Short‐term effects of pH and aluminium on mineral nutrition in maize varieties differing in proton and aluminium tolerance

In short‐term (24 h) nutrient solution experiments, the influence of different proton (pH 6.0 and pH 4.3) and aluminium (Al) (0, 20, and 50 μM) concentrations on root and coleoptile elongation, dry weight, and the uptake of selected mineral nutrients was studied in maize (Zea mays L.) varieties that differ in acid soil tolerance under field conditions. The acid‐soil‐tolerant maize varieties, Adour 250 and C525M, proved to be hydrogen (H⁺) ion sensitive, but Al tolerant, while the acid soil tolerant variety BR201F was H⁺ tolerant but Al sensitive. The acid soil sensitive variety HS 7777 was affected by both H⁺ and Al toxicity. The proton‐induced inhibition of root elongation was closely related to the proton‐induced decrease of the specific absorption rates (SAR) of boron (B), iron (Fe), magnesium (Mg), calcium (Ca), and phosphorus (P). In contrast, only the specific absorption rate of B (SARB) was significantly correlated to the Al‐induced inhibition of root elongation. It is concluded, that alterations of nutrient uptake may play an important role in H⁺ toxicity, while at least after short‐term exposure to Al, alterations of Ca, Fe, Mg, or P uptake do not seem to be responsible for Al‐induced inhibition of root elongation. Further attention deserves the Al‐B interaction, moreover taking into account that a highly significant correlation between Al‐induced increase of callose concentration in root tips and Al‐induced decrease of SAR_B could be established.     

Hydrogen and aluminum inhibition of soybean root extension from limed soil into acid subsurface solutions

Soybean [Glycine max (L.) Merr. cv. ‘Ransom'] root elongation under varying concentrations of solution hydrogen (H) and aluminum (Al) was investigated in a vertical split‐root system. Roots extending from a limed and fertilized soil compartment grew for 12 days into a subsurface compartment with solutions adjusted to either different pH values from 3.7 to 5.5 or a factorial combination of pH (4.0,4.6, and 5.2) and Al (0,7.5, 15, and 30 μM) levels. Ionic forms of Al were estimated with GEOCHEM and solution Al was determined with ferron. Boron (B) (18.5 μM) and zinc (Zn) (0.5 μM) were supplied to all solution treatments, in addition to 2000 μM Ca, after preliminary studies at pH 5.2 without Al indicated that their omission inhibited length of tap roots and their laterals in the subsurface compartment. Both H⁺ and Al inhibited the length of lateral roots more than tap roots. Lateral roots failed to develop on tap roots at pH<4.3 or in treatments with 30 μM Al. Relative tap root length (RRL) among treatments receiving Al correlated with Al as measured by reaction with ferron for 30s. Ferron‐reactive Al was correlated to GEOCHEM‐predicted Al³⁺ activity (r=0.99). A 50% reduction in RRL occurred with either 2.1 μM Al³⁺ activity or 4.9 uM ferron‐reactive Al. The absence of shoot and soil‐root biomass differences among solution treatments in the split‐root system indicated that differences in root growth in the subsurface compartment were not directly confounded with differences in top growth.     

Aluminum tolerance,mineral nutrition,and growth of ‘Helleri’ holly in solution culture

‘Helleri’ holly (Ilex crenata Thunb. ‘Helleri') plants were grown in solution culture at aluminum (Al) concentrations of 0, 6, 12, 24, and 48 mg.L^‐1 for 116 days. Aluminum did not affect root or crown index, stem length growth, plant dry weight, or leaf area. Aluminum treatments significantly increased Al uptake and reduced nutrient uptake of magnesium (Mg), calcium (Ca), zinc (Zn), and copper (Cu) on some sampling dates. Iron (Fe) and manganese (Mn) uptake decreased on most sampling dates but increased on some with Al treatments. Potassium (K), phosphorus (P), and boron (B) uptake were significantly affected by Al, decreasing and increasing at different sampling dates. Although plants preferentially took up ammonium‐nitrogen (NH₄ ⁺‐N) in all treatments (including 0 Al controls), neither NH₄ ⁺‐N nor nitrate‐nitrogen (NO₃ ‐N) uptake were affected by Al. Tissue concentrations of P, K, B, Zn, and Al increased with Al treatment; whereas tissue Ca, Mg, and Cu concentrations decreased with increasing Al. Iron and Mn tissue concentrations exhibited increases and decreases in different tissues. Results indicated that ‘Helleri’ holly was tolerant of high concentrations of Al.     

