Soil Chemistry Notes PDF

⚛️ Soil Chemistry Notes Tags - The notes are organized thoroughly with base on the PPT order - I have included relevant questions and notes from the question Description examples file as well as...

⚛️ Soil Chemistry Notes Tags - The notes are organized thoroughly with base on the PPT order - I have included relevant questions and notes from the question Description examples file as well as the learning outcomes mentioned on the PPT Done? Parent item 👣 Soil Physics & Chemistry Status In progress Fundamentals of Soil Chemistry Soil Constituents O 46,5%; Si 27,5%; Al 8%; Fe 4,5%; C; Ca 3,5%; Na 3%; K 2,5%; Mg 2%; Ti 0,4% The solid phase Rock and Mineral fragments Primary: Silicates, quartz Secondary: Oxides, Hydroxides, Clay Minerals Organic Material: Biomass, humus Primary Minerals are those who come from the parent material: Silicate, Quartz Soil Chemistry Notes 1  Secondary Minerals are those that came from weathering: Oxides, Clay minerals Aluminium oxides Iron oxides Carbonates Gibbsite Al(OH)3 Goethite α-FeOOH Calcite Bohemite γ-AlO(OH) Hematite α-Fe2O3 Dolomite Diaspore α-AlO(OH) Magnetite Fe3O4 Magnesite Aluminium Oxide Bernalite Fe(OH)3 Al2O3 PseudoboehmiteAlOOH Silicates Primary: Secondary: With Al: Clay Minerals (all with Al) Feldspars (Orthiklas, Albit, Iliite Anothit Vermiculite Micas (Muscovit, biotit) Smectite Pyroxenes: Augite Amphibole: Hornblende Without Al Olivin, Quartz, SiO2 Main Processes Organic processes that influence components Soil Chemistry Notes 2  uptake by plants / microorganisms Efflux from plant roots / microorganisms Exudates from plant roots/microorganisms Precipitation Dissolution Co-precipitation Co-dissolution Complexation Chelation Chelation is the formation or presence of two or more separate bindings between a polydentate (multiple bonded) ligand and a single central atom Usually these ligands are organic compounds, and are called chelants, chelators, chelating agents, or sequestering agents The Ion Exchange Surface reaction in which ions absorbed to the solid phase are replaced by ions of the same charge Exchangeable cations: Soil minerals and soil organic matter (SOM) are predominantly negatively charged. The exchange equilibrium with major cations in soil solution is attempted to be reached. Major cations = Ca2+, Mg2+, K+, Na+ (“base”cations, dominate at moderate to high pH) Al3+, Fe3+, Mn2+, H+ (“acidic”cations, dominate at low pH) Extraction (exchange) by neutral salt solution containing relatively high concentrations of a competitive cation (Ba2+, NH4+) Soil Chemistry Notes 3  In Austria: 0.1 M BaCl2 extraction for 2 hours (end-over-end shaking), filtration and measurement of the major cations in the filtrate; solution : soil ratio 100 mL : 5 g Cation exchange capacity (CEC): back change of the Ba2+ ions by adding 100 mL 0.2 M HCl to the soil remaining in the filter --> 2 hours shaking and filtration --> measurement of Ba in the filtrate. CEC is calculated as the charge equivalent of Ba2+ ions (mmolc/kg soil). CEC is a measure of the negative surface charges available for cation sorption in soil Related to the specific surface area Depends on mineral composition and SOM content CEC at soil pH (“effective”) versus CEC obtained by buffers (pH 8.2) BaCl2 solution (“potential” CEC) Buffer solution: triethanolamine + 0.2 M HCl (> pH 6.5 required to avoid dissolution of Ca from carbonates) Base saturation: Percentage of “base” cations relative to CEC (%) Significance of CEC Measure of the buffer capacity of a soil for cations Nutrient and pollutant binding Hydrolysis = the cleavage of chemical bonds through the addition of water. The reverse reaction is a condensation reaction. Hydrolysis is a process in the degradation of substances. Example: X-Y + H-OH X-H + Y-OH (hydrolyses of a molecule X-Y) 💡 How does it affect element concentration in soil solution? Soil Chemistry Notes 4  Through hydrolysis cations (e.g.: K, Ca, Mg, Fe) in a rock-framework are replaced by H+ protons --> the framework structure becomes instable and collapses. Isomorphic substitution. Hydrolysis requires the presence of water, which is the main transporter for soluble nutrients in soils. Hydrolysis has an effect on the free ions. ->the element concentration in solution gets higher Properties Chemical Speciation Chemical speciation refers to the occurance of different chemical forms (species) of an element within each of the three soil phases (solid, soil solution, soil air). For instance in soil solution, an element may occur as "free" cation, various hydrolysis species, inorganic ion complexes and ion pairs, organic complexes. 