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Item Amplifying the power of proximal sensing techniques to assess the cadmium concentration in agricultural soils : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand(Massey University, 2023) Shrestha, GautamCadmium (Cd) accumulation in agricultural soils due to long-term phosphate fertiliser applications has raised concerns in New Zealand and globally due to the potential toxicity of Cd in food products. Elevated soil Cd concentration can enhance Cd availability for plant uptake, increasing the risk of food chain transfer. Cadmium management is generally achieved through reference laboratory methods to estimate Cd concentration in soil and plant samples. Reference laboratory methods of Cd analysis are precise; however, sample preparation and associated resource cost make them expensive. As a complementary method, proximal sensing techniques including visible-near-infrared (vis-NIR: 350–2500 nm), mid-infrared (MIR: 4000–400 cm-1) reflectance and portable X-ray fluorescence (pXRF: 0–40 keV) spectroscopy have been successfully used to monitor elevated Cd levels in mining areas and in plants showing stress or toxicity symptoms due to Cd. However, application of such technologies in agricultural soils with low Cd concentration are relatively understudied. Hence, this study was conducted to amplify the power of three proximal sensing techniques to quantify Cd in soil samples from diverse soil orders, climatic conditions, land uses, and vegetations and plant samples for cost-effective Cd monitoring at regional to farm scale. In this doctoral study, soil and plant samples were scanned using vis-NIR, MIR, and pXRF sensors. Topsoil samples were obtained from (1) the Otago-Southland regional survey (n=622), (2) a pastoral farm survey (n=87) including dairy and sheep and beef farms with long-term phosphate fertiliser application history, and (3) two independent glasshouse experiments using Pallic and Allophanic soils amended with increasing soil Cd concentrations, and with or without a model forage herb, chicory (Cichorium intybus L.). In both experiments, chicory aboveground biomass and root samples were scanned using the three sensors, along with a periodic collection of vis-NIR spectra from soil and plant in-situ. Total Cd was determined in all samples, while the distribution of Cd among geochemical fractions was studied in the pastoral farm survey samples only. Reference laboratory results and spectral information were combined to develop models for accurate Cd predictions. For regional survey samples (n=622, 0.01–0.56 mg Cd/kg) including agricultural soils (47%), validation (v) results (n=124, 0.01–0.43 mg Cd/kg) showed Granger-Ramanathan model averaging of outputs from models using individual pXRF, vis-NIR, and MIR data as input for partial least squares (PLS) – support vector machine regression performed optimally to quantify total soil Cd with normalised root mean square error (nRMSEv) of 37% and concordance correlation coefficient (CCCv) of 0.84. For agricultural soils (n=84, 0.10–1.20 mg Cd/kg), cross-validation (cv) results of models using individual vis-NIR, MIR, and pXRF data as input for PLS performed with nRMSEcv of 26%, 30%, and 31% and CCCcv of 0.85, 0.77, and 0.75 respectively. For acid soluble (0.01–0.27 mg Cd/kg) and organic matter bound (0.02–0.27 mg Cd/kg) Cd, models using vis-NIR data performed with nRMSEcv of 11% and 33% and CCCcv of 0.97 and 0.84, respectively. For exchangeable (0.003–0.25 mg Cd/kg) Cd, a model using MIR data as input performed with nRMSEcv of 40% and CCCcv of 0.57. Using the Otago and Southland regional survey soil samples spectra as a soil spectral library (SSL), Cd concentration in the local set (agricultural soil samples) were quantified. A model using MIR data from the regional SSL pastoral soil subset (n=283, 0.01–1.31 mg Cd/kg) spiked with selected local set samples (n=12) with weights (×4) as input for LOCAL algorithm quantified local soil Cd with nRMSE of 38% and CCC of 0.78. In the glasshouse experiments, Cd translocation factor (TF) values for chicory were calculated using proximal sensor data and the results showed a significant relationship (R2=0.74, p<0.001) between measured and predicted TF values. A model using in-situ leaf clip vis-NIR spectra showed optimal performance to assess Cd concentration in aboveground chicory biomass with nRMSEcv of 28% and CCCcv of 0.93. Among vegetation indices calculated ‘blue green index 2’ showed a significant (p<0.01) R2 value (0.19, 0.36) in both experiments. Models using pXRF spectra as input showed optimal performance to predict chicory root (n=28, 0.86–25.79 mg Cd/kg) and Allophanic soil (n=112, 0.41–4.81 mg Cd/kg) Cd with nRMSEcv of 16% and 9% and CCCcv of 0.95 and 0.99, respectively. A model using laboratory vis-NIR spectra showed optimal performance to quantify Pallic soil Cd (n=336; 0.17–5.45 mg