Lecture 13 Soil Formation and Chemistry - soest. 13 Soil Formation and Chemistry ... Most abundant organic component, improves soil physical properties, exchanges nutrients, reservoir of fixed N

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<ul><li><p>1</p><p>GG425/625 Wk 7 L13, S2018</p><p>Lecture 13</p><p>Soil Formation and Chemistry</p><p>Please read Manahan Chapter 14 AND 15 (for this week and next).</p><p>Today</p><p>1. Weathering the context</p><p>2. Clay Minerals the materials</p><p>3. Organic solids the special sauce</p><p>4. Some soil examples</p><p>5. Inorganic reactions/transformation in soils</p><p>GG425/625 Wk 7 L13, S2018</p><p>Soils Intro</p><p>Important substrate and growth medium </p><p>for terrestrial biosphere. </p><p>Susceptible to many anthropogenic </p><p>effects.</p><p>They contain many materials in a </p><p>gradient between</p><p>organic rich surface deposits </p><p>deeper inorganic deposits called saprolite. </p><p>Saprolite with original rock textures preservedhttp://www.nicholas.duke.edu/eos/geo41/</p><p>saprolite</p><p>organic </p><p>rich topsoil</p><p>http://www.teara.govt.nz/en/photograph/12319/organic-soil</p></li><li><p>2</p><p>GG425/625 Wk 7 L13, S2018</p><p> On a gentle slope, rock is altered in place, </p><p>sometimes to form soil.</p><p> On a steep slope, weathered solids are whisked </p><p>away by wind or water and deposited elsewhere, resulting in sediment accumulation elsewhere.</p><p>Soils Intro</p><p>How do soils form? Initially, physical and chemical breakdown of surficial rocks (weathering) produces secondary materials.</p><p>GG425/625 Wk 7 L13, S2018</p><p>WeatheringThe breakdown of rock to form secondary deposits is controlled by</p><p> Physical</p><p> Chemical</p><p> and biological processes</p><p>Chemical and biological weathering are almost always mediated by H2O. </p><p>During weathering new solid materials are formed and the composition of the </p><p>mediating H2O is modified. </p><p>The rates of alteration, and thus rates of soil (or sediment) accumulation and </p><p>maturation, are governed by climate:</p><p> temperature,</p><p> the availability of H2O</p><p> biome factors (flora/fauna and the DOC they produce)</p><p>The formation of a soil is also dependent upon </p><p> the bedrock type in the area</p><p> physical factors (such as rock porosity and texture)</p><p> mineralogic factors (solubility) </p></li><li><p>3</p><p>GG425/625 Wk 7 L13, S2018</p><p>Weathering Primary minerals can be weathered from the source rock intact (mineralogically) or </p><p>dissolved (recall congruent and incongruent dissolution)</p><p>Mineral dissolution susceptibility is related to stability at the P, T and pE conditions of </p><p>Earths surface. </p><p>The higher their T and P, or more reducing the pE of formation, the </p><p>more susceptible to weathering their minerals are.</p><p>Many crustal rocks were formed at elevated P and/or T, and lower pE, in the lower </p><p>crust or upper mantle. They were then "moved" to their present location at the surface </p><p>through the combined processes of tectonics and erosion.</p><p>The Bowen's reaction series (a </p><p>gross generalization of mineral </p><p>stability as a function of magma </p><p>temperature) can also be used to </p><p>understand weathering of many </p><p>silicate minerals, because high </p><p>temperature minerals are the first to </p><p>form from a crystallizing magma </p><p>and are more susceptible to </p><p>weathering.</p><p>GG425/625 Wk 7 L13, S2018</p><p>Sequence of events for </p><p>weathering common rock </p><p>forming minerals</p><p>The most soluble chemical </p><p>elements are transported in the </p><p>aqueous state to a new location </p><p>(eventually the sea) </p><p>The least soluble elements are </p><p>mostly left behind.