💡 What is the ecological Significance of chemical speciation? These different species have different diffusion coefficients (effect of size and charge), react different with charged soil surfaces (sorption) and may selectively be taken up (or not) through biological membranes (of plant roots and soil microorganisms) Examples of what it might affect: element transport and (diffusional) fluxes in soil leaching and buffering by the soil matrix uptake by plants and microorganisms (typically "free" ions are taken up most readily) Soil Chemistry Notes 5  toxicity (e.g. "free" Al3+ is more toxic than organic complexes of Al) Chemical Concentration Concentration is the abundance of a constituent divided by the total volume of a mixture. The activity of a material is a measure of its effective concentration in a chemical reaction. 🡪 Generally the concentration of a chemical substance does not represent its actual reactivity. This difference between the activity (or effective concentration) and concentration of a substance is due to interactions (eg: repulsion/attraction between ions). Chemical Equilibrium It is reasonable to assume equilibrium for fast reactions such as most reactions in soil solution such as hydrolysis and formation of inorganic complexes; non- specific sorption and cation exchange are typically also fast reactions for which the equilibrium assumption may approximately apply in many cases; however, soil is an open system in which inputs (rainfall, litter fall, crop residues..) and outputs (plant uptake, leaching..) can at least locally and periodically disturb the equilibrium; precipitation / dissolution reactions are often kinetically limited and may therefore not reflect equilibrium conditions. Chemical equilibrium is the state in which both reactants and products are present at concentrations which have no further tendency to change with time. Usually, this state results when the forward reaction proceeds at the same rate as the reverse reaction. Video 12.01 Min 43:10 : The most soluble mineral controls the concentration= The mineral with the highest equilibrium concentration. The solution is supersaturated for the less Soil Chemistry Notes 6  soluble minerals and they will precibitate. But Mineral A in soil solution will be depleted over time, now B is the most soluble. The least soluble mineral remains. Mass Action Law concentration based constants vary with ionic composition / ionic strength activity constants are real constants A + B = C + D 💡 Complete a chemical equation and apply the mass action law to calculate the log activity Chemical Activity refers to a measurement of the “effective concentration” of a chemical species in a mixture. The species' chemical potential depends on the activity of a real/ideal solution. The activity (a) is treated as a dimensionless quantity. The activity of an ion is particularly influenced by its surroundings. Activities should be used to define equilibrium constants but, in practice, concentrations are often used instead. The same is often true of equations for reaction rates. However, there are circumstances where the activity and the concentration are significantly different and, as such, it is not valid to approximate with concentrations where activities are required. a = y ∗ C Activity Coefficient The activity coefficient is a measure for the reactivity at a given concentration. Soil Chemistry Notes 7  💡 Within which limits does the activity coefficient range? Activity coefficient y = aA / cA (aA= activity of substance A; cA= concentration of substance A) The coefficient (y) is not a constant, it varies due to the physico- chemical conditions. In heavily diluted solutions (ideal case would be infinite dilution) the activity coefficient approaches the value of 1 ( aA = cA). It ranges between 1 (infinite) and 0 (theoretically). Debye-Hückel Equation Estimates activity coefficient KaTeX parse error: Can't use function '$' in math mode at position 1: $̲$ log(yi) = - A… Ionic Strength The ionic strength of a solution is a measure of the concentration of ions in that solution. Concentration-based constants vary with ionic composition/ionic strength. Generally multivalent ions contribute strongly to the ionic strength. Soil Chemistry Notes 8  💡 Know how to calculate the ionic strength 💡 Calculate the activity 1. Calculate the Ionic Strength 2. Calculate the activity coefficient with the Debye-Hückel Equation 2 log(yi) = −A ∗ (Z i) ∗ μ ​ 3. Calculate