Chemical Properties of Soils PDF

CHEMICAL PROPERTIES OF SOILS Lesson 4 Objectives 1. Explain the soil chemical properties. 2. Analyze soil chemical properties 3. Interpret soil chemical data 4. Recommend specific practices CONTENTS I. Soil Colloids II. Silicate clays III.Organic colloids IV. Factors affecting St...

CHEMICAL PROPERTIES OF SOILS Lesson 4 Objectives 1. Explain the soil chemical properties. 2. Analyze soil chemical properties 3. Interpret soil chemical data 4. Recommend specific practices CONTENTS I. Soil Colloids II. Silicate clays III.Organic colloids IV. Factors affecting Strength of Absorption of ions in soil colloids V. Cation Exchange Capacity VI. Base Saturation (BS) and Exchangeable Sodium Percentage (ESP) VII.Soil pH VIII.Liming IX. Soil salinity and sodicity Chemical Nature of Soil Constituents Three phases: a. Solid - Skeletal framework of soils - Mixture materials of inorganic and organic b. Liquid - Soil solution, carries nutrients and moves dissolved c. Gas - Soil air: O2, N and CO2 Chemical Nature of Soil Constituents Percentage Composition by Volume  Soil Colloids I  Soil Colloids ❖ The most reactive components of the soil. Two Types: 1. Organic Colloids - include highly decomposed organic matter generally called (humus) most stable product of organic matter decomposition. 2. Inorganic Colloids – include silicate clays, oxides and hydrous oxides and amorphous clays. Size: very small particles less than 0.001 mm in size. too small to be seen by be seen by naked eye can only be seen under an electron microscope Surface Area: Total surface area may range from 10 m2/g to more than 800 m2/g depending on the external and internal surfaces of the colloid. Surface Charges Soil colloids carry negative or positive charges on their external and internal surfaces. Some soil colloids in very acid soils have net positive charge. For most soil colloids, net negative charge predominates. Arrangement of Atoms Basic Structural unit of silicate clays Silicate clay minerals: basic chemical structure is made up of Si and Al Basic structural unit: 1. Silica tetrahedron 2. Alumina octahedron Classification of silicate clays (based on of number tetrahedral to octahedral sheet): 1:1Type: Unit Layer Classification of silicate clays (based on of number tetrahedral to octahedral sheet): 2:1Type: Unit Layer Silicate clay Electro- microscopic clay minerals, which diameter less than 2 microns. It is the textural classes of soil and developed more in the horizontal axis than the vertical axis of soil profile. It is the characteristics minerals of the earths near surface environments. They form soil and sediments. E.g: Kaolinite, Micas, Vermiculite, Chlorite etc.  Classification of Silicate Clays 1:1 type – KAOLINITE- strong not expanding Two sheets are held together by oxygen atoms that shared by Si and Al Unit layers are held together tightly by H- bonding ► restricts expansion >limits the reactive area to external ►specific surface area: 10-20 m2/g Classification of Silicate Clays  Classification of Silicate Clays 2:1 expanding type: MONTMORILLONITE – weak expanding type Most common member of smectite group Unit layers are loosely held together by weak oxygen-to-oxygen and high in cation exchange. When saturated with water, 20A (2.0nm) basal spacing between layers can approach Under dry conditions it may be reduced to less than 1 OA (1.0nm) Classification of Silicate Clays Classification of Silicate Clays  Classification of Silicate Clays 2:1 expanding type: MONTMORILLONITE Specific surface (total surface area) is large due to contribution of internal surface Specific surface: 700-800 mlg Derives most of the negative charge from isomorphous substitution of Mg2+ for Al3+ in the octahedral sheet CEC: 60-100 cmol/kg High shrink swell capacity ►Tendency for crack formation ►General instability of the surface Soils dominated by smectites are: ► Very difficult to cultivate because they form aggregates or clods that are very hard ► Poor bases for roadbeds and building foundations  Classification of Silicate Clays 2:1 non-expanding type: ILLITE (fine-grained micas) ► Chemical composition is similar to muscovite, but contains more SiO2 and less K ►Contains interlayer K, the unit layers are bonded more strongly than montmorillonite and vermiculite hence no expansion ►CEC: 20-40 cmol/kg ►Specific surface:40-180 m/g  Classification of Silicate Clays 2:1 expanding type: VERMICULITE ►Layer structure resembles that of mica from which it is derived. ►Capable only of limited expansion; swells less than montmorillonite because of higher layer charge Al 3+ substituting for Si4+ in the tetrahedral sheet Mg2+ and Fe2+ as octahedral cations ►Water molecules