ABSTRACT

Liming acid soils is fundamental for agricultural activity such as the study and characterization of alternative liming material. The objective of this study was to characterize biological, sedimentary, and metamorphic limestones. The following limestones were studied: biogenic (BL), sedimentary (SL), commercial (CL), and marble (ML). This study evaluated total relative neutralizing power (TRNP), chemical composition, specific surface area, and Ca²⁺and Mg²⁺release at concentrations of 0.10, 0.50, 1.00, and 2.00 mol L^-1 HCl, in addition to the effects on the soil 90 days after limestone application. The Si identified contributed to the lower TRNP of BL (73.7%) and CL (74.4%). At the lowest HCl concentrations, BL was the most reactive, but with an increase in concentration, ML became the most reactive. The highest CaO (58.32%) and MgO (13.05%) levels were observed in SL and ML, respectively, which contained the highest Ca²⁺(3.95 cmol_cdm^-3) and Mg²⁺ (1.09 cmol_cdm^-3) concentrations when applied to the soil. All the limestones increased pH and lowered soil H+Al. Soil Ca²⁺and Mg²⁺were influenced by the composition of the amendments.

Keywords:
liming; lithothamnium; soil acidity

RESUMO

A correção da acidez é fundamental para a atividade agrícola em solos ácidos, assim como a caracterização e estudo do efeito no solo de fontes alternativas de corretivos. Objetivou-se caracterizar os calcários de origem biológica, sedimentar e metamórfica. Os calcários estudados foram: biogênico (CB), sedimentar (CS), comercial (CC) e o mármore (CM). Foram avaliados: poder relativo de neutralização total (PRNT); composição química; área superficial específica e liberação de Ca²⁺ e Mg²⁺ nas concentrações de 0,10; 0,50; 1,00 e 2,00 mol L^-1 de HCl e os efeitos no solo, após 90 dias da aplicação dos calcários. O Si identificado contribuiu para o menor PRNT do CB (73,7%) e CC (74,4%). Nas menores concentrações de HCl, o CB mostrou-se mais reativo, porém com o aumento da concentração do HCl, o CM passou a ser mais reativo. Os maiores teores de CaO (58,32%) e MgO (13,05%) foram observados no CS e CM, respectivamente, que contribuíram com maiores aportes de Ca²⁺ (3,95 cmol_c dm^-3) e Mg²⁺ (1,09 cmol_c dm^-3), quando aplicados ao solo. Todos os calcários promoveram elevação do pH e redução do H+Al no solo. Os teores de Ca²⁺ e Mg²⁺ no solo foram influenciados pela composição dos corretivos.

Palavras-chave:
lithothamnium; calagem; acidez do solo

INTRODUCTION

Acid soils present high saturation by Al, low saturation by base, and lower availability of P (Fageria, 2011Fageria NK (2011) Efeito da calagem na produção de arroz, feijão, milho e soja em solo de cerrado. Pesquisa Agropecuária Brasileira, 11:1419-1424. ), all of which limit the development of most agricultural crops (Bortoluzzi et al., 2014Bortoluzzi ED, Parize GL, Korchagin J, Silva VR, Rheinheimer DS & Kamins J (2014) Soybean root growth and crop yield in reponse to liming at the beginning of a no-tillage system. Revista Brasileira de Ciência do Solo , 38:262-271.; Singh et al., 2017Singh SDK, Tripathi SSS, Sharma NK, Dubey DK & Chauhan MV (2017) Toxicity of aluminium on various levels of plant cells and organism: a review. Environmental and Experimental Botany, 137:177-193. ). Liming stands out as an efficient and less costly practice to raise pH, contents of Ca, Mg, base saturation, and P availability and to reduce toxic Al in soil (Fageria, 2011Fageria NK (2011) Efeito da calagem na produção de arroz, feijão, milho e soja em solo de cerrado. Pesquisa Agropecuária Brasileira, 11:1419-1424. , Rheinheimer et al. 2018Rheinheimer DS, Tiecher T, Gonzatto R, Zafar M & Brunetto G (2018) Residual effect of surface-applied lime on soil acidity properties in a long-term experiment under no-till in a Southern Brazilian sandy Ultisol. Geoderma, 313:07-16.). These effects result in a significant improvement in the production capacity of the soils, which allows better yields to the farmer in addition to avoiding the opening of new areas (Crusciol et al., 2016Crusciol CAC, Artigiani ACCA, Arf O, Carmeis Filho ACA, Soratto RP, Nascente AS & Alvarez RCF (2016) Soil fertility, plant nutrition, and grain yield of upland rice affected by surface application of lime, silicate, and phosphogypsum in a tropical no-till system. Catena, 137:87-99.; Inagaki et al., 2016Inagaki TM, de Moraes Sá JC, Caires EF & Gonçalves DRP (2016) Lime and gypsum application increases biological activity, carbon pools, and agronomic productivity in highly weathered soil. Agriculture Ecosystems & Environment, 231:156-165. ).