Cd/kg) with nRMSEcv of 22% and CCCcv of 0.97. Optimal prediction models using proximal sensor data can potentially be used for rapid cost-effective analysis of Cd concentration in soil and plant samples. Quantitative models for soil Cd using a combination of complementary proximal sensors data and chemometrics could feasibly be deployed for long-term monitoring of soil Cd at concentrations below pXRF detection limits and with reduced matrix interference from organic matter when compared to the individual techniques alone. The use of proximal sensing techniques to determine total soil Cd concentration in New Zealand agricultural soils has the potential to improve the scale and scope of long-term repeated monitoring of soil Cd concentration required under the framework of the national Tiered Fertiliser Management System. Reflectance spectroscopy could potentially be implemented to monitor plant-available and potentially-available soil Cd fractions to minimise plant Cd uptake. The use of a large soil spectral library to assess the local Cd concentration in agricultural soils could reduce the analytical cost to the farmers and allow intensive spatial and temporal monitoring of pastoral farms based on spectral analysis only. The use of in-situ and laboratory proximal sensor data to calculate bioconcentration and translocation factors could potentially support the evaluation of Cd food chain transfer risks. The spectral library developed from this doctoral study, including soil and plant root and aboveground biomass pXRF, vis-NIR, and MIR spectra with a wide range of Cd concentration can be used as reference materials for field level and airborne remote sensing studies.Item Plant associated soil mechanisms of cadmium uptake and translocation in chicory and plantain : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Environmental Science at Massey University, Palmerston North, New Zealand(Massey University, 2021) Ubeynarayana, NilushaCadmium (Cd) is a non-essential trace element that is extensively distributed in the environment. Cadmium is effectively absorbed by plant roots and transported to its aerial parts and plants growing in soils with high Cd concentration can accumulate Cd in their roots and shoots to levels which can threaten human and animal health. Elevated Cd concentrations in New Zealand agricultural soils are a function of the country’s long-term history of using Cd-contaminated phosphate fertiliser. Recent studies have identified that two forage species chicory (Cichorium intybus L.) and plantain (Plantago lanceolata L.), which are increasingly used in New Zealand agriculture, accumulate a significantly higher shoot Cd concentration than traditional pasture species. The variation in Cd accumulation between forage species suggests that different plants have different abilities to absorb Cd in roots and translocate this trace element from roots to shoots. Thus, Cd uptake and the potential translocation of Cd to aerial tissues deserves more research, particularly for forage species of economic importance to countries such as New Zealand, where agriculture is dependent on pastoral grazing systems. Information from such studies will be useful in mitigating the continuing risk of Cd transfer into the food chain. The overall aim of this thesis is to better understand Cd uptake and translocation mechanisms in chicory and plantain. Cadmium uptake by plant roots is a function of rhizosphere soil chemistry and the interaction between plant roots and soil solution. Plants exude Low Molecular Weight Organic Acids (LMWOA) into soil solution and these play a key role in regulating Cd bioavailability. A pot trial was conducted to evaluate the influence of increasing soil Cd concentration on the secretion of LMWOAs by chicory and plantain roots and to analyse their impact on plant Cd uptake. Chicory and plantain were grown under increasing Cd levels and showed variable secretion of oxalic, fumaric, malic and acetic acids as a function of Cd treatment. Results revealed that the primary cause for the significant increase of shoot and root Cd concentration in both chicory and plantain, as a function of treatment level, is the significantly greater bioavailable Cd concentration in soil solution with increasing Cd treatment level. The significantly higher shoot Cd accumulation in chicory (18.63 mg Cd/kg DW) than plantain (4.22 mg Cd/kg DW) at the highest tested soil Cd concentration (1.6 mg Cd/kg) can be explained by increased acetic acid and reduced fumaric acid excretion from chicory relative to plantain. Increased understanding of Cd translocation mechanisms in plants requires knowledge of the free Cd2+ ion concentration in xylem saps. However, the determination of low concentrations of free Cd2+ ions in a low volume of xylem sap poses an analytical challenge. To overcome this limitation, a thiosalicylic-acid-modified carbon-paste electrode was