Simulation of the long-term soil response to acid deposition in various buffer ranges

A soil acidification model has been developed to estimate long-term chemical changes in soil and soil water in response to changes in atmospheric deposition. Its major outputs include base saturation, pH and the molar Al/BC ratio, where BC stands for divalent base cations. Apart from net uptake and net immobilization of N, the processes accounted for are restricted to geochemical interactions, including weathering of carbonates, silicates and Al oxides and hydroxides, cation exchange and CO₂ equilibriums. First, the model's behavior in the different buffer ranges between pH 7 and pH 3 is evaluated by analyzing the response of an initially calcareous soil of 50 cm depth to a constant high acid load (5000 mol_c ha^?1 yr^?1) over a period of 500 yr. In calcareous soils weathering is fast and the pH remains high (near 7) until the carbonates are exhausted. Results indicate a time lag of about 100 yr for each percent CaCO₃ before the pH starts to drop. In non-calcareous soils the response in the range between pH 7 and 4 mainly depends on the initial amount of exchangeable base cations. A decrease in base saturation by H/BC exchange and Al/BC exchange following dissolution of Al³⁺ leads to a strong increase in the Al/BC ratio near pH 4. A further decrease in pH to values near 3.0 does occur when the A1 oxides and/or hydroxides are exhausted. The analyses show that this could occur in acid soils within several decades. The buffer mechanisms in the various pH ranges are discussed in relation to Ulrich's concept of buffer ranges. Secondly, the impact of various deposition scenarios on non-calcareous soils is analyzed for a time period of 100 yr. The results indicate that the time lag between reductions in deposition and a decrease in the Al/BC ratio is short. However, substantial reductions up to a final deposition level of 1000 mol_c ha^?1 yr^?1 are needed to get Al/BC ratios below a critical value of 1.0.     

PH-Verteilung und Pufferung von Böden

PH-distribution and buffering of soils A frequency distribution of the pH values of 927 soils clearly reflects the three soil buffer ranges (carbonate, pH-dependent charge and aluminium release) by higher frequencies within these ranges and lower frequencies between them.     

Chemische Eigenschaften von Waldböden im Nordwestdeutschen Pleistocän und deren Gruppierungen nach Pufferbereichen

Chemical properties of forest soils in the pleistocene of Northwest Germany and their classification based on soil buffering systems In order to characterize the chemical characteristics of forest soils of Hamburg, 800 soil samples and 400 root samples from 172 sites were used to obtain suitable soil chemical and ecochemical parameters. A strong and deep reaching soil acidification was observed on all sites with exception of those on till. The sulfate concentration in the equilibrium soil solution allows the conclusion that deposition of acid plays a significant role in this acidification. Classification of sites based on edaphic factors did not provide useful information on the chemical status of soils. However, grouping of soil horizons using pH measured in 0.01 M CaCl₂ lead to a stratification according to the soil buffering systems. Distinction between the exchanger and the aluminium buffer ranges was, however, not quite satisfactory.     

Changes in aluminum pools of andisols due to soil acidification

Abstract

Aluminum concentrations in soil solutions are not only controlled by inorganic clay minerals but also by organically bound aluminum. The objective of this study was to determine which pools contribute to Al dissolution. Soil samples were taken at various distances from tree trunks and at various depths at the Rolling Land Laboratory (RLL), Hachioji, Tokyo. Selective dissolution techniques were used to analyze the changes in pools of solid-phase aluminum. Soil pH values around Hinoki cypresses were in the aluminum buffer range. Exchangeable aluminum contents in soils under Hinoki cypresses were 104 mmol_c kg-^?1 on the average. This value was similar to that of the cation exchange capacity (CEC) of Andisols at RLL at a soil pH of 4. The relationship between the soil pH and exchangeable, organically bound, and amorphous aluminum pools showed that dissolved aluminum ions in the soil solution were primarily derived from the amorphous Al pool. Dissolved aluminum ions were substituted with base cations of soils, resulting in the increase of the content of exchangeable Al and/or the formation of complexes with organic matter which increased the proportion of organically bound Al pools. Increase in the proportion of organically bound Al pools indicated the importance of complexation with soil organic matter for controlling the aluminum concentration in the soil solution.     

