WEATHERING • Physical and Chemical • Physical weathering- Changes in the degree of consolidation with little or no chemical and mineralogical changes in rocks and minerals. Frost wedging, salt weathering in arid climates, thermal expansion, plant and animal disruption. • Chemical weathering - chemical changes rocks and mineralogical composition via dissolution, hydration, hydrolysis, acidolysis, chelation, and oxidation/reduction. • Dissolution • minerals most affected - salts, sulfates, carbonates. • H2O + CO2 H+ + HCO3- H2CO3 • CaCO3 is a congruent reaction - entire mineral is weathered and results in completely soluble products • CO2 from microbes contribute to breakdown of silicates in soils
Hydration and hydrolysis: Hydration - incorporation of water molecules into minerals which results in structural and chemical change. CaSO4 + 2 H2O CaSO4 . 2H20 anhydrite gypsum Gypsum is relatively soluble and can undergo dissolution whereas anhydrite is less soluble. Hydrolysis - incorporation of H+ or OH- in to mineral. KALSi3O8 + H+ HALSi3O8 + K+ (incongruent) Feldspar FeOOH + 3H+ Fe3+ + 2H2O (congruent) goethite
Acidolysis: Similar to reaction of hydrolysis where H+ is used to weather minerals, however, H+ supply not form water but organic and inorganic acids. Humic and fulvic acids, carbonic, nitric, sulfuric and low molecular weight organic acids such as oxalic acid. Chelation: Organic acids can also cause weathering by chelation. A chelator is a ligand capable of forming multiple bonds with metal ions such as fe, Al, Ca resulting in ring-type structures with the metal incorporated. Large complex acids in soils can strip metals from minerals. EDTA (ethylene diamine tetraacetic acid) - common artificial chelator used in labs.
Oxidation and reduction: Oxidation and reduction reactions weather minerals by the transfer of electrons. Minerals containing elements that can have multiple valence states such as Fe, Mn, S are susceptible. Fe3+ + 2H2O FeOOH + 3H+ Goethite • Mg-olivine (Fosterite) • Mg2SiO4+ 4CO2 + 4H2O 2Mg++ + 4HCO3+ H4SiO4 • 2Mg++ + 4HCO3- 2MgCO3+ 2CO3-+ H2O • HCO3- is a good indication of weathering. • Mg precipitates as magnesite, thus 4 moles of CO2 are taken from air, 2 return when Mg precipitates. So, for every mole of fosterite--2moles of CO2 get fixed into carbonate.
Minerals in soils are divided into primary and secondary minerals. • Primary minerals, which occur in igneous rocks, metamorphic , and sedimentary rocks, are inherited by soil from the parent material. • Secondary minerals form in soils and include layer-silicate clays, amorphous (non-crystalline) minerals, carbonates, phosphates, sulfide, sulfates, oxides, hydroxides, and oxyhydroxides. • Primary minerals are typically larger than secondary - high surface area of secondary minerals make them extremely reactive in soils.
Basic tetrahedral unit –Si ion shares charge equally with four oxygen ions.
Second most common building block of phyllosilicates is the Al octahedral polyhedron.
Feldspar • Na-Feldspar (Albite) alkaline solution + kaolinite • 2NaAlSi3O8 + 2CO2 + 11H2O Al2Si2O5(OH)4 + 2Na+ + 2HCO3- + 4H4SiO4 • Ca-Feldspar (Anorthite) kaolinite, 1 mole Ca, 2 moles HCO3-, but no H4SiO4 • Ca2+ +2HCO3- CaCO3 +CO2 +H2O, 1 mole of CO2 returned to atmosphere • In addition to carbonic acid, other organic acids -citric acid from plant roots, • phenolics (tannins) - decomposition, • fungi - oxalic acid • fulvics and humics weathering
Also chelation (e.g., Fe & Al) combine with fulvic acids -- can percolate to lower profile. • Fe and Al are relatively insoluble, found as crystalline and hydrous oxides. • Hydrous oxide • Fe--hematite • Al--boehmite • Crystal oxide • Fe-- goethite • Al-- gibbsite • Oxides are common in tropical soils where high temperatures and decomposition leave little humics to chelate.
Can use ratio of Si to Al as indicative of weathering • kaolinite - 1:1 ratio - more weathered • montmorillonite - 2:1 ratio • some 2:1 clay minerals hold H2O and NH4 in crystal lattice. Can represent 10% of total N. • Bauxite - Residual weathering mineral composed of more than 50% of Al, Fe, and Ti oxides and hydroxides. • The primary Al ore occurs in 2 varieties • 1. Lateritic-occurs with Al-Si rocks: granite, basalt. • (most widespread) • 2. Karst-occurs with carbonate rocks: limestone, dolomites
Structure of a 1:1 (kaolinite) and a 2:1 (montmorillonite) layer-silicate clay mineral.