Solubility control of KCl extractable aluminum in soils with variable charge

Abstract

Solubility and kinetic data indicated that concentrations of aluminum (Al) extracted with 1 M KCl are determined by the solubility of a precipitated A1(OH)₃ phase in soils dominated by variable charge minerals. Kinetic studies examining the release of Al on non‐treated and KCl treated residues indicated the precipitation of an acid‐labile Al phase during the extraction procedure. The log ion activity products estimated for the KCl extracts ranged between 8.1–8.6 for the reaction Al(OH)₃ + 3H⁺ < = > Al³⁺+ 3H₂O, which was similar to the solubility product of several Al(OH)₃phases. The mechanism proposed for Al precipitation indicated that Al released by exchange with added K⁺ hydrolyzed and released H⁺ that was readily adsorbed on surfaces of variable charge minerals. The increased ionic strength of the extracting solution further increased the amount of H⁺adsorbed to the variable charge surface and reduced the H⁺ concentration in the aqueous phase. Consumption of H⁺ induced further hydrolysis of Al, resulting in supersaturation of the extracting solution and formation of polynuclear hydroxy Al species. It was concluded that the 1 M KCl extraction does not quantitatively extract salt exchangeable Al from variable‐charge soils.     

Changes of H+ pumps of tonoplast vesicle from wheat roots in vivo and in vitro under aluminum treatment and effect of calcium

Wheat seedlings were treated with 0.1 mM Al for 7 days. Tonoplast vesicles were isolated from apical segments of roots and activities of H⁺‐ATPase and H⁺‐PPase measured. Compared with the control, 0.1 mM Al increased H⁺‐ATPase activity and decreased H⁺‐PPase activity. High external supply of calcium (Ca) (5 mM) diminished the extent of the stimulation of H⁺‐ATPase activity and alleviated the reduction of H⁺‐PPase activity under Al treatment. However, 0.1 mM Al treatment in vitro resulted in the inhibition of H⁺‐ATPase activity with the decrease of Vmax and Km for ATP. In vitro treatment with 0.1 mM Al also decreased the H⁺‐PPase activity, but increased the Km for PPi.     

Simulated effects of acid deposition on podzolic soils: Consequences of process and parameter aggregation

Solution cation concentrations and base cation leaching were simulated for a homogenous soil block and a soil showing five horizons of a podzolic forest soil. The dynamic model ACIDIC simulated water flow, nutrient uptake for tree growth, and cation exchange between H⁺, Al³⁺, Ca²⁺, Mg²⁺ and K⁺ in forest soil. In the multi-layer simulations exchangeable base cation concentrations changed most in the O horizon. The subsoil had a decisive effect on the pH of the runoff and base cation leaching from the soil. The one-layer model underestimated Ca and Mg leaching and overestimated H⁺ and Al concentrations in the runoff. In the eluvial and the top of illuvial horizon the solution Al / (Ca + Mg) ratio exceeded that in one-layer structure more than 10-fold. Cases with the horizon-specific cation exchange coefficient values and mean coefficient values for all layers showed only minor differences in Al / (Ca + Mg) ratio. The vertical variation in the soil chemical properties should be accounted for even if some details of processes and parameters were unavailable.     