developed as an alternative and reliable measurement tool for the detection of free Cd2+ ions in environmental samples, including xylem saps. Compared to other Cd2+ ion ligands used to develop Cd2+-ion-specific electrodes in literature, thiosalicylic acid is a readily available solid, which is stable to air, making it a conveniently handled ligand. The developed electrode showed a lower detection limit of 11 μg Cd/L (0.1 10-6 mol Cd/L) with a linear range from 20 to 100 μg Cd/L (0.18 10-6 to 0.88 10-6 mol Cd/L). To the best of my knowledge, this is the first time a Cd2+ ion-specific electrode was developed to determine free Cd2+ ion concentration in plant xylem sap. The modified electrode has the ability to distinguish between total Cd and free Cd2+ in solution and measure only the free Cd2+ ions in environmental samples, including xylem sap, with high precision (RSD<5%). Subsequent analysis using the thiosalicylic acid modified electrode showed that Cd is mainly in a complex form in chicory and plantain xylem sap. Therefore, a glasshouse experiment was set up with six increasing Cd concentrations in hydroponic solution to assess the impact of LMWOA on xylem sap Cd translocation and shoot accumulation in chicory and plantain. Results revealed that both chicory and plantain showed variable production of oxalic, fumaric, citric, malic and acetic acids with increasing Cd concentration in the hydroponic media. The higher shoot Cd accumulation (by 28-208%) in chicory compared to plantain can be explained in terms of variations in LMWOA production between chicory and plantain. Functional relationship analysis showed that the primary cause for higher shoot Cd concentration in chicory relative to plantain is fumaric acid production in chicory xylem sap which may bind with Cd in chicory and translocate the metal towards shoots. To explore the specific role of fumaric and acetic acids on Cd uptake and translocation in chicory, a glasshouse experiment was conducted with the external addition of fumaric and acetic acid into the hydroponic solution. Increasing fumaric acid concentration in the hydroponic solution showed the ability to reduce Cd uptake and translocation in chicory with a maximum reduction achieved at 10 mg/L and 50 mg/L fumaric acid treatment for root and shoot Cd accumulation, (respectively) for a solution concentration of 1 mg/L Cd. The shoot Cd concentration significantly increased at lower acetic acid treatment levels (1 mg/L) and reduced with increasing acetic acid concentrations from 10 mg/L to 50 mg/L in the presence of 1 mg Cd/L solution concentration. However, the root Cd accumulation increased as a function of acetic acid concentration in the hydroponic solution up to 50 mg/L acetic acid treatment. The root: shoot Cd concentration ratio showed a significant positive correlation (R=0.729 P<0.05) with acetic acid treatments (up to 50 mg/L treatment). Chicory biomass significantly reduced at all LMWOA treatments compared to the control treatment in the presence of 1 mg Cd/L Cd level, showing that there was a limited potential ameliorative effect of LMWOA on Cd toxicity at any concentration for the experimental conditions used in this study. This study highlights that variations in plant root LMWOA secretion and xylem sap LMWOA production between chicory and plantain can explain the different shoot Cd accumulation characteristics of these two forage species. This work shows that fumaric acid plays a fundamental role in both Cd uptake and translocation in chicory, while such a role is not clear for plantain. Low secretion of fumaric acid by roots and production of fumaric acid in chicory xylem sap aid to increase shoot Cd accumulation in chicory compared to plantain while low acetic acid secretion by chicory roots supports the high shoot Cd accumulation in chicory compared to plantain. Future work is recommended to develop a new cultivar of chicory which express traits of variations in fumaric acid production and acetic acid production. Such work may yield new cultivars of chicory which restrict the translocation of Cd from roots to shoots in this important forage species. The future application of this work is to help develop strategies which could assist in mitigating high Cd accumulation in offal to maintain the standards of New Zealand’s food production.Item Distribution of cadmium in long-term dairy soils, its accumulation in selected plant species, and the implications for management and mitigation : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand(Massey University, 2017) Stafford, Aaron DavidAccumulation of cadmium (Cd) in agricultural soils, and in pasture, fodder and horticultural crop species is an on-going management concern for New Zealand agriculture. Recent implementation of a new National Cd Management Strategy (MAF, 2011) has increased awareness of this issue. Of key concern are long-term dairy farms in some of