Soil Colloids Overview CHEM 133

Soil Colloids Overview Definition: Fine particles ( Ca²⁺ > Na⁺). Phosphates are adsorbed in high pH due to Ca fixation; in low pH due to Al and Fe fixation 5. Non-Permeability: Cannot pass through semi-permeable membranes. 6. Cohesion & Adhesion: Cohesion binds particles; adhesion help...

Soil Colloids Overview Definition: Fine particles ( Ca²⁺ > Na⁺). Phosphates are adsorbed in high pH due to Ca fixation; in low pH due to Al and Fe fixation 5. Non-Permeability: Cannot pass through semi-permeable membranes. 6. Cohesion & Adhesion: Cohesion binds particles; adhesion helps retain water. 7. Swelling & Plasticity: Swelling increases volume, while plasticity allows shaping. Inorganic: silicate clays are dominant in temperate; Fe and Al hydrous oxides dominant in tropical. Organic: dominant in temperate Silicate Clay Minerals 1. Two-Layer Type (1:1): One silica and one alumina sheet (e.g., kaolinite). Non-expanding, low cation exchange. 2. Three-Layer Type (2:1): Two silica sheets and one alumina sheet (e.g., montmorillonite). Expands, high cation exchange. Clay Types Comparison (From Table 7.4) highly weathered highly developed Property Kaolinite Montmorillonite Illite Structure 1:1, Non-expanding 2:1, Expanding 2:1, Non-expanding Size (microns) 0.1–5 (Coarse) 0.01–1 (Fine) 0.1–2 (Medium) Surface Area 5–20 m²/g 700–800 m²/g 11–120 m²/g Plasticity Low High Medium Swelling Low High Medium Capacity substitution of Al by Mg Substitution of Si by Al Substitution None or Fe Non exchangeable None Mg K cations High Low Medium Porosity & Permeability Significance Enhances soil fertility by holding water and nutrients. Affects soil structure, aeration, and water retention. Soil Colloids, Their Chemical Nature, and Properties Key Concepts 1. Definition and Types of Soil Colloids: ○ Inorganic: Crystalline silicate clays: e.g., kaolinite, montmorillonite, illite. Non-crystalline silicate clays: Amorphous. Iron and aluminum oxides: Common in tropical soils. ○ Organic: Humus: Dominant in temperate soils. 2. Chemical Composition: ○ Comprised of silica (SiO₂), alumina (Al₂O₃), and associated nutrients (e.g., Mg²⁺, Ca²⁺). ○ Contains negative charges primarily due to isomorphous substitution or pH-dependent charges. 3. Layering of Silicate Clays: clay is the general term for inorganic colloids ○ 1:1 Clays (e.g., Kaolinite): Stable, low CEC, good physical properties, suitable for construction. ○ 2:1 Expanding Clays (e.g., Montmorillonite): High CEC, swelling/shrinking, nutrient-rich but poor physical structure. (other examples: smectite, vermiculite) ○ 2:1 Non-expanding Clays (e.g., Illite): Intermediate properties. aka hydrous mica 4. Properties Imparted by Soil Colloids: ○ Chemical: High cation exchange capacity (CEC) facilitates nutrient availability. Buffering capacity stabilizes soil pH. Adsorption aids in retaining water and nutrients. ○ Physical: Large surface area enhances reactivity. Cohesion, adhesion, and plasticity support soil structure. Swelling and shrinking influence soil aeration and water retention. 5. Uses and Benefits: ○ Enhance soil fertility by retaining nutrients and water. ○ Stabilize soil pH and improve soil structure for better plant growth. ○ Specific clays (e.g., kaolinite) are used in construction and ceramics due to stability. Study Questions: 1. How do colloids benefit soil? ○ Retain water/nutrients, improve structure, stabilize pH. 2. What are their uses? ○ Kaolinite: Construction. ○ Montmorillonite: Fertile soils. ○ Illite: Moderate farming. 3. Why are they important? ○ They enhance soil fertility and structure, supporting plant and microbial life. Origin of Charges in Soil and Ion Exchange Key Points to Review: 1. Sources of Charges in Soil Colloids: ○ Isomorphous Substitution: Permanent negative charges arise when ions like Al³⁺ replace Si⁴⁺ in the tetrahedral layer or Mg²⁺ replaces Al³⁺ in the octahedral layer of clays. this is common in montmorillonitic clays ○ Ionization of Hydroxyl Groups: pH-dependent charges are generated when hydroxyl (-OH) groups at clay edges ionize, adding positive or negative charges. common in tropics ○ Organic Functional Groups: Groups like -COOH and -OH in humus ionize based on pH, contributing pH-dependent charges. 