Potential acidity determination for soils with high soil organic matter

ABSTRACT

Lime applications are used in acid soils to correct pH and eliminate Al³⁺ toxicity in crops. The lime rates are determined based on the estimated soil potential acidity (H+Al), using soil incubations with calcium carbonate (CaCO₃) for 180 days, being a time-consuming and laborious process. An alternative method with calcium hydroxide [Ca(OH)₂] for 96 h incubation may efficiently estimate H+Al, but it needs further studies. Also, soils from specific regions in southern Brazil have high organic matter (SOM) contents (>5 %), medium clay contents (20-40 %), and are stony, lacking research to improve liming recommendations. The study aimed to determine potential acidity and adjust lime rates for acidity correction in medium-textured soils with high SOM and stoniness in the Serra Gaúcha region (RS), Brazil, and to test the efficiency of the short incubation alternative method. For this purpose, 20 native forest soils were sampled in the 0.00-0.20 m layer. Samples were dried, sieved, and subjected to short and long incubations. The H+Al values obtained through the incubations were compared with the Manual de calagem e adubação para os Estados do Rio Grande do Sul e de Santa Catarina (CQFS-RS/SC, 2016) equation. The H+Al values estimated by CQFS-RS/SC were lower than those determined by the long incubation, indicating lime rates ranging from 2.0 to 5.1 Mg ha^-1 lower than in the long incubation. The short incubation showed a positive correlation (0.93***) with the long incubation. The potential acidity for soil water pH 6.5 can be obtained by long [H+Al (cmol_c dm^-3) = 1354.9e^-0.855TSM] and short [H+Al (cmol_c dm^-3) = 3763.8e^-1.086TSM] incubations, in which TSM is the Tampão Santa Maria index. On average, 34 % of the soil volume was composed of rock fragments (>2 mm). We recommend adjusting the limestone rates for soils with stoniness, considering only the percentage of soil in the diagnostic layer (SDL%).

Keywords
stoniness soils; soil organic carbon; lime requirement; acidity neutralization

INTRODUCTION

Acid soils cover approximately 30 % of the Earth surface (von Uexküll and Mutert, 1995; Bian et al., 2013). Since they have low pH values, high Al³⁺ contents are present in them, which is toxic to plants (Singh et al., 2017; Rahman and Upadhyaya, 2021). Also, they have low contents of cations, such as calcium (Ca), magnesium (Mg), and potassium (K), which may have been lost through chemical weathering and leaching processes (Goulding, 2016; Lu et al., 2022). This can be reflected in low Ca-Mg-K saturation values and high Al saturation values (Miotto et al., 2020; Antonangelo et al., 2022).

Soil acidity is composed of active acidity and potential acidity (H+Al). Active acidity is characterized by the concentration of H⁺ ions in solution and is measured by the soil pH(H₂O). Its value is also used to define whether or not lime needs to be applied to the soil. Potential acidity is the largest portion of soil acidity, which can be exchangeable (mainly represented by Al³⁺) and non-exchangeable (H+Al) (Rheinheimer et al., 2018; Teixeira et al., 2020). Potential acidity, represented by the H+Al value (CQFS-RS/SC, 2016), is composed of H⁺ and Al³⁺ that are specifically adsorbed to the soil functional groups present in SOM and clay minerals. Stoichiometrically, 1.0 cmol_c dm^-3 of acidity is neutralized by the same value of base, corresponding to 1.0 Mg ha^-1 of calcium carbonate (CaCO₃) in the 0.00-0.20 m layer, assuming the soil mass of 2,000,000 kg (d = 1.0 kg dm^-3) (Kaminski et al., 2002).

Agricultural practices in the Serra Gaúcha region occur in soils with low to medium weathering degree, formed by intermediate to acidic rocks (basalt, rhyolite, and rhyodacite), with poorly developed profiles, with Neossolos and Cambissolos predominating (Modena et al., 2016; Santos et al., 2018), which correspond to Entisols and Inceptisols (Soil Survey Staff, 2014). One of the characteristics of these soils is the presence of parent material in the surface layers of the soil, which reduces the amount of useful soil in the 0.00-0.20 m layer, implying nutrient accumulation in the profile (Zheng et al., 2021). Also, the accentuated slope in many cultivated areas in this region favors erosive processes and increases stoniness and rockiness in the surface layers (Streck et al., 2018). In this case, it is important to consider that in stony soils, not all of their volume can be considered useful for reacting with the applied acidity corrective. This can cause an overestimation of the corrective rate and raise the soil pH(H₂O) above the value considered adequate for the crop of interest, in addition to generating unnecessary costs with the acquisition of limestone.

