Soil Si Research Articles

In Vitro Digestibility and Chemical Composition of Kikuyugrass as Influenced by Soil Silicon, Liming, and Genotype

Kikuyugrass (Pennisetum clandestinum Hochst. ex Chiov.) is an active silicon (Si) accumulator, and Si can reduce the organic‐matter digestibility of forage grasses. Calcium carbonate (crushed coral) applications to a low Si Maile silt loam soil (Acrudoxic Hydrudands) had no effect on the in vitro digestible organic matter (IVDOM) concentration of 3‐week‐old (4.5‐leaf‐per‐tiller stage of development) ‘Whittet’ kikuyugrass regrowth, whereas calcium silicate (wollastonite) applied at the calcium equivalent of the calcium carbonate rates produced a curvilinear (crescent‐shaped quadratic) IVDOM response. Whittet IVDOM decreased when grass Si concentration exceeded about 22 g kg−1 or when 0.5 M acetic acid–extractable soil Si exceeded about 230 mg kg−1. Common and ‘Hosaka’ kikuyugrass regrowth on the Maile soil had less Si and greater IVDOM concentrations than those observed for high‐Si Kohala (Humic Dystrustepts) and Waialua (Pachic Haplustolls) silty clay soils. Whittet had greater crude protein and IVDOM concentrations and lesser fiber concentrations than common.

Effect of Silicon on Growth, Biochemical Constituents, and Mineral Nutrition of Cowpea

The effect of different concentrations (50, 100, 200, 400, and 800 mg kg−1) of silicon (Si) added to the soil was studied on growth performance, biochemical constituents, and nutritional status of cowpea [Vigna unguiculata (L.) Walp.]. Lower applications of Si resulted in an enhancement in relative yield, leaf area, chlorophyll, and iron (Fe) contents and a reduction in proline contents. A significant (P = 0.01) direct relationship between relative yield and lower applied Si concentrations was observed. A significant dependence of tissue Si concentration on soil Si concentration was observed. The results indicate a beneficial effect of Si on cowpea plants.

Relationship between mineral composition or soil texture and available silicon in alluvial paddy soils on the Shounai Plain, Japan

To evaluate the relationship between the amount of available Silicon (Si) in paddy soils and their mineral properties on the Shounai Plain in Japan, which is formed from several parent materials, we evaluated the amount of available Si, the particle size distribution, the oxide composition of crystalline minerals and the amount of oxalate-extractable Si (Sio), iron (Feo) and aluminum (Alo) in the soil. The amount of available Si in the soil and the oxide content of the crystalline minerals differed among four soil groups that were distinguished by their clay mineral composition. There was no difference in the particle size distribution among the soil groups. The amount of available Si was positively related to the SiO2/Al2O3 ratio of clay, the CaO concentration of silt and fine sand, and the amounts of Sio, Feo and Alo in the soil. The amount of available Si in the soils was negative correlated with the Na2O and K2O concentrations of silt, the K2O concentration of fine sand, and the coarse sand content. These results suggest that the amount of available Si in soils is affected by the weathering resistivity of their minerals and that the particle size distribution and mineral composition are related to the available Si of the soils. Mineralogical properties, including the particle size distribution and mineral composition such as the SiO2/Al2O3 ratio × clay fraction content and the amounts of CaO and MgO in silt-sized particles, were positively correlated with the amount of available Si in the soil, but these correlations were not found for fine sand-sized particles. The Sio, Feo and SiO2/Al2O3 ratio × clay fraction contents contributed approximately 50% to the amount of available Si in the soils. The amount of available Si in the soil was divided into two groups according to the location of the paddy field. The amount of soil-available Si in the alluvial plain was affected by the geology upstream through the mineral composition.