2. Ion Exchange in Soil: ○ Cation Exchange Capacity (CEC): Reflects the soil's ability to hold and exchange positively charged ions. measure of negative charge in the soil; increases as pH increases ○ Anion Exchange Capacity (AEC): Indicates the soil's ability to exchange negatively charged ions, influenced by organic matter and pH. 3. Importance of pH: Higher pH increases negative charges, enhancing soil fertility by retaining essential cations like K⁺, Ca²⁺, and Mg²⁺. Types of Ion Exchange and Their Importance to Soil Ion Exchange Overview Ion exchange is a reversible process where cations and anions are interchanged between the solid and liquid phases in soil. This process is central to soil fertility, nutrient availability, and soil structure stability. There are two types of ion exchange: 1. Cation Exchange (Base Exchange) ○ Involves the interchange of positively charged ions (cations) between soil colloids and the soil solution. ○ Cation exchange capacity (CEC) determines soil’s ability to retain essential nutrients like Ca²⁺, Mg²⁺, K⁺, and NH₄⁺. Base saturation increases as pH increases, base saturation increases as hardness increases 2. Anion Exchange (Acid Exchange)  ○ Involves the interchange of negatively charged ions (anions) between soil colloids and the soil solution. ○ Occurs primarily in acidic soils with hydrous oxides of iron and aluminum that exhibit positive charges. Cation Exchange: Benefits and Mechanisms 1. Benefits of Cation Exchange: ○ Major source of essential plant nutrients like K⁺ and Mg²⁺. ○ Retains fertilizers (K⁺, NH₄⁺) and reduces nutrient losses by leaching. ○ Adsorbs harmful metals like Cd²⁺ and Pb²⁺, cleaning percolating water. ○ Regulates soil pH through lime application, especially in acidic soils. ○ Enhances soil structure by binding soil particles into aggregates. 2. Mechanism of Cation Exchange: ○ Cations adsorbed on soil colloids (clay or organic matter) are exchanged with cations in the soil solution. H+>Al3+>Ca2+=Mg2+>K+=NH4+>Na+ ○ Adsorption follows an order: H⁺ > Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ > Na⁺. In kaolinite, monovalent cations are more adsorbed. In montmorillonite, divalent cations are more adsorbed. Anion Exchange: Significance and Fixation 1. Importance of Anion Exchange: ○ Plays a crucial role in phosphate ion availability and fixation. ○ Anions like H₂PO₄⁻ are strongly adsorbed, whereas NO₃⁻ and SO₄²⁻ are prone to leaching at neutral or alkaline pH. OH->H2PO4->SO42->NO3-=Cl ○ Fixation of phosphate ions reduces immediate availability but allows slow nutrient release with lime application. fixed phosphates are not available for anion exchange. if fixation occurs at strongly acidic conditions, it becomes irreversible and totally unavailable for plants. 2. Mechanism of Anion Exchange: ○ Positive charges on hydrous oxides of iron and aluminum attract anions. ○ Adsorbed anions are replaced or fixed, often forming insoluble compounds like hydroxyl phosphates, which are less available to plants. Study Questions 1. Benefits of Cation Exchange in Soil: Supplies key nutrients (K⁺, Mg²⁺, Ca²⁺) for plants. Improves fertilizer retention and reduces nutrient leaching. Regulates soil pH and lime requirements. Purifies water by adsorbing toxic metals. Enhances soil structure and water retention through aggregation. 2. Isomorphous Substitution: Definition: Replacement of ions in clay minerals, causing a permanent negative charge. Conditions: Happens during clay mineral formation, e.g., Si⁴⁺ replaced by Al³⁺. Outcome: Increases soil’s cation exchange capacity (CEC), improving nutrient retention and fertility. Cation Exchange Capacity (CEC) and Base Saturation Cation Exchange Capacity (CEC) Definition: The total capacity of soil colloids (clay and organic matter) to absorb and exchange cations. Significance: ○ Determines soil's ability to retain essential nutrients like Ca²⁺, Mg²⁺, and K⁺. ○ Higher CEC = higher fertility and nutrient retention. Factors Affecting CEC: ○ Soil Type: Clay soils and those rich in organic matter have higher CEC than sandy soils. ○ pH Influence: Acidic soils (low pH) have lower CEC, while lime application increases CEC by raising pH. Base Saturation Definition: The percentage of CEC occupied by base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺). Calculation: Importance: ○ High base saturation (e.g., >50%) correlates with neutral or alkaline soils, indicating better nutrient availability. ○ Low base saturation (2 ppm @420 nm ○ Quantification: Colorimetric methods (Molybdenum Blue, Molybdovanadate). for 0.1-1.0 ppm range, @660 nm arsenate and silicates cause + error while Cl and F causes - 3. Potassium (K⁺) Sources and Pools: ○ Primary minerals: Feldspar, micas. ○ Forms: Non-exchangeable (within mineral lattices), exchangeable (readily available), water-soluble. Transformations: ○ Weathering of minerals releases K⁺. ○ Fixation: K trapped in clay layers, released by wetting/drying. Effects on Availability: ○ Excess lime competes with K uptake (via Ca²⁺ competition). Determination Methods: ○ Total K: Decomposed with HF and H₂SO₄. ○ Exchangeable K: Extracted with NH₄OAc. ○ Quantitative Analysis: Flame photometry, Atomic Absorption Spectroscopy (AAS), or Inductively Coupled Plasma (ICP). Semi-Macronutrients in Soil – Sources and Methods of Analysis 1. Sulfur (S) Sources: ○ Minerals: Sulfates and sulfides (e.g., FeS, pyrite in acid sulfate soils). ○ Atmosphere: Acid rain and plant absorption. ○ Organic Matter: Derived from amino acids/proteins. Methods of Analysis: ○ Turbidimetric Method: Extract SO₄²⁻ with BaCl₂ to form BaSO₄, quantified by turbidity. for available S ○ Johnson and Nishita Method: Converts SO₄²⁻ to H₂S, detected colorimetrically via methylene blue complex. ○ Acid Digestion: Uses HClO₄ or NaOBr for total S analysis. 2. Calcium (Ca²⁺) and Magnesium (Mg²⁺) Sources: ○ Calcium: Found in feldspars, calcite (CaCO₃), and liming materials.  ○ Magnesium: Derived from ferro-magnesian minerals, olivines, and organically complexed pools. Methods of Analysis: ○ Exchangeable Ca and Mg: Extract using acetate salts (e.g., NH₄OAc) or chloride salts. Used for determining exchange sites and CEC. ○ Total Ca and Mg: Na₂CO₃ fusion followed by quantitative analysis. ○ Quantitative Determination: AAS/FES: Calcium (absorption/emission mode), Magnesium (absorption mode). Titration: EDTA titration using indicators (e.g., EBT or calcon). Ion Chromatography: Employs cation exchange methods. 3. Deficiency Corrections Calcium Deficiency: ○ Use gypsum (CaSO₄·H₂O), Ca-bearing P fertilizers, or Ca liming materials. Magnesium Deficiency: ○ Apply Epsom salts (MgSO₄·7H₂O) or Mg-liming materials like MgCO₃. Micronutrients in Plants Micronutrients are essential elements required in trace amounts for plant growth and development. Despite their small concentration, they play critical roles in various biochemical and physiological processes. The seven primary micronutrients are: Cationic Micronutrients: Iron (Fe), Manganese (Mn), Copper (Cu), Zinc (Zn) Anionic Micronutrients: Boron (B), Molybdenum (Mo), Chlorine (Cl) Roles of Micronutrients Cu, Mo, Fe - chlorophyll formation and carriers 1. Iron (Fe): Aids in chlorophyll formation, acts as an electron carrier, and is essential in enzyme systems. 2. Manganese (Mn): Involved in nitrogen utilization, especially in nitrate reduction. 3. Copper (Cu): Facilitates respiration and iron utilization, and acts as a co-enzyme. 4. Zinc (Zn): Stimulates growth hormone production. 5. Boron (B): Helps in sugar translocation and cell wall synthesis. 6. Molybdenum (Mo): Critical for nitrogen transformation in both symbiotic and non-symbiotic systems. 7. Chlorine (Cl): Contributes to osmoregulation and photosynthesis. Effects of Micronutrient Deficiency Deficiencies can impair plant metabolism and growth: Iron (Fe): Causes chlorosis (yellowing of leaves). Manganese (Mn): Leads to interveinal chlorosis and reduced nitrogen assimilation. Copper (Cu): Results in stunted growth and wilting. Zinc (Zn): Causes rosetting of leaves and reduced growth. Boron (B): Leads to brittle tissues, poor sugar transport, and growth retardation. Molybdenum (Mo): Affects nitrogen fixation and results in yellow, spotted leaves. Chlorine (Cl): Rarely deficient but can lead to reduced plant vigor. Effects of Toxicity Micronutrient excess can also harm plants: Low pH soils: Micronutrients like Fe and Mn may become toxic. Excessive applications of fertilizers: Can lead to imbalances and induce deficiencies in other nutrients. Methods of Analysis Micronutrient analysis is crucial for assessing soil and plant health. Common methods include: 1. Extraction Techniques: ○ Water or salt solutions (e.g., deionized water, 1N KCl, NH4CH3COO). ○ Weak and strong acids (e.g., 0.1N HCl for Fe, Mn, Cu, Zn; hot water for B). 