Although studies on the field of acidity correction and improvement in soil fertility show great progress, there is a need to seek new agricultural inputs, which are sustainable and provide greater productivity. In this context, it is important to characterize and understand the behavior of non-conventional sources for acidity correction in the soil, such as marine biogenic limestone.

Limestone produced from the extraction of calcareous algae receives the name of marine biogenic limestone or lithothamnium, alluding to the genus of Lithothamnium algae, which are the most extracted ones. The group of calcareous algae presents over 30 genera and about 500 species (Dias, 2000Dias GTM (2000) Granulados bioclásticos: Algas calcárias. Revista Brasileira de Geofísica, 18:307-318. ). Marine biogenic limestone represents a renewable short-term source. According to Dias (2000Dias GTM (2000) Granulados bioclásticos: Algas calcárias. Revista Brasileira de Geofísica, 18:307-318. ) and Melo & Moura (2009Melo TV & Moura AMA (2009) Utilização da farinha de algas calcáreas na alimentação animal. Revista Archivos de Zootecnia, 58:99-107.), as long as there is light, algae grow naturally in the marine environment up to approximately 200 m of depth. Since the seventeenth century, calcareous algae have been used for the correction of acid soils in Europe (Dias, 2000Dias GTM (2000) Granulados bioclásticos: Algas calcárias. Revista Brasileira de Geofísica, 18:307-318. ). However, in Brazil, its agricultural use and exploration are still incipient.

According to Alvarez et al. (1999Alvarez VVH, Dias LE, Ribeiro AC & Souza RB (1999) Calagem. In: Ribeiro AC, Guimaraes PTG & Alvarez VVH (Eds.) Recomendação para o uso de corretivos e fertilizantes em Minas Gerais. Viçosa, Comissão de Fertilidade do Solo do Estado de Minas Gerais. p.43-60.), the reactivity of a limestone depends somewhat on its geological nature, in which those of sedimentary origin and of a less crystalline nature are more reactive than the metamorphic ones, with a more crystalline structure. Gallo & Catani (1954Gallo JR & Catani RA (1954) Solubilidade de alguns tipos de calcários. Bragantia, 13:63-74.) reported that sedimentary rocks are more soluble than metamorphic rocks of the same chemical composition. However, Bellingieri et al. (1989Bellingieri PA, Alcarde JC & Souza ECA de (1989) Eficiência relativa de diferentes frações granulométricas de calcários na neutralização da acidez dos solos, avaliada em laboratório. Anais da Escola Superior de Agricultura Luiz de Queiroz, 46:303-317.) reported that the efficiency of limestones in soil acidity correction is not influenced by their geological nature, provided they have the same particle size. Such information gives rise to a demand for knowledge on the performance of biogenic, sedimentary, and metamorphic limestone in agriculture.

Limestones of sedimentary origin are formed through the disintegration and decomposition of preexisting rocks by weathering agents, in which the Ca in solution is conducted to continental or marine accumulation sites. Evaporation and temperature variations can reduce the CO₂ content in the water and precipitate CaCO₃ due to saturation conditions (Sampaio & Almeida, 2008Sampaio JA & Almeida SLM (2008) Calcário e dolomito. In: Luz AB da & Lins AF (Eds) Rochas e Minerais Industriais: Usos e Especificações. Rio de Janeiro, CETEM/MCTI. p.363-387.). Metamorphic rocks, represented by marbles, can also be used as acidity correctives. Moreover, they are composed of calcite and/or dolomite and result from the recrystallization of limestone rocks, mostly of sedimentary nature by means of high temperatures and pressure (Sampaio & Almeida, 2008Sampaio JA & Almeida SLM (2008) Calcário e dolomito. In: Luz AB da & Lins AF (Eds) Rochas e Minerais Industriais: Usos e Especificações. Rio de Janeiro, CETEM/MCTI. p.363-387.).