Calcium alleviation of hydrogen and aluminum inhibition of soybean root extension from limed soil into acid subsurface solutions

Alleviation by calcium (Ca) of inhibition of soybean [Glycine max (L.) Merr. cv. ‘Ransom'] root elongation by hydrogen (H) and aluminum (Al) was evaluated in a vertical split‐root system. Roots extending from a limed and fertilized soil compartment grew for 12 days into a subsurface compartment containing nutrient solution with treatments consisting of factorial combinations of either pH (4.0, 4.6, and 5.5) and Ca (0.2, 2.0, 10, and 20 mM), Al (7.5, 15, and 30 μM) and Ca (2.0,10, and 20 mM) at pH 4.6, or Ca (2, 7, and 12 mM) levels and counter ions (SO₄ and Cl) at pH 4.6 and 15 μM Al. Length of tap roots and their laterals increased with solution Ca concentration and pH value, but decreased with increasing Al level. Length of both tap and lateral roots were greater when Ca was supplied as CaSO₄ than as CaCl₂, but increasing Ca concentration from 2 to 12 mM had a greater effect on alleviating Al toxicity than Ca source. In the absence of Al, relative root length (RRL) of tap and lateral roots among pH and Ca treatments was related to the Ca:H molar activity ratio of solutions (R²≥0.82). Tap and lateral RRL among solutions with variable concentrations of Al and Ca at pH 4.6 were related to both the sum of the predicted activities of monomeric Al (R²≥0.92) and a log‐transformed and valence‐weighted balance between activities of Ca and selected monomeric Al species (R²≥0.95). In solutions with 15 μM Al at pH 4.6, response of tap and lateral RRL to variable concentrations of CaSO₄ and CaCl₂ were related to predicted molar activity ratios of both Ca:Al³⁺ (R²≥0.89) and Ca:3 monomeric Al (R²≥0.90), provided that AISO₄ and AI(SO₄)₂ species were excluded from the latter index. In all experiments H and Al inhibited length of lateral roots more than tap roots, and a greater Ca:H or Ca:Al concentration ratio was required in solutions to achieve similar RRL values as tap roots.     

Salts and acids as determinants of the solution composition of acidic forest soils — anion effects

The study aimed at evaluating whether salt-induced mobilization of acidity may be modified by the type of anion. For this purpose, the effects of different neutral salts on the solution composition of acid soils were investigated. The results were compared with those of the addition of acids. Two topsoil (E and A) and two subsoil horizons (Bs and Bw) were treated with NaCl, Na₂SO₄, MgCl₂, MgSO₄, HCl, and H₂SO₄ at concentrations ranging from 0 to 10 mmol dm^?3. With increasing inputs of Cl^? the pH of the equilibrium soil solution dropped, the concentrations of Al and Ca increased, and the molar Ca/(Al³⁺ + AlOH²⁺ + Al(OH)₂⁺) ratios decreased. These effects were the least pronounced when NaCl was added and the most at the HCl treatments. According to the release of acidity, the topsoils were more sensitive for salt-induced soil solution acidification whereas on base of the molar Ca/(Al³⁺ + AlOH²⁺ + Al(OH)₂⁺) ratios, the salt effect seems to be more important for the subsoils. Addition of S0₄^2? salts and H₂SO₄ induced higher pH and lower Al concentrations than the corresponding Cl^? treatments due to the SO₄^2? sorption, especially in the subsoils. The Ca/(Al³⁺ + AlOH²⁺ + Al(OH)₂⁺) ratios were higher than those of the corresponding Cl^? treatments. In subsoils even after H₂SO₄ additions these ratios were not higher than those of the NaCl treatments. The results indicate (I) that speculation about the effects of episodic salt concentrations enhancement on soil solution acidification not only need to consider the ionic strength and the cation type but also the anion type, (II) that salt-induced soil solution composition may be more crucial in subsoils than in topsoils, and (III) that in acid soils ongoing input of HNO₃ due to the precipitation load may induce an even more acidic soil solution than the inputs of H₂SO₄ of the last decade.     