Real estimation of H+Al and the need for limestone to reach a certain soil pH(H₂O) value can be determined from the incubation of soils with CaCO₃ and subsequent determination of the SMP Index (Shoemaker et al., 1961; Gama et al., 2013) or TSM (Toledo et al., 2012), which have a direct relationship with potential acidity. However, the soil incubation process requires a long period, about 180 days (Kaminski et al., 2002), and, therefore, can also be conceptualized as long incubation (Brunetto et al., 2019). Thus, the determination process is slow, laborious, and, consequently, more expensive. An alternative to this method is wet incubation, in which the soil is incubated for only 96 h, with rates of calcium hydroxide [Ca(OH)₂] (Liu et al., 2004, 2005), which is called short incubation (Brunetto et al., 2019).

Studies have been conducted to estimate potential acidity in heterogeneous soil groups in southern Brazil, typically using long incubation (Kaminski et al., 2002; Predebon et al., 2018). However, in some soils, the corrective rates indicated by CQFS-RS/SC (2016) are underestimated (Brignoli et al., 2020) and, in others, overestimated (Brunetto et al., 2019) due to the variability of soils and their chemical properties, specific to each region. Thus, there is also a need to adjust the H+Al equations for more homogeneous soil groups, especially those present at the extremes of the calibration curves, such as soils with low clay and low SOM (low H+Al) or high SOM contents (high H+Al), between the TSM and H+Al index values (Gama et al., 2013; Predebon et al., 2018; Brunetto et al., 2019). This may be the case for soils with higher organic matter (SOM) content (high class: >5.0 % SOM) and medium texture (medium class: 21-40 % clay) (CQFS-RS/SC, 2016), such as those located in the Serra Gaúcha Region of Rio Grande do Sul (RS). This region is the largest grape and peach producing region in Brazil (Silva and Rodrigues, 2018; Seapdr, 2023), in addition to other temperate and subtropical fruit trees.

Accurate estimation of limestone rates to correct soil acidity improves the chemical environment for root growth of commercial crops or soil cover crops species (Chaudhari et al., 2019; Miotto et al., 2020; Gurmessa, 2021; Rahman and Upadhyaya, 2021). This also decreases heavy metal availability, such as copper (Cu) and zinc (Zn), which are observed in excess in orchard and vineyard soils due to frequent foliar fungicide applications (Miotto et al., 2017; Hummes et al., 2019; Bossolani et al., 2021), reducing the potential toxicity of these metals to plants (Ambrosini et al., 2015; Rosa et al., 2020). This study aimed to determine potential acidity and adjust H+Al equations for acidity correction in soils with medium texture, high organic matter content, and stoniness, characteristic of the Serra Gaúcha region (RS), in addition to testing the efficiency of the short incubation alternative method.

MATERIALS AND METHODS

Soil characterization

The study was conducted on soil samples collected from municipalities in the Serra Gaúcha region of Rio Grande do Sul, southern Brazil (Figure 1). According to the Köppen-Geiger classification system, the climate is classified as Cfa, with hot summers, and Cfb, with temperate summers (Alvares et al., 2013). The region has heterogeneous natural characteristics, with rugged relief and high variability of soil types (Sarmento et al., 2006, 2012; Streck et al., 2018). Soil samples were collected from native forest areas, without human action or agricultural cultivation history, in the 0.00-0.20 m layer, totaling 20 soils. Samples, with approximately 50 kg of soil, were air-dried, ground, and passed through a 2 mm mesh sieve. The sieved volume (<2 mm) was quantified and considered as usable soil; in other words, the soil that was able to react with the acidity corrective added to the soil and was considered as the percentage of soil in the diagnostic layer (SDL%). The portion retained in the sieve (>2 mm) was considered as the non-reactive fraction (gravels, cobbles, stones, and boulders), and its percentage could be disregarded when calculating the lime requirement, which allows reducing the acidity corrective rate to be applied. Chemical and physical characterization of the soils are presented in table 1.

