Subsurface Acidity Research Articles

Nitrate leaching stimulates subsurface root growth of wheat and increases rhizosphere alkalisation in a highly acidic soil

Subsurface acidity is a major factor limiting crop yield in some agricultural soils. The surface application of lime has limited effect on the subsurface acidity due to the slow downward movement, while deep incorporation of lime is costly. This paper tested the concept of biologically ameliorating subsurface acidity in a highly acidic soil through the net uptake of anions by plant roots. Nitrogen was supplied to the top soil (0–10 cm) as Ca(NO3)2 at rates equivalent to 30–240 kg N ha−1. Four water levels were imposed (40, 60, 80 and 100% of field capacity). Aluminium-tolerant wheat was grown for 58 days. The high N and high water treatments stimulated root growth below 15 cm, which in turn increased N capture, resulting in a greater excess anion uptake over cations and thus alkalisation of subsurface soil layers. This study suggests that it is feasible to exploit the process of nitrate uptake by an aluminium-tolerant wheat genotype to increase pH in acidic subsoil.

Resposta da caramboleira à calagem

A correção da acidez superficial e subsuperficial do solo, em especial para culturas perenes, é fundamental para garantir o rápido estabelecimento do pomar e a precocidade da produção de frutos. O presente trabalho objetivou avaliar os efeitos da calagem na implantação e formação de um pomar de caramboleiras (Averrhoa carambola) e seus reflexos no solo, na planta, na produção de frutos, bem como determinar a duração do efeito residual da aplicação inicial do calcário. O experimento foi realizado na Estação Experimental de Citricultura de Bebedouro, São Paulo, em um Latossolo Vermelho distrófico (V inicial = 26% na camada de 0-20cm), no período de agosto/1999 a julho/2006. As doses de calcário empregadas foram: 0; 1,85; 3,71; 5,56 e 7,41 t ha-1. Durante 78 meses após a aplicação do corretivo, foram realizadas análises químicas de solo, bem como a avaliação do estado nutricional e da produção de frutos durante cinco safras agrícolas. A calagem proporcionou melhoria das características químicas do solo, elevando o pH, Ca, Mg, V% e diminuindo o H+Al, até 60 cm de profundidade. Os teores foliares de Ca e Mg aumentaram, correlacionando-se positivamente com a produção de frutos. A maior produção acumulada de carambolas esteve associada à saturação por bases de 40 e 53% na linha e na entrelinha do pomar de caramboleiras, respectivamente.

Mobilidade de íons em solo ácido com aplicação de calcário, ácido orgânico e material vegetal em superfície

Há controvérsias quanto à eficiência da correção da acidez da camada subsuperficial pela aplicação de calcário na presença de materiais vegetais de plantas de cobertura e faltam informações sobre os mecanismos envolvidos no processo. O objetivo deste estudo foi quantificar a contribuição dos materiais vegetais de plantas de cobertura, com ênfase nos seus conteúdos de ácidos orgânicos de baixa massa molar e nutrientes solúveis, sobre a mobilização, no perfil do solo, dos produtos da dissolução do calcário aplicado em superfície. Foram desenvolvidos dois experimentos em laboratório, usando colunas de PVC de 30 cm de altura com amostras deformadas de um Latossolo Vermelho textura muito argilosa. Os tratamentos constituíram-se da aplicação isolada ou conjunta de calcário para atingir 80 % de saturação por bases (6,1 t ha-1) e de materiais vegetais (20 t ha-1) de nabo forrageiro e aveia-preta, bem como de soluções de ácidos orgânicos e sais neutros aproximadamente equivalentes ao conteúdo dessas espécies. A calagem isolada ou em associação com os materiais vegetais promoveu a redução da acidez somente na camada superficial (0-8 cm). A baixa taxa de recuperação dos ácidos orgânicos adicionados (< 7,2 %) indica que essas substâncias presentes na massa seca do nabo forrageiro foram rapidamente degradadas por microrganismos ou adsorvidas aos colóides do solo, tendo pouca influência na mobilização de cátions. Parte substancial do efeito sobre a mobilização de íons pela aplicação de material vegetal de nabo forrageiro se deveu à composição química do material, em vista da alta solubilidade em água dos cátions (65 a 71 %) e ânions (84 %). Além disso, a solução de sais foi o tratamento mais eficiente para deslocar o Al do solo para as soluções percoladas. A adição de materiais vegetais teve pouca influência na mobilização no perfil do solo dos produtos da dissolução do calcário aplicado em superfície.

