an interaction and effect on Mg in sandy soil (p-value: 0.0437). When testing the significance of the results, treatments T2 and T3 showed significant differences compared to T1 and T4, as shown in Figure 8.

Figure 8 highlights the result obtained in sandy soil regarding magnesium content after treatment with T2 (DCAN), which differed significantly from T3 (DCAS) at 60 days.

The Mg levels obtained after 90 days in both T2 and T3 are classified as medium [31]. In contrast, in T1 (control) and T4 (DCAG), Mg contents were very low

Regarding the results on the interaction between treatments and study periods in clay soil, there was no interaction or effect on soil Mg (p-value: 0.2566). However, when testing the significance of the results, the values obtained for T3 (at 30 and 60 days) and T2 (at 30 days) showed significant differences compared with the other results. Additionally, T4 showed significant differences relative to T1 during the 30- and 60-day periods.

According to Figure 9, the highest Mg levels were observed at 30 days of incubation for all treatments. Despite the reductions in Mg content across the different treatments applied in the present study, all levels are classified as high [31,34].

In the research conducted by Samudio [43], who applied doses of 2400 and 4000 kg/ha of dolomitic calcareous amendment to sandy and clay soils, respectively, incubated for 90 days, a significant increase in Mg content was observed with the application of dolomitic calcareous amendment in both soil types. Regarding the incubation period, the author observed a significant increase in clay soil up to 90 days; however, in sandy soil, the effect was much faster but shorter in duration, with the effect observed at 30 days. The amendment increased Mg content to 1.55 cmol_c/dm³ (at 90 days) and 0.75 cmol_c/dm³ (at 30 days) in clay and sandy soils, respectively. These results contrast with those of the present study, since the reaction was faster in clay soil, reaching the highest Mg content at 30 days. Furthermore, the Mg concentrations obtained in clay soil in the present study (at 90 days) are higher than those reported in the cited work, as values of 2.46 and 2.29 cmol_c/dm³ were obtained after applying 2122 kg/ha of DCAS and DCAN, respectively; notably, this dose is lower than that used by the aforementioned author.

3.2.5. Effect on Phosphorus (P)

Figure 10 and Figure 11 show the results obtained for phosphorus and the interaction between treatments and study periods in sandy and clay soils, respectively.

From the statistical analysis and the interaction between treatments and study periods, it is confirmed that there was an interaction and effect on P in sandy soil (p-value: 0.0037). When testing the significance of the results, the values obtained for treatments T1 (60 days), T2 (60 and 90 days), and T3 (90 days) showed significant differences compared to those obtained for T1 and T4, as can be seen in Figure 10.

Figure 10 highlights the results obtained in sandy soil regarding P content after 60 days of study. An increase in P content can be observed with the application of treatments T2 (DCAN), T3 (DCAS), and T4 (DCAG). All values obtained are classified as high [31].

Some authors indicate that as soil pH becomes less acidic (pH > 5.5), phosphorus availability or solubility increases [44]. This explains the increase in P content in T3 observed after 60 days, since during this period, the pH of soil treated with DCAS increased to 5.63.

Regarding the results on the interaction between treatments and studied periods in clayey soil, there was interaction and effect on soil P (p-value: 0.0008). When testing the significance of the results, the value obtained in T1 (at 60 days) showed a significant difference compared to the other results (Figure 11).

In Figure 11, the P levels obtained at 60 days are noteworthy, as they increased compared with the 30-day period, even though pH values during this period were the lowest found in this study, below 5.0. Concentrations then decreased again at 90 days. P levels are classified as high for T1 and medium for T2, T3, and T4 at the end of the study period [31,34]. According to the literature, phosphorus retention at pH below 5.5 is mainly due to its reaction with iron and aluminum, precipitating as iron phosphate (FePO₄) and aluminum phosphate (AlPO₄) [45].

Table 4. Most significant effects of each treatment in the studied soils.