Soybean root growth in relation to ionic composition in magnesium-amended acid subsoils: Implications on root citrate ameliorating aluminum constraints

Abstract

Hydroponic studies with soybean (Glycine max [L.] Merr.) have shown that µmol L^?1 additions of Mg²⁺ were as effective in ameliorating Al rhizotoxicity as additions of Ca²⁺in the mmol L^?1 concentration range. The objectives of this study were to assess the ameliorative effects of Mg on soybean root growth in acidic subsoils and to relate the soil solution ionic compositions to soybean root growth. Roots of soybean cultivar Plant Introduction 416937 extending from a limed surface soil compartment grew for 28 days into a subsurface compartment containing acid subsoils from the Cecil (oxidic and kaolinitic), Creedmoor (montmorillonitic) and Norfolk (kaolinitic) series. The three Mg treatments consisted of native equilibrium soil solution concentrations in each soil (50 or 100 µmol L^?1) and MgCl₂ additions to achieve 150 and 300 µmol L^?1 Mg (Mg150 and Mg300, respectively) in the soil solutions. Root elongations into Mg-treated subsoils were compared with a CaCO₃ treatment limed to achieve a soil pH value of 6. Subsoil root growth responses to the Mg treatments were less than for the lime treatments. Root length relative to the limed treatments for all subsoils (RRL) was poorly related to the activity of the soil solution Al species (Al³⁺ and Al-hydroxyl species) and Mg²⁺. However, the RRL values were more closely related to the parameters associated with soil solution Ca activity, including (Ca²⁺), (Al³⁺)/(Ca²⁺) and (Al³⁺)/([Ca²⁺] + [Mg²⁺]), suggesting that Ca could be a primary factor ameliorating Al and H⁺ rhizotoxicity in these subsoils. Increased tolerance to Al rhizotoxicity of soybean by micromolar Mg additions to hydroponic solutions, inducing citrate secretion from roots to externally complex toxic Al, may be less important in acid subsoils with low native Ca levels.     

High monosilicic acid supply rapidly increases Na accumulation in maize roots by decreasing external Ca2+ activity

Both calcium (Ca²⁺) and silicon (Si) improve plant performance under salt (NaCl) stress. Although these two mineral elements share numerous similarities, the information on how their extracellular interactions in the root apoplast affect uptake of sodium (Na⁺) is still lacking. Here, we investigated the effect of high Si supply in the bioavailable form of monosilicic acid (H₄SiO₄) on the activity of Ca²⁺ in the external root solution, and subsequent root uptake and compartmentation of Na in maize (Zea mays L.). In the short‐term experiments (6 h), 14‐d‐old maize plants were exposed to various concentrations of Ca²⁺ at three different pH‐values (6.5, 7.5, and 8.5) and two Si concentrations, i.e., low (1 mM) and high (4 mM) supply of H₄SiO₄. The activity of Ca²⁺ and Na⁺ in the external solution as well as the root concentrations of total and cell sap and BaCl₂‐exchangeble apoplastic fractions of both elements were analyzed. The pH of the nutrient solution affected neither the ion activities nor the root accumulation of both Ca²⁺ and Na⁺. At higher pH values (7.5 and 8.5) the interactions of Ca²⁺ and Si at high Si supply led to a decrease of Ca²⁺ activity and, hence, an increase of Na⁺ : Ca²⁺ activity ratio in the external root solution. Concomitantly, despite the elevated exchangeable apoplastic fraction of both Ca²⁺ and Na⁺, the total and cell sap concentrations were remarkably decreased for Ca²⁺ and increased for Na⁺ by the addition of 4 mM H₄SiO₄. This work demonstrates that at high Si supply extracellular Ca‐Si interactions leading to lowered activity of Ca²⁺ might rapidly compromise the ameliorative effect of Ca²⁺ on Na⁺ accumulation in roots. Practically, Si over‐fertilization of saline and, in particular, sodic soils may further promote the accumulation of Na⁺ in root tissues hours after Si application and, hence, increase a potential risk of Na⁺ toxicity.     