积盐对设施栽培土壤酸度及酸组成的影响

[目的]探讨盐渍化对土壤酸度的可能影响,为了解设施栽培土壤的酸化过程提供依据。[方法]采集了不同酸化特征的设施栽培土壤、露天栽培土壤和自然酸性土壤等3类表层土壤和剖面分层土壤样品,通过化学分析和室内添加肥料盐及土壤洗盐模拟试验,比较研究设施栽培土壤、露天栽培土壤与自然酸性土壤中活性酸、潜性酸、盐基饱和度之间关系差异及其受土壤盐分积累的影响。[结果]与自然酸性土壤相比,设施栽培土壤的酸是人为输入式,其酸化主要发生在表层,土壤剖面呈自上而下下降。在相同交换性酸水平的条件下,设施栽培土壤的pH值最低,其次为露天栽培土壤,而自然酸性土壤的pH值相对较高。在相同土壤pH值的情况下,自然酸性土壤的盐基饱和度明显低于设施栽培土壤和露天栽培土壤,而设施栽培土壤的盐基饱和度高于露天栽培土壤;设施栽培土壤的交换性酸中活性酸组成比例高于自然酸性土壤。增加中盐的积累可显著降低设施栽培土壤pH值;设施栽培土壤的盐分淋洗过程在降低土壤盐分的同时也降低了土壤的活性酸(提高了土壤的pH值)。[结论]盐分的积累增强了设施栽培土壤中潜性酸向活性酸的转化,高量施用化肥不仅可直接通过酸性物质的输入促进土壤pH值的下降,同时由此引起的盐分也可在一定程度上进一步降低土壤的pH值。     

Kurzfristige pH-Pufferung von Böden

Short-time pH buffering of soils Changes in pH of 60 soils after HCl addition were related to reaction time and soil characteristis. Between 80 and 100% of the added protons were taken up by the soils within a few seconds, resulting in the release of exchangeable Ca, Mg and Al in strongly acid soils. The decrease of proton activity between 0.25 and 30-70 h can be formally described as a diffusion process. pH buffering depended on soil pH and organic C content but not on clay content. Buffering decreased from pH 3.5 to about 6 and increased again up to pH 7. Increase of C_org increased buffering mainly from pH 6-7.     

Acid precipitation effects on soil pH and base saturation of exchange sites

The typical values and probable ranges of acid-precipitation are evaluated in terms of their theoretical effects on pH and cation exchange equilibrium of soils characteristic of the humid temperature region. The extent of probable change in soil pH and the time required to cause such a change are calculated for a range of common soils. Hydrogen ion input by acid precipitation is compared to cation inputs from nutrient cycling and other sources. For example it can be calculated that 100 yr of acid precipitation (10000 cm at pH 4.0) could be expected to shift the percentage base saturation in the top 20 cm of a typical midwestern forest soil location exchange capacity of 20 meq 100 g^?1 downward 20%, thus lowering the pH of the Al horizon by approximately 0.6 units, if there are no countering inputs of basic materials.     

The sensitivity of Swiss forest soils to acidification and the risk of aluminum toxicity