The reserve of weatherable primary silicates impacts the accumulation of biogenic silicon in volcanic ash soils

Banana plantlets (Musa acuminata cv Grande Naine) cultivated in hydroponics take up silicon proportionally to the concentration of Si in the nutrient solution (0-1.66 mM Si). Here we study the Si status of banana plantlets grown under controlled greenhouse conditions on five soils developed from andesitic volcanic ash, but differing in weathering stage. The mineralogical composition of soils was inferred from X-ray diffraction, elemental analysis and selective chemical/mineralogical extractions. With increasing weathering, the content of weatherable primary minerals decreased. Conversely, clay content increased and stable secondary minerals were increasingly dominant: gibbsite, Fe oxides, allophane, halloysite and kaolinite. The contents of biogenic Si in plant and soil were governed by the reserve of weatherable primary minerals. The largest concentrations of biogenic Si in plant (6.9-7 g kg(-1)) and soil (50-58 g kg(-1)) occurred in the least weathered soils, where total Si content was above 225 g kg(-1). The lowest contents of biogenic Si in plant (2.8-4.3 g kg(-1)) and soil (8-31 g kg(-1)) occurred in the most weathered desilicated soils enriched with secondary oxides and clay minerals. Our data imply that soil weathering stage directly impacted the soil-to-plant transfer of silicon, and thereby the stock of biogenic Si in a soil-plant system involving a Si-accumulating plant. They further imply that soil type can influence the silicon soil-plant cycle and its hydrological output.

Leaf silicon content in banana (Musa spp.) reveals the weathering stage of volcanic ash soils in Guadeloupe

Several plant species accumulate silicon, which is taken up by roots in soil solution. The Si concentration in soil solution can be governed by silicate dissolution and formation, and thus soil constitution. Here, we study the Si leaf content of mature banana plants (Musa acuminata cv Grande Naine) cropped on soils derived from andesitic ash in Guadeloupe through standard foliar analysis. The soils strongly differ in weathering stage and total Si content. The most desilicated soils (Andosol-Nitisol Ferralsol) occur in the wettest areas, on the Eastern slopes (Es) of the volcano exposed to rain bearing winds. Least weathered soils (Andosol-Cambisol) occur on Western slopes (Ws). The average leaf Si concentration ranges from 2.7 to 3.9 g kg(-1) for bananas cropped in Es soils, and from 7.7 to 9.6 g kg(-1) in Ws soils. The leaf Si concentrations are lowest for the Es gibbsite-rich Andosols and Ferralsols. The leaf Si concentration is positively correlated with soil CaCl2-extractable Si content, soil Si content and total reserve in weatherable minerals. The silicon content of banana leaves thus reveals the weathering stage of volcanic ash soils in Guadeloupe.

Effects of Soil Type, Source of Silicon, and Rate of Silicon Source on Development of Gray Leaf Spot of Perennial Ryegrass Turf.

Silicon amendments have been proven effective in controlling fungal diseases of various crops. However, effects of silicon amendments on gray leaf spot (Magnaporthe oryzae) of perennial ryegrass are not known. Studies were conducted in controlled-environment chambers and microplots where perennial ryegrass pots were buried among perennial ryegrass turf to determine the effects of silicon amendments on gray leaf spot development. Plants were grown in two soil types: peat:sand mix (soil Si = 5.2 mg/liter) and Hagerstown silt loam (soil Si = 70 mg/liter). Both soil types were amended with two sources of silicon-wollastonite and calcium silicate slag-at 0, 0.5, 1, 2, 5, and 10 metric tons/ha and 0, 0.6, 1.2, 2.4, 6, and 12 metric tons/ha, respectively. Nine-week-old perennial ryegrass was inoculated with M. oryzae. Gray leaf spot incidence and severity were assessed 2 weeks after inoculation. Gray leaf spot incidence and severity of perennial ryegrass significantly decreased by different rates of wollastonite and calcium silicate slag applied to both soils under both experimental conditions. Tissue silicon content increased consistently with increasing amount of silicon in the soils, while disease incidence decreased consistently with increasing tissue silicon content in all four soil and source combinations under both experimental conditions. These findings suggest that silicon amendments may be utilized in integrated gray leaf spot management programs on perennial ryegrass.