A Scanning Electron Microscopy Study of a Calcium‐Polygalacturonate Network in the Presence of Aluminum: A Model to Investigate Soil‐Root Interface Processes

Abstract

The objective of this study was to investigate the effects of aluminum (Al³⁺) on a calcium‐polygalacturonate (Ca‐PG) network used as a soil‐root interface model. Calcium‐PG networks were exposed to Al³⁺ solutions at different concentrations (100, 200, 400, and 800 µM) at pH 3.50. In the present study, the scanning electron microscopy technique was used to evaluate morphological variations induced by Al³⁺sorption. Results showed how aluminum (Al) sorption induces conformational changes of the Ca‐PG complex. The Ca‐PG complex shows a regular structure with a honeycomb‐like pattern. Interlacing fibrils form a porous system, which can easily allow sorption and/or passage of nutrients as well as toxic elements. As Al becomes the predominant reticulating ion, the pores decrease in size and lose their regular shape. The scanning electron micrographs have in fact shown that Al sorption damages the Ca‐PG complex, leading to its collapse; the Ca‐Al‐PG networks exhibit an irregular uneven structure.     

Short‐term response of the Ca cycle of a montane forest in Ecuador to low experimental CaCl2 additions

The tropical montane forests of the E Andean cordillera in Ecuador receive episodic Sahara‐dust inputs particularly increasing Ca deposition. We added CaCl₂ to isolate the effect of Ca deposition by Sahara dust to tropical montane forest from the simultaneously occurring pH effect. We examined components of the Ca cycle at four control plots and four plots with added Ca (2 × 5 kg ha^–1 Ca annually as CaCl₂) in a random arrangement. Between August 2007 and December 2009 (four applications of Ca), we determined Ca concentrations and fluxes in litter leachate, mineral soil solution (0.15 and 0.30 m depths), throughfall, and fine litterfall and Al concentrations and speciation in soil solutions. After 1 y of Ca addition, we assessed fine‐root biomass, leaf area, and tree growth. Only < 3% of the applied Ca leached below the acid organic layer (pH 3.5–4.8). The added CaCl₂ did not change electrical conductivity in the root zone after 2 y. In the second year of fertilization, Ca retention in the canopy of the Ca treatment tended to decrease relative to the control. After 2 y, 21% of the applied Ca was recycled to soil with throughfall and litterfall. One year after the first Ca addition, fine‐root biomass had decreased significantly. Decreasing fine‐root biomass might be attributed to a direct or an indirect beneficial effect of Ca on the soil decomposer community. Because of almost complete association of Al with dissolved organic matter and high free Ca²⁺ : Al³⁺ activity ratios in solution of all plots, Al toxicity was unlikely. We conclude that the added Ca was retained in the system and had beneficial effects on some plants.     

Influence of surface‐applied poultry manure on topsoil and subsoil acidity and salinity: A leaching column study

It has been suggested that surface applications of animal manure can ameliorate both top and subsoil acidity. For that reason, the effects of surface incorporation (0–5 cm) of a high rate of poultry manure to an acid soil on pH and exchangeable and soluble Al in the top‐ and subsoil were investigated in a leaching column study. During the experimental period of 108 d, columns received a total of 875 mm with leaching events occurring after 9, 37, 58, and 86 d. Incorporation of poultry manure into the surface 5 cm resulted in a large rise in pH measured in both 1M KCl and in soil solution. This liming effect was attributed primarily to the substantial CaCO₃ content of poultry manure. In the 15–45 cm layer, pH_KCl was not significantly different between poultry manure and control treatments but surprisingly, soil‐solution pH was substantially less in the poultry‐manure treatments. Exchangeable Al was significantly less in poultry manure than in control in all soil layers although the effect was most marked in the 0–5 cm layer. However, although concentrations and activities of monomeric Al (Al_mono), and the proportion of total Al present as Al_mono, in soil solution were lower under poultry manure than in control in the 0–5 cm layer, the reverse was, in fact, the case in lower soil horizons. This was attributed to a soluble‐salt effect, originating from the large cation content of poultry manure, displacing exchangeable Al³⁺ and H⁺ back into soil solution. Indeed, electrical conductivity and concentrations of Ca²⁺, Mg²⁺_, K⁺, and Na⁺ in soil solution were substantially higher in the poultry‐manure than in the control treatments at all soil depths. Poultry‐manure applications also resulted in substantial increases in the concentrations of Ca²⁺_, Mg²⁺_, K⁺, Na⁺, Al_mono, NH , and NO in leachates, particularly at the fourth leaching. It was concluded that although surface application of poultry manure can raise soil pH in the topsoil, increases in soluble‐salt concentrations in soil solution can greatly modify this effect in the subsoil.     