the Activity Base Saturation Base saturation is defined as the percentage of “base” cations (Ca2+, Mg2+, K+ or Na+) in relation to CEC [%]. Furthermore it can be defined as the amount of positively charged ions (excluded hydrogen and aluminum ions) that are absorbed on the surface of soil particles. Base saturation is positively related to soil pH because a high base saturation value would indicate that the exchange sites on a soil particle are dominated by non-acidic ions. Index of soil weathering. Sorption Adsorption Desorption Soil Chemistry Notes 9  Transfer of an ion from the soil Desorption refers to the transfer of an solution to the solid phase ion from the solid phase to the soil This process creates a film of the solution. In order for desorption to adsorbate on the surface of the take place the particle must have adsorbent. enough energy or enough energy must be provided to the particle to overcome the bond energy. Cations are preferably adsorbed in most temperate climate soils that contain permanent negatively-charged clay minerals (e.g. illite, vermiculite) and organic matter as both exhibit net negative charges on their surfaces within the common range of pH in soils. 💡 Adsorption vs Desorption Chemical reaction = process that leads to the transformation of one set of chemical substances to another. They typically comprise changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei. Ion exchange = surface reaction in which ions adsorbed to the solid phase, are replaced by ions of the same charge (i.e. cation by cation or anion by anion). Ion exchangers are either cation exchangers or anion exchangers. Non specific adsorption vs specific Non-specific sorption: electrostatic (Coulomb) forces (e.g.: attached to a clay mineral, outer sphere complex) Specific sorption: mainly covalent bonds (e.g.: in a clay mineral, inner sphere complex) Soil Chemistry Notes 10  However, as compared to chemical reactions (e g dissolution or precipitation of soil minerals), the equilibrium constant of adsorption reactions is not independent of the composition of the system but needs to be experimentally determined for each soil (sorption envelops) Sorption isotherm The sorption isotherm describes the relation between the concentration of an ion/compound in the solid phase to that in the liquid phase (=soil solution). Freundlich equation: Q = A ∗ Cn  Langmuir equation: Q = (b ∗ k ∗ C )/(1 + k ∗ C )  Chemical reaction vs Sorption At equilibrium, chemical reaction can be treated using the mass action law. The equilibrium constant K is independent of the composition of the system. In contrast, K changes as function of availability or occupation of surface sites and concentration of the ion in soil solution during the reaction and is soil- specific. Chemical reaction = process that leads to the transformation of one set of chemical substances to another. They typically comprise changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei. Sorption= transfer of an ion from/to the soil solution to/from the solid phase. This process creates a film of the adsorbate on the surface of the adsorbent. The sorpted ions can be exchanged. One substance becomes attached to another. Precipitation is the creation of a solid from a solution. When the reaction occurs in a liquid solution, the solid formed is called the 'precipitate'. Soil Chemistry Notes 11  The dissolution of gases, liquids, or solids into a liquid or other solvent is a process by which these original states become solutes (dissolved components), forming a solution of the gas, liquid, or solid in the original solvent Species Free ions (anion, cation), ion-pairs, ion-complexes, organic-complexes, hydrolysis-species, redox-species, molecules, chelates Ion Pairs Ion Complexes = Outer sphere complex = Inner sphere complex are created by ion impact Is an aggregate of an ion with one or Each pair consists of a free electron more molecules and a positive ion. Ion-pairs are formed when a cation and anion come together. Free Ions A free ion is an ion whose properties (spectrum and magnetic moment) are not significantly affected by other atoms/ions/molecules nearby. Free ions interact to/with ion-pairs/ion-complexes/organic substances/colloids. The ability of a solvent to form free ions will dictate the reactivity of the propagating cationic chain. In order for a plant to absorb nutrients, the nutrients must be dissolved (as ions). 