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Sci. agric. (Piracicaba, Braz.) vol.60 no.2 Piracicaba 2003
SOILS AND PLANT NUTRITION
Relationship between acidity and chemical properties of Brazilian soils
Relações entre acidez e propriedades químicas de solos brasileiros
Cássio Hamilton Abreu Jr.I; Takashi MuraokaII; André Fernando LavoranteIII
IUSP/CENA - Lab. de Nutrição Mineral de Plantas, C.P. 96 - 13400-970 - Piracicaba, SP - Brasil
IIUSP/CENA - Lab. de Fertilidade do Solo
IIIUSP/CENA - Lab. Química Analítica
ABSTRACTIn soils of tropical climate regions the high acidity and the presence of exchangeable aluminum (Al3+), associated to low fertility, are the main restrainting factors for agricultural production. A laboratory experiment was conducted using 26 soils of different Brazilian regions, to investigate soil acidity components, giving emphasis to Al and their relations with chemical properties. The pH correlated positively with P, Ca, Mg, K, BS, CEC and V% values, and negatively with Al saturation. The Al3+ was the predominant exchangeable cation in 32% of the soils with pH below 5.6. The KCl titratable H+ represents the hydroxi-Al(OH)x complex with low stability and the Wolf-Morgan extracted Al corresponds to the exchangeable + non-exchangeable Al species. The Al3+ and low stability hydroxi-Al decreased quickly with increasing pH up to 5.5. The non-exchangeable Al increased up to pH 4.5, then decreased to pH 5.5 and had a small increment from 7.0 to 7.5.
Key words: total acidity, exchangeable acidity, pH, aluminum in soil
Nos solos de regiões de clima tropical, a elevada acidez e a presença de alumínio trocável (Al3+), aliadas à baixa fertilidade, são os principais fatores a restringir a produção agrícola. Investigaram-se os componentes da acidez, com ênfase ao alumínio, e suas relações com as propriedades químicas de 26 solos de regiões brasileiras. O pH correlacionou positivamente com os valores de P, Ca, Mg, K, SB, CTC e V%, e negativamente com a saturação de Al. O Al3+ foi o cátion trocável predominante em 32 % dos solos com pH inferior a 5,6. O H+ titulável em KCl representa formas hydroxi-Al(OH)x de baixa estabilidade e o Al obtido pelo extrator de Wolf-Morgan corresponde ao Al trocável + não-trocável. As formas Al3+ e hydroxi-Al de baixa estabilidade diminuíram rapidamente com o pH até 5,5. O Al não-trocável aumentou até pH 4,5, diminuiu a seguir até pH 5,5 e aumentou lentamente com o pH de 7,0 a 7,5.
Palavras-chave: acidez total, acidez trocável, pH, alumínio no solo
For soils of tropical and humid subtropical climate regions, the high acidity and high exchangeable aluminum content, associated to low fertility, are the main constraints for agricultural production (McLean, 1965; Pavan, 1983; Nachtigall & Vahl, 1989). Soil acidity is characterized by its intensity and quantity aspects (Kinjo, 1983). The intensity factor is given by the soil solution hydrogen (H+) ion activity, and the quantity factor, by the amount of H, bound to the exchange complex, which the soil can liberate to solution. There is also the soil acidity capacity factor, which is the resistance to the variation of pH resulting from the addition of acids or bases, i.e., the soil buffering capacity.
The characterization of soil acidity components is given by the active acidity (intensity factor), usually expressed as soil pH (-log[H+] in solution), and by the potential acidity (quantity factor), of which more actual terminology, and hereafter used, is the total acidity (at pH 7) (Raij et al., 2001). The total acidity, by its turn, is the sum of exchangeable acidity and non-exchangeable acidity. The exchangeable acidity is given by the aluminum ion electrostatically retained by colloid surfaces with pH dependent negative charges, also called exchangeable aluminum (Al3+). The non-exchangeable is related to the content of H covalently bound to colloids, and monomers and polymers of aluminum in soil (Kinjo, 1983; Thomas & Hargrove, 1984; Raij et al., 1987; Nachtigall & Vahl, 1989; Raij et al., 2001).