What is limiting canola yield in southern New South Wales? A diagnosis of causal factors

During the 1990s there was considerable evidence from grower surveys and other regional statistics to suggest that canola (Brassica napus) yields were declining in the medium and high rainfall areas of southern New South Wales (NSW). A paddock survey was conducted across three regions of southern NSW over three consecutive seasons (2003–05) to explore the importance of disease and other possible causes of low productivity. Under-performing paddocks were identified by comparing measured paddock yields with simulated potential yields. The causes of the resultant yield gaps were identified by analysis of the survey results with growers and consultants and from insights provided by the simulation analysis. Seasonal water supply and emergence date accounted for around 70% of the variation in yield across the survey, although no dependence on these variables was evident in 2005 as a result of high spring rainfall. A majority (95) of the 132 paddocks surveyed yielded to within 20% of the simulated potential yield. Disease, while significant in some paddocks, was limited by the dry seasons, there was no evidence for widespread micronutrient deficiencies and most crops were adequately supplied with nitrogen. There was no single cause of under-performance and the impact of those causes varied across regions and seasons. Subsoil constraints (seven paddocks) and late season water stress (six paddocks) were the most common factors associated with under-performance, while five paddocks had inexplicable yield gaps. Restrictions to taproot growth were widespread, especially in the southern region where around 60% of paddocks had significantly restricted taproots in all seasons (>3 on a 0–5 scale). Survey paddocks in which significant root restriction was found were between 10 and 50% below potential yield. Subsequent soil profile analysis identified a range of possible subsoil constraints including high soil strength, sodic or saline subsoils or subsurface acidity and further research is warranted to determine their impact on canola productivity in the region.

Phalaris persistence under rotational grazing on a highly acidic soil on the south-west slopes of New South Wales

Phalaris (Phalaris aquatica L.)-based pastures were established with and without lime in 1992 as a part of a long-term pasture–crop rotation experiment (Managing Acid Soils Through Efficient Rotations). Pre- and post-grazing pasture dry matter, phalaris basal cover and proportion of phalaris in sward were measured since 1992. In general, phalaris persisted well and its productivity was high on the highly acidic soil studied in the current experiment, and this was improved on the limed treatment. After establishment in 1992, the average proportion of phalaris in spring 2001 was 32.1% in the limed treatment and 15.6% in the unlimed treatment. Basal cover at the end of summer 2002 was 4.5% and 2.0% for the limed and unlimed treatments, respectively. The results from the current experiment showed that subsurface acidity (low pHCa and high exchangeable aluminium percentage in the 10–30 cm soil depth) had significant impacts on phalaris persistence. It is concluded that subsurface pH was one of the major constraints for the persistence of phalaris. The long-term management of soil acidity should aim to eliminate the exchangeable aluminium from the soil profile by maintaining a high pHCa (5.5 or above) in the 0–10 cm soil depth. Rainfall during growing season had no direct effect on phalaris persistence. Nevertheless, feed scarcity in dry years due to moisture stress often exacerbated grazing pressure on phalaris, which may affect the phalaris persistence indirectly. It is the grazing management in autumn and summer that had significant effects on phalaris persistence. It is suggested that rotational grazing plus strategic rest if possible in autumn could prolong the life of phalaris-based pastures. Repeated heavy grazing should be avoided during summer, particularly after light to moderate summer rainfall events have stimulated sprouting.

Modelling yield losses of aluminium-resistant and aluminium-sensitive wheat due to subsurface soil acidity: effects of rainfall, liming and nitrogen application

Subsurface soil acidity reduces the growth of roots, which can potentially decrease crop yields. However, the magnitude of these yield reductions is dependent on interactions between factors such as the depth and severity of subsurface soil acidity, plant resistance to acidity, and water and nutrient availability. The Agricultural Production Systems Simulator (APSIM) was used to examine effects of these factors and their interactions on wheat yields in the Mediterranean climatic regions of Western Australia. The model was linked to historical meteorological data of the region (up to 90 different seasons), and was run for three locations representing low, medium and high rainfall zones and three constant but contrasting soil acidity profiles in a deep sandy soil with two wheat cultivars differing in aluminium (Al) resistance. The simulated results showed inherently high variability between seasons in grain yield, rooting depth and nitrogen leaching. Subsurface soil acidity could decrease average grain yields by up to 60%, particularly in soil profiles with acidity in deep layers. The adverse effects of acidity on wheat yields were greater in the high than the low rainfall zone. Amelioration of acidity by 75% in the entire profile or in the top 20-cm layer improved the yield of the Al-sensitive wheat cultivar. Growing Al-resistant wheat partially eliminated the negative effects of acidity on yields in soils with severe subsurface acidity and almost fully eliminated these negative effects in soils with moderate subsurface acidity. The yield benefits arising from growing Al-resistant wheat were greater than those from ameliorating acidity in the 0–20 cm layer by liming. Increasing nitrogen input increased yields of both Al-sensitive and Al-resistant wheat grown in acid soils in all the rainfall zones, but the yield increments were much greater in the high than the low rainfall zones. Applications of nitrogen fertilisers mitigate the effect of acidity on yields of Al-sensitive wheat in soils with shallow (10–40 cm) subsurface acidity. Furthermore, the improved yield by growing Al-resistant wheat and amelioration of acidity was correlated with increased rooting depth and was associated with decreased nitrogen leaching. Possible agronomic management options to combat the subsurface acidity problem are discussed.