NZ’s most productive farming districts, where future land-use and trade limitations might be apparent due to their intensive phosphorus (P)-fertiliser application history. The research described in this thesis was undertaken to improve national understanding of; i) soil Cd variability within long-term dairy farms, ii) variability in Cd accumulation between different forage plant species and the consequent risk to livestock grazing these forages, and iii) soil / environmental factors and management / mitigation options that can influence Cd phytoavailability. Two long-term dairy farms on contrasting soils in the Waikato and Canterbury regions of New Zealand showed wide variability in soil total Cd concentrations (Waikato: 0.48-1.64 mg kg-1, Canterbury: 0.15-0.64 mg kg-1). The strong correlation (R2 = 0.84-0.85) between soil total Cd and total P concentrations indicated the importance of P fertilisation history on soil Cd variability. However, within blocks of common P fertiliser management history, there was also a strong effect of soil type on soil Cd concentration. Slope class only exerted an influence on soil total Cd concentration when slope exceeded 15°, while application of dairy shed effluent did not appear to have any consistent influence on soil Cd accumulation. All paddocks should be tested independently (based on predominant soil type) to allow Cd-enriched zones to be identified for remediation or alternative management purpose. Individual results can be areaweighted to provide a property-mean soil Cd concentration, where this is required. For both properties, soil Cd concentrations decreased with depth, however this was effect was stronger and more consistent in the Waikato due to its lack of tillage history. Models developed from visible and near-infrared reflectance spectroscopy (NIRS) scanning of intact soil cores (collected to 400-600 mm depth) were able to successfully predict soil total carbon (C) (R2 = 0.91-0.95) and total nitrogen (N) concentrations (R2 = 0.91-0.92). This technique shows promise for identifying paddock-specific tillage history, and based on the strong correlation between measured soil total Cd and total C and/or total N within each property (R2 = 0.83-0.90), for identifying Cd distribution within the soil profile. Such information could be used to quantify the tillage depth required to dilute Cd-enriched topsoil to a desired target. A glasshouse trial on 12 common animal forage species revealed that chicory and plantain accumulated significantly (P < 0.05) greater tissue Cd concentrations than other plant species. A subsequent survey of commercial farms across New Zealand validated these findings, with mean tissue Cd concentrations decreasing in the order chicory (1.82 mg kg-1 DM) > plantain (0.80 mg kg-1 DM) > ryegrass (0.11 mg kg-1 DM) > white clover (0.07 mg kg-1 DM). A very large range in tissue Cd concentrations for chicory and plantain (0.40-4.50 and 0.23-2.40 mg kg-1 DM, respectively) indicating the sensitivity of these species to soil Cd phytoavailability, although only chicory tissue Cd concentration could be satisfactorily explained (R2 = 0.745) by the variables soil total Cd concentration, pH and total carbon content. Although soil redox potential is known to influence Cd solubility, a pot trial on two different soils types (Kereone (Allophanic) and Topehaehae (Gley)) revealed that there was no significant difference in 0.05 M CaCl2 soil extractable Cd or plantain tissue Cd concentrations between cyclical flooded (3 days flooded, 11 days drained) and non-flooded (continuously drained) irrigation regimes. However, there was a large difference in soil extractable Cd concentration between the two soils types, with this difference appearing to be driven by differences in soil pH and organic matter content (and possibly clay mineralogy). Ultra-fine elemental sulphur and hydrated lime soil amendments were used to produce a wide range in soil pH (approximately 5.0-6.5) in two field trials on contrasting soils. There was a strong negative (linear) correlation (R2 = 0.64-0.82) between 0.05 M CaCl2 soil extractable Cd concentration and soil pH. Plant tissue Cd concentration was poorly explained by soil pH (chicory R2 = 0.35-0.52, ryegrass R2 = 0.19-0.42) and 0.05 M CaCl2 soil extractable Cd concentration (chicory R2 = 0.11, ryegrass R2 = 0.28). Perennial ryegrass Cd concentrations remained low (<0.3 mg kg-1) regardless of soil pH, suggesting that animal Cd accumulation risk is low when grazing this plant species, even in Cd-enriched soils at low pH. However, soil pH should be increased to a minimum of 6.5 to decrease livestock dietary Cd exposure when grazing chicory. Mean chicory Cd concentrations were significantly (P < 0.05) greater following a period of increased soil moisture, consistent with the increases in Cd solubility observed in the pot trial following soil re-wetting. This research highlights