Silicon fractions in Histosols and Gleysols of a temperate grassland site

AbstractThe importance of silicon (Si) in nutrition is currently being recognized by its beneficial effects on many crops. Therefore, it is important to determine the soil Si status and to examine different extractants for testing plant‐available Si. Little information is available about the Si status of Histosols and Corg‐rich Gleysols in temperate climate. This study was undertaken (1) to characterize different Si pools in Corg‐rich groundwater soils of an experimental site and (2) to study the influences of small‐scale variability on element distribution. At the experimental site, the thickness of the Corg‐rich layer ranges between 4 and 5 dm overlying fine‐sandy fluvial sediments. Four extractants were evaluated: 0.01 M CaCl2, 0.5 M acetic acid, 0.1 M sodium pyrophosphate, and 0.1 M Tiron (C6H4Na2O8S2 · H2O). Further, total element content was determined following HNO3/HF digestion. Calcium chloride–soluble Si shows no significant relations to other parameters analyzed. On the basis of published data, the soils investigated could be classified as Si‐deficient. The Si fraction extracted with acetic acid displays relations to Corg content of the soil and a weak correlation to CaCl2‐soluble Si, indicating that both solutions extract overlapping but not the same fractions. Sodium pyrophosphate extracts mainly organo‐mineral Fe and Al complexes in the soils studied, which is reflected in a highly positive correlation to Corg. Pyrophosphate‐soluble Si showed a negative relationship to Corg, which means a closer relation of this Si fraction to mineral matter than to Corg. The Tiron solution extracted most Si of all extractants, but this amounts only 1% of the total Si content. Taking into account the element‐specific relationship between pyrophosphate and Tiron‐extractable Fe, Al, and Si, it can be concluded that Tiron dissolves mainly the opaline silica present in Histosols and Corg‐rich Gleysols. The distribution of Corg and ash content shows clear spatial trend at the experimental site, which is correlated to pyrophosphate‐extractable as well as total Si. This small‐scale variability of soil parameters itself is related to a distinct microrelief.

Acúmulo de silício em plantas de arroz do ecossistema de várzea submetido à aplicação de diferentes fontes

A baixa produtividade brasileira de arroz decorre, principalmente, da reduzida fertilidade do solo e alta suscetibilidade a algumas doenças. A adubação com silício pode ser uma alternativa no sentido de amenizar, sobretudo, o impacto das doenças sobre a produtividade. No intuito de identificar fontes eficientes quanto ao fornecimento de Si para a cultura do arroz, foi instalado um experimento em vasos, utilizando-se Neossolo Quartzarênico Órtico típico, em delineamento em blocos casualizados. As fontes de Si utilizadas foram siligran e silicon, nas formas granulada e pó, na dose de 200 mg kg-1 de Si. A curva de resposta a Si foi estabelecida pela aplicação de uma fonte-padrão (Wollastonita) nas doses de 0, 50, 100, 200, 400 mg kg-1 de Si, visando obter uma curva de resposta. Após a semeadura da cultivar de arroz Rio Formoso, avaliou-se a produção de massa seca da parte aérea, massa de panículas, produção de grãos, Si acumulado na planta e teores de Si no solo. A fonte-padrão Wollastonita apresentou comportamento linear, aumentando o Si disponível no solo e a absorção pelo arroz com o aumento das doses aplicadas. Os maiores aumentos, nos teores de Si na parte aérea, casca, Si acumulado e na massa seca total foram proporcionados pela Wollastonita. A fonte que proporcionou maior aumento na produtividade de grãos foi silicon na forma de pó. O extrator ácido acético extraiu maiores quantidade de Si do solo em relação ao cloreto de cálcio, superestimando os resultados.

Modelling the biogeochemical cycle of silicon in soils: Application to a temperate forest ecosystem

We investigated the biogeochemical cycling of silicon (Si) in an acidic brown soil covered by a coniferous forest (Douglas fir). Based on published and original data, we constructed a conceptual model and used a modified version of the reactive transport code MIN3P for model testing and quantification purposes. The model was first calibrated and further validated with respect to biomass data and Si-concentrations in capillary solutions, which were collected monthly over several years by means of suction-cup lysimeters placed at different soil depths. Following sensitivity tests, the model was calibrated quite accurately (limited to a 10% concentration error) by the adjustment of kinetic constants, longitudinal dispersion, and apparent activation energy for K-feldspar dissolution. Calibrated parameter values were constrained by ranges reported in the literature, when available. Mass balance calculations indicate that an average of 60% of the biogeochemical cycle of Si was controlled by biological processes (i.e. Si-uptake and dissolution of phytoliths). Sensitivity analyses suggest that no more than 55% of the Si-cycle is controlled by weathering of primary silicates. Such a large contribution of biological turnover to Si-cycling may be explained by the combined effects of a relatively large Si-content in the litter fall (i.e. specifically in the needles) and high biomass productivity of the coniferous species considered. In addition to potential implications for the global Si cycle, this investigation raises several fundamental questions concerning the nature of Si-uptake mechanisms and physiological use of Si by trees in natural systems.