The detrimental effects of acidity on plant growth depend on the H+ and Al3+ ions activities in the soil solution, which may be related to the activities and exchangeable contents of calcium (Ca2+), magnesium (Mg2+) and potassium (K+) cations, of orthophosphate (H2PO4-), nitrate (NO3-) and sulphate (SO42-) anions, and with organic matter (MO) content (Pavan, 1983; Thomas & Hargrove, 1984). With the neutralization of part of the soil total acidity by lime application, negative charges of the soil exchange complex are released, and then occupied by Ca2+, Mg2+ and K+ (Oates & Kamprath, 1983), improving the soil fertility and the conditions for agricultural production.
The objective of this work was to evaluate the acidity components and their relationship with chemical properties, for improving crop management in soils from different regions of Brazil.
MATERIAL AND METHODS
The experiment was carried out in Piracicaba (SP), Brazil, using samples collected from the surface layer (0 – 0.2 m depth) of 26 soils from different Brazilian regions (Table 1). Soil samples with reaction close to neutrality were also included with the objective of evaluating the extraction of non-exchangeable forms of aluminum and to obtain low total acidity values.
Air dried samples of each soil were homogenized, divided in three subsamples, identified, ground in a porcelain crucible and passed through 0.5 mm mesh sieve and packed. For the chemical characterization of soil samples (Table 2), methods described in Raij et al. (1987; 2001) were used, except for the exchangeable sodium which was extracted with 0.05 mol L-1 HCl + 0.0125 mol L-1 H2SO4 solution in 1:5 (v/v) soil/extractant ratio and determined in flame photometer.
Soil acidity components were characterized as follows: the active acidity was determined in 0.01 mol L-1 CaCl2 solution, 1:2.5 (v/v) soil/solution ratio, through pH measurement; the total acidity (H + Al3+) directly through the extraction with 1 mol L-1 ammonium acetate solution at pH 7, followed by titration, and indirectly by SMP method (Raij et al., 1987; 2001). The exchangeable acidity (Al3+ + H+tit ) and the exchangeable aluminum (Al3+) were extracted by 1 mol L-1 KCl solution, 1:10 (v/v) soil/solution ratio, and determined by titration of 25 mL KCl extract with 25 mmol L-1 NaOH, using 1 g L-1 phenolphthalein as indicator, and by back-titration, after acidification with 40 g L-1 NaF, with 25 mmol L-1 HCl, respectively (routine methodology of Soil Fertility Lab., USP/CENA, adapted from McLean, 1965). The difference between titratable exchangeable acidity and titratable aluminum gave the socalled titratable hydrogen (H+tit) content. The soluble Al (WMAl) was extracted with Wolf-Morgan solution (0.73 mol L-1 sodium acetate and 0.0001 mol L-1 DTPA, pH 4.8) and determined through the aluminon method, (Wolf, 1982). The total Al content was determined in ICPAES, after HClO4 + HNO3 + HF digestion. The nonexchangeable acidity was determined by the difference between the total acidity and exchangeable acidity obtained by titrimetric method.
Results were submitted to descriptive statistics for estimations of mean, median, upper and lower quartiles, and correlation and regression analysis for selected variables.
RESULTS AND DISCUSSION
The pH values in CaCl2 (Table 3), varied from 3.78 to 7.86, with 4.58 as the most frequent value, and 75% of values between 4.25 and 5.52. According to the limits established by Raij et al. (1987) for soils of São Paulo State, Brazil, 31% of samples presented very high active acidity (pHCaCl2 £ 4.3), 38% high (4.4 to 5.0), 4% medium (5.1 to 5.5), 8% low (5.6 to 6.0), and 19% very low (> 6.0).