Long-term benefits of limestone applications to soil properties and to cereal crop yields in southern and central New South Wales

The cost of buying, carting and spreading limestone, relative to the value of broadacre crops, makes investment in liming a questionable proposition for many farmers. The longer the beneficial effects of limestone persist, however, the more the investment in liming becomes economically favourable. We re-established previous lime trials with the aim of measuring the long-term effects of limestone on surface acidity (pH run-down), subsurface acidity (lime movement) and grain yield. The study made use of experiments where there was adequate early data on soil chemical properties and cereal yields. We report data from 6 trials located at 4 sites between Dubbo and Albury in New South Wales. The rate of surface soil (0–10 cm) pH decline after liming was proportional to the pH attained 1 year after liming. That is, the higher the pH achieved, the more rapid the rate of subsequent pH decline. Since yields (product removal) and nitrification (also acid producing) may both vary with pH, the post-liming pH acts as a surrogate for the productivity and acid-generating rate of the soil–plant system. The apparent lime loss rate of the surface soils ranged from the equivalent of nearly 500 kg limestone/ha.year at pH approaching 7, to almost zero at pH approaching 4. At commercial application rates of 2–2.5 t/ha, the movement of alkali below the layer of application was restricted. However, significant calcium (Ca) movement sometimes occurred to below 20 cm depth. At rates of limestone application exceeding the typical commercial rate of 2.5 t/ha, or at surface pH greater than about 5.5, alkali and Ca movement into acidic subsurface soil was clearly observed. It is therefore technically feasible to ameliorate subsurface soil acidity by applying heavy rates of limestone to the soil surface. However, the cost and risks of this option should be weighed against the use of acid-tolerant cultivars in combination with more moderate limestone rates worked into the surface soil.There was a positive residual benefit of limestone on cereal grain yield (either barley, wheat, triticale, or oats) at all sites in both the 1992 and 1993 seasons. While acid-tolerant cultivars were less lime responsive than acid-sensitive ones, the best yields were generally obtained using a combination of liming and acid-tolerant cultivars.The long-term residual benefits of limestone were shown to extend for beyond 8–12 years and indicate that liming should be profitable in the long term.

Limestone, Gypsum, and Magnesium Oxide Influence Restoration of an Abandoned Appalachian Pasture

When restoring abandoned pastures on acidic hill‐land soils to productivity, it is important to bring soil Ca and Mg to adequate levels. Gypsum is a readily available Ca amendment that is sufficiently soluble to move rapidly into the soil when surface‐applied. Gypsum has been shown to reduce detrimental effects of subsurface acidity in soils of the southeastern USA. A 4‐yr experiment was initiated to measure effects of surface gypsum application on forage production and to evaluate Mg‐containing amendments to avoid gypsum‐induced Mg deficiency. The study site was a southern West Virginia Gilpin silt loam (fine‐loamy, mixed, mesic, Typic Hapludult) where abandoned hill‐land pasture was being restored to productivity. Treatments included 0, 1000, 8000, 16 000, and 32 000 kg/ha flue gas desulfurization coal combustion by‐product gypsum (gypsum) together with dolomitic limestone and five additional treatments to evaluate sources of supplemental Mg. Application of 16000 kg/ha gypsum together with limestone increased forage yields of mixed orchardgrass (Dactylis glomerataL.) and tall fescue (Festuca arundinaceaSchreb) pasture during establishment by 42% and production by 11% compared with limestone alone. About 8% of the mean 790 kg/ha yield increase could be attributed to acidity‐neutralizing effects of alkaline constituents in the gypsum by‐product. Plants in higher gypsum treatments had higher concentrations of K and P, but gypsum application decreased soil and plant Mg concentrations. This indicated that gypsum should not be applied on typical acid soils without supplemental Mg.