The objective of this study was to comparatively characterize limestone of biological, sedimentary, and metamorphic origin for use in agriculture.

MATERIAL AND METHODS

The experiment was carried out in the Geochemistry Laboratory and in a greenhouse belonging to the Department of Soils in the Universidade Federal de Viçosa, Viçosa, MG - Brazil, from March to July 2014. The limestones used in the experiment were the following: marble (ML), biogenic limestone (BL), commercial limestone (CL), and sedimentary limestone (SL). Figure 1 shows the location of the municipalities from where the corrective samples were collected.

Blocks of the studied materials were collected in each location. In the laboratory, ML and SL were washed with distilled water, broken into smaller blocks with the aid of a geological hammer, and ground in a disc mill. After that, they were standardized in the particle size of less than 0.074, for characterization and experiment assembly.

The total relative neutralizing power (TRNP) and the CaO and MgO contents were determined according to Mapa (2014Mapa - Ministério da Agricultura, Pecuária e Abastecimento (2014) Manual de métodos analíticos oficiais para fertilizantes minerais, orgânicos, organominerais e corretivos. Brasília, MAPA. 141p.), considering four replications. The determination of Ca²⁺ and Mg²⁺ elements was obtained using an atomic absorption spectrometer. Total contents of Ca, Mg, Al, K, P, Na, and Si contents were determined by the alkaline fusion method, following the procedure described by Pansu & Gautheyrou (2003Pansu M & Gautheyrou J (2003) L'analyse du sol minéralogique, organique et minérale. Springer, France. 993p.), adapted by Guerra et al. (2013Guerra MBB, Neto EL, Prianti MTA, Pereira Filho ERP & Schaefer CEGR (2013) Post-fire study of the Brazilian Scientific Antarctic Station: Toxic element contamination and potential mobility on the surrounding environment. Microchemical Journal, 110:21-27.). The dosage of the above elements was performed in inductively coupled plasma - optical emission spectrometry (ICP-OES). The B.E.T. method was used to determine the specific surface area (SSA). (Brunauer, Emmett and Teller) using the NOVA instrument (Quantachrome Instruments version 10.01) (Gregg & Sing, 1982Gregg SJ & Sing KSW (1982) Adsorption, Surface area and Porosity. New York, Academic Press. 303p.).

Regarding the solubility test in hydrochloric acid, the methodology adapted from Amaral Sobrinho et al. (1992Amaral Sobrinho NMB, Costa LM, Oliveira C & Velloso ACX (1992) Metais pesados em alguns fertilizantes e corretivos. Revista Brasileira de Ciência do Solo , 16:271-276.) was used, which consists of weighing 1 g of limestone in 125-mL Erlenmeyer. Then, 50 mL HCl was added at the concentrations of 0.01, 0.10, 0.50, 1.00, and 2.00 mol L^-1, with four replicates. The reaction time was 12 h, in which the samples were shaken for 15 min every hour in a shaker of 200 oscillations per minute. After the reaction period, the solutions containing limestone samples were filtered on slow filtering quantitative filter paper. Then, contents of the Ca²⁺ and Mg^{2 +} were determined in the filtrate by atomic absorption spectroscopy. The filter papers containing the residues were taken to the muffle at 500 °C for 1 h. By mass difference before acid attack and post muffle, it was possible to determine the limestone residue for each sample.