Figure 1
Location of sampled soils collected to determine potential acidity (H+Al) in the Serra Gaúcha region in Rio Grande do Sul State, Southern Brazil.

Table 1
Physical and chemical properties of 20 native soils collected in the Serra Gaúcha region in Rio Grande do Sul State

Soil	Altitude	Sand(1)	Silt(1)	Clay(1)	SOM(2)	C/N Ratio(2)	pH (H2O)	pH (TSM)(3)	P(4)	K(4)	Ca2+(5)	Mg2+(5)	Al3+(5)	CECpH7.0(6)	SDL(7)
	m	—————— g kg-1 ——————					1:1 ratio	1:1.5 ratio	— mg dm-3 —		—————— cmolc dm-3 ——————				%
1	330	291	482	228	58.8	9.2	5.6	5.8	41.7	245.0	16.2	3.7	0.08	25.6	63
2	555	363	421	216	64.8	11.1	5.3	5.6	6.4	322.3	11.0	2.4	0.16	20.0	64
3	428	334	368	297	49.8	10.3	4.6	5.2	3.6	131.7	7.1	1.1	0.62	18.2	55
4	575	312	448	240	64.7	10.0	6.1	6.2	5.5	398.3	13.3	2.7	0.08	40.9	76
5	550	336	398	266	51.8	10.3	4.9	5.3	3.2	306.3	7.8	1.9	0.14	18.0	75
6	496	365	362	273	50.6	9.1	4.3	4.9	3.3	205.0	5.3	1.2	1.32	19.0	69
7	385	352	431	217	102.8	9.2	5.6	5.7	8.6	354.3	27.0	4.0	0.28	36.5	49
8	557	228	483	290	80.1	9.8	4.3	4.6	4.0	203.7	7.8	1.8	0.96	14.7	71
9	619	328	402	270	61.0	11.5	4.6	5.2	3.5	198.3	7.4	1.4	0.25	16.8	61
10	583	435	294	272	51.4	10.2	5.7	6.0	6.1	377.0	14.4	3.0	0.15	21.9	65
11	578	447	381	172	42.3	10.0	5.0	5.6	3.8	159.7	7.0	1.6	0.21	15.2	83
12	716	210	458	333	66.7	11.0	4.3	5.0	3.6	138.3	4.5	1.8	1.82	20.8	83
13	759	185	396	419	74.9	12.9	3.5	4.0	5.3	98.3	0.4	0.5	5.51	36.9	60
14	495	361	383	256	71.4	10.4	4.6	5.1	3.6	278.3	10.0	2.4	0.22	23.0	71
15	661	308	396	295	67.6	10.6	4.9	5.4	5.2	335.7	8.5	2.4	0.17	18.1	75
16	254	221	513	267	45.3	8.7	5.4	5.8	1.6	291.7	8.9	2.0	0.14	15.7	70
17	612	396	309	295	50.2	8.9	4.7	5.3	6.0	231.7	9.5	2.7	0.24	20.1	63
18	537	346	381	274	76.4	9.1	5.2	5.5	7.2	261.0	1.9	1.1	5.61	36.9	62
19	172	162	574	265	48.2	8.9	5.7	6.0	1.2	275.7	11.7	2.7	0.22	25.4	53
20	505	382	380	239	75.2	9.6	5.4	5.8	11.3	369.0	12.2	2.2	0.04	19.0	77

• ⁽¹⁾
  Particle size distribution determined by the Pipete method.
• ⁽²⁾
  Nitrogen and SOM (Soil Organic Matter) determined by dry combustion in an Elemental Analyzer (Flash EA1112, Thermo Electron Corporation, Milan, Italy).
• ⁽³⁾
  pH-TSM determined by Toledo et al. (2010) method;
• ⁽⁴⁾
  P and K were extracted by Mehlich-1.
• ⁽⁵⁾
  Ca²⁺, Mg²⁺, and Al³⁺ were extracted by KCl 1 mol L^-1.
• ⁽⁶⁾
  CEC_pH7.0: cation exchange capacity [Ca²⁺ + Mg²⁺ + K⁺ + (H+Al)].
• ⁽⁷⁾
  SDL%: percentage of soil in the diagnostic layer after sieving in 2 mm diameter mesh.