Phosphorus- • Limited in supply to plants, apatite contains P. • Ca5(PO4)3 OH + 4H2CO3-- 5Ca++ + 3HPO4-- + 4CO3- +H2O • Initially non-occluded then taken up and bound by Al & Fe oxides-- “occluded” • Animal P • 1. Hydroxyapatite (Bones) • 2. Flouroapatite (Teeth)
Cation Exchange Capacity • Silicate clay minerals in temperate soils have a net negative charge. • 1. Mg++ substitute for Al+++ in montmorillonite- unsatisfied negative charge in crystalline lattice • 2. OH- radicals along edge of clay particles. • Depending on pH, H+ can be more or less reactive with the radical • Organic matter phenolic (-OH) and organic acid (-COOH) radicals contribute.
Diagram illustration the development of positively and negatively charged sites on surfaces of soil constituents, at low and high pH.
CEC = Total negative charge (meq/100g soil) • Assuming equal molar concentrations, cations are held in sequence and displace one another: • Al+++>H+>Ca++>Mg++>K+>NH+>Na+ Ca2+ forms bases of Ca(OH)2 • Si:Al ratio - 2:1 indicates greater exchange capacity • than 1:1 ratio • Tropical soil has no exchange capacity except via organic matter. • Tropical soils have strong CEC - thus, can resist acid rain • However, acid rain in Northeast U.S. can dissolve gibbsite Al2O3-leading to Al+++ which is toxic to organisms.
Anion Adsorption Capacity Soils dominated by oxides and hydrous oxides of Fe and Al have variable charge depending upon pH. a. low pH adsorbs H+ from solution, results in positive charge - mineral AlOH+ b. high pH adsorption H+dissociation AlOH H+ c. pH>9 - additional H+ dissociates - surface becomes negative - AlO-H+
PO43->SO4-->Cl->NO3- There is a low availability of P in soils Soils with poorly developed crystalline forms of Fe and Al oxides have greater anion adsorption capacity (AAC). Tropical soils all controlled by organic matter.
SOILS SOIL LAYERS A.Forest Floor L-Layer or Oi--undecomposed litter F-Layer or Oe--fungi and bacteria (Fermentative) H-Layer or Oa--humus amorphous organic matter, increase in % mineral
In tropics decomposition occurs rapidly so there is little time for development of soil structure (L-Layer, F-Layer, H-Layer are all the same) O-Horizon -- all organic; A Horizon--alluvial processes (removal). Organic matter and minerals - chelation with organic acids - Fe and Al percolate down from forest floor - referred to as podzolization, not common in tropics because the decomposition is too complete. E-Horizon-some minerals remain, podzolization is less intense; B-Horizon--illuvial (deposition); C-Horizon -- lowest soil layer least weathered and most similar to parent rock material
Grassland soils (i.e., tallgrass prairie at base of Rocky Mountains). • Classified as Mollisols - high organic content and base saturation • In forests precipitation exceeds evapotransportation, however, not typically in grasslands. Thus, less H2O, slower decomposition, high pH and Ca - same as in East African grasslands. • Clays complexed with organic acids • In great plains- leaching is so limited CaCO3 precipitates and accumulates in calcic horizons.
Spodosols - intense podzolization (common in temperate northeast U.S soils) Alfisols - low podzolization Utisols - southeast yellowish, deep-reddish color in B horizon Soils in Tropics are many meters thick and have endured millions of years without disturbance
Deserts • Entisols - recent with little profile development • Chemical weathering proceeds slowly • Soils in southwest U.S. deposited by alluvial transport from adjacent Mountain Ranges • CaCO3 - horizons called caliche • NaCl • can contain illuvial clays - eolian dust
Typical soil horizon sequence for a Spodosol developed under a coniferous forest (left) and a Mollisol developed under grasses and herbaceous plants.
The term soil organic matter (SOM) is generally used to represent the organic constituents in the soil, including undecomposed plant and animal tissues, their partial decomposition products, and the soil biomass. Thus, this term includes: 1.identifiable, high-molecular-weight organic materials such as polysaccharides and proteins, 2.simpler substances such as sugars, amino acids, and other small molecules, 3.humic substances.
Soil is a complex, multicomponent system of interacting materials, and the properties of soil result from the net effect of all these interactions. One of the major problems in the field of humic substances is the lack of precise definitions for unambiguously specifying the various fractions. The term humus is used by some soil scientists synonymously with soil organic matter, that is to denote all organic material in the soil, including humic substances.
SOM consists of humic and nonhumic substances. Nonhumic substances are all those materials that can be placed in one of the categories of discrete compounds such as sugars, amino acids, fats, etc. Humic substances are the other, unidentifiable components. Even this apparently simple distinction, however, is not as clear cut as it might appear.
Humic acids - the fraction of humic substances that is not soluble in water under acidic conditions(pH < 2) but is soluble at higher pH values. Humic acids are the major extractable component of soil humic substances. They are dark brown to black in color. Fulvic acids - the fraction of humic substances that is soluble in water under all pH conditions. They remains in solution after removal of humic acid by acidification. Fulvic acids are light yellow to yellow-brown in color. Humin - the fraction of humic substances that is not soluble in water at any pH value. Humins are black in color.