along with other ions act as bridges unit layers together rather than apart ►Specific surface: 500-700 m2 ►Exhibits high CEC: 101-160 cmol/kg Classification of Silicate Clays  Classification of Silicate Clays 2: 1:1 or 2:2 non-expanding type – CHLORITE ►2:1 layers alternate with hydroxide sheet commonly dominated by magnesium ► No water adsorption between the unit layers hence its non expansive in nature ► CEC: 10-40cmol/kg  Classification of Silicate Clays Allophane ►Also silicates but are non-crystalline or amorphous ►Abundant in soils derived from volcanic ash deposits ► Have high phosphate fixation capacity ► Most soils containing allophane have black A horizons, extremely high in OM  Structure of Organic Colloid Consists of large organic molecules whose chemical composition varies. Structure contains complex series of C chains and ring structures with many functional groups- carboxyl, phenolic, and alcoholic groups. negative or positive charges on the humus colloid develop as Ht ions are either lost or Gained by this group.  Origin of charge on soil colloidal surfaces Negative charges 1. lsomorphous sustitution ►Substitution of one ion for another of similar size within a crystal lattice. ►Major source of negative charges in 2:1 layer silicates > A for Si? in the tetrahedral sheet > Mg?', Fe?" for AI in the octahedral sheet ►The resulting negative charge is permanent, since it will change with changing pH, and is balanced by cations. Montmorillonit Msis(A14»Mg»)O2o(OH)4  Origin of charge on soil colloidal surfaces Negative charges 2. Ionization of exposed hydroxyl groups ►At high pH, the hydrogen of these exposed hydroxyls, dissociates slightly and the surface of the clay is left with the negative charge of the oxygen ions. Alkaline medium: -AI-OH + OH- = -Al-O: + H2O ►Called variable or pH-dependent charge and its magnitude varies with pH and type of colloid ►An important type of charge for 1:1 layer silicates, iron, and aluminum oxides and organic colloids  Origin of charge on soil colloidal surfaces pH dependent charges on oxides  Origin of charge on soil colloidal surfaces Positive charge 1. Protonation of exposed hydroxyl groups ►These exposed OH can also be adsorb or gain protons particularly in strongly acidic media and create positive charges > Acidic medium: -Al-OH + H - - Al-OH'  Origin of charge on soil colloidal surfaces Importance of negative charges in the soils ►Enables the soil to store nutrients, specifically the negative charged ions or cations. ► The cations adsorbed and kept from being washed away by water passing through the soil column Cation Exchange Basic Concepts: Agriculturally important soils generally possess net negative charge. Negatively charged colloids attract cations from soil solution which become adsorbed on the surface.  Anion (-) Silicate Nitrate  Diagram showing the net negative charges on edges of soil colloids and the cations which are attracted onto the colloid by electrical attraction.  These adsorbed cations can undergo reactions with: cations in the soil solution, or cations in the medium that comes in contact with the colloids exchange Adsorbed cations are also known as exchangeable cations Ca ,Mg ,Na' ,and K - exchangeable bases Cation Exchange The process whereby cations adsorbed on the surface of soil colloids are exchanged for those in the soil solution or in any medium that comes in contact with the soil colloids. Cation exchange is a continuous process and is related to a number of important soil processes: a. Weathering - Chemical weathering releases nutrients from minerals. If nutrients are not adsorbed by soil colloids, they are subject to loss through leaching. b. Nutrient retention Low CEC, capacity, Nutrients, low nutrient retention, low supply of cationic. c. Dispersion and Flocculation Na-rich soils are in dispersed state Ca-rich soils are flocculated Dispersion productive is not a desirable characteristic soils.  Characteristics of Cation Exchange Reactions: Exchange reactions are stoichiometric in equivalent amounts: 1 meq of Mg is replaced by 1 meq of Ca2+ 1 cmol replaced of Mg2+ is by 1 cmol of Ca 2+ 1 meq of K is replaced by 1 meq of Ca 2+ 2 cmol of K is replaced by 1 cmol of Ca 2+ Sample Problem: Centimole: 1 cmol = 1/100t of a mole Ca2 = 40 g/mole 0.40 g/cmol 0.20g/cmolc H + = 1 g/mole = 0.01 g/cmol = 0.01 g/cmol Sample Problem: Calculate the weight (g) of Ca? needed to replace 1 g of H' Using me: 1 me Ca2 will replace 1 me H 1 me Ca2 = 0.02 g 1 me H = 0.001 gH xg Ca2+ = 0.02 g Ca Sample Problem: Calculate the weight (g) of Ca2+ needed to replace 1 g of H‘ Using cmol: 1cmol Ca2 will replace 2 cmol Ht 1cmol Ca2 = 0.40.g 1cmol H = 0.01 g Practice Problem Using both