Treatment	Sandy Soil
	pH H2O	Al	Ca	Mg	P
		cmolc/dm3			mg/dm3
T1(control)	4.42 (90 d)	0.75 (90 d)	0.46 (90 d)	0.14 (30 d)	6.52 (90 d)
T2 (DCAN)	6.42 (90 d)	0 (60 d)	1.81 (90 d)	0.97 (60 d)	6.55 (60 d)
T3 (DCAS)	6.06 (90 d)	0 (60 d)	1.41 (60 d)	0.80 (30 d)	6.59 (60 d)
T4 (DCAG)	4.59 (60 d)	0.71 (90 d)	0.64 (60 d)	0.11 (90 d)	6.03 (60 d)
Treatment	Clayey Soil
	pH H2O	Al	Ca	Mg	P
		cmolc/dm3			mg/dm3
T1(control)	4.73 (90 d)	0.23 (90 d)	3.49 (30 d)	1.93 (30 d)	7.24 (60 d)
T2 (DCAN)	5.45 (30 d)	0 (30 d)	3.80 (30 d)	3.18 (30 d)	5.93 (60 d)
T3 (DCAS)	5.58 (30 d)	0 (30 d)	4.18 (30 d)	3.21 (30 d)	6.64 (60 d)
T4 (DCAG)	4.61 (90 d)	0.21 (90 d)	3.34 (30 d)	2.61 (30 d)	6.64 (60 d)

presents the most significant effects (values) of each treatment in the studied soils. It also shows the time at which these effects were observed.

Nutrient availability for plants is strongly correlated with soil pH. Major plant nutrients such as N, P, K, Mg, and Ca are severely limited in acidic soils [46]. In Table 4, it can be observed that in treatments T2 and T3, as pH increased, aluminum concentration decreased significantly while phosphorus availability increased. This behavior was observed in both soils studied; soluble aluminum in ionic form precipitates into non-toxic forms for plants, such as aluminum hydroxides [40]. Depending on soil pH, phosphorus solubility can be controlled by aluminum, iron, or calcium. In neutral and calcareous soils (mainly composed of calcium carbonate), phosphorus precipitates with calcium or is fixed on the surfaces of clay minerals and calcium carbonate. In this particular case, phosphorus increases its availability and/or solubility with the rise in pH [44].

According to Díaz-Poveda & Sadeghian [36], who evaluated the efficiency of commonly used amendments in Colombian coffee cultivation to correct soil acidity, Al³⁺ decreased with increasing pH, reaching analytically undetectable levels for pH values above 5.31. This correlates with the findings of the present study, since in sandy soil, as pH increased at 60 days in T2 and T3 (5.08 and 5.62, respectively), aluminum became analytically undetectable. In clayey soil, this result was obtained at 30 days, reaching pH values of 5.45 and 5.58 for T2 and T3, respectively. In these periods, the higher pH values observed in T3 could be attributed to the higher calcium content in the amendment. Thus, DCAS not only eliminates Al³⁺ but also supplies Ca to the soil, improving plant nutrition and cation balance in the soil.

The reaction mechanism of DCAS allows the neutralization of soil acidity when it comes into contact with water. The calcium (Ca²⁺) released from the amendment does not neutralize acidity directly; instead, it displaces hydrogen (H⁺) and aluminum (Al³⁺) ions from the cation exchange sites on soil particles [47]. These ions, once in the soil solution, are neutralized by hydroxide ions (OH⁻) derived from the hydrolysis of the amendment anions, such as carbonate. In this way, toxic aluminum in the soil solution is neutralized, and soil pH increases, improving fertility and conditions for plant growth.

4. Conclusions

Based on the statistical analysis, it was confirmed that there was an interaction and an effect over time for the variables pH, exchangeable aluminum, and available phosphorus in both sandy and clay soils.

The dolomitic calcareous amendment saturated with nutrients from hydroponic effluent (DCAS-T3) showed good affinity with both types of soils studied, highlighting its ability to improve soil chemical properties. In sandy soil, the pH increased up to 90 days. This increase influenced the concentration of exchangeable aluminum and available phosphorus; aluminum content decreased to analytically undetectable levels, while during the same 60-day incubation period, phosphorus concentration increased, confirming its effectiveness in neutralizing acidity and improving soil properties, thus enhancing conditions for plant development. In clayey soil, pH, exchangeable aluminum, calcium, and magnesium reacted rapidly after 30 days of study. The highest phosphorus levels were observed at 60 days.

The use of DCAS represents a viable alternative for improving the chemical properties of acidic soils in the context of sustainable and circular agriculture. However, field studies are required to validate its agronomic effectiveness, assess its persistence over time (residual effect), and rule out potential environmental risks associated with continuous application.