that Cd accumulation in soil and plants poses a real risk to New Zealand’s primary production industries. Existing animal Cd accumulation models indicate that when grazing Cd-accumulating forage species such as chicory and plantain, lamb kidneys may exceed food standard maximum levels in animals much younger than the current meat industry 30-month offal discard age. Understanding Cd accumulation in Cd-sensitive species such as chicory and plantain is important for farmers to be able to manage livestock dietary Cd exposure. Manipulation of soil pH to decrease soil Cd phytoavailability, and utilisation of deep inversion-tillage to bury Cd-enriched topsoil stand out as the most practical management strategies available to farmers. Animal grazing trials on Cd-enriched chicory crops are recommended to evaluate partitioning of ingested Cd, to validate and/or improve the predictions of existing animal Cd accumulation models. Plant breeding opportunities should be a priority focus, to produce chicory / plantain varieties that accumulate lower Cd concentrations in their vegetative tissues.Item Cadmium management in New Zealand's horticultural soils : a thesis presented in partial fulfilment of the requirements for the degree of Master of Environmental Management, Massey University, Palmerston North, New Zealand(Massey University, 2017) Thompson-Morrison, HadeeCadmium (Cd) is a heavy metal trace element which presents risks for the horticultural industry in New Zealand (NZ). This element is added to soils through phosphate fertiliser application, and once there may be available for plant uptake and food chain transfer. When food products exceed international standards for Cd concentrations, these products may be excluded from international markets upon which NZ relies to maintain its economy. This presents a reputational risk for NZ’s horticultural exports. Soil pH and organic matter (OM) content are the two key drivers influencing Cd’s bioavailability, and field trials are currently being undertaken in four horticultural sites throughout NZ – Pukekawa, Manawatu, and two adjacent sites at Lincoln – to test the efficacy of the use of lime and compost amendments to influence these soil variables and thus reduce Cd plant uptake from soils. Potatoes are grown at all sites while Lincoln also includes wheat. This research aimed to characterise these soils, including total and plant-exchangeable Cd concentrations, pH, OM content, cation exchange capacity, total and plant-exchangeable Zn concentrations, aluminium and iron oxide content, total phosphorus and total nitrogen content. Findings indicated that total Cd concentrations varied among sites, with the highest (0.52 mg kg-1) at Pukekawa, followed by Manawatu (0.26 mg kg-1) and Lincoln Wheat and Potato sites (both 0.13 mg kg-1). Exchangeable Cd concentrations were low at all sites (0.01-0.02 mg kg-1) indicating little risk of plant uptake from these soils. The mitigation strategy tested in this work focuses on pH as a key soil variable that can be readily changed to restrict Cd uptake. However, the effectiveness of amendment rates to effect target pH values is dependent on soil chemistry and rates will vary across sites. Incubation experiments were conducted to determine amendment rates for lime and sulphur, and to compare the pH of amended soil in a laboratory situation with the in-field situation. Incubation and field situations were found to be similar, with no significant differences between pH values after a period of 274 days in the incubator and 169 days in the field. The accuracy of the calculated amendment rates at achieving target pH values was assessed with extended incubation experiments. The results here varied between soils, with the sulphur application rate proving more accurate in the Pukekawa soil, however too high for the Manawatu and Lincoln potato soils. The calculated liming application rate similarly resulted in a higher-than-target pH, however after a period of 231-274 days the pH reduced and approached the target value. A cost-benefit analysis was undertaken to determine the economic viability of the proposed mitigation strategy at each potato site. Results proved the strategy to be a viable option, which would remain viable in the face of varying uncertainty and reductions in potato yields. Practical considerations including timing and weather conditions, and compost availability were considered. Implementation of this strategy within NZ’s current framework of the Tiered Fertiliser Management System, which focuses on total rather than exchangeable Cd concentrations, may present difficulties, and thus there is a clear need for risk-based, soil and crop specific guidelines for Cd management within a NZ context. Considering the apparent difficulties in designing pH amendments strategies, a model to convert pH buffer curve-generated lime application rates which can be derived in as little