Acumulação de silício em arroz em diferentes condições de pH da rizosfera

A quantidade de Si acumulada pelas plantas depende do teor de Si disponível, e este, do pH do solo. É possível que a absorção e acumulação de Si em gramíneas possam estar associadas ao pH rizosférico. Este trabalho teve como objetivo estudar o efeito do pH de rizosfera na disponibilidade do Si no solo, na acumulação por plantas de arroz de sequeiro (vr. Epagri 109) e na produção de massa seca. Para isso, instalou-se um ensaio em casa de vegetação, em parcela subdividida, no esquema fatorial 5 x 2. Os cinco tratamentos (TR) foram constituídos de dose única de N (200 mg kg-1), aplicada nas seguintes relações de N-NH4+e N-NO3-: TR1 = 0,0:1,0; TR2 = 0,25:0,75; TR3 = 0,50:0,50; TR4 = 0,75:0,25; e TR5 = 1,0:0,0. O fator subparcela consistiu da aplicação de 200 mg kg-1 de Si. As fontes de N usadas foram nitrato de cálcio e sulfato de amônio, e a fonte de Si foi o tetracloreto de Si, aplicadas em uma amostra de Neossolo Quartzarênico. Para inibir a nitrificação foram utilizados 10 mg kg-1 de nitrapirina nos tratamentos em que amônio foi empregado. Os valores de pH rizosférico decresceram proporcionalmente ao aumento da concentração de N-NH4+ e influenciaram de forma significativa o teor de Si no solo: quanto maior o pH rizosférico, maior a disponibilidade de Si e maior a absorção pela planta de arroz.

Benefits of organic residues and chemical fertilizer to productivity of rain-fed lowland rice and to soil nutrient balances

Low yields and high risk characterize many rain-fed lowland rice environments, including those in Laos. Drought and fluctuating soil-water conditions (from aerobic to anaerobic states) can limit productivity and the efficient use of applied nutrients. Although addition of organic matter may improve the efficiency of fertilizer use, on-farm residues, for example farmyard manure (FYM), rice straw and rice hulls, are, currently, poorly utilized in these systems. Single and multi-year experiments were designed to evaluate the effect of these residues on rice productivity and efficiency of fertilizer use at four sites. Rice yield without fertilizer but with addition of residues ranged from 1.1 to 1.7 t ha−1 across sites and years. In response to fertilizer, yields increased on average by 1.4 t ha−1. For all sites and years there was a significant response of yield to organic residues applied without fertilizer, with responses ranging from 0.2 to 1.4 t ha−1. In 58% of cases there was no residue×fertilizer interaction (benefits of residues when applied with fertilizer were additive). In 38 and 4% of cases the interaction was negative (no response to residues if fertilizer was already applied) or positive (synergistic), respectively. In the multi-year studies, the type of interaction varied between years, suggesting that seasonal events, rather than soil type, determine the type of interaction. The greatest benefits of applying organic and chemical fertilizers together were observed in years when soil-water conditions were unfavorable (fluctuating anaerobic–aerobic conditions). The long-term effects of these different management strategies on soil nutrient balances suggest that N, P, and K balances were maintained as a result of balanced commercial fertilizer management but that addition of residues further enhanced these balances. All residues, when applied alone, resulted in positive soil Si balances; only with FYM were long-term N, P, and K balances maintained or positive, however. For resource-poor farmers, applying on-farm residues can be a sustainable approach to increasing productivity.

Effect of long-term intensive rice cultivation on the available silica content of sawah soils: Java Island, Indonesia