An incubation experiment was carried out, by using samples of Red-Yellow Latosol (Embrapa, 2018Embrapa - Empresa Brasileira de Pesquisa Agropecuária (2018) Sistema Brasileiro de Classificação de Solos. 5ª ed. Rio de Janeiro, Embrapa Solos . 180p.), collected in the city of Viçosa, MG, in the 0-20 cm layer. The results of the soil chemical analysis were as follows: pH (H₂O), 5.24; Al³⁺ (cmol_c dm^-3), 1.4; H + Al³⁺ (cmol_c dm^-3), 9.70; Ca²⁺ (cmol_c dm^-3), 0.63; Mg⁺² (cmol_c dm^-3), 0.14; K⁺ (mg dm^-3), 43.00; P (mg dm^-3), 1.2. The physical characterization indicated sand, silt, and clay contents of 130, 190, and 680 g kg^-1, respectively. The methods used in these analyzes are described by Embrapa (2011). Based on the results of chemical analysis of soil and limestone, the need for liming (NL) was determined for the soil, using the Al³⁺ neutralization method and raising of contents of Ca²⁺ and Mg²⁺ (Alvarez et al., 1999Alvarez VVH, Dias LE, Ribeiro AC & Souza RB (1999) Calagem. In: Ribeiro AC, Guimaraes PTG & Alvarez VVH (Eds.) Recomendação para o uso de corretivos e fertilizantes em Minas Gerais. Viçosa, Comissão de Fertilidade do Solo do Estado de Minas Gerais. p.43-60.). For the purpose of calculating the NL, the parameters of the corn crop were considered. To calculate the amount of limestone to be applied, the TRNP found for the different materials studied was considered.

The limestones were applied to the dry soil and homogenized throughout the volume. The treatments, arranged in a factorial scheme (4 × 4) + 1, consisted of four acidity limestones (ML, BL, CL, and SL), four NL levels (30, 60, 100 and 150%), and one control, with no limestone application, and were distributed in randomized blocks, with three replications. Each experimental unit consisted of a plastic bag, with a capacity of 1 dm³ of soil, distributed on a bench, inside the greenhouse. The experimental units were maintained with humidity between 60 and 80% of the field capacity, through weighing and replenishment of water. The plastic bags were kept slightly open, using polypropylene tubes, to allow gas exchanges.

The sample units were incubated for 90 days. At the end of the incubation period, the fine air-dry soil of each experimental unit was obtained. From the fine air-dry soil, the following chemical and physical-chemical features were determined: pH in water, in the 1:2.5 soil:solution ratio; Ca^{2 +} and Mg²⁺ contents (extraction with KCl 1 mol L^-1); exchangeable acidity by titration (extraction with KCl 1 mol L^-1); and potential acidity, also by titration (extraction with Ca(Ac)₂ 0.5 mol L^-1at pH 7, 1:15 soil:solution ratio) (Embrapa, 2011Embrapa - Empresa Brasileira de Pesquisa Agropecuária (2011) Manual de métodos de análises de solos. 2ª ed. Rio de Janeiro, Embrapa Solos. 230p.).

The means obtained for 100% of the NL at the agronomic interest level were compared by the classification proposed by the Fifth Approximation Recommendation of Corrective and Fertilizers (Ribeiro et al., 1999Ribeiro AC, Guimaraes PTG & Alvarez VVH (1999) Recomendação para o uso de corretivos e fertilizantes em Minas Gerais. 5ª ed. Viçosa, Comissão de Fertilidade do Solo do Estado de Minas Gerais . 359p.) to relate the results experimentally obtained for the different limestones with those targeted in the agricultural practices.

Data were subjected to analysis of variance. In relation to the qualitative factor, the means were compared using the Tukey test, adopting the 5% probability level. To compare the control to the other treatments, Dunnett’s test was used, adopting the 5% probability level. For the quantitative factor, regressions were adjusted, and the models were chosen based on the significance of the regression coefficients using the t test at the 1% probability level, in the coefficient of determination (R² = S.S Regression/ S.S Treatment) and the behavior of the phenomenon under study. Because of the great variability, the descriptive statistical analysis was chosen for the chemical characterization of the different acidity correctives under study. SAEG 9.1 was used to support the statistical analysis (Universidade Federal de Viçosa, 2007Universidade Federal de Viçosa (2007) SAEG: Sistema para Análises Estatísticas e Genéticas. Versão 9.1. Viçosa, Fundação Arthur Bernardes. CD-ROM.).