Long incubation

Experimental units were composed of portions of one kilogram of soil, which were placed in plastic containers with a capacity of two liters. Treatments used were based on five fractions of the lime requirement to achieve the soil pH(H₂O) to 6.5, estimated by the TSM Index (CQFS-RS/SC, 2016), in addition to an uncorrected control (0, 100, 150, 200, 250, and 300 %). The experimental design was completely randomized, with three replications. After adding the soil to the containers, the moisture was maintained at around 70 % of the field capacity. For this, the soils were moistened with distilled water until the estimated weight was reached. The long incubation was carried out with calcium carbonate (CaCO₃) applied in the solid state and homogenized with the soil. Each container was closed with a plastic lid, containing holes for gas circulation. Weekly, the soil in each container was stirred to accelerate the chemical reaction and allow the removal of CO₂ excess. Every 30 days, the initial weights were adjusted to standardize the moisture content, and soil samples were collected to monitor the pH(H₂O) values. Soil was dried and ground and pH(H₂O) (soil:solution ratio 1:1) was determined (Tedesco et al., 1995). The incubation was finalized after 180 days, with the stabilization of the pH(H₂O) value. Subsequently, the soils were removed from the flasks, dried, and passed through a 2 mm mesh sieve. The pH(H₂O) and pH-TSM values were determined (Figure 2a) (Toledo et al., 2010). We highlight that the potential acidity (H+Al) obtained through long incubation with CaCO₃ (standard method) was considered as the real potential acidity (Real H+Al), and will be discussed in the text.

Figure 2
Soil 2 exemplifying the potential acidity (H+Al) determination using the pH evaluations after 180 days in long incubation with CaCO₃ (a), after 96 hours in short incubation with Ca(OH)₂ (b), and the rate of Ca(OH)₂ after 72 hours to achieve the reference pH 7.0 (c) that represents pH(H₂O) of 6.5.

Short incubation

Soil samples were subjected to short incubation with calcium hydroxide [Ca(OH)₂] (Liu et al., 2004, 2005). Portions of 40 g of dried soil were weighed and added to 150 mL beakers. Then, five rates equivalent to the lime requirement to correct the H+Al (Equation 1) were added to the samples, in addition to a control treatment (0, 50, 75, 100, 125, and 150 %) and performed in a completely randomized experimental design with three replicates. The rates were composed of a Ca(OH)₂ solution (0.022 mol L^-1), according to the H+Al value estimated by pH-TSM (CQFS-RS/SC, 2016), up to the H+Al limit of 8.0 cmol_c dm^-3. In soils with H+Al >8.0 cmol_c dm^-3, Ca(OH)₂ was added in the solid state, in an amount corresponding to the estimated potential acidity, according to Liu et al. (2005), to increase the pH(H₂O) to 6.5. In addition to the Ca(OH)₂ rate, when necessary, distilled water was added to complete the solution volume to 80 mL. In each replication, 1.0 mL of chloroform was added to inhibit the activity of the microbial population and possible changes in pH values. Subsequently, the containers were covered with plastic film containing a 5 mm diameter hole to allow gas exchange and reduce solution evaporation. Then, they were subjected to an oven with forced air circulation at 35 °C. The solution pH was measured at 0, 24, 48, 72, and 96 hours. The Ca(OH)₂ rate used to reach the pH value of 7.0 at 72 hours was considered the rate to neutralize the H+Al, increasing the soil pH(H₂O) to 6.5.

Exemplification of incubation methods

Results obtained in the incubation of soil were used to exemplify the pH(H₂O) and pH-TSM evaluations performed in relation to the CaCO₃ rates (Figure 2a). The percentage of 100 % of the rate recommended by CQFS-RS/SC (2016) increased the soil pH(H₂O) to 6.5. Also, the pH evaluation of the samples subjected to Ca(OH)₂ rates was performed (Figure 2b), and it was observed that the percentage of 114 % of the rate recommended by CQFS-RS/SC (2016) was able to increase the solution pH to 7.0 (Figure 2c), which represents the correction of the soil pH(H₂O) to 6.5. In both incubations, the corrective rate to reach pH 6.5 was observed (Figures 2a and 2c), and after calculating the H+Al values determined in each method.