The dramatic increases in rice productivity and cultivation intensity through the implementation of green revolution (GR) technology using high yielding varieties (HYVs) of rice and chemical fertilizers were not long lasting in Indonesia. The stagnancy of rice productivity in recent years without any scientific reasons presents a challenge for agronomists and soil scientists in Indonesia. This study describes the effects of long-term intensive rice cultivation on the change in available silica (Si) in sawah soil. The term sawah refers to a leveled and bounded rice field with an inlet and an outlet for irrigation and drainage. Soil samples collected by Kawaguchi and Kyuma in 1970 and new samples taken in 2003 from the same sites or sites close to the 1970 sites were analyzed and compared. From 1970 to 2003, the average content of available Si decreased from 1,512 ± 634 kg SiO2 ha−1 to 1,230 ± 556 kg SiO2 ha−1 and from 6,676 ± 3,569 kg SiO2 ha−1 to 5,894 ± 3,372 kg SiO2 ha−1 in the 0–20 cm and 0–100 cm soil layers, respectively. Cultivation intensity differences between seedfarms planted with rice three times a year and non-seedfarms rotating rice and upland crops appeared to affect the changing rates of available Si within the study period. In the 0–20 cm soil layer, the average content of available Si decreased from 1,646 ± 581 kg SiO2 ha−1 to 1,283 ± 533 kg SiO2 ha−1 (−22%) and from 1,440 ± 645 kg SiO2 ha−1 to 1,202 ± 563 kg SiO2 ha−1 (−17%) in seedfarms and non-seedfarms, respectively. Differences in topographical position also influenced the decreasing rate of available Si in this study. Using similar management practices and cultivation intensity, upland sampling sites lost more Si compared with lowland sites. Planted rice under a rain fed system with no Si addition from rain water in an upland position may be a reason for the higher loss of Si, particularly in non-seedfarms. The Si supply from irrigation water might have contributed to the slowdown in the decreasing rate of available Si in Java sawah soils.

Silicon pools and fluxes in soils and landscapes—a review

AbstractSilicon (Si) is the second‐most abundant element in the earth's crust. In the pedosphere, however, huge spans of Si contents occur mainly caused by Si redistribution in soil profiles and landscapes. Here, we summarize the current knowledge on the different pools and fluxes of Si in soils and terrestrial biogeosystems. Weathering and subsequent release of soluble Si may lead to (1) secondarily bound Si in newly formed Al silicates, (2) amorphous silica precipitation on surfaces of other minerals, (3) plant uptake, formation of phytogenic Si, and subsequent retranslocation to soils, (4) translocation within soil profiles and formation of new horizons, or (5) translocation out of soils (desilication). The research carried out hitherto focused on the participation of Si in weathering processes, especially in clay neoformation, buffering mechanisms for acids in soils or chemical denudation of landscapes. There are, however, only few investigations on the characteristics and controls of the low‐crystalline, almost pure silica compounds formed during pedogenesis. Further, there is strong demand to improve the knowledge of (micro)biological and rhizosphere processes contributing to Si mobilization, plant uptake, and formation of phytogenic Si in plants, and release due to microbial decomposition. The contribution of the biogenic Si sources to Si redistribution within soil profiles and desilication remains unknown concerning the pools, rates, processes, and driving forces. Comprehensive studies considering soil hydrological, chemical, and biological processes as well as their interactions at the scale of pedons and landscapes are necessary to make up and model the Si balance and to couple terrestrial processes with Si cycle of limnic, fluvial, or marine biogeosystems.

Silicon pools and fluxes in soils and landscapes—a review

AbstractSilicon (Si) is the second‐most abundant element in the earth's crust. In the pedosphere, however, huge spans of Si contents occur mainly caused by Si redistribution in soil profiles and landscapes. Here, we summarize the current knowledge on the different pools and fluxes of Si in soils and terrestrial biogeosystems. Weathering and subsequent release of soluble Si may lead to (1) secondarily bound Si in newly formed Al silicates, (2) amorphous silica precipitation on surfaces of other minerals, (3) plant uptake, formation of phytogenic Si, and subsequent retranslocation to soils, (4) translocation within soil profiles and formation of new horizons, or (5) translocation out of soils (desilication). The research carried out hitherto focused on the participation of Si in weathering processes, especially in clay neoformation, buffering mechanisms for acids in soils or chemical denudation of landscapes. There are, however, only few investigations on the characteristics and controls of the low‐crystalline, almost pure silica compounds formed during pedogenesis. Further, there is strong demand to improve the knowledge of (micro)biological and rhizosphere processes contributing to Si mobilization, plant uptake, and formation of phytogenic Si in plants, and release due to microbial decomposition. The contribution of the biogenic Si sources to Si redistribution within soil profiles and desilication remains unknown concerning the pools, rates, processes, and driving forces. Comprehensive studies considering soil hydrological, chemical, and biological processes as well as their interactions at the scale of pedons and landscapes are necessary to make up and model the Si balance and to couple terrestrial processes with Si cycle of limnic, fluvial, or marine biogeosystems.