RESULTS AND DISCUSSION

In the chemical limestone characterization (Table 1), except for Ca and Mg, the highest concentrations of Al, Na, Zn, Fe, Cu, Mn, B, K, P, and Si elements were observed in BL (Table 1). According to Melo & Moura (2009Melo TV & Moura AMA (2009) Utilização da farinha de algas calcáreas na alimentação animal. Revista Archivos de Zootecnia, 58:99-107.), the main characteristics that potentiate calcareous algae are attributed to the greater availability of micronutrients, adsorbed on cell walls, easily assimilated by plants and animals. In addition, Dias (2000Dias GTM (2000) Granulados bioclásticos: Algas calcárias. Revista Brasileira de Geofísica, 18:307-318. ) indicated that calcareous algae are composed mainly of CaCO₃ and MgCO₃ and varying concentrations of elements such as Fe, Mn, B, Ni, Cu, Zn, Mo, Se, and Sr.

A larger concentration of Mg and Ca was observed in CM and in SL, respectively, due to the inherent characteristics of each reserve. Jordt-Evangelista & Viana (2000Jordt - Evangelista H & Viana DJ (2000) Mármores da região de Itaoca (ES) e escarnitos no contato com diques máficos e félsicos: Mineralogia e petrogênese. Geonomos, 08:61-62.) observed that the marbles that compose the collection region of the ML are predominantly composed of dolomite, with small intrusions of calcite, whereas limestones with low dolomite content compose the collection SL region.

Due to the particle size standardization in a sieve smaller than 0.074 mm, the TRNP will be equal to the NP (neutralization power). The limestone NP was: ML = 98.9%; BL = 73.7%; CL = 74.4%; and SL = 96.3%. The lowest NP values were observed in the BL and CL, a fact that may be associated with the presence of silicate minerals in the composition of the correctives. In the chemical characterization, 65.72 and 57.19 mg g^-1 Si were observed in BL and CL correctives, respectively. The neutralizing capacity of limestones considers the chemical purity of the rock of origin and the fineness of grinding (Quaggio, 2000Quaggio JA (2000) Acidez e calagem em solos tropicais. Campinas, Instituto Agronômico. 111p.). For the same granulometry, the higher the chemical purity of the rock, the higher the TRNP of the corrective.

The CaO and MgO contents maintained the same behavior of the total contents of Ca and Mg in the correctives. The CaO and MgO contents were as follows: ML with 35.40% CaO and 13.05% MgO; BL with 43.90% CaO and 2.91 MgO; CL with 30.80% CaO and 6.63% MgO; and SL with 58.32% CaO and 0.51% MgO.

The largest SSA was observed in BL, with a value of 4.05 m² g^-1. According to Melo & Moura (2009Melo TV & Moura AMA (2009) Utilização da farinha de algas calcáreas na alimentação animal. Revista Archivos de Zootecnia, 58:99-107.), calcareous algae present high porosity that provides a greater surface area of action. The ML, CL, and SL presented SSA of 0.09, 1.68, and 2.08 m² g^-1, respectively. The smaller SSA observed in the ML and CL may be attributed to the higher crystallinity of these rocks.

According to Gonçalves et al. (2011Gonçalves JRP, Moreira A, Büll LT, Crusciol CAC & Villas Boas RL (2011) Granulometria e doses de calcário em diferentes sistemas de manejo. Acta Science Agronômica, 33:369-75.) and Rodrighero et al. (2015Rodrighero MB, Barth G & Caires EF (2015) Aplicação superficial de calcário com diferentes teores de magnésio e granulometrias em sistema plantio direto. Revista Brasileira de Ciência do Solo , 39:1723-1736.), neutralization speed depends on the surface area of the corrective in contact with the soil and, due to the low solubility of the limestones, it requires its incorporation in the soil to promote a better reaction (Alleoni et al., 2005Alleoni LRF, Cambri MA & Caires EF (2005) Atributos químicos de um Latossolo de cerrado sob plantio direto, de acordo com doses e formas de aplicação de calcário. Revista Brasileira de Ciência do Solo, 29:923-934. ). Acidity correctives with a large contact surface may present great potential in areas where limestone has not been incorporated such as in the no-tillage system. According to Alleoni et al. (2005Alleoni LRF, Cambri MA & Caires EF (2005) Atributos químicos de um Latossolo de cerrado sob plantio direto, de acordo com doses e formas de aplicação de calcário. Revista Brasileira de Ciência do Solo, 29:923-934. ), in the no-tillage system, as time passes by, the intervention with limestone application is necessary because there is a decrease in pH and, consequently, an increase in Al³⁺ on the superficial layer. In these cases, the use of limestones with higher reaction velocity associated with greater residual power becomes interesting. In this way, BL could be used combined with other limestones with a smaller contact surface to reduce the reaction time and to increase the residual effect.