💡 Usually the most readily available to plants and microorganisms Organic Complexes Soil Chemistry Notes 12  Redox Species Hydrolosis Species Fate of elements Nitrogen 1. Mineralisation + N : NH , N O2 4 ​ ​ Organic nitrogen to ammonium nitrogen Organic forms of nitrogen 95 of nitrogen in soil Organic nitrogen not taken up by plants but gradually transformed by soil microorganisms to ammonium (NH 4 Ammonium not leached to a great extent as it is a cation which is attracted to and held by the negatively charged clay minerals Ammonium is available to plants 2. Nitrification + − NH : NO 4 3 ​ ​ Ammonium nitrogen to nitrate nitrogen Rapid transformation of ammonium to nitrate (NO 3 in well drained, warm soils Nitrate is the principle form of nitrogen used by plants The negatively charged nitrate is leached from soil to groundwater as it is not attracted to clay minerals 3. Immobilisation − + NO ; NH : OrganicN 3 4 ​ ​ Soil Chemistry Notes 13  Nitrate or ammonium to organic nitrogen Rapid transformation of ammonium to nitrate ( N O3 −in well drained, warm ​ soils Nitrate is the principle form of nitrogen used by plants The negatively charged nitrate is leached from soil to groundwater as it is not attracted to clay minerals 4. Denitrification − NO => N2 , N2 O 3 ​ ​ ​ Nitrate nitrogen to gaseous nitrogen In unaerobic soils (e g Gleysols, soils with stagnic properties), microorganisms use the oxygen from NO 3 in place of that in the soil air and rapidly convert nitrate to nitrous oxide ( N2O) and nitrogen gases (N 2 Both gases escape to the atmosphere and are not available to plants This transformation can occur within two or three days in poorly aerated soil and can result in substantial loss of nitrate type fertilisers 5. Ammonia volatilisation + NH => N H3 4 ​ ​ Ammonium nitrogen to ammonia gas At high pH 7 5 large amounts of NH 4 are lost from soil by conversion to NH 3 gas and volatilisation (NH 4 NH 3 H+) To minimize these losses, solid ammonium type fertilizers, urea and anhydrous ammonia should be incorporated below the surface of a moist soil Soil Chemistry Notes 14  💡 Name the three most abundant forms of Nitrogen Amonium; Nitrate; Nitrite (toghether make ~5% of soil N) The N bound in organic molecules makes the other ~95% 💡 How do plants take up organic nitrogen? Generally organic nitrogen is not taken up by plants. Exception: Carnivorous plants consume animals Other ways to take up nitrogen: Symbiotic associations: with microorganisms who can bind the atmospheric N direct uptake from soil: take N up with roots in form of NH4+ and/ or NO3- (H+ protons excreted to the external solution in equimolar ratio to the NH4+ uptake). Nitrate is the principle form of nitrogen used by plants Phosphorus 💡 Name the two main inorganic chemical forms of phospfhorus HPO4-; H2PO4- HP O − 4 e H2 P O4− são as formas disponíveis no solo ​ ​ ​ A primeira está disponível ao máximo em ph4 A segunda está disponível a ph9 pH7 é onde há mais disponibilidade de fósfuro, uma vez que há igual concentração das duas Soil Chemistry Notes 15  💡 Name the two main inorganic chemical forms in the pH range In natural systems like soil and water, P will exist as phosphate, a chemical form in which each P atom is surrounded by 4 O atoms. Orthophosphate, the simplest phosphate, has the chemical formula PO4-3. In water, orthophosphate mostly exists as H2PO4- in acidic conditions or as HPO42- in alkaline conditions. “The solubility of the various inorganic phosphorus compoundsdirectly affects the availability of phosphorus for plant growth The solubility is influenced by the soil pH Soil phosphorus is most available for plant use at pH values of 6 to 7 When pH is less than 6 plant available phosphorus becomes increasingly tied up in aluminum phosphates As soils become more acidic (pH below 5 phosphorus is fixed in iron phosphates 💡 Apply the mass action law to the dissolution reaction of monocalcium phosphate Soil Chemistry Notes 16  💡 List the main forms of inorganic phosphorus in the soil in the solid phase soils with high pH (alkaline): Ca- and Mg- phosphates including apatite soils with low pH (acidic): Phosphates adsorbed to clay or Al-/Fe oxyhydroxides e.g. variscite, strengite Phosphate adsorbed to soil minerals (oxyhydroxides of Fe and Al, clay minerals) phosphates occluded/diffused in (oxy)hydroxides of Al, Fe, Mn 💡 Estimate the proportion of organic P up to 40-60% (>80% in organic soils such as fens and forest floors) Main compounds include phytin (2-50%), phospholipids (1-5%), nucleic acids (0.2-0.5%) Smaller amounts of phosphoproteins and metabolic phosphates P content in soil organic compounds in the range of 10 – 30 g kg-1 Small amounts of organic P in soil solution Microbial mineralization to inorganic P (orthophosphate) Arsenic Soil Chemistry Notes 17  💡 Which is the main form of arsenic in the solid phase of aerobic soils (at high redox potential? Fe- (oxy-) hydroxides, As-V min 82 in präsi 26.01 Adsorbed: low pH AsO2HO -> higher pH AsOOH Dissociation increases with increase of pH In soil solution: In oxygen-rich environments and well-drained soils, arsenate (+V) species dominate (H2AsO4- in acidic soils and HAsO42- in alkaline ones). 