Toxicity factors in acidic forest soils: attempts to evaluate separately the toxic effects of excessive Al3+ and H+ and insufficient Ca2+ and Mg2+ upon root elongation

The acidic soils of temperate forests and some pastures on former forest land characteristically contain large amounts of aluminium and hydrogen ions and small amounts of calcium and magnesium ions. The relative importance of these potential toxicity factors are assessed from published data from soils collected in the United States and Europe. Activities of ions in the soil solutions and at the surfaces of root‐cell plasma membranes were computed with electrostatic models. Activities of Al³⁺ in soil solutions ({Al³⁺}) peaked at pH 4.1, and Al³⁺ activities at the surface of the plasma membrane ({Al³⁺}₀) achieved a broad maximum between pH 4.1 and 4.8; thus, Al³⁺ intoxication is likely to be more severe in soils at pH 4.1 than in more acidic ones. Intoxication (assessed by root elongation) correlated somewhat ambiguously with ion activities, but Al³⁺‐ and H⁺‐induced depletion of Ca²⁺ and Mg²⁺, or both, from the cell surface appears to play a role in toxicity. By contrast, experiments in solution culture, where intercorrelation among {Al³⁺}, {H⁺}, and {Ca²⁺} could be avoided, clearly demonstrated the following extrinsic and intrinsic effects. 1 The ions Al³⁺ and H⁺ are intrinsic toxicants. 2 They are also extrinsic toxicants because of the electrostatic displacement of Ca²⁺ from the surface of the plasma membrane. 3 They are extrinsic ameliorants because each electrostatically displaces the other from the surface of the plasma membrane. 4 The ion Ca²⁺ is an extrinsic ameliorant because of the electrostatic displacement of Al³⁺ and H⁺ from the surface of the plasma membrane. 5 It is an intrinsic ameliorant of intrinsic H⁺ toxicity, but not intrinsic Al³⁺ toxicity. 6 It meets an intrinsic requirement. 7 The ion Mg²⁺ resembles Ca²⁺ in item 4 but not items 5 and 6 in short‐term cultures. In acidic soils, Al³⁺ may prevent H⁺ from becoming an intrinsic toxicant (item 3) and may induce an insufficiency of Ca²⁺ and Mg²⁺ (item 2). These findings have implications for the mechanisms by which woodland plants tolerate very acidic soils.     

Influence of Different Forms of Iron and Aluminum on the Nature of Soil Acidity of Some Inceptisols,Alfisols, and Ultisols

Abstract

Fifteen acid soils of Mizoram representing Ultisols and Inceptisols, and Madhya Pradesh, representing Alfisols, were studied to characterize the nature of acidity in relation to different forms of iron (Fe) and aluminum (Al). The mean contents of Fe and Al were extracted by various extracting reagents and were found to be in descending order as followed: dithionite>oxalate>pyrophosphate>ammonium acetate>KCl. The electrostatically bonded EB‐H⁺ and EB‐Al³⁺ acidity comprised 28.3 and 71.7% of exchangeable acidity whereas EB‐H⁺, EB‐Al³⁺, exchangeable, and pH‐dependent acidities comprised 9.8, 30.7, 40.5, and 59.5% of total potential acidity. All forms, of acidity showed significant correlation with pH_k and organic carbon. Among the different forms, Fe and Al caused most of the variations in different forms of soil acidity but the effect of different forms of Al are more active and directly participate in the formation of EB‐H⁺, EB‐Al³⁺, and exchangeable acidity.     