Solubility of the different limestones in hydrochloric acid

Among the evaluated limestones (Table 2), BL was more reactive at the concentrations of 0.01 and 0.1 mol L^-1 HCl; however, as the HCl concentration was increased, the carbonate consumption occurred, providing greater residue generation in BL as well as in CL treatments as both limestones presented the lowest NP values among them. The ML and SL presented higher chemical purity, consequently, higher NP values and a smaller amount of residue.

By using the adjusted equation of the variable limestone residue as a function of the concentration (Figure 2A), it was observed that concentrations greater than 0.31 mol L^-1 HCl solubilize the carbonates present in BL. In CL, values greater than 0.29 mol L^-1 HCl were obtained, that is, from those concentrations, the concentration does not affect the generation of limestone residue anymore. A likely influence of the specific surface area was observed in ML and SL. In SL, the surface area is 30 times larger than in ML, with a faster consumption of carbonates. On the other hand, because ML is a more crystalline rock and, consequently, presents a smaller SSA, HCl found greater difficulty to solubilize the carbonates. The lower solubility of ML may also be related to its chemical constitution; when a change occurs in the crystalline structure of limestones, through the substitution of Ca ions with Mg ions, the solubility of limestones decreases (Grunwaldt et al., 2016Grunwaldt HS, Zimina A, Göttlicher J, Steininger R & Grunwaldt JD (2016) Study of the relation between Mg content and dissolution kinetics of natural lime stone using μXRF, μXRD and μXAS J. Journal of Physics Conference Series, 712:10.1088.). Rodrighero et al. (2015Rodrighero MB, Barth G & Caires EF (2015) Aplicação superficial de calcário com diferentes teores de magnésio e granulometrias em sistema plantio direto. Revista Brasileira de Ciência do Solo , 39:1723-1736.) observed that the solubility of limestones containing a large Ca content was higher than that of dolomitic limestones with the same particle size.

In relation to Ca²⁺ release, it was observed that at the concentration of 0.01 mol L^-1, the correctives ML, CL, and SL presented a greater release of Ca²⁺ in relation to BL; at the concentration of 0.10 mol L^-1 HCl, the limestones that released more Ca²⁺ to the solution were CL and SL; and at the concentration of 0.50, the limestones ML and SL stood out. Only at the 1.00 and 2.00 mol L^-1 HCl concentrations, the Ca²⁺ release followed the same trend observed in the chemical characterization. The limestones presented CaO percentage and Ca²⁺ release in the following order: SL> BL> ML> CL. In relation to the release of Mg²⁺, it followed the same trend observed in the chemical characterization, in which the limestones ML and CL presented the greatest percentages of MgO and greater release of Mg to the solution (Table 2).

For the adjusted values of Ca²⁺ release (Figure 2B), it was observed that as HCl concentration increased, a greater Ca²⁺ release occurred in limestones with higher CaO, SL followed by BL. Similar behavior was observed for Mg²⁺ release when ML and CL also released more Mg^{2 +} since they presented a higher percentage of MgO (Figure 2C).

Effects of limestones on soil physicochemical characteristics

It was observed with the analysis of variance that only the levels of NL had a statistically significant influence on soil pH (p <0.05), whilst the variables H+Al, Ca²⁺ _, and Mg²⁺ were statistically influenced by the different sources of limestone and liming levels.