Statistical analysis

The H+Al values reached in the short incubation, as well as by the CQFS-RS/SC (2016) equation (Equation 1) were correlated with the H+Al reached by long incubation of soils that received CaCO₃, these long incubation values being considered as the real H+Al.

$H+AI\left({\mathrm{cmol}}_{c}d{m}^{-3}\right)={\mathrm{e}}^{\left(10,665-\left(1,{1483}^{\ast }TSM\right)\right)}/10$ Eq. 1

Each soil was considered an experiment and the CaCO₃ or Ca(OH)₂ rates were considered as treatments. The pH-TSM and H+Al values were correlated with the respective reference pH(H₂O) (5.5, 6.0, and 6.5), using regression analysis to obtain the mathematical model to determine H+Al or the lime requirement to better adjust the data. The data were statistically compared by the Tukey’s test (p<0.05) and presented graphically with a confidence interval (p<0.05) for the equation coefficients.

RESULTS AND DISCUSSION

The evaluated soils were characterized as medium clay class (210-400 g kg^-1), ranging from 172 to 419 g kg^-1 (Table 1). The SOM contents were interpreted as high (>50 g kg^-1), ranging from 42.3 to 102.8 g kg^-1. The average H+Al values determined by long, short, and estimated incubation by the TSM index (CQFS-RS/SC, 2016) were 12.7, 10.4, and 8.2 cmol_c dm^-3, respectively, with the H+Al value indicated by CQFS-RS/SC underestimated in relation to long incubation (Table 2). The H+Al value of the short incubation did not differ statistically from the long incubation.

Table 2
Potential acidity (H+Al) determined through long (180 days) and short (96 hours) incubations, and by CQFS-RS/SC equation, in soils from Serra Gaúcha (RS) to achieve a soil pH of 6.5

Soil	Long Incubation (Real H+Al)(1)	Short Incubation(2)	CQFS-RS/SC (2016)
	———————————— cmolc dm-3 ————————————
1	12.38	6.25	7.92
2	9.28	7.31	6.18
3	12.16	10.03	7.57
4	22.57	26.15	13.59
5	10.32	9.46	7.45
6	13.36	11.88	9.41
7	9.88	7.27	4.69
8	15.43	7.22	12.54
9	10.85	9.03	7.06
10	6.50	4.34	4.57
11	10.63	7.48	5.26
12	18.48	15.92	8.92
13	23.58	24.33	22.27
14	16.40	14.99	8.46
15	12.40	10.17	6.85
16	8.70	6.39	4.62
17	10.85	10.16	8.36
18	13.30	9.74	7.89
19	6.53	3.84	4.82
20	9.35	6.83	4.51
Standard error	±1.04	±1.32	±0.93
Mean(3)	12.65a	10.44ab	8.15b

• ⁽¹⁾
  Values obtained by long incubation of soils with CaCO₃ rates for 180 days.
• ⁽²⁾
  Values obtained by short incubation of soils with Ca(OH)₂ rates for 96 hours.
• ⁽³⁾
  Means with equal letters do not differ from each other by the Tukey test (p<0.05).

The H+Al values estimated by CQFS-RS/SC (2016) were underestimated in relation to the real H+Al values obtained by long incubation (Figure 3a). However, there was a positive correlation (0.87***) with the real H+Al (Supplementary Material). The short incubation with Ca(OH)₂ also showed a positive correlation (0.93***) with the real H+Al (Supplementary Material). The exponential equations for determining the H+Al values for the soil pH(H₂O) of 6.5 according to long [H+Al (cmol_c dm^-3) = 1354.9e^-0.855TSM] and short [H+Al (cmol_c dm^-3) = 3763.8e^-1.086TSM] incubations were obtained, and could be compared with the CQFS-RS/SC equation [H+Al (cmol_c dm^-3) = 4283e^-1.148*TSM] in figure 3b.