Effects of reduced atmospheric deposition on soil solution chemistry and elemental contents of spruce needles in NE—Bavaria,Germany

The decrease in anthropogenic deposition, namely SO₄^2— and SO₂, in European forest ecosystems during the last 20 years has raised questions concerning the recovery of forest ecosystems. The aim of this study was to evaluate if the long term data of element concentrations at the Fichtelgebirge (NE‐Bavaria, Germany) monitoring site indicates a relationship between the nutrient content of needles and the state of soil solution acidity. The soil at the site is very acidic and has relatively small pools of exchangeable Ca and Mg. The trees show medium to severe nutrient deficiency symptoms such as needle loss and needle yellowing. The Ca and Mg concentrations in throughfall decreased significantly during the last 12 years parallel to the significant decline in the throughfall of H⁺ and SO₄^2— concentrations. Soil solution concentrations of SO₄^2—, Ca and Mg generally decreased while the pH value remained stable. Aluminum concentrations decreased slightly, but only at a depth of 90 cm. Simultaneously a decrease in the molar Ca/Al and Mg/Al ratios in the soil solution was observed. Ca and Mg contents in the spruce needles decreased, emphasizing the relevance of soil solution changes for tree nutrition. The reasons for the delay in ecosystem recovery are due to a combination of the following two factors: (1) the continued high concentrations of NO₃^— and SO₄^2— in the soil solution leading to high Al concentrations and low pH values and, (2) the decreased rates of Ca and Mg deposition cause a correlated decrease in the concentration of Ca and Mg in the soil solution, since little Ca and Mg is present in the soil's exchangeable cation pools. It is our conclusion that detrimental soil conditions with respect to Mg and Ca nutrition as well as to Al stress are not easily reversed by the decreasing deposition of H⁺ and SO₄^2—. Thus, forest management is still confronted with the necessity of frequent liming to counteract the nutrient depletion in soils and subsequent nutrient deficiencies in trees.     

Effect of varying solution calcium or magnesium concentrations in the presence or absence of aluminum on yield and plant calcium or magnesium concentrations in wheat

The effect of varying solution calcium (Ca) and magnesium (Mg) concentrations in the absence or presence of 10 μM aluminum (Al) was investigated in several experiments using a low ionic strength (2.7 × 10^‐3 M) solution culture technique. Aluminium‐tolerant and Al‐sensitive lines of wheat (Triticum aestivum L.) were grown. In the absence of Al, top yields decreased when solution Ca concentrations were <50 μM or plant Ca concentrations were <2.0 mg/g. Top and root yields decreased when solution Mg concentrations were <50 μM or plant Mg concentrations were <1.5 mg/g. There were no differences between the lines in solution or plant concentrations at which yield declined. Increasing solution Ca concentrations decreased plant Mg concentrations in the tops (competitive ion effect) but increased plant Mg concentrations in the roots of wheat. This suggests that Ca is competing with Mg when Mg is transported from the roots. Increasing solution Mg concentrations decreased plant Ca concentrations in the tops and the roots (competitive ion effect). In the roots, increasing solution Mg concentrations decreased plant Ca concentrations at a lower solution Ca concentration in the Al‐sensitive line than the Al‐tolerant line. In the presence of Al, increasing solution Ca and Mg concentrations increased yield (Ca and Mg ameliorating Al toxicity). Yield increased until the sum of the solution concentrations of the divalent cations (Ca+Mg) was 2,000 μM for the Al‐tolerant line or 4,000 μM for the Al‐sensitive line. The exception was that yield decreased when solution Mg concentrations were > 1,500 μM and the solution Ca concentration was 100 μM (Mg exacerbating Al toxicity). The ameliorative effects of solution Ca or Mg on Al tolerance were not related to plant Ca or Mg concentrations per se.     