as 24 hours, to field applicable application rates which target a specific soil pH was developed for the Pukekawa soil. A similar model was not achieved for the Lincoln Wheat soil, and thus the development of such a model is not possible for all soil types. Where possible, the development of this model would be an innovative and useful tool for farmers with which to accurately and quickly determine required lime application rates to achieve a targeted soil pH. This would be of great benefit in the implementation of a Cd mitigation strategy using lime amendments, and would allow greater control over, and management of, soil pH in a horticultural context.Item The influence of zinc and copper fertilizer application on zinc, copper and cadmium concentration in mixed pasture : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Soil Science at Massey University(Massey University, 1996) Khan, Md. Afiqur RahmanThere has been considerable debate about the accumulation of cadmium (Cd) in agricultural soils and its subsequent uptake by pasture plants due to phosphate fertilizer application. Ruminants grazing pastures absorb a small fraction of this Cd, and some of this is subsequently accumulated in the liver and kidney. Although tissue accumulation of Cd in grazing livestock is generally small (< 1 mg Cd kg-1 fresh tissue), but any reduction in plant uptake is beneficial in reducing such accumulation further, especially in the kidneys. Uptake of Cd by pasture may be affected by the concentration of other nutrient cations, such as zinc (Zn) and copper (Cu). In addition, since Zn and Cu are complexed by the same metal binding protein (metallothionein) as Cd, a change in the ratio of these nutrients in pasture may also reduce Cd accumulation rates by interfering with Cd accumulation. In order to assess the effects of Zn and Cu on Cd uptake by pasture, a field experiment was conducted, using three pairs of pasture plots with low ( 0.2 mg Cd kg-1) and high (0.6 mg Cd kg-1) background Cd status. Twelve sub-plots (l.44 m2) were laid out in each plot and increasing levels of Zn (0, 5, 15 and 40 kg ha-1) and Cu (0, 2, 5 and l0 kg ha-1) were added as ZnSO4. 7H2O and CuSO4.5H2O respectively. Pasture samples were collected at regular intervals and analysed for dry matter yield, botanical composition and Zn, Cu and Cd uptake. Soil samples were extracted with 0.01M CaCl2 and 0.lM HCl solution to measure the plant available Zn, Cu and Cd. It was found that the plots with a high background Cd status in the soil resulted in a higher Cd concentration in mixed pasture (0.22 mg Cd kg-1 DM) than those with a low background Cd status (0.10 mg Cd kg-1 DM) at the first harvest (after 73 days). The Cd concentration in the mixed pasture was higher during the summer (December) period than in the early spring (September). Application of Zn fertilizer increased the Zn concentration in pasture from 37 to 150 mg kg-1 DM at the first harvest. Excessive amounts of Zn lead to a decrease in DM yield. The growth of pasture was controlled principally by the amount of plant available Zn, which depended on the amount of both added Zn and added Cu. The effect of the added Cu was to increase the toxicity of the addd Zn. Application Cu fertilizer increased the Cu levels from 9 to 16 mg kg-1 DM at the first harvest. The Cu concentration in pasture continued to decrease with time following the addition of fertilizers. The legumes are more tolerant of Cu than grass. The Cu concentration in harvest 4 (after 159 days) ranged from 6.9 to 7.0 mg kg-1 DM in grass and 8.9 to 9.9 mg kg-1 DM in legumes. The Cd concentration in the pasture decreased with increasing Zn concentration in the pasture at the first harvest. The effect of Zn on Cd uptake was more pronounced on plots with a high background Cd status in the soil. The effect of Zn on Cd concentration depends on the external Zn concentration levels. There was no consistent effect of Cu concentration on Cd concentration. The effect of the addition of Cu and Zn in fertilizer was to lower the Cd:Cu and Cd:Zn ratios in the herbage. There was a good relationship between soil available Zn as extracted by 0.1M HCl and Zn concentration in the herbage. A similar observation was obtained for Cu. But there was no consistent relationship between 0.01M CaCl2 extractable Cd and the Cd concentration in pasture. The results indicated that pasture and soil analysis for Cd and Zn may provide useful guides to situations where Cd concentrations in pasture may be decreased by Zn applications.Item Adsorption of zinc and cadmium by soils and synthetic hydrous metal oxides : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University(Massey University, 1981) Roberts, Antony Hugh ColebyThis study involved an investigation into the adsorption ot Zn and Cd by soils and synthetic hydrous metal oxides, and the effect of several factors on the adsorption reactions in