In the context of pollution‐control strategies to minimize the detrimental effects of soil acidification, there is a need to know how and to what extent soils respond to acidifying substances. The purposes of this study were to assess the sensitivity of soil to acidification, in particular to a decrease in pH and in base saturation (BS), and the risk of Al toxicity for vulnerable plants using chemical indicators. These indicators were derived from soil data (pH, exchangeable cations, amount of fine earth) measured in the mineral horizons of 257 soil profiles throughout Switzerland. Based on the analysis of the distribution of pH and BS values in the soil collective, we assessed the sensitivity of soils to a decrease in pH and in BS. Soils that were considered sensitive to a decrease in pH had pH values between 4.8 and 7.0. The degree of sensitivity was estimated with the proportion of fine earth in the critical pH range to a depth of 100 cm. Soils that were considered sensitive to a decrease in BS had pH values between 3.6 and 5.5 and a BS between 10% and 95%. Since the effective cation‐exchange capacity (CEC_eff) of the fine earth might dampen the decrease in BS when acidity is added, the disposition for a decrease in BS was related to the relative amount of fine earth in the sensitive BS and to the mean CEC_eff of this fine‐earth fraction. The risk of Al toxicity for vulnerable plants was estimated using the ratio of base cations to Al at the cation‐exchange sites (BC : Al_exc). A BC : Al_exc of 0.2 was taken as a threshold value below which the risk for sensitive plants increases. The degree of risk was based on the proportion of fine earth in the critical BC : Al range (≤0.2) in the soil profile. These indicators taking into account the various aspects of soil acidification are derived from usually available data and represent therefore a cost‐effective tool to assess the sensitivity of soils to an input of acidity.     

Effect of ph on amount of phosphorus extracted by 10 mm calcium chloride from three Rothamsted soils

Abstract

Three silty clay loams from the classical field experiments at Rothamsted Experimental Station (UK) with low phosphorus (P) status were treated with phosphate fertilizer and incubated for 15 days at field capacity with added acid [hydrochloric acid (HCl)] or base [sodium hydroxide (NaOH)] to give pH ranges measured in 1:5 suspensions of soil in 10 mM calcium chloride (CaCl₂) of 6.3–8.7 (Hoosfield A, pH 8.2), 3.8–8.2 (Hoosfield B, pH 6.3), and 2.8–7.1 (Geescroft, pH 5.3). Extractable ? was measured by the 10 mM CaCl₂ extraction procedure. For the Hoosfield ? and Geescroft soils without added phosphate fertilizer, extractable ? was decreased at both ‘low’ and ‘high’ pH values, the maximum being at about 5.4 in both soils. In the calcareous Hoosfield soil, extractable ? decreased with increasing pH over the range studied. These changes in extractable ? were magnified in soils treated with phosphate fertilizer but the maxima and trends were unaltered. The results indicate that native soil ? appears to be changed by pH in the same way as added ? to the soil. In the calcareous Hoosfield A soil, added acid did not reduce soil pH to less than 6.4 (because of its high buffer capacity) and so the pH level for maximum ? solubility was not found.     

Impacts of Acid Rain on Base Cations,Aluminum, and Acidity Development in Highly Weathered Soils of Thailand

The impacts of simulated acid rain on leachability of major plant nutrients, toxic element [aluminum (Al)], and acidity development in highly weathered tropical soils of Thailand were studied. Leaching experiments were conducted on soil columns with acidic solutions of pH 5.0, 4.0, 3.0, 2.0, and with water of pH 7.0 as a control treatment. Leaching losses of base cations from all soils increased with the decrease in pH associated with simulated acid rain (SAR) additions, and were found to be quite high under SAR with pH 2.0. The leaching removal of these cations was lesser at pH 3.0, 4.0, and 5.0 but greater than that in pH 7.0. The leaching of base cation from the soils depended not only on acid rain pH but also on soil properties, especially cation exchange capacity, soil texture, and initial base content. The significant losses of major plant nutrients [such as potassium (K⁺), calcium (Ca²⁺), and magnesium (Mg²⁺)] from the plant root zone over extended periods could cause nutrient imbalance and lower soil productivity.     

Calcium requirements of citrus

Abstract

A close relationship was found between the pH of soil suspensions in the SMP buffer solution (pH_smp) and the potential acidity of soils (H + Al) extracted by a neutral calcium acetate solution (r = 0.98), for twenty six soil samples of the State of Sao Paulo, Brazil, This relationship was represented by the equation lnY = 7.76 ‐ 1.053X, which allowed for the calculation of H + Al directly from the values of pH_smp.