Understanding the realised niche of an amphibious softwater plant, Eleocharis multicaulis

The West European amphibious plant Eleocharis multicaulis, characteristic of softwater lakes, is rare and endangered in many regions. The present study aimed to evaluate several presumed niche features suggested in syntaxonomic studies, to reveal other important niche variables affecting the cover of this species and to quantify these responses. A dataset of 724 environmental and vegetation variables was built from a survey of 59 plots in The Netherlands and Belgium. Discriminant analysis, Pearson goodness-of-fit calculations, weighted averages and presence profiles were used. We found that Eleocharis multicaulis performs best in base-poor environments with an or- ganic top layer and an established vegetation. The species is apparently N limited and profits from N input as long as NO3 - is dominant over NH4 + . A high atmospheric N deposition is detrimental, because the resulting strong acidification leads to dominance of NH4 + over NO3 - . Intense acidification in agricultural areas has probably been an important extinction factor in the recent past. We hypothesise that current variability of soil Si, a very good predictor for the species' performance, might reflect these acidifi- cation processes. This hypothesis is supported by the fact that Si is correlated with trophic status, which is likely higher in agricultural regions. Nowadays, Eleocharis multicaulis suffers from agricultural P, K and alkalinity inputs as well as reduced oxy- gen supply (e. g. by reduced water-level dynamics), all leading to a lower redox state. These factors could bring about competitive suppression by other species, a high NH4 + /NO3 - ratio and P mobilisation.

The role of phytolith formation and dissolution in controlling concentrations of silica in soil solutions and streams

By integrating the results of recent studies on the concentration and flux of Si in European forest soils (podzols and acid brown soils) with published evidence on the amounts of Si cycled through forest vegetation, we conclude that phytoliths must be the principal immediate source and sink of silica in soil solution, although mineral weathering is the ultimate source. During the growing season, forest vegetation takes up most of the Si made available by weathering of soil minerals and by phytolith dissolution, and converts it into phytoliths. Phytoliths will be the principal source of Si leached from the soil during winter rains and spring snowmelt. This work accounts for the plateau concentrations of Si observed in laboratory equilibrations of Bs horizon soils, for the maximum Si concentration reported for B horizons in the field, for a deep sink for Si in Bs horizons, and for the fact that the concentrations of Si in soil drainage water and in streams all fall in, or below, the range expected for phytolith solubility. Concentrations of Si in surface horizons can exceed these levels during droughts. The common assumption that the concentration of Si in soil solution is a result of a dynamic balance between Si released by mineral weathering and that lost by leaching is untenable for Bs horizons.

Evaluation of Silicon Availability in Fertilizers by Water-Resin Extraction Method and Adequate Recommendations for Si Application Based on Available Si in Fertilizers and Soils

Evaluation of Silicon Availability in Fertilizers by Water-Resin Extraction Method and Adequate Recommendations for Si Application Based on Available Si in Fertilizers and Soils

SOIL SILICON EXTRACTABILITY WITH SEVEN SELECTED EXTRACTANTS IN RELATION TO COLORIMETRIC AND ICP DETERMINATION

The importance of silicon (Si) in nutrition is currently being recognized by its beneficial effects in many crops. At present, various procedures are being used to extract Si and to determine soil Si status. This study was undertaken to evaluate the relationships among various extractants and to compare inductively coupled plasma (ICP) Si measurement with the molybdenum blue colorimetric method for possible incorporation of Si determination into existing routine soil and water testing procedures. Seven extractants were evaluated on 30 Louisiana soils. These were: deionized (D.I.) water, 0.5 M acetic acid, 1 M sodium acetate buffer (pH 4.0), 0.5 M ammonium acetate (pH 4.8), 0.1 M hydrochloric acid, 0.5 M citric acid, and Mehlich III. Soil-extractable Si determination by ICP correlated highly with that by colorimetric analysis in HCl, citric acid, acetic acid, acetate buffer, and ammonium acetate extractions (R 2 ≥ 0.972, P citric acid > HCl > acetic acid > acetate buffer > NH 4 OAc > D.I. water, as determined by colorimetry. Silicon extracted by different extractants was well correlated among citric acid, HCl, acetic acid, acetate buffer, and NH 4 OAc (R 2 ≥ 0.611, P < 0.001). Water and Mehlich III showed poor correlations with other extractants (R 2 ≤ 0.430). The results suggest that these seven extractants characterize different pools of Si-supplying capacity of the soil: extractable by water, extractable by any of HCI, citric acid, acetic acid, acetate buffer, and NH 4 OAc, and extractable by Mehlich III.