controlled laboratory experiments. Initially, in order to determine low concentrations of Zn and Cd in solution, a concentration procedure involving solvent extraction was developed. The procedure utilised a chelating agent, dithizone, and an organic solvent, carbon tetrachloride. Zinc and Cd were back-extracted from the organic solvent into hydrochloric acid to achieve a ten-fold increase in concentration. For soils and Fe gel, adsorption of both Zn and Cd was characterised by an initial, rapid removal from solution which was followed by a much slower, continuing decrease in solution concentration. Isotherms (40 hr) for Zn and Cd adsorption from 3 x 10-2M NaCl by soils, synthetic hydrous ferric oxide (Fe gel) and allophane, were described using two Langmuir equations. Values for the derived free energies of adsorption, for each region of adsorption, were similar for both Zn and Cd on all soils and on Fe gel. Allophane, however, had higher free energies of adsorption than either the soils or Fe gel. For all adsorbents, Langmuir constants derived from adsorption data indicated that the adsorbing surface contained a relatively small number of sites with a high free energy of adsorption and a much larger number of sites with a lower adsorption energy. Iron gel appeared to provide a satisfactory model for describing Zn and Cd adsorption by soils. Amounts of Zn and Cd adsorbed by Fe gel increased as pH increased. Adsorption of Zn and Cd by Fe gel in each Langmuir region appeared to be affected similarly by changes in pH, although the pH50 values (the pH at which 50% of the initial amount added was adsorbed) were higher for Cd than for Zn. Experimental data obtained in the study are consistent with a two-mechanism model for both Zn and Cd adsorption. It is proposed that the first mechanism involves the formation of a bidentate complex. Adsorption of Zn and Cd into this region involves the release of two protons for each Zn or Cd ion adsorbed, resulting in a bond of higher energy than when Zn or Cd is adsorbed by the second mechanism. In this latter case, it is proposed that a monodentate complex is formed with one proton released for each cation adsorbed. In addition to proton release, data in support of the two-mechanism system was obtained in isotopic exchangeability and desorption studies. For example, the mole ratios (i.e., number of protons released for every Zn or Cd ion adsorbed) for Zn and Cd were non-integer values. For Zn the mole ratios ranged between 1.31 and 1.67, and for Cd from 0.80 to 1.12, at pH 6.4. There was no obvious trend in mole ratios for Zn either with increasing amounts of Zn adsorbed or increasing pH, but for Cd mole ratios increased with increasing amounts of Cd adsorbed of increasing pH. The isotopic exchangeability of Zn was similar at all levels of Zn adsorbed, but decreased with increasing pH (pH 5.85 - 6.65) from 58% exchangeability to 27%, possibly due to an increased proportion of more-tightly bound Zn. Cadmium, by contrast, had a lower exchangeability at low levels of Cd adsorbed (55 - 76%) than at higher levels (80 - 85%) but was more exchangeable (55 - 85%) than Zn at equivalent pH values and lower surface coverages, indicating that a greater proportion of adsorbed Cd was less tightly bound compared to Zn. Sequential desorption of Zn by calcium ions followed by copper ions showed that a proportion of Zn (12 - 24%) was retained by the surfaces against desorption, further indicating the presence of two adsorption mechanisms of different binding strength. Only at low levels of adsorbed Zn did the amount of desorbed Zn closely approximate the amounts of Zn calculated (from Langmuir constants) to be in region II. These desorption data, together with the exchangeability data for Zn, point to the possible limitations in the use of Langmuir equations for quantifying the amount of Zn absorbed by each mechanism. The Langmuir isotherm studies, together with proton release, exchangeability and desorption data, indicated two mechanisms of adsorption for Zn and for Cd. However, the greater exchangeability and fewer protons released per mole adsorbed suggest that more Cd is held by the mechanism involving monodentate bonding than is the case for Zn. Zinc or Cd was not observed to be absorbed or "occluded" by synthetic ferric oxide gel or goethite. There was evidence to suggest that Zn and Cd might diffuse into cracks or defects in the crystal structure of natural goethite, developed by grinding. Although some fraction of adsorbed Zn or Cd was non-exchangeable and non-desorbable (by copper) for all three adsorbents, this Zn and Cd was not in the absorbed phase. Long term (up to thirty year) additions of superphosphate fertiliser to three soils did not produce measurable accumulations of Zn or Cd in the soils. The calculated additions agreed well with actual increases measured in total Zn, but not with actual increases in total Cd.