With the values of H + Al and the sum of bases, calcium, magnesium and potassium, the cation exchange capacity (CEC), and the base saturation (V) were calculated. Relationships between the base saturation of the soils and the active acidity of soil suspensions were close, both for pH determined in water (r=0.94) and pH determined in 0.01M CaCl₂ solution (r ‐ 0.97). Thus the lime requirement (LR) of soils could be calculated, for given values of pH or base saturation, using the equation LR = CEC (V₂ ‐ V₁)/100, in which V₁ is the base saturation of the soil and V₂ is the expected value upon liming.

The predicted values for lime required to increase the soil pH in water to either 5.5 or 6.0 were comparable to those obtained by the direct use of the SMP buffer method, and were, respectively, two and four times higher than the amounts required to neutralize exchangeable aluminum, considering the criterion LR = Al × 1.5.

The proposed method to determine lime requirement of soils is described in detail and the advantages of its use are discussed.     

Aluminum toxicity in corn at near neutral soil pH levels

Aluminum toxicity to plants is often responsible for yield reductions under acid soil conditions. Despite the fact that Al becomes insoluble in the slightly acid to neutral pH range, there are indications that it can still be taken up and become toxic to plants. Corn plants were grown in the greenhouse on three highly weathered soils (Oxisol and Ultisols) containing substantial quantities of exchangeable Al. The soils were limed at various rates up to pH values near or above neutrality. On all soils a positive yield response to lime occurred at low pH values while on two of the soils, yields decreased significantly as the pH approached neutrality. Yield and Al content of the tissue were exponentially related irrespective of the level of phosphate applied (r² = 0.34 and 0.71). The mechanism by which Al becomes increasingly available to the plant as the pH approaches neutrality is not clear and needs further investigation. Strong Al‐Mg and Al‐P antagonisms occur at the high and low pH values. The existance of Al toxicity at near‐neutral pH values can be invoked to explain many of the anomolous results reported in the literature concerning yield depressions previously ascribed a posteriori to such factors as micro‐element or P deficiencies.     

Laboratory prescreening of Bradyrhizobium japonicum for low pH,Al and Mn tolerance can be used to predict their survival in acid soils

We examined whether strains of Bradyrhizobium japonicum selected for growth on acid media in vitro would also survive and grow better in acid soils. Four agar screening media for acid-tolerant rhizobia, which differed in the number of acid soil stresses imposed (pH, low calcium (Ca) and phosphorus (P), high aluminum (Al) and manganese (Mn)), were assessed for their effects on the survival of 14 Indonesian strains and two commercial strains of B. japonicum. Survival of B. japonicum in the agar media was compared with that in two acid soils. A repeat stab inoculation method which provided a declining range of inoculum cell number to 10³ cells per stab was used to assess the daily growth of the strains on the screening media at 5 pH levels (3.8, 4.2, 4.5, 5.0, and 6.8). The growth and survival of the 16 strains were then measured at days 1, 8, 18, and 28 after inoculation in two acid soils (pH 4.24 and 4.35) sterilized using γ-irradiation at 5.0 Mrad. Selectivity of the agar media improved as more acid stress factors were incorporated in the media. Those strains of Bradyrhizobium identified as acid, Al and Mn-tolerant in acidic agar media, also had better survival in the low pH soils. There was no relationship between acid or alkali production on agar media and acid tolerance on agar or in soil. There was no apparent relationship between symbiotic performance and acid tolerance, and one acid-tolerant strain was as effective as the commercial inoculant strain CB1809. The most acid-tolerant strain was also the most ineffective.     