Pedogenic Silica Accumulation in Chronosequence Soils, Southern California

Chronosequential analysis of soil properties has proven to be a valuable approach for estimating ages of geomorphic surfaces where no independent age control exists. In this study we examined pedogenic silica as an indicator of relative ages of soils and geomorphic surfaces, and assessed potential sources of the silica. Pedogenic opaline silica was quantified by tiron (4,5‐dihydroxy‐1,3‐benzene‐disulfonic acid [disodium salt], C6H4Na2O8S2) extraction for pedons in two different chronosequences in southern California, one in the San Timoteo Badlands and one in Cajon Pass. The soils of both of these chronosequences are developed in arkosic sediments and span 11.5 to 500 ka. The amount of pedogenic silica increases with increasing duration of pedogenesis, and the depth of the maximum silica accumulation generally coincides with the maximum expression of the argillic horizon. Pedogenic silica has accumulated in all of the soils, ranging from 1.2% tiron‐extractable Si (Sitn) in the youngest soil to 4.6% in the oldest. Primary Si decreases with increasing duration of weathering, particularly in the upper horizons, where weathering conditions are most intense. The loss of Si coincides with the loss of Na and K, implicating the weathering of feldspars as the likely source of Si loss. The quantity of Si lost in the upper horizons is adequate to account for the pedogenic silica accumulation in the subsoil. Pedogenic silica was equally effective as pedogenic Fe oxides as an indicator of relative soil age in these soils.

Using magnetic susceptibility to facilitate more rapid, reproducible and precise delineation of hydric soils in the midwestern USA

Standard field indicators, currently used for hydric soil delineations [USDA-NRCS, 1998. Field indicators of hydric soils in the United States, Version 4.0. In: G.W. Hurt et al. (Ed.), United States Department of Agriculture-NRCS, Fort Worth, TX], are useful, but in some cases, they can be subjective, difficult to recognize, or time consuming to assess. Magnetic susceptibility (MS) measurements, acquired rapidly in the field with a portable meter, have great potential to help soil scientists delineate and map areas of hydric soils more precisely and objectively. At five sites in Illinois (from 5 to 15 ha in area) with contrasting soil types and glacial histories, the MS values of surface soils were measured along transects, and afterwards mapped and contoured. The MS values were found to be consistently higher in well-drained soils and lower in hydric soils, reflecting anaerobic deterioration of both detrital magnetite and soil-formed ferrimagnetics. At each site, volumetric MS values were statistically compared to field indicators to determine a critical MS value for hydric soil delineation. Such critical values range between 22×10 −5 and 33×10 −5 SI in silty loessal or alluvial soils in Illinois, but are as high as 61×10 −5 SI at a site with fine sandy soil. A higher magnetite content and slower dissolution rate in sandy soils may explain the difference. Among sites with silty parent material, the lowest critical value (22×10 −5 SI) occurs in soil with low pH (4.5–5.5) since acidic conditions are less favorable to ferrimagnetic mineral neoformation and enhance magnetite dissolution. Because of their sensitivity to parent material properties and soil pH, critical MS values must be determined on a site specific basis. The MS of studied soil samples (0–5 cm depth) is mainly controlled by neoformed ultrafine ferrimagnetics and detrital magnetite concentrations, with a minor contribution from anthropogenic fly ash. Neoformed ferrimagnetics are present in all samples but, based on high χ FD% (∼5% to 10%), are most prevalent in high pH Mollisols of northeastern Illinois. Scanning electron microscope images display significantly more detrital magnetite alteration in hydric soils, substantiating that reductive dissolution of magnetite (aided by microorganisms) is a primary cause for lower MS. Fly ash comprises 8–50% of the >5 μm strongly magnetic particles and typically accounts for 5–15% of the total MS signal. The proportion of fly ash in >5 μm strongly magnetic fractions is greater in hydric soils because of lower natural magnetite contents, possibly combined with historical topsoil accumulation in lower landscapes. Magnetic fly ash particles are also more altered in low MS soils, implying that significant magnetite dissolution can occur in less than 150 years.