Aluminium-tolerant wheat uses more water and yields higher than aluminium-sensitive one on a sandy soil with subsurface acidity

Subsurface acidity with high levels of toxic aluminium (Al) is a major limiting factor in cereal production in many parts of the world. While surface liming is a common practice to ameliorate topsoil acidity, growing Al-tolerant crops may provide an additional strategy to combat the subsurface acidity problem. The present study compared the growth, water use and yield of two near-isogenic wheat genotypes differing only in Al tolerance in response to subsurface acidity in the field. The trial was conducted on a sandy soil in the low-rainfall region of Western Australia, and received 130 mm rainfall during the growing season (May–October 2000). The soil had pH 4.4 (0.01 M CaCl 2) and extractable Al 5.2 mg kg −1 in the 0.1–0.4 m layer. The trial consisted of seven irrigation treatments and two wheat genotypes (Al-tolerant ET8 and Al-sensitive ES8). The water treatments were natural rain, weekly, fortnightly and monthly irrigation of 0.3 or 0.6 of the pan evaporation. Genotype ET8 produced more shoot biomass than ES8 from 34 days after sowing (DAS). At maturity, ET8 produced 51% higher yield than ES8 under natural rain. Under irrigation, ET8 produced up to 26% higher yield than ES8 but the yield difference was greater in monthly irrigation treatments. While both genotypes had similar root length density in the topsoil, root length density in the 10–40 cm layer was 20–50% higher in ET8 than in ES8. Soil water was depleted faster under ET8 than under ES8 in soil layers between 10 and 110 cm. Under irrigation, the decrease in water content was not evident below 70 cm for ES8 and below 110 cm for ET8, indicating that ET8 roots grew deeper than those of ES8. The results suggest that the higher yield of Al-tolerant wheat than Al-sensitive wheat grown with subsurface acidity results from the greater root proliferation and hence water use in the subsurface layers, and that growing Al-tolerant wheat provides an additional strategy to liming to combat subsurface acidity.

Limestone, Gypsum, and Magnesium Oxide Influence Restoration of an Abandoned Appalachian Pasture

When restoring abandoned pastures on acidic hill-land soils to productivity, it is important to bring soil Ca and Mg to adequate levels. Gypsum is a readily available Ca amendment that is sufficiently soluble to move rapidly into the soil when surface-applied. Gypsum has been shown to reduce detrimental effects of subsurface acidity in soils of the southeastern USA. A 4-yr experiment was initiated to measure effects of surface gypsum application on forage production and to evaluate Mg-containing amendments to avoid gypsum-induced Mg deficiency. The study site was a southern West Virginia Gilpin silt loam (fine-loamy, mixed, mesic, Typic Hapludult) where abandoned hill-land pasture was being restored to productivity. Treatments included 0, 1000, 8000, 16 000, and 32 000 kg/ha flue gas desulfurization coal combustion by-product gypsum (gypsum) together with dolomitic limestone and five additional treatments to evaluate sources of supplemental Mg. Application of 16000 kg/ha gypsum together with limestone increased forage yields of mixed orchardgrass (Dactylis glomerata L.) and tall fescue (Festuca arundinacea Schreb) pasture during establishment by 42% and production by 11% compared with limestone alone. About 8% of the mean 790 kg/ha yield increase could be attributed to acidity-neutralizing effects of alkaline constituents in the gypsum by-product. Plants in higher gypsum treatments had higher concentrations of K and P, but gypsum application decreased soil and plant Mg concentrations. This indicated that gypsum should not be applied on typical acid soils without supplemental Mg.

Root Elongation of Soybean Genotypes in Response to Acidity Constraints in a Subsurface Solution Compartment