基于不同方法测定土壤酸性磷酸酶活性的比较

土壤酸性磷酸酶与有机磷的矿化及植物的磷素营养关系最为密切。目前国内学者在测定酸性磷酸酶活性时主要参照关松荫《土壤酶及其研究法》中以磷酸苯二钠为基质的测定方法,而国外学者主要参照Dick《Methods of Soil Enzymology》中以对硝基苯磷酸二钠为基质的测定方法(PNPP)。但是,在以磷酸苯二钠为基质测定生成物的过程中,常出现显色程度不明显的问题;另外,采用不同基质测定酸性磷酸酶活性也造成了测定方法选择的困难。为合理选择土壤酸性磷酸酶活性的测定方法,本研究选用酸性、中性和碱性土壤各10个土样,分别采用以磷酸苯二钠为基质,且在显色阶段分别加入pH5.0醋酸盐缓冲液(DPP 1)和pH9.4硼酸盐缓冲液(DPP 2)的方法,以及PNPP方法测定土壤酸性磷酸酶活性。同时也研究了不同pH缓冲液和苯酚浓度对生成物显色反应的影响。结果表明:以磷酸苯二钠为基质、在显色反应阶段加入pH≤6的缓冲液时,苯酚和2,6-二溴苯醌氯亚胺不显色;当加入pH≥8的缓冲液时,两者之间显色且苯酚浓度和吸光值的Pearson相关系数极显著。这说明pH低是导致高苯酚浓度和2,6-二溴苯醌氯亚胺显色效果差的一个主要原因。此外,采用PNPP方法测定时,在酸性、中性和碱性土壤中,10个样本酸性磷酸酶活性的变异系数分别较DPP 2增加了70.04%、42.44%和21.17%;极差分别是DPP 2的27.18倍、26.85倍和39.43倍。总之,如果选用磷酸苯二钠为基质测定土壤酸性磷酸酶活性,应在显色阶段加入碱性硼酸盐缓冲液;选用对硝基苯磷酸二钠为基质,是更为简单和灵敏的方法。     

Efficiency of gypsum application to acid Andosols estimated using aluminum release rates and plant root growth

Abstract

A great deal of information on the efficiency of gypsum or phosphogypsum to ameliorate acidity in highly weathered soils is available, but only limited information is available on the efficiency in acid Andosols, which possess large amounts of active aluminum (Al). We examined the effectiveness of gypsum application to non-allophanic Andosols (one humus-rich A horizon and two B horizons poor in humus) using extractable soil Al analyses (batch and continuous extraction methods) and a cultivation test using burdock (Arctium lappa). With gypsum amendment, pH(H₂O) values of the soil decreased from 4.5–4.7 to 4.2–4.4, whereas the treatment made almost no difference to the values of pH(KCl). Total active Al (acid oxalate-extractable Al) was hardly affected by gypsum for all samples. Potassium chloride-extractable Al definitely decreased with the addition of gypsum in all soils; however, the decrease was small (0.1–1.4 cmol_c kg^?1) and the values still exceeded “the threshold of 2 cmol_c kg^?1” for inducing Al toxicity in sensitive plants (4.4–8.6 cmol_c Al kg^?1). The change in Al solubility with gypsum application represented by Al release rates from soils using continuous extraction methods with a dilute acetate buffer solution (10^?3 mol L^?1, pH 3.5) differed greatly among the soil samples: The release rate of one of the B horizon samples decreased by 71%, certainly showing the insolubilization of Al compounds, whereas the release rates of the A horizon sample showed almost no change. These changes in Al solubility were well correlated with the plant root growth. Root growth was improved with gypsum in the B horizon sample, whereas improvement was not observed in the A horizon soil. The decrease in the rate of Al release of another B horizon soil with gypsum treatment was smaller (by 20–34%), possibly because of lower pH values after gypsum application (pH[H₂O] of 4.2–4.3). In the B horizon soil, root growth improved only slightly. Thus, the effectiveness of gypsum application to acid Andosols appeared to be largely influenced by soil humus contents and slight differences in soil pH values, and corresponded to a decrease in Al release rates using the continuous extraction method.     