Figure 3
Regression equations (a) between real potential acidity (H+Al) of the soil at pH 6.5 by long incubation and potential acidity estimated by short incubation method and by CQFS-RS/SC (2016) equation. The H+Al obtained by the long incubation was considered as Real H+Al (a). Gray bands represent 95 % confidence intervals. Exponential regressions (b) between TSM index and soil potential acidity by short and long incubation methods, and by CQFS-RS/SC (2016) equation, in 20 soils from Serra Gaúcha region of RS.

The short incubation showed lower H+Al values than the long incubation (Figure 3). This may have occurred due to the high SOM contents of the soils, which, when incubated for 96 h, were not affected by microbial activity. Soil organic compounds have a great influence on acidity buffering, directly affecting the soil cation exchange capacity (Gurmessa, 2021) and Al³⁺ complexation (Brown et al., 2008). This is because soils with higher pH(H₂O) and exchangeable Ca and low C/N ratio (12.8) form organic complexes with a higher degree of SOM humification, thus forming highly stable Al-complexes (Eimil-Fraga et al., 2015). In the evaluated soils, an average C/N ratio (10.2) was observed in the SOM and high exchangeable Ca and Mg contents (>4.0 and >1.0 cmol_c dm^-3), respectively (Table 1). In addition, increasing CaCO₃ rates and pH(H₂O) can reduce soil carbon contents due to increased SOM mineralization (Jakšic et al., 2021), favoring microbial efficiency in C use (Grover et al., 2017). Thus, the long incubation process for 180 days favored the activity of the microbial population that consumed part of the C from the SOM. As a result, larger amounts of H⁺ and Al³⁺ were released that were adsorbed to the coordination compounds of the SOM, causing greater consumption of CaCO₃, increasing the soil H+Al values (Gurmessa, 2021). In addition, the small number of soils with H+Al >20 cmol_c dm^-3 may have negatively affected the comparison of the tested methods. Despite this, studies with soils from different regions of Brazil (Giuliani, 2015) and in sandy soils with low SOM contents (Brunetto et al., 2019) showed that short incubation was efficient in estimating H+Al values, equivalent to the long incubation method. Thus, short incubation is a promising method for determining H+Al in soils due to its low cost or speed.

Based on the H+Al values obtained by long incubation, the lime requirement rates to raise the soil pH(H₂O) to 5.5, 6.0, and 6.5 were suggested, according to the TSM Index (Table 3). The rates were also established based on the short incubation for the pH(H₂O) of 6.0 and 6.5. It was not possible to establish the H+Al values for the pH(H₂O) of 5.5 through short incubation, as most native soils already had pH(H₂O) values above 5.0. In these cases, smaller incubation rates are required than the 50 % of the rate recommended by CQFS-RS/SC (2016). The lime rates to reach the pH(H₂O) of 6.5, according to CQFS-RS/SC (2016), are underestimated compared to long incubation by about 2.0 to 5.1 Mg ha^-1. The lime rates to reach the pH(H₂O) of 6.5, according to short incubation, are underestimated in relation to long incubation by about 0.5 to 2.7 Mg ha^-1.

Table 3
Estimation of the lime requirement (LR) to raise soil pH to 5.5, 6.0, and 6.5 in the 0.00-0.20 m layer by long and short incubation and by the Index TSM used in the equation of the CQFS-RS/SC (2016), and the lime adjustment according to the percentage of reactive soil to soils from Serra Gaúcha region (RS)