For soil samples with medium to very high acidity (pH < 5.6), the sum of bases (SB), the cation exchange capacity (CEC) and base saturation (V%) (Table 2) varied from 3.3 to 66.7 mmolc dm-3, from 37.7 to 117.8 mmolc dm-3 and from 4.5 to 66.0%, respectively. There were positive correlations between the values of P (resin), Ca2+, Mg2+, K+, SB, CEC and V% and soil pH (Table 4), and a negative correlation between aluminum saturation (m%), showing the importance of soil reaction on soil fertility and the conditions for crop production. The chemical properties values (Tables 2 and 3) were similar to those obtained by Pavan (1983) and Nachtigal & Vahl (1989) for soils in the same pH range.
The titratable exchangeable acidity (Al3+ + H+tit ) varied between 0 to 18 mmolc dm-3, and the exchangeable aluminum (Al3+) between 0 to 15 mmolc dm-3 (Table 3). This demonstrated that the exchangeable acidity was constituted mostly of Al3+. In samples with pH lower than 5.6, the contribution of H+tit was highly significant, representing 50% or more of titrated exchangeable acidity in 35% of all samples.
Aluminum (Al3+) was the predominant exchangeable cation (m% > 30) in 23% of the analyzed soils, and in 32% of those with pH lower than 5.6. This result differs from that verified by Pavan (1983) in acid soils of Paraná, and explains that even in soil with high acidity, the Al3+ may not be present and Al3+ is a function of parent material and of soil mineralogy. The high Ca2+ and Mg2+ content (Table 2) and the negative correlation between these cations with Al3+ (Table 4) may indicate that a significant fraction of soluble Al could have been neutralized by the liming.
In acid mineral soils, the H+tit present in the KCl soil extract is not derived from Al3+ displaced from exchange sites; it is rather the result of pH dependent hydrolysis reactions that involve the hydroxi-Al(OH)x forms, the organic matter, and the Al and Fe oxides (Kissel et al., 1971; Thomas & Hargrove, 1984). Hiradate et al. (1998), using the nuclear magnetic resonance technique for the Al speciation of acid soil samples in KCl solution, verified that 92 to 96% of Al3+ were made of electrically symmetric octahedral Al (monomer and dymer of hydroxi-Al) and of organically complexed Al, respectively. However, the behavior of hydrogen bound to exchange complex depends on the nature of soil colloids. When associated to constant negative charge of 2:1 clay minerals, with planar surface, the H+ ion is retained by electrostatic forces, i.e., as exchangeable cation; when associated to variable negative charge of organic matter, kaolinite, allophane and iron and aluminum oxides, the hydrogen is retained by covalent bound, i.e., non-exchangeable cation (Kinjo, 1983). Therefore, in acid mineral soils, the H+tit present in non-buffered KCl soil solution extract does not represent the soil exchangeable acidity, but the low stability hydroxi-Al forms, except in soils with high organic matter content (Oates & Kamprath, 1983; Raij et al., 1987; 2001).
The concentration of low stability hydroxi-Al forms in soil samples from different regions of Brazil (H+tit contained in non-buffered KCl solution) varied from 0 to 4.7 mmolc dm-3 (Table 3) and was dependent on Al3+ content (Figure 1) and of sample pH (Table 4). In acid soils, there are predominance of Al3+ over the Al(OH)2+ and Al(OH)+2, however with increasing pH, there gradually occurs the increment of OH/Al relation and of polymerization of hydroxi-Al forms. The formed polymers, of variable size and charges, neutralize negative charges but are not displaced in non-buffered saline solutions (Thomas & Hargrove, 1984). Therefore, in acid mineral soil containing 1:1 clay minerals and Al, Fe oxides, the lower the hydroxi-Al forms content, the higher is the proportion of their hydrolysable forms, resulting from lower polymerization (stability) of these Al forms, which produces H+ in KCl extract (Kissel et al., 1971).