Vereinfachte Bestimmung der Basenneutralisationskapazität (BNK) stark saurer Böden

Simplified method to determine the base-neutralizing capacity of strongly acidified soils The base-neutralizing capacity (BNC) is an important property to characterize acid soils. The BNC can be reliably determined by soil-base titrations (BNC_tit). But these methods are work-expensive, and a large amount of soil material is needed. Such a large amount of soil material is not available if the soil is sampled by a hand-auger (e.g. mapping, areal investigations). Known buffer methods require less soil material, but have some shortcomings. Therefore, the well known Schachtschabel-method with calcium acetate buffer was developed for the application to strongly acidified soils. BNC values measured by the developed calcium acetate method (BNC_ac) agreed nearly perfectly with BNC_tit values of 59 mineral soils. The BNC_ac slightly overestimated (< 10%) the BNC_tit of 10 samples of humus layers. Furthermore, small soil samples (down to 1 g soil), wide soil:solution ratios, and integration of pH (H₂O) measurements into the determination of the BNC_ac yielded reliable BNC_ac values. Thus, the BNC_ac-method may be recommended as a simple and reliable method to determine the BNC of acid soils, especially if only a small amount of soil material is available.     

Acid- and base titration behaviour of finnish mineral soils

The acid- and base-buffering properties of 84 non-calcareous surface soil samples were studied by batch titration with HCI or KOH at a constant ionic strength of I = 0.1. The soil samples were classified according to their pH of zero point of titration (ZPT). Differential buffer values, dB(H) or dB(OH) (H⁺ or OH^? as meq kg^?1 needed to reduce or increase the soil pH sequentially by 0.5 units, respectively), were introduced to describe the course of titration curve and the intensity of buffer action. In all soils, the first acid-buffer value, dB(H)_0→0.5, varied from 8 to 78 meq kg^?1 and the second one, dB(H)_0.5→1, from 10 to 138 meq kg^?1. The corresponding base-buffer values, dB(OH)_0→0.5 and dB(OH)_0.5→1, ranged from 10 to 48 and from 14 to 44 meq kg^?1, respectively. The most acid soils were most strongly buffered against acid, and the soils with the highest initial pH against base. The results reveal the acid-buffering by exchange reactions to be very important. In the soils with ZPT≦5.4, the first acid-buffer value was dependent on the content of organic matter and oxalate-soluble Al, whereas in the more acid soils the role of clay became significant. Thus, it was concluded that at higher pHs the foremost inactivation of H⁺ is attributable to soil components of pH-dependent charges, and the significance of constituents of permanent charges to increase with proceeding acidification. In strongly acid soils (ZPT≦4.8) the very effective buffering seemed to be primarily due to the dissolution of Al-hydroxides and, thus, to exert detrimental effects on the edaphic environment. The general rank of soil factors explaining the variation in the base-buffer values was in accord with the neutralization sequence, i.e. the strongest acid in the soil being neutralized first. In the strongly acid soils (ZPT≦4.8) the base-buffer values seemed to depend on the clay as well as KCl- and NH₄OAc-extractable Al, whereas in the soils with higher initial pH mostly on organic C.     

Cation exchange in forest soils: the need for a new perspective

The long‐term sustainability of forest soils may be affected by the retention of exchangeable nutrient cations such as Ca²⁺ and the availability of potentially toxic cations such as Al³⁺. Many of our current concepts of cation exchange and base cation saturation are largely unchanged since the beginnings of soil chemistry over a century ago. Many of the same methods are still in use even though they were developed in a period when exchangeable aluminium (Al) and variable charge were not generally recognized. These concepts and methods are not easily applicable to acid, highly organic forest soils. The source of charge in these soils is primarily derived from organic matter (OM) but the retention of cations, especially Al species, cannot be described by simple exchange phenomena. In this review, we trace the development of modern cation exchange definitions and procedures, and focus on how these are challenged by recent research on the behaviour of acid forest soils. Although the effective cation exchange capacity (CECe) in an individual forest soil sample can be easily shown to vary with the addition of strong base or acid, it is difficult to find a pH effect in a population of different acid forest soil samples. In the very acidic pH range below ca 4.5, soils will generally have smaller concentrations of adsorbed Al³⁺. This can be ascribed to a reduced availability of weatherable Al‐containing minerals and a large amount of weak, organic acidity. Base cation saturation calculations in this pH range do not provide a useful metric and, in fact, pH is modelled better if Al³⁺ is considered to be a base cation. Measurement of exchangeable Al³⁺ with a neutral salt represents an ill‐defined but repeatable portion of organically complexed Al, affected by the pH of the extractant. Cation exchange in these soils can be modelled if assumptions are made as to the proportion of individual cations that are non‐specifically bound by soil OM. Future research should recognize these challenges and focus on redefining our concepts of cation retention in these important soils.     