2. Chelating Agents: ○ DTPA, EDTA, or TEA for micronutrient extraction. 3. Quantitative Determination: ○ Spectroscopy: Atomic Absorption Spectroscopy (AAS), Inductively Coupled Plasma Emission Spectroscopy (ICPES). ○ Colorimetric Methods: Using specific reagents to form measurable complexes. 4. Soil Testing: ○ Targeted tests for individual micronutrients like B (hot water-soluble) or Fe and Mn (extraction with HCl or NH4OAC). Micronutrient Interactions Deficiency or toxicity of one micronutrient can affect the availability of others. For example: ○ High phosphorus levels may induce zinc and iron deficiencies. ○ Excess manganese can suppress iron uptake and vice versa. liming causes B deficiency; Cu/Fe accumulation causes Zn deficiency Practical Recommendations Regular soil and plant tissue testing are essential for effective nutrient management. Maintain optimal soil pH to ensure balanced micronutrient availability. Use targeted fertilizers and amendments to address specific deficiencies. Deficiency Symptoms 1. Nitrogen: Yellowing (chlorosis) starting on older leaves, stunted growth, poor lateral branching. 2. Phosphorus: Dark green leaves with purple undertones, stunted growth, thin stems. 3. Potassium: Chlorosis on leaf margins, necrotic spots, weak stalks. 4. Calcium: Tip burn, blackened roots, blossom-end rot. 5. Magnesium: Interveinal chlorosis on older leaves. 6. Micronutrients: ○ Iron: Interveinal chlorosis in young leaves. ○ Boron: Poor fruit and seed set. ○ Zinc: Stunted growth, interveinal chlorosis. ○ Copper: Small, misshapen leaves. Excess Symptoms 1. Nitrogen: Excessive vegetative growth, poor fruit set. 2. Phosphorus: Interference with micronutrient uptake. 3. Potassium: Impedes uptake of magnesium and calcium. 4. Micronutrients: Toxicity leads to discoloration, leaf burn, or inhibited growth. Module 7 - Fertilizers (CHEM 133) I. Introduction Fertilizers are materials added to soil to supplement deficient nutrients. They significantly enhance agricultural productivity and are subject to strict regulations to ensure quality and proper use. III. Categories of Fertilizers Fertilizers are categorized by: ○ Number of nutrient elements they supply. ○ Method of manufacture. ○ Effects on soil. ○ Supplementary Reading explains more. IV. Fertilizer Inspection and Control Regulated to ensure quality and inform end-users via proper labeling. Key details in fertilizer labels include: 1. Fertilizer guarantee: Specifies nutrient content. 2. Fertilizer recommendation: Suggests proper application methods. Learn More. V. Fertilizer Analysis Sample Preparation: Standard procedures for reliable nutrient determination. Moisture Content (Gravimetric Method): ○ Assumes weight loss during drying is due to moisture. Nitrogen Content: ○ Devarda’s Alloy reduces nitrate to ammonia for accurate measurement. Details and Tolerance Limits. VI. Regulatory Policies Detailed fertilizer regulatory policies can be found in the FPA Blue Book. Study Questions & Brief Answers 1. Categories of fertilizers? Based on nutrient elements supplied, manufacture type, and soil effects. 2. Details on fertilizer labels? ○ Nutrient guarantee and application recommendations. 3. Difference: fertilizer guarantee vs. recommendation? ○ Guarantee: Declares nutrient content. ○ Recommendation: Advises application specifics. 4. Common fertilizer sample preparation steps? Homogenization, grinding, and accurate weighing. 5. Gravimetric method assumption? Weight loss on drying equals moisture content. 6. Role of Devarda’s Alloy in nitrogen determination? Reduces nitrate to ammonia for accurate nitrogen quantification.