TSM Index	Long incubation (Real H+Al)			Short incubation		Equation of CQFS-RS/SC (2016)			Lime adjustment
	5.5	6.0	6.5	6.0	6.5	5.5	6.0	6.5	6.5
	LR = 36565e -1.666*TSM	LR = 2318.9e -1.021*TSM	LR = 1354.9e -0.855*TSM	LR = 18723e -1.465*TSM	LR = 3763.8e -1.086*TSM	LR = 10629e -1.484*TSM	LR = 7692e -1.326*TSM	LR = 4283e -1.148*TSM	LR = (1354.9e -0.855*TSM) * DSL%
	——————————————— Mg ha-1 (1) ———————————————
4.4	24.0	26.0	31.5	29.7	31.7	15.5	22.5	27.4	20.8
4.5	20.3	23.4	28.9	25.7	28.4	13.4	19.7	24.4	19.1
4.6	17.2	21.2	26.5	22.2	25.5	11.5	17.3	21.8	17.5
4.7	14.5	19.1	24.4	19.1	22.9	9.9	15.1	19.4	16.1
4.8	12.3	17.3	22.4	16.5	20.5	8.6	13.2	17.3	14.8
4.9	10.4	15.6	20.5	14.3	18.4	7.4	11.6	15.4	13.6
5.0	8.8	14.1	18.8	12.3	16.5	6.4	10.2	13.8	12.4
5.1	7.5	12.7	17.3	10.7	14.8	5.5	8.9	12.3	11.4
5.2	6.3	11.5	15.9	9.2	13.3	4.7	7.8	10.9	10.5
5.3	5.3	10.4	14.6	7.9	11.9	4.1	6.8	9.8	9.6
5.4	4.5	9.4	13.4	6.9	10.7	3.5	6.0	8.7	8.8
5.5	3.8	8.4	12.3	5.9	9.6	3.0	5.2	7.8	8.1
5.6	3.2	7.6	11.3	5.1	8.6	2.6	4.6	6.9	7.4
5.7	2.7	6.9	10.4	4.4	7.7	2.3	4.0	6.2	6.8
5.8	2.3	6.2	9.5	3.8	6.9	1.9	3.5	5.5	6.3
5.9	2.0	5.6	8.7	3.3	6.2	1.7	3.1	4.9	5.8
6.0	1.7	5.1	8.0	2.9	5.6	1.4	2.7	4.4	5.3
6.1	1.4	4.6	7.4	2.5	5.0	1.2	2.4	3.9	4.9
6.2	1.2	4.1	6.8	2.1	4.5	1.1	2.1	3.5	4.5
6.3	1.0	3.7	6.2	1.8	4.0	0.9	1.8	3.1	4.1
6.4	0.9	3.4	5.7	1.6	3.6	0.8	1.6	2.8	3.8
6.5	0.7	3.0	5.2	1.4	3.2	0.7	1.4	2.5	3.5
6.6	0.6	2.7	4.8	1.2	2.9	0.6	1.2	2.2	3.2
6.7	0.5	2.5	4.4	1.0	2.6	0.5	1.1	2.0	2.9
6.8	0.4	2.2	4.0	0.9	2.3	0.4	0.9	1.7	2.7
6.9	0.4	2.0	3.7	0.8	2.1	0.4	0.8	1.6	2.5
7.0	0.3	1.8	3.4	0.7	1.9	0.3	0.7	1.4	2.3

• ⁽¹⁾
  Limestone with relative total neutralization power (RTNP 100 %).
• LR: lime requirement; TSM: Tampão Santa Maria; SDL%: percentage of soil in the diagnostic layer after sieving in 2 mm diameter mesh, in this case, was used 0.66, but we recommend estimating it in the soil evaluated to improve lime recommendation.

In addition to the difference in corrective rates observed by long incubation and by the CQFS-RS/SC (2016) equation, the soil stoniness factor, if not considered, can cause overestimation errors of the rate, raising pH(H₂O) levels above the desired. This is because the study region has soil with low weathering and stoniness on the surface. On average, 34 % of the volume of the soils sampled in the 0.00-0.20 m layer did not pass through a 2 mm diameter sieve (Figure 4a). The >2 mm fraction (gravels, cobbles, stones, and boulders) has a low contribution to acidity and, if not disregarded in the calculation of the corrective acidity rate, will result in the application of quantities higher than the ideal, raising the pH(H₂O) above the range suitable for the crop of interest in the soil layer to be corrected. In this example, it is recommended that the corrective rate, obtained by long or short incubation, be multiplied by 0.66, which is the percentage of soil in the diagnostic layer (SDL%) sieved (<2 mm diameter mesh). In this way, for greater assertiveness, we recommend estimating the amount of rock fragments in the layer where the acidity is intended to be corrected, and that the lime rate be adjusted according to the SDL% (Table 3). There are several methods that can be used to define the percentage of rock fragments, which can help in this estimate, including the visual method, using images of the soil profile (Jiang et al., 2020). Another way is to collect a known soil volume, dry and sieve it (2 mm mesh), and weigh the fractions (Figures 4b and 4c).