Aluminum‐tolerant germplasm is needed to overcome subsurface acidity constraints to root growth and plant access to water and nutrients. Root elongation of four soybean [Glycine max (L.) Merr.] genotypes exposed to varying concentrations of Al, H, and Ca were compared in two experiments using a vertically split root system. Roots extending from a limed surface soil compartment grew for 12 d into a subsurface compartment with nutrient solution treatments. In Exp. 1 root growth for cv. Ransom and Plant Introduction 416937 (PI) were compared in solutions with factorial combinations of pH (4.2, 5.2) and Al (0, 7.5, 15 μM) with Ca maintained at 10 mM In Exp. 2 soybean line N93‐S‐179 (N93), PI, and cultivars Ransom and Young were compared in solutions with factorial combinations of Ca (2 and 10 mM) and Al (7.5 and 15 μM) maintained at pH 4.6. Ransom and PI had similar responses in tap and lateral root elongation to solution pH and Al treatments in Exp. 1, but mean tap root length of Ransom in the subsurface compartment exceeded that of PI by 22%. Aluminum inhibited the length of lateral roots more than tap roots in both experiments. Molar activity ratios between Ca and Al3+ {Ca/Al3+} accounted for most of the differences in root elongation response among solution treatments in Exp. 2. A 50% reduction in relative length of tap roots for all genotypes occurred with a {Ca/Al3+} value of 891. Values of {Ca/Al3+} for 50% reductions in relative length of lateral roots differed among genotypes and were 1.6 to 3.5 times greater than for tap roots. On the basis of the {Ca/Al3+} indices for lateral root length, line N93 and Ransom exhibited greater tolerance to subsurface solution Al than PI and Young.

Amelioration of subsurface acidity in the south-west of Western Australia: downward movement and mass balance of surface-incorporated lime after 2-15 years

Twenty-one lime trial sites in the south-western agricultural region of Western Australia (WA) ranging in duration from 2 to 15 years were sampled in 1995 to a depth of 40 cm in 10-cm increments. In each case, samples were taken from both control plots and those with highest lime rates; additionally, samples were taken from plots with intermediate lime rates at 3 trial locations. Lime had been incorporated at rates of 2–12 t/ha within the top 0–10 cm layer at 20 of the sites, and at 15 t/ha within 0–20 cm at the remaining site. The following measurements were made: all samples, pH (1 : 5 0.01 M CaCl 2 ) and exchangeable cations (1 : 10 0.1 M BaCl 2 ); selected depths, undissolved carbonate; selected samples, total Ca (1 : 10 0.2 M HCl) and exchangeable Ca (1 : 10 0.1 M BaCl 2 buffered to pH 8.2). The pH of the unlimed subsurface was between 4.0 and 4.5 at 10–20 cm for 15 sites, at 20–30 cm for 7 sites, and at 30–40 cm for 2 sites. In addition, 3 of the unlimed sites had a pH =4.0 at all of these depths. Where lime had been applied, the pH was significantly (P < 0.05) higher than in the controls by at least 0.2 units for 13 sites at depths of 10 cm below the depth of lime incorporation. The same significantly elevated pH was found for 5 sites at depths of 20 cm and for 4 sites at depths of 30 cm. At 5 sites that had also been sampled 3 years previously, the subsurface pH had increased within 4–7 years of liming with 2.5–5 t/ha. Where added lime could be accounted for, the amount of lime added explained 65% of the variation in the residual lime effect at a depth of 10–40 cm. Over all of the wheatbelt trials this represented an average downward movement of lime to a maximum depth of 40 cm of at least 8.3 kg/ha.year.t CaCO 3 applied (range 0–22.5). Increased exchangeable Ca corresponded with a decrease in other base cations in only 2 trials. Decreases in exchangeable Al corresponded with increases in both pH and exchangeable Ca, but were less than the Ca increases. Precipitation of exchangeable Al represented an average of about one-sixth of the pH buffering capacity. Mass balance for Ca was achieved in 11 out of 20 cases in the wheatbelt, if Ca loss as nitrate was assumed to occur at rates of leaching published for WA. Lime recovery on the basis of pH increase was lower. The quantity of lime accounted for by Ca in the added and the residual undissolved lime, plus the increase in exchangeable Ca, ranged from <10% to >90%, with <50% in 9 of 19 cases at 15 trials. Non-exchangeable Ca in HCl extracts of a subset of surface samples was from 1% to 60% of Ca not accounted for. In 6 cases the Ca deficit exceeded plausible rates of acidification, and at 2 of these marked decreases of about 1 t/ha undissolved CaCO3 corresponded with episodes of high wind speed and observations of soil erosion. The implications of possible losses of lime by erosion are discussed.