</p><p>Elements of any solubility may be </p><p>dissolved during weathering, </p><p>redeposited by the aqueous </p><p>solution somewhere down its </p><p>flow path, and then re-dissolved </p><p>in a new, later episode of </p><p>weathering later.</p></li><li><p>4</p><p>GG425/625 Wk 7 L13, S2018</p><p> Inorganic constituents: Minerals stable at high temperatures and pressures are broken down into hydrous sheet silicates (clays) and oxide minerals (Fe, Al and Mn oxides)</p><p>Organic constituents: derived from flora, and soil microorganisms.</p><p>Org.-Inorg. Proportion: Typical composition is 95% inorganic material and 5% organic matter -highly variable though.</p><p>Soil Composition Basics</p><p>GG425/625 Wk 7 L13, S2018</p><p>Three main forms:</p><p>Very resistant Primary minerals</p><p>Alteration minerals (incongruently formed clays/oxides)</p><p>Precipitation minerals (mostly carbonates/hydroxides)</p><p>Inorganic Solids in Soils</p></li><li><p>5</p><p>GG425/625 Wk 7 L13, S2018</p><p>Alteration minerals:The structure and composition of </p><p>these solids is important because </p><p>they modify soil water and affect </p><p>the availability of nutrients to </p><p>plants.</p><p>The types of secondary minerals </p><p>formed from the weathering and </p><p>hydrolysis of common primary </p><p>minerals are given below.</p><p>The mineral names are not </p><p>important here, except to note that</p><p> both clays and oxides can be </p><p>formed</p><p> ion exchange with water is </p><p>involved</p><p> CECs are variable.</p><p>GG425/625 Wk 7 L13, S2018</p><p>Simple oxide/ hydroxide examples are goethite and gibbsite.</p></li><li><p>6</p><p>GG425/625 Wk 7 L13, S2018</p><p>Clay minerals:</p><p>Clays are structurally more complex.</p><p>They are composed of layered matrices of Si, Al and Mg </p><p>bonded to O. </p><p>Of the 3 basic clay types (platy, fibrous and </p><p>amorphous), the most important in soils are the </p><p>platy "phylosilicate" clays</p><p>The layers are of two types: </p><p>tetrahedral and octahedral</p><p>These occur in clay minerals in 2 layer and 3 layer </p><p>varieties:</p><p>2 Layer Clays - T-ORepeated units of 1 tetrahedral and 1 octahedral layer</p><p>3 Layer = T-O-T)Repeated units of 2 tetrahedral and 1 octahedral layers</p><p>GG425/625 Wk 7 L13, S2018</p><p>tetrahedral (Si surrounded by tetrahedrally-arranged O)</p><p>SiO4 tetrahedra share 3 basal oxygens </p><p>with neighboring tetrahedra, forming a </p><p>sheet structure. The Si:O ratio = 1 to 1 </p><p>lone O + 3 50% shared oxygens =</p><p>1: (1 + 3 x 0.5) = 1:2.5 = 2:5</p><p>octahedral (Al or Mg surrounded by octahedrally-arranged O as hydroxyl groups). </p><p>Octahedral Mg clays are commonly formed </p><p>only during alteration of magnesian rocks. </p><p>Octahedral layers of pure Al and Mg occur in </p><p>the minerals gibbsite, Al(OH)3, and brucite, </p><p>Mg(OH)2.</p><p>each Al(OH)6 (or Mg(HO)6) octahedron </p><p>shares all of its oxygens with neighboring </p><p>octahedra. Al:O ratio of 3, as in gibbsite.</p><p>T and O layers combine by sharing the non-</p><p>basal O of the silica tetrahedra with the one of </p><p>the octahedral O atoms on each Al (or Mg).</p></li><li><p>7</p><p>GG425/625 Wk 7 L13, S2018</p><p>T-O clays:</p><p>we can think of each Al as having effectively lost one O atom to a </p><p>Si, and Al:O goes from 1:3 to 1:2 (octahedral O atoms are </p><p>actually in hydroxide form). </p><p>Kaolinite, the simplest T-O clay, has Si:O of 2:5, </p><p>Al:Si of 1:1 (or 2:2) and Al:OH of 1:2 (or 2:4). </p><p>This gives the formula Al2Si2O5(OH)4.</p><p>T-O-T clays:</p><p>Similar arguments can be made to show that T-O-T clays have </p><p>Si:Al of 2:1</p><p>The simplest chemical formula is Al2Si4O10(OH)2 (pyrophyllite).</p><p>GG425/625 Wk 7 L13, S2018</p><p>Solute-Solids interactionsStill and through flowing water interacts with solids to exchange compositional attributes:</p><p>3 mechanisms of compositional exchange with water operate, </p><p>as discussed last week.