💡 Which is the main form of arsenic in the solid phase of aerobic soils (at low redox potential)? Low redox soils (e.g. water logging): As-III (lower oxidation state), As-Fe-Sulfides, As-Sulfides In soil solution H3AsO3 Under reducing conditions, such as regularly flooded soils, arsenite (+III) is the stable oxidation state, but elemental arsenic and arsine (III) can also be present in strongly reducing environments. Fe-arsenate(Fe3(AsO4)2) Soil Chemistry Notes 18  💡 57) Which oxidation state of arsenic is the most important in aerobic (oxic) soils? As-V 58) Which oxidation state of arsenic is the most important in anaerobic (anoxic) soils? As-III Cadmium Forms and reactions solubility controlled by adsorption on clay minerals, hydroxides (alumnium/iron) and organic matter unstable in small quantities Forms complexes with organic matter in the soil solid phase Species Free Cd 2 ++( is the predominant species between pH 3 and 8 this is the form typically taken up by plants and microorganisms Hydrolysis species such as Cd(OH) are not important In calcareous soils CdHCO 3 CdCO 3 0 may dominate At higher chloride (Cl concentrations ( salt affected soils) significant contribution of CdCl and CdCl 2 0 complexes At higher sulfate (SO 4 2 concentrations, Cd forms CdSO 4 0 complexes (e g in salt affected soils, soils derived on gypsum or after application of gypsum) Soil Chemistry Notes 19  Depending on the concentration and properties of soluble organic matter ( Cd forms soluble Cd SOM complexes 💡 How is cadmium solubility influenced by soil pH and how can you explain this? Präsi 2.2 min 28:30 why is Cd more soluble? Like any cation you can expect that protons that are more active in low pH exchange Cd from surfaces, Cd2+ will be transferred into soil solution, proton is sorbed to negatively charged surfaces Increasing H+ activity more and more Cd2+ becomes released in soil solution. Luvisol higher in Cd2+ , lower om content less Cd2+ is absorbed to the solid phase, there may be cases where high om enhances higher in Cd2+ solubility, this is the case if you have a high proportion of soluble om (like in Podsols), mostly effect of solid om is stronger When Cd forms complexes with for example Cl- the solubility gets higher because Cd2+ is removed from the solution, more Cd2+ can get dissolved, same with sulfate SO42-, not so much with nitrate and other anions High content of soluable organic matter (SOM) in soil- forms also soluble complexes (Podsols) Cd normally present in traces, so normally no precipitation because there is never a too high conc. (supersaturation) 💡 Which is the main chemical form species of cadmium in most soil solutions? Free Cd2+ (hydrated) (most common between ph 3-8 Soil Chemistry Notes 20  Soil analysis Basic characteristics pH EC (Electrical conductivity) EC (Exchangeable cations) Soil minerals and soil organic matter (SOM) are predominantly negatively charged. The exchange equilibrium with major cations in soil solution is attempted to be reached. Major cations = Ca2+, Mg2+, K+, Na+ (“base”cations, dominate at moderate to high pH) Al3+, Fe3+, Mn2+, H+ (“acidic”cations, dominate at low pH) Extraction (exchange) by neutral salt solution containing relatively high concentrations of a competitive cation (Ba2+, NH4+) In Austria: 0.1 M BaCl2 extraction for 2 hours (end-over-end shaking), filtration and measurement of the major cations in the filtrate; solution : soil ratio 100 mL : 5 g Cation Exchange capacity Cation exchange capacity (CEC [meq/100g]) is defined as a calculated value that is an estimate of the soils ability to attract, retain and exchange cation elements. It is reported in millequivalents per 100 grams of soil. Is a measure of the negative surface charges available for cation sorption in soil Larger CEC values indicate that a soil has a greater capacity to hold cations. Therefore, it requires higher rates of fertilizer or lime to change a high CEC soil. A high CEC soil requires a higher soil cation level to provide adequate crop