Figure 4
Percentage of soil in the 0.00-0.20 m layer (a) from the Serra Gaúcha region after sieving (Ø <2 mm mesh), and volumetric ring representing the presence of stones (b, c).

We highlight that the calculations used in liming recommendations assume a soil density of 1.0 kg dm^-3. However, under field conditions, densities in the upper 0.00-0.20 m layer can be observed. Therefore, the calculations of corrective rates can also be adjusted when this estimate is available, i.e., the rates should be higher to neutralize the H+Al values of this layer. Also, for fruit trees, when possible, deeper layers can be corrected, such as 0.00-0.30 or 0.00-0.40 m (Brunetto et al., 2020). In these cases, it is recommended to multiply the limestone rates by 1.5 or 2.0 times, respectively.

The method of applying corrective material is a crucial aspect to consider. Whenever possible, it is advisable to incorporate the lime rate into the soil during its preparation for the establishment of the cropping system. This is because the low solubility and low mobility of lime in the soil profile limit the corrected area, even after long periods (44 months) after application (Miotto et al., 2020). In stony soils, such as those in this study, which often limit the incorporation of lime, alternatives seek to minimize the effects of Al³⁺ in the subsurface (0.20-0.40 m). When the Al saturation >10 % and/or exchangeable Ca²⁺ <3.0 cmol_c dm^-3, such as using agricultural gypsum, may be indicated to increase the interest crop yield (Tiecher et al., 2018). However, there is still a deficiency of information on reference values for gypsum application in perennial crops such as fruit trees, since these species can expand their root system to deeper layers compared to annual crops, and also present cultivars that may be tolerant to the effects of Al³⁺ stress (Setotaw et al., 2015). In addition, soils that receive initial adequate applications of deep acidity corrective material do not require reapplications of incorporated lime or gypsum applications (Hammerschmitt et al., 2021).

It was possible to update the estimate of potential acidity (H+Al) based on long incubation (CaCO₃) for soils with high SOM contents in the Serra Gaúcha region - RS. This is important information, as the equation currently used by CQFS-RS/SC (2016) underestimates H+Al values. On the other hand, the H+Al values determined by short incubation [Ca(OH)₂] showed a high correlation with the long incubation values, being a fast alternative method for determining potential acidity. However, for greater assertiveness in the lime recommendation, it is recommended to adjust the corrective rate, disregarding the rock fragment fraction in stony soils.

CONCLUSION

The current estimate of potential acidity (H+Al) by CQFS-RS/SC (2016) underestimates lime corrective rates for soils with high organic matter contents in the Serra Gaúcha region (RS), compared to the real H+Al determined by long incubation. Potential acidity for soil pH(H₂O) 6.5 can be obtained through long incubation [H+Al (cmol_c dm^-3) = 1354.9e^-0.855*TSM]. However, this estimate does not consider the amount of rocky material present in the soil. For greater efficiency in lime recommendation and, at the same time, cost reduction, we recommend estimating stoniness and adjusting corrective rates, considering only the percentage of soil in the diagnostic layer (SDL%) in the layer to be corrected. Potential acidity obtained by short incubation showed a high correlation with long incubation, and its equation [H+Al (cmol_c dm^-3) = 3763.8e^-1.086*TSM] can be used for soils with high SOM contents.

ACKNOWLEDGMENT

We would like to thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq (Academic Doctorate for Innovation – MAI/DAI, CNPq Public Call No. 12/2020) and the Cooperativa Vinícola Aurora Ltda for the financial support.

• How to cite: Grando DL, Deponti LP, Rodrigues ML, Martins CG, Natale W, Schmitt DE, Siqueira GN, Palermo NM, Kaminski J, Brunetto G. Potential acidity determination for soils with high soil organic matter. Rev Bras Cienc Solo. 2025;49:e0240146. https://doi.org/10.36783/18069657rbcs20240146

DATA AVAILABILITY

All data was generated or analyzed in this study.

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Edited by

• Editors:
  José Miguel Reichert https://orcid.org/0000-0001-9943-2898 and Leônidas Azevedo Carrijo Melo https://orcid.org/0000-0002-4034-4209

Publication Dates

• Publication in this collection
  31 Mar 2025
• Date of issue
  2025

History

• Received
  19 July 2024
• Accepted
  03 Oct 2024