Increases in pH and soluble salts influence the effect that additions of organic residues have on concentrations of exchangeable and soil solution aluminium

It has been suggested that additions of organic residues to acid soils can ameliorate Al toxicity. For this reason the effects of additions of four organic residues to an acid soil on pH and exchangeable and soil solution Al were investigated. The residues were grass, household compost, filter cake (a waste product from sugar mills) and poultry manure, and they were added at rates equivalent to 10 and 20 t ha^?1. Additions of residues increased soil pH measured in KCl (pH_(KCl)) and decreased exchangeable Al³⁺ in the order poultry manure > filter cake > household compost > grass. The mechanism responsible for the increase in pH differed for the different residues. Poultry manure treatment resulted in lower soil pH measured in water (pH_(water)) and larger concentrations of total (Al_T) and monomeric (Al_mono) Al in soil solution than did filter cake. This was attributed to a soluble salt effect, originating from the large cation content of poultry manure, displacing exchangeable Al³⁺ and H⁺ back into soil solution. The considerably larger concentrations of soluble C in soil solution originating from the poultry manure may also have maintained greater concentrations of Al in soluble complexed form. There was a significant negative correlation (r = ?0.94) between pH_(KCl) and exchangeable Al. Concentrations of Al_T and Al_mono in soil solution were not closely related with pH or exchangeable Al. The results suggest that although additions of organic residues can increase soil pH and decrease Al solubility, increases in soluble salt and soluble C concentrations in soil solution can substantially modify these effects.     

Patterns of soil solution composition in acid forest soils: Differences between undisturbed and bulk samples

In acid forest soils pores draining under conditions of low water tension contain a solution usually differing in composition from the equilibrium solution of the bulk soil. For tree nutrition, the former was described as comparable to the bulk soil solution or less favourable. Our recent work on solution quality contradicted the universal validity of these findings. It turned out that the methods usually recommended tend to consistently reproduce the pattern of more advanced acidification of coarse pores. We applied our combination of methods to a range of soils to find out whether our previous finding of more favourable solution conditions in undisturbed soil samples as compared to the bulk soil is common or rather an exception. Undisturbed cores and bulk soil were sampled from four forested sites differing in pH and base saturation. We used an iterative procedure to adjust the equilibrium soil solution of disturbed soil and a water extraction with a soil:solution ratio of 1:2. To extract solution from undisturbed soil cores only small volumes of water were added, and we distinguished analytically between total (by ICP) and ionic (by CE) concentrations of cations. We calculated Ca/Al molar ratios and the fraction of (Al³⁺ + H⁺) on total cationic charge in solution as criteria for solution quality. Whereas with total concentrations, there always appeared to be less favourable conditions in the soil pore solution, free ionic concentrations allowed a differentiation between the soils. In view of plant nutrition, the quality of the soil solution from undisturbed samples was better, equal to, or worse than that of the bulk soil, suggesting that forest sites represent a continuum with respect to these chemical gradients. Even in soils where gradients are not observed it is not necessary to assume an equilibrium achieved by internal processes to exist between coarse pores and the bulk soil.     

Interactions of aluminium and fulvic acid in moderately acid solutions: stoichiometry of the H+/Al3+ exchange

Complexation with organic matter controls the activity of dissolved Al³⁺ in many soils. The buffering intensity of these soils is largely dependent on the H⁺/Al³⁺ exchange ratio, i.e. the number of protons consumed by the solid phase when one Al³⁺ is released. Here, the H⁺/Al³⁺ exchange ratio was determined from batch titrations using solutions of fulvic acid (FA) as a model for soil organic matter. Aluminium was added, from 1.04 to 6.29 mmol Al per g FA, which is within the range of humus‐bound Al found in the upper B horizon of podzolized soils. Furthermore, pH was varied with NaOH to give values between 3.5 and 5.0. The H⁺/Al³⁺ exchange ratio ranged between 1.49 and 2.23 with a mean of 1.94. It correlated positively with pH and the total concentration of Al present. Theoretically, this can be explained with a partial hydrolysis of bound Al. The slope of logAl (log₁₀ of Al³⁺ activity) against pH generally underestimated the actual exchange ratio, which can partly be attributed to the systems being diluted (100 mg FA l^?1). However, where 4 mmol Al or more had been added per g FA, the logAl slope gradually approached ?3 between pH 4.5 and 5.0. This might be the result of a shift from Al³⁺ activity control by humus complexation to control by Al(OH)₃(s).