nutrition. The particular CEC of a soil is neither good nor bad, but knowing it is a valuable management tool. Soil Chemistry Notes 21  💡 Describe the principles of measuring exchangeable cations and cation exchange capacity back change of the Ba2+ ions by adding 100 mL 0.2 M HCl to the soil remaining in the filter --> 2 hours shaking and filtration --> measurement of Ba in the filtrate. CEC is calculated as the charge equivalent of Ba2+ ions (mmolc/kg soil). Carbonate content Soils may contain substantial amounts of carbonates mainly of Ca and Mg (e.g. calcite, CaCO3; dolomite, CaCO3*MgCO3). A-C soils on carbonate or carbonate- silicate bedrock (Rendsina, Pararendsina, Chernozem on loess…). Secondary carbonates (e.g. in C horizons of Chernozems, Luvisols) Method of Scheibler (pressure calcimeter) Diluted HCl is added to wetted soil; the evolution of CO2 is measured volumetrically using the Scheibler apparatus; at given temperature, the measured volume of CO2 can be converted to mass equivalent of CaCO3 which then is expressed as g CaCO3 equivalent kg-1 soil Carbonate content: CaCO3 + 2H+ Ca2+ + CO2 + H2O Significance: Pedogenesis / soil formation (loss & redistribution of carbonates) pH buffer (carbonate buffer system) Dominance of Ca2+ on exchange complex Implications for micronutrient availability, especially for iron, zinc, manganese, copper (very low in calcareous soils) liming required for soils below optimal pH range (if pH drops below 5,5 to 6) Soil Chemistry Notes 22  Total Carbon & Nitrogen Inorganic carbon in carbonates Organic carbon in soil humus / organic matter Total C measurement by dry combustion and measurement of evolving CO2 Detection of CO2 for instance by infrared spectrometry Calculation of total C in g C kg-1 soil Organic Carbon & SOM Organic C calculated from total C minus C in carbonates SOM can be calculated by multiplying organic C by the factor 1.724 which is derived from the average content of C in SOM Significance of organic C and SOM Soil Nutrient & Pollutant analysis Total nutrient and pollutant contents 💡 Describe and explain the principal of total nutrient and pollutant analysis in soil Acid digestion of soil on a heating plate or block and measurement of the diluted and Filtered digests for the elements of interest Calculation of total nutrient/pollutant concentrations (e.g., mg P/kg soil) through acidic digestion with strong acids (e.g. Aqua regia, mixture of concentrated HCl and HNO3). Acid is added to soil, weighed into glass tubes, incubated at up to 140 °C and slow cooling down. Digest is transferred to marked tubes and filled up to e.g. 100 ml, filtrated and nutrients/pollutants are measured in the filtrate using AAS (atomic absorption spectrometry). Soil Chemistry Notes 23  Available Nutrients 💡 Describe and explain the principle of the analysis of available nutrients Principle: Extraction with water or a relatively week salt solution For micronutrients, complexing solutions such as EDTA are used to account for the action of plant root exudates Similarly for P and C Calibration Measurement Labile pollutants 💡 Describe and explain the principle of the analysis of labile/mobile pollutant elements Extraction with water or a relatively weak salt solution (e.g. 1 M NH4NO3 or 0.1 M CaCl2) Calibration: In pot and field experiments, the labile pollutant concentrations are compared to pollutant concentrations in soil solution (risk of leaching to groundwater) and uptake in plants and related toxicity symptoms; similarly, extractable pollutant levels can be related to the activity of soil microorganism or to absorption by animals and humans after ingestion From such studies, toxicity thresholds or risk values can be derived Soil Chemistry Notes 24  Measured pollutant extractability in a given soil can then be compared to such thresholds to decide about 💡 Why is EDTA used to extract plant available micronutrients from soil? EDTA = Ethylenediaminetetraacetic acid; it is a chelating agent. For micronutrients, complexing (chelating) solutions such as EDTA are used to account for the action of plant root exudates. It can bind 6 times to a cation, very stable complexes also with cations (charge number of at least +2) for example calcium with extremely low tendency to form complexes. Complexes of this type = chelate complexes, especially stable complexes with Cu2+, Ni2+, Fe3+ and Co2+. Acidic pH: EDTA is predominantly an undissociated acid reform, the acid dissolves slowly, especially at low pH, the salts are very soluble in water. Soil Chemistry Notes 25