The soluble Al content obtained by Wolf-Morgan extractant (WMAl) varied from 0.2 to 21.8 mmolc dm-3 (Table 3), with most frequent value of 7.0 mmolc dm-3 (8.2 mmolc dm-3 in samples with pH < 5.6) and were superior or near the Al3+ content in KCl neutral solution, except in LA-3 sample. Similar values of Al extracted with 1 mol L-1 NH4AOc (pH 4.8), to obtain the exchangeable + non-exchangeable Al, was reported by Pavan (1983) in soil samples from Paraná with the same range of Al3+ of this work. This indicates that the Wolf-Morgan solution extracts exchangeable Al and non-exchangeable Al as well, which may be toxic to plants (Noble et al., 1988; Hiradate et al., 1998). Therefore, the Al obtained by difference between WMAl and Al3+ constitute the non-exchangeable Al (Table 3), and those values varied from 0 to 11.7 mmolc dm-3. Similar results were observed by Pavan (1983) in soil samples with the same variation in Al3+.
The Al contents of exchangeable and of low polymerization degree hydroxi forms diminish rapidly with the sample pH increment up to 5.5 value; through the non-exchangeable form increased up to pH 4.5, decreased thereafter up to pH 5.5, and increased slowly with the pH of 7.0 to 7.5 (Figure 2), according to the stage of polymerization of hydroxi-Al forms which are functions of pH and of organic matter (McLean, 1965; Kissel et al., 1971; Pavan, 1983; Thomas & Hargrove, 1984).
The determined total acidity was 8.6 to 91.9 mmolc dm-3 and those obtained by SMP pH, 7.9 to 95.2 mmolc dm-3, with most frequent values of 39.1 to 39.4 mmolc dm-3, respectively. The indirect determination of total acidity (Raij et al., 1987) is based on SMP pH (buffered at pH 7.5) variations in consequence of H+, Al3+ and OH ions released by soil colloids and organic matter, which allows to establish the relationship between the pH of SMP solution and the H+ + Al3+ content in the soil sample.
The analysis of the relationship between exchangeable aluminum with total acidity and base saturation of the soils (Figure 3) revealed that a small increase in Al3+ content (0 to 0.2 mmolc dm-3) results in a quick evolution of total acidity (8.6 to 21.1 mmolc dm-3). This generated acidity, for soils with pH ³ 5.6, is exclusively attributed to H+ ions (Table 3). Thereafter, when Al3+ > 0.2 mmolc dm-3, the total acidity increased 5.1 mmolc dm-3, on average, for each released mmolc dm-3 of Al3+. Simultaneously, the base saturation reduced drastically with increasing Al3+ content from 0 to 4 mmolc dm-3; above this boundary value, the effect of increasing Al3+ was much smaller in reducing soil V%.
There was a very narrow relationship between the total acidity obtained directly and indirectly (Figure 4a) and the obtained exponential relationship between SMP pH and the determined total acidity (Figure 4b) was much similar to that used by laboratories of soil analyses, mainly in the State of São Paulo (Raij et al., 1987; 2001). This indicates that the SMP pH method can be used for the evaluation of total acidity of soils practically from all Brazilian regions. Recently, Nascimento (2000) and Silva et al. (2000) established second degree equations for the calculation of H+ + Al3+ by SMP pH method in soils of Pernambuco and of Semi-Arido of Brazilian Northeast, respectively.
Although, unlike Pavan's (1983) paper, significant correlations have not been verified among the variables associated to the soil acidity and the organic matter content (Table 4), except for non-exchangeable acidity, it is known that in acid soils the organic matter can play an important role in Al3+ complexation (McLean, 1965; Pavan, 1983; Oates & Kamprath, 1984; Hiradate et al., 1998), reducing the toxic effect of aluminum to plants. This fact may have an important role in agricultural systems with management of crop residues or of organic fertilizers, for which the liming would be reduced without affecting productivity, increasing the cost/benefit relationship. The relationship with organic matter explains also the Al3+ content reduction along the acid soil profile (McLean, 1965).
There were correlations (P < 0.001) between the soil Al saturation (m%) and other acidity associated variables (Table 4), except for total Al. The total Al only correlated with the CEC of soil with pH bellow 5.6, evidencing different mineralogy among the studied soil samples.
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Cássio Hamilton Abreu Jr.
Received August 19, 2002