approaches (me and cmol) calculate how many grams of Ca needed to replace 1 g NH4+ Given: 1 g NH4+ Required: me and cmol Equation: Practice Problem Using both approaches (me and cmol) calculate how many grams of Ca needed to replace 1 g NH4+ Equation: Solution: wt of NH4 = __ 18 __ = 0.018 g/me 1 x 1000 wt of Ca = __ 40 __ = 0.02 g/me 2 x 1000  Practice Problem Using both approaches (me and cmol) calculate how many grams of Ca needed to replace 1 g NH4+ Equation: Using cmol: 1cmol Ca will replace 1g cmol NH4+ 1cmol Ca = 0.02 g 1cmol NH4 = 0.018 g __0.20 g Ca2+___ = ___x g Ca2+__ Answer: 1.11 g Ca 0.18 g NH4+ 18 g NH4+  Cation Exchange Is Instantaneous Exchange reactions involving kaolin minerals take place more rapidly than involving smectites. lllites require even longer time due to presence of exchange sites between firmly held layers of the mineral Exchange Reactions Are Reversible Reactions They attain equilibrium state where rate of forward reaction reaction. Because of their is equal to rate of reverse reversibility, exchange reactions can be driven in either the forward or reverse direction Base Saturation Proportion of the CEC occupied by basic cations such as: (Ca++ , Mg++ , K + , Na+ , NH4 + , etc) Percentage base saturation is an important soil property because it is inversely related to soil acidity. Generally as percentage base saturation increases, soil pH increases. Calculated by taking the ratio of the bases with CEC. %BS = me of bases x 100 CEC Chemical Properties of Soil Percentage Base Saturation (% BS) Chemical Properties of Soil Percentage Base Saturation (% BS) Chemical Properties of Soil Percentage Base Saturation (% BS) Chemical Properties of Soil Percentage Base Saturation (% BS) Exchangeable Sodium Percentage (ESP) Proportion of the exchange sites by sodium High ESP values (greater than 15%), soil becomes highly dispersed which leads to poor aeration and drainage or permeability to water. Sodic soils: soils with excessive amounts of soluble sodium. Computed by taking the ration of the me of NA+ and that of CEC. Problem Solving: % Base Saturation = meq bases + CEC x 100 % Hydrogen Saturation = meq H + CEC x100 Example: Ap Soil Horizon Cations – H Ca? Mg? K Na 9.4 14 3 0.5 0.1 Answer: % BS = 17.6 / 27 x 100 = 65% % ESP = 9.4 / 27 x100 = 35% Soil pH Is the degree of acidity and alkalinity of the soil. Determined by the relative concentration of H and OH ions. By replacing Na+ in the exchange sites of colloids with Ca++ (Source : Gypsum) and then washing out the Na+. Methods for measuring: soil pH pH Meter Soil Test Kit  pH is a 'master' variable Affects chemical, physical, and biological properties of soils ►Nutrient availability ►Metal toxicity and solubility ►Microbial activity (especially important in the N cycle) ✓ The wide portion of the bands indicate the zone the most readily available nutrients. ✓ When pH is too low (below pH 5.0), Al, Zn, Cu, Fe, and Mn become more available. ✓ Most of the micronutrients(except Mo) become unavailable at high pH.  Soil pH Effects of Soil pH: Microbial activity - Fungi: unaffected - Bacteria and Actinomycetes: pH5 Availability of nutrients - N- decrease at pH 8.5 Soil is highly dispersed Dispersed humus carried upwardly by capillary gives the soil black color (black alkali)  Reclamation of Saline Soils Leaching of excess salts out of the root zone (internal and surface drainage and salt disposal dump areas) Retardation of evaporation (use of sulfate mulch) Use of salt tolerant crops. Use of special planting procedures that minimize salt accumulation around the seed.  Reclamation of Saline Soils Replacement of excess Na on the exchange sites by Ca and leaching of exchanged Na out of the root zone Gypsum cheapest and most commonly used S is likely to be used when the soil contains free lime in addition to too much Na Addition of OM Deep plowing  Chemistry of Flooded Soils Flooded soils are submerged soils used to grow rice; paddy soils Profile: - Plow layer is a puddle system which results from plowing and harrowing while the soil is wet. - Underneath is a hardpan which prevents or minimizes the percolation of water into the subsoil. Effects of Flooding on Soil pH and Redox Potential pH value - acid soils increases on submergence - calcerous and sodic soils decreases most soils attained fairly stable pH of 6.5 to 7.0 after several weeks of submergence  Effects of pH Change Acid soils, increase in Ph - decreases Fe, Al, Mn toxicity - increases P, Si and Mo availability - enhances microbial activity (pH near neutrality favors microbial activity)  Effects of pH Change alkaline soils, decrease in pH - Increases availability of P, Zn, Cu - enhances microbial activity Thank you…….