Management of soil acidity in long-term pastures of south-eastern Australia: a review

Acidification of non arable soils under long-term pasture presents a major agricultural problem in the high rainfall areas (≥600 mm/year) of central and southern New South Wales and north-eastern Victoria. Some of these soils were already strongly acid to depth before agriculture. Evidence suggests that persistence of pasture species tolerant of acidic soils is being affected adversely on a number of severely acidic soils. Acidification processes are well understood but the capacity for grazing enterprises to pay for amendment by lime application is a major constraint in long-term pasture areas. In addition, soil acidification is likely to have substantial off-site effects on water quantity and quality and as a result, on dryland salinity. However, there is a paucity of scientific evidence to link soil acidity and dryland salinity in this way. Production from a grazing enterprise can be maintained by selecting plants for tolerance of acidity, surface application of lime or a combination of both. Responses by subterranean clover, lucerne and perennial grass (mainly phalaris and cocksfoot)-based pastures to incorporated lime are reported, but there is limited evidence of responses to surface applied lime. The movement of the lime effect into the soil from surface application is suggested as a major factor in controlling lime responses by plants. There is a need for more confidence in the benefits of topdressed lime in non arable soils before producers are likely to adopt the practice. High subsurface acidity in many soils is a major limitation to the range of species that can be grown. In the longer term, the use of lime may remove constraints on the use of productive species such as lucerne. Other options for acidic soils where slope is less than 10% are for the grazing system to be modified or intensified, or for crop or horticultural production. Costs of lime could be justified through more profitable enterprises than traditional grazing operations. Low input systems based on native grasses are intrinsically appealing, however, this is only possible where a premium is paid for such produce (such as super fine wool). Forestry is an option where suitable land and infrastructure are present and should slow soil acidification and minimise off-site impacts. Land retirement may be a useful option for some parts of the landscape that contribute disproportionately to environmental problems. Private and government funded land retirement may have a role to play.

Ameliorating acid soils with organic materials: effects of amendments derived from coal on yield and composition of young wheat plants grown on an acid red podzol

Acidity in surface and subsurface soil horizons limits crop and pasture production over considerable areas of the world, and lime is the traditional amendment used to neutralize surface soil acidity. Earlier reports have indicated that calcium (Ca)‐saturated organic materials, produced by the oxidation of coal, may be of value as ameliorants for subsurface acidity. Two of them (OXPR and OXFU) were evaluated and compared with lime in pot studies with wheat (Triticum aestivum). In the first experiment, lime, OXPR and OXFU, each at four rates, were applied to an acid red podzolic soil, high in extractable aluminum (Al) and were compared for their ability to improve seedling root growth. In the second experiment, the three amendments were applied to the same soil and shoot yields and mineral content were measured after five weeks growth. The three amendments raised soil pH, lowered extractable Al and increased root elongation. OXFU at the highest rate (251 ha‐1) inhibited root growth possibly due to the high concentration of solutes as indicated by high electrical conductivity of the soil solution. In the second experiment the three amendments improved shoot yields, with significant increases up to 0.81 Ca ha‐1 application. Yields were in general higher with lime than with OXPR and OXFU at equivalent rates of added Ca. However, a significant asymptotic relationship was observed between yield and calcium carbonate (CaCO3) equivalence of each of the applied amendments. Yield was highly correlated with measures of soil Al {r2 for [Al] in 0.01M calcium chloride (CaCl2), 0.915; [Al] in 0.1M barium chloride (BaCl2)/ammonium chloride (NH4C1), 0.873; [Al] in water, 0.779; [Al] in water/EC, 0.904}. Plant manganese (Mn) concentrations were decreased by lime and OXPR and there were linear correlations between plant Mn concentration and soil Mn extractable in 0.01 M CaCl2 With OXFU, which has a high concentration of water soluble Mn, plant Mn was decreased below the unamended controls only at the higher rates of application. .Leaf tissue zinc (Zn) concentrations were high in treatments receiving the two coal‐derived amendments. In an effort to determine the role of ion complexation in the remediation process, a third study evaluating the Al complexing ability of the two coal derived products was compared to that of a water‐soluble fraction extracted from chestnut (Castanea saliva) litter which is known to have a high affinity for Al. The two coal derived materials had considerably lower affinities for Al than the chestnut extract supporting the thesis that complexing reactions were of minor importance in the observed responses. The results indicate that field testing of these products is warranted particularly at sites where plant growth is constrained by subsurface soil acidity.