Solution chemistry in acid forest soils: Are the BC : Al ratios as critical as expected in Switzerland?

The molar ratio of base nutrient cations to total dissolved aluminum (BC : Al_tot) in the soil solution was measured at six forest sites in Switzerland in acid mineral soils to determine whether the ratio measured in the field was lower than the critical value of 1, as predicted by the mapping of exceedances of critical loads of acidity. The soil chemistry was then related to the soil solution composition to characterize the typical effective base saturation (BS) and BC : Al ratio in soil leading to critical BC : Al_tot in the soil solution. The median BC : Al_tot ratio in the soil solution never reached the critical value in the root zone at any sites for the whole observation period (1999–2002), suggesting that the BC : Al_tot ratios measured in the field might be higher than those modeled for the determination of critical loads of acidity. The gibbsite model usually applied for the calculation of critical loads was a poor predictor of the Al³⁺ activity at the study sites. A curvilinear pH‐pAl³⁺ relationship was found over the whole range of pH (3.8–6.5). Above a pH of 5.5, the slope of the pH‐pAl³⁺ relation was close to 3, suggesting equilibrium with Al(OH)₃. It decreased to values smaller than 1.3 below a pH of 5.5, indicating complexation reactions with soil organic matter. The BS and the BC : Al ratios in the soils were significantly correlated to the BC : Al_tot ratios in the soil solution. The soil solutions with the lowest BC : Al_tot ratios (≤ 2) were typically found in mineral soils with a BS below 10 % and a BC : Al ratio in the soil lower than 0.2. In acid pseudogleyed horizons overlying a calcareous substrate, the soil solution chemistry was strongly influenced by the composition of the underlying soil layers. The soil solutions at 80 cm had pH values and BC : Al_tot ratios much higher than expected. This situation should be taken into account for the calculations of critical loads of acidity.     

Calibration of the single‐ and double‐buffer SMP lime requirement methods by root elongation bioassay

Abstract

Chemical and biological lime requirement (LR) reference values of 154 soils were obtained by six months of incubation of each soil with five levels of calcium carbonate (CaCO₃). Levels of CaCO₃ addition differed among soils according to their characteristics. Chemical LR values were based on the individual neutralization curves to achieve a desired pH (pH_d) value of 5.5, 6.0, and 6.5 in water or 5.0, 5.5, and 6.0 in 0.01M calcium chloride (CaCl₂). Biological LR values were estimated to achieve 90% relative root elongation on each soil after a growth period of 48 h using wheat cv Abe. Chemical values of LR suggest that SMP method is valid for a wide range of mineral soils from different geographic regions. However, the proportion of soil acidity reacting with the buffer is not constant. Results indicate that values obtained with the routine methods need to be calibrated with equations different from the originals. The use of curvilinear models to adjust one single pH of the soil‐buffer system improved substantially its accuracy, allowing the single‐buffer (SB) results to be comparable to the more time consuming and labourious double‐buffer (DB) technique. No advantage was noticed with the use of curvilinear equations for DB technique. The adoption of pH_d in 0.01M CaCl₂ leads to an increase of precision of the predicted LR. Regression equations are provided for calculating LR rates to different pH_d values. Accuracy is high (r²=0.887) even for pH values (5.0 in 0.01M CaCl₂) lower than normally considered in methods based on buffer solutions. An overestimation of biological LR values was observed with both SMP methods. Notwithstanding, a calibration can be also made with the root bioassay, adjusting the chemical values to lime rates based on biological constraints related with aluminum (Al) toxicity. Regression equations are provided. Once more, the use of quadratic model for SB method allows an accuracy (r²=0.836) comparable with the DB technique (r²=0.850).