It was observed that the adjusted pH values as a function of NL increased linearly as the levels of NL increased (Figure 3A). Similar results were also observed by Kurihara et al. (1999Kurihara CH, Maeda S, Hernani LC & Silva WM (1999) Eficiência relativa de frações granulométricas de calcários Sul-Mato-Grossenses. Pesquisa Agropecuária Brasileira, 34:1443-1449.), in which at the end of 100 days of incubation, the increase in pH was similar between sedimentary and metamorphic limestones with a particle size within the 50-70 mesh interval. The treatments that received acidity correctives showed significant (p<0.05) and greater results than the control by the Dunnett test (Table 3). The pH of the control was classified as low (4.5-5.4); however, regardless of the soil acidity corrective applied to the soil at the level of agronomic interest, the pH reached values classified as good (5.5-6.0), and at the level of 150% of liming requirement, it reached values classified as high (6.1-7.0).

In relation to the active acidity, ML and BL reduced Al³⁺ from 1.23 cmol_c dm^-3, observed in the control, to 0.48 cmol_c dm^-³ at 30% of NL. The value observed when the CL and SL were applied was 0.58 cmol_c dm^-3. At 60% of NL, values of Al³⁺ observed with the application of the BL and SL correctives were 0.10 cmol_c dm^-3, ML of 0.19 cmol_c dm^-3, and CL of 0.29 cmol_c dm^-3. The pH range reached at 100 and 150% of liming requirement, regardless of the limestone applied, was sufficient to reduce Al^{3 +} to zero. The exchangeable acidity has an inverse relationship with soil pH; therefore, the rise in pH is one of the alternatives to reduce the toxic effect of Al (Sousa et al., 2007Sousa DMG De, Miranda LN De & Oliveira ESA De (2007) Acidez do solo e sua correção. In: Novais RF, Alvarez VVH, Barras de NF, Fontes RL, Cantarutti RB & Lima JC (Eds) Fertilidade do Solo. Viçosa, SBCS. p.206-262.). According to the same authors, when the pH of the soil, determined in water, is close to 5.5, Al^{3 +} is reduced to almost zero.

The application of increasing levels of limestone promoted a reduction in values of H + Al (Figure 3B). In relation to the behavior of the different limestones within each liming level (Table 3), at the 30% level, the BL acidity corrective presented the lowest average H + Al values. At 60 and 100% levels, the lowest mean values were observed in ML-treated samples. At the 150% level, there was no statistical difference between limestones. Regardless of the evaluated limestone, at the liming level of agronomic interest, limestones were effective in reducing the potential acidity of the very high level presented in the control (> 9.00 cmol_c dm^-3) for values classified as medium (2.51-5.00 cmol_c dm^-3). According to Dunnett’s test, all treatments that received lime presented lower and statistically significant H + Al values (p<0.05) compared with the control (Table 3).

The average content of Ca²⁺ in the soil increased linearly as the levels of the applied liming requirement levels increased (Figure 3C). The BL and SL presented a tendency to contribute with more Ca²⁺ in relation to ML and CL. It is likely that such behavior is related to the fact that SL presents a greater percentage of CaO and second largest specific surface. Although ML presented TRNP very close to SL, which results in the application of very close acidity corrective amounts, the ML contributed with less Ca²⁺ in the soil. Similar behavior was observed between BL and CL, although the TNRP was very close, BL displayed a larger CaO percentage, which resulted in a greater release of Ca²⁺ in contact with the soil. Despite presenting a higher percentage of CaO in relation to CL, in the soil, ML behaved in an opposite manner, possibly due to the largest specific surface area of CL, 18 times larger than ML, which promoted the highest Ca ²⁺ release.

In relation to the behavior of the different limestones within each liming level, at the 30% level, BL and SL were more efficient in releasing Ca^{2 +} to the soil, while for the other evaluated liming levels, SL offered a greater release of this nutrient to the soil. The limestones ML and CL presented similar behavior at the 60% liming requirement level, with a lower contribution of Ca²⁺ to the soil. At the 100 and 150% liming levels, the smallest mean values of Ca ²⁺ release were observed in ML (Table 3).

A linear increase occurred in the average content of Mg²⁺ in the soil as the liming requirement levels increased (Figure 3D). The b₁ values of the equations presented in this figure indicate that each unit of liming requirement applied to the soil causes an increase of 0.01, 0.002, 0.007, and 0.00009 cmol_c dm^-3 of Mg²⁺ when the soils were incubated with ML, BL, CL, and SL. The largest contribution of ML and CL of Mg²⁺ was expected since, among the evaluated limestones, they were those with the largest percentages of MgO.