Subsurface acidity and liming affect yield of cereals

In 2 field experiments, varying lime rates were applied to acidic soils and incorporated to 10 or 20 cm depth by rotary hoeing. In Expt 2, soil was also limed in 10-cm layers to 40 cm depth, and the profile was reconstructed. The aim was to remove acidity as a constraint to plant growth. In both experiments, 3 cereal cultivars (1 barley and 2 wheat) of varying aluminium (Al) tolerance were sown. Grain yield was monitored in 4 seasons (Expt 1) or 5 seasons (Expt 2). Incorporation of lime to 20 cm rather than to 10 cm improved yields of the cereals in 2 of 4 seasons in Expt 1, and improved yields markedly in a drier season in Expt 2. In Expt 2, cereal yield was demonstrated to be a function of soil pHCa in both the 0-10 and 10-20 cm layers. There was no advantage of amendment to 40 cm depth compared with 20 cm depth at this site. At both sites, the 10-20 cm soil (corresponding to the upper A2 layer) was very acidic, and the incorporation of lime to 20 cm removed this barrier. The effect of amendment or pHCa increase in the 10-20 cm layer is clearest and most consistent for barley, the most Al-sensitive species, and least consistent for the tolerant wheat cultivars. It is probably not realistic in commercial agriculture to incorporate lime to 20 cm depth. The alternative strategies are to wait for the effects of amendment of the 0-10 cm layer to move down the soil profile, or to amend part of the soil below 10 cm depth by lime injection. Until the subsurface soil is amended, the best strategy is to combine surface liming (0-10 cm) with the use of Al-tolerant cultivars.

Evaluation of two coal‐derived organic products in ameliorating surface and subsurface soil acidity

SummaryAcidity in the soil surface and subsurface is of major concern in horticulture, cropping and pasture production systems in southern Australia. Broadcast applications of lime to the surface have proved to be ineffective in ameliorating subsurface acidity in the short term. Two calcium‐saturated coal‐derived organic products, an ‘oxi‐product’ (OXPR) and an ‘oxi‐fulvate’ (OXFU), were evaluated for use as liming materials with specific consideration given to amelioration of subsurface acidity. The effects of these two organic products and of lime on the chemical composition of the A1 and A2 horizons of an acid red podzol were investigated in leaching columns, 20 cm long. OXPR and OXFU supplying 80 or 160 g Ca m−2 were compared to 160 g Ca m−2 applied as CaCO3 and a control receiving no amendments. The effects of CaCO3 on exchangeable Al and pH were limited to the surface 2 cm. In contrast, the two organic amendments were effective in decreasing exchangeable Al and increasing pH and exchangeable Ca to depth, the extent being a function of amendment and rate applied. The formation of inorganic and organic complexes were assumed to be responsible for the movement of Al out of the column in the leachate, although significant quantities of Al were precipitated in the column. Significant linear regressions were observed between the ionic strength and Al in the leachate. Significant quantities of Mg and K were displaced by Ca and leached from the column; consequently, this loss must be considered against the benefits of decreased exchangeable Al when assessing fertility for plant production.

Effect of surface applications of lime, gypsum and phosphogypsum on the alleviating of surface and subsurface acidity in a soil under pasture

Changes in the chemistry of an acidic grey massive earth soil in response to various ameliorant treatments (gypsum and phosphogypsum in the presence or absence of lime) were investigated in a subterranean clover-based pasture in the southern highlands of New South Wales. Lime, gypsum, and phosphogypsum, or lime in combination with gypsum and phosphogypsum were broadcast at 2500 kg ha-1 on the surface of the soil in May 1990. Pasture production was determined and the soil was sampled to 25 cm depth, 6 and 18 months after treatment application. Surface soil pH was increased to 6-1 by the application of lime, gypsum plus lime and phosphogypsum plus lime treatments in the 0-5 cm depth interval, but remained unchanged when gypsum or phosphogypsum was used alone. Calcium chloride extractable aluminium increased down the soil profile under all treatments to 10-15 cm, but was highest in the gypsum treatment at depth. The application of phosphogypsum increased the 0.01 m CaCl2 extractable fluoride in the surface 5 cm from 26 to 43 �M. In contrast, fluoride concentrations were decreased to between 5.3 and 7.3 �M in the lime, gypsum plus lime and phosphogypsum plus lime treatments. Gypsum and phosphogypsum decreased the concentration of Al3+ in solution and on the exchange sites in the 0-5 cm depth interval. However, the effectiveness of the amendments to reduce toxic Al3+ concentrations were confined to the surface 5 cm. The concentration of aluminium and the activity of Al3+ in the 0-5 cm soil layer at both soil samplings were decreased by the amendments. Lime, and gypsum or phosphogypsum in combination with lime, were the most effective treatments for reducing the activity of Al3+. The activity of Al3+ increased with depth in all treatments. The pH and activity of Al3+ measured in the 0.01 m CaCl2 extracts plot near the gibbsite solubility line and suggest that Al solubility was controlled by this mineral. Exchangeable Ca in the 0-5 cm soil layer was significantly increased by the application of lime whereas exchangeable aluminium was decreased by lime, gypsum and phosphogypsum. There was no significant change in exchangeable cations other than Al below the surface 5 cm which suggests limited leaching of lime, gypsum and phosphogypsum in the profile within the 18 month study period. Pasture yield was significantly increased by the addition of lime and was related to the decrease in the activity of Al3+ in the surface soil.

Amelioration of subsurface acidity in sandy soils in low rainfall regions .2. Changes to soil solution composition following the surface application of gypsum and lime

Two field trials were sampled to investigate the changes to soil solution chemical properties of a yellow sandplain soil with an acidic subsoil following the application of gypsum and lime to the soil surface in 1989. The soils were sandy textured and located in a region of low annual rainfall (300-350 mm). Soil was sampled annually to a depth of 1 m and changes in soil solution composition were estimated by extraction of the soil with 0.005 M KCl. Gypsum leaching caused calcium (Ca), sulfate (SO4) and the ionic strength to increase substantially in both topsoil and subsoil by the end of the first year. Continued leaching in the second year caused these properties to decrease by approximately one-half in the topsoil. Gypsum appeared to have minimal effect on pH or total Al (Al-T), although the amount of Al present as toxic monomeric Al decreased and the amount present as non-toxic AlSO+4 ion pairs increased. Magnesium (Mg) was displaced from the topsoil by gypsum and leached to a lower depth in the subsoil. In contrast, lime caused pH to increase and Al to decrease substantially in the topsoil, but relatively little change to any soil solution properties was observed in the subsoil. There was an indication that more lime may have leached in the presence of gypsum in the first year after application at one site. Wheat yields were best related to the soil acidity index Al-T/EC (where EC is electrical conductivity of a 1:5 soil:water extract), although the depth at which the relationship was strongest in the subsoil varied between sites. The ratio Al-T/EC was strongly correlated with the activity of monomeric Al species (i.e. the sum of the activities of Al3+, AlOH2+ and Al(OH)+2 in the soil solution. An increase in the concentration of sulfate in the subsoil solution (which increased the ionic strength, thereby decreasing the activity of Al3+, and also increased the amount of Al present as the AlSO+4 ion pair) was probably the most important factor decreasing Al toxicity to wheat. The results indicated that gypsum could be used to increase wheat growth in aluminium toxic subsoils in sandy soils of low rainfall regions and that a simple soil test could be used to predict responses.

Amelioration of subsurface acidity in sandy soils in low rainfall regions .1. Responses of wheat and lupins to surface-applied gypsum and lime

Amelioration of subsoil acidity using gypsum (CaSO4.2H2O) or lime (CaCO3) was studied on sandy textured soils with low water holding capacity in a low rainfall environment. Field trials were established in 1989 at two sites on yellow sandplain soils to investigate whether different rates, sources and combinations of gypsum and lime application could be used to increase wheat and lupin yields. Gypsum increased wheat yields by up to 45% in the first two growing seasons whereas lime increased wheat yields by up to only 15% in the second season. The highest yields were generally recorded when gypsum and lime were applied together. The response of wheat to the various treatments varied both regionally and temporally and it is suggested that the inherent soil solution composition affected the magnitude and rapidity of wheat responses to gypsum. The rate of gypsum application affected the longevity of the wheat responses, with a low application rate (1 t ha-1) increasing yields for only one season. No differences in wheat yields were recorded between different sources of gypsum or application rates higher than 3 t ha-1. In contrast to wheat, lupin yields were substantially lower on gypsum-treated plots. The yield decline did not appear to be related to any simple nutritional factor and the gypsum effect was generally minimized when lime was added with the gypsum. The results indicated that lower rates of gypsum than used in previous subsoil amelioration studies were suitable for increasing wheat yields on sandy soils in low rainfall environments, and that gypsum should not be used if lupins are to be grown within at least 2 years of its application to the soil surface.

Surface and subsurface acidity of faujasite-type zeolites in relation to their composition: an XPS and TPD of ammonia study

A combination of temperature-programmed desorption (TPD) of NH3 and X-ray photoelectron spectroscopy (XPS) has been used for the qualitative and quantitative study of the surface acidity of HY zeolites with different Si : Al molar ratios (ca. 2.7, 5.8, 24). The N 1s binding energies of the probe molecule showed two types of sites, Brønsted and Lewis, whose relative proportions varied with pretreatment temperature, as well as dissociative adsorption of NH3 following dehydroxylation. The measurement of the NH3 concentration in zeolites as a function of desorption temperature enabled the site distribution to be determined in terms of strength, for the external surface of crystals and for a subsurface which is partially free of the very slightly acidic extraframework alumina and silica species that cover it.