For the variable Mg²⁺, it was observed that at the 30% NL level, no difference was found between the application of the different limestones. At the 60% NL level, the application of ML, BL, or CL resulted in a higher Mg²⁺ input to the soil when compared with SL. At the levels of 100 and 150% NL, ML and CL contributed with the largest input of Mg²⁺ to the soil (Table 3).

Treatments that received limestone presented higher and statistically significant (p<0.05) average values of Ca²⁺ and Mg²⁺ levels than the control by Dunnett’s test (Table 3), except for the variable Mg²⁺ in the soil that received SL, a fact attributed to chemical characteristics of the rock that has been previously discussed.

Regarding the NL level of agronomic interest, the evaluated limestones increased the level of Ca²⁺ from very low (0.41-1.20 cmol_c dm^-3) to the good level (2.41-4.00 cmol_c dm^-3). For Mg²⁺, although its level in the soil was very low (≤ 0.15 cmol_c dm^-3), the incubation with ML increased it to a very good level (> 1.50 cmol_c dm^-3). Biological and commercial limestones were used for the good level (0.91-1.50 cmol_c dm^-3), while SL increased it to 0.20 cmol_c dm^-3, classified as low level of Mg²⁺ (0.16-0.45 cmol_c dm^-3).

Limestones should be chosen not only for their acid neutralization power but mainly for the Ca:Mg ratio in the material. Among the limestones of acidity under study, the Ca:Mg ratio was 3:1 for ML, 15:1 for BL, 4:1 for CL, and 114:1 for SL. Currently, most studies consider Ca:Mg ratios between 4:1 for most cultivated plants and 8:1 as adequate for some less-demanding plants (Medeiros et al., 2008Medeiros JC, Albuquerque JÁ, Mafra AL, Rosa JD & Gatiboni LC (2008) Relação cálcio:magnésio do corretivo da acidez do solo na nutrição e no desenvolvimento inicial de plantas de milho em um Cambissolo Húmico Álico. Semina: Ciências Agrárias, 29:799-806.). Excess of Ca²⁺ in relation to Mg²⁺ in the soil solution may impair the absorption of the later (Malavolta et al., 1997Malavolta E, Vitti GC & Oliveira AS (1997) Avaliação do estado nutricional de plantas: Princípios e aplicações. Piracicaba, Potafos. 308p.). The interrelationship between nutrients in plant nutrition is related to their close chemical properties, such as ionic radius, valence, degree of hydration, and mobility, causing competition for soil adsorption sites and root absorption (Medeiros et al., 2008Medeiros JC, Albuquerque JÁ, Mafra AL, Rosa JD & Gatiboni LC (2008) Relação cálcio:magnésio do corretivo da acidez do solo na nutrição e no desenvolvimento inicial de plantas de milho em um Cambissolo Húmico Álico. Semina: Ciências Agrárias, 29:799-806.; Júnior et al., 2013Júnior MPAG, Santos AC, Dos Santos Araújo A, Oliveira LBT, Rodriguês MOD & Martins AD (2013) Relação Ca:Mg do corretivo da acidez do solo e as características agronômicas de plantas forrageiras. Revista Brasileira de Saúde e Produção Animal,14: 460-471.). Medeiros et al. (2008Medeiros JC, Albuquerque JÁ, Mafra AL, Rosa JD & Gatiboni LC (2008) Relação cálcio:magnésio do corretivo da acidez do solo na nutrição e no desenvolvimento inicial de plantas de milho em um Cambissolo Húmico Álico. Semina: Ciências Agrárias, 29:799-806.) observed the influence of the high Ca:Mg ratio of the correctives on nutritional imbalance, through a reduction in the growth and productivity of forage plants.

CONCLUSIONS

When applied to soil, biogenic, metamorphic, and sedimentary limestones are able to correct the soil acidity. The increase in the levels of Ca²⁺ and Mg ²⁺ in the soil are influenced by the chemical composition of the limestones. The limestones of sedimentary and metamorphic origin account for the greater concentration of Ca²⁺ and Mg²⁺ in the soil.

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