</p></li><li><p>8</p><p>GG425/625 Wk 7 L13, S2018</p><p>Ion Substitution:</p><p>Ion substitution for Si, Al and Mg gives clays exchangeable ion </p><p>sites that can exert a compositional control on aqueous solutions </p><p>contacting them. The degree of substitution depends on the </p><p>environment of their formation. </p><p> Octahedral replacement is by ions such as Fe+3, Fe+2, Cr+3, </p><p>Zn+2, Li+.</p><p> Tetrahedral Si replacement is less common and mostly limited</p><p>to Al-for-Si substitution. </p><p>Structural substitutions result in a charge imbalance on the clay </p><p>backbone that is balanced by addition of interlayer (non-</p><p>structural) ions and accounts for the CEC of clays (as discussed </p><p>last week).</p><p>GG425/625 Wk 7 L13, S2018</p><p>Charge on clay particles:In addition to cation exchange, clays and oxides take charges in </p><p>natural waters (discussed earlier this semester). </p><p>The sign of the charge </p><p>is a function of pH and </p><p>the density of charge is </p><p>a function of the </p><p>structure</p></li><li><p>9</p><p>GG425/625 Wk 7 L13, S2018</p><p>Al (OH)3</p><p>Na, Ca containing clay Clay mineral depleted </p><p>in Alkalis &amp; Alkali </p><p>Earths: Al2Si2O5(OH)4</p><p>Increased water flow during weathering leads to increased leaching of cations </p><p>which lowers CEC and charge on clays.</p><p>GG425/625 Wk 7 L13, S2018</p><p>The gain or loss of chemical constituents in saprolite records the progress of weathering/ soil formation in the absence of significant DOC.</p><p>In practice, Al is the least soluble element during weathering followed by Ti and Fe.</p><p>Please note that % metal oxide </p><p>is a way of expressing bulk </p><p>composition of a rock. Many of </p><p>these oxides are not actually </p><p>present in the rock as oxides.</p><p>SOIL INORGANIC SOLIDS saprolite development</p><p>mineralogical</p><p>changes that </p><p>occur during </p><p>weathering</p><p>elemental</p><p>changes that </p><p>occur during </p><p>weathering</p></li><li><p>10</p><p>GG425/625 Wk 7 L13, S2018</p><p>Elements removed during saprolite formation have high concentration in soil</p><p>and ground waters.</p><p>Si is removed slower than Ca and Na. Lower but still significant Si </p><p>concentrations remain at high % Al.</p><p>Fe and Ti continually increase with Al, suggesting that a totally weathered rock </p><p>would be mostly Al, Fe, Ti and Si (-the Si curve eventually flattens out as some </p><p>of the Al is found in Kaolinite, Al2Si2O5(OH)4). </p><p>Remember that Fe2+ is soluble and Fe3+ is not. The typical accumulation of </p><p>Fe in saprolites indicates that this process takes place at fairly high pe.</p><p>Most species decrease as % Al2O3 increases.</p><p>EXCEPTIONS: Ti and in some cases Fe.</p><p>The faster the rate of decrease, the more </p><p>mobile the element is.</p><p>Note that Ca and Na are removed very quickly </p><p>(at relatively low Al2O3) and then K and Mg are </p><p>removed. </p><p>GG425/625 Wk 7 L13, S2018</p><p>SOIL WATER</p><p>Depth profiles of elemental concentrations in soil water, provide </p><p>insight into geochemical processes during soil formation/ </p><p>weathering and in bio-availability of some important nutrient </p><p>elements.</p><p>It is important to examine the total amount of ion present and the </p><p>relative proportions of free versus DOC-complexed ions.</p><p> inorganic solubility during saprolite formation</p><p> solubility in the presence of DOC/POC higher up in the </p><p>soil column.</p><p>The common rock-forming chemical elements are found in soil </p><p>waters as a function of:</p></li><li><p>11</p><p>GG425/625 Wk 7 L13, S2018</p><p>Al: Weathering-resistant. Conc. increases with depth. No significant DOC complexation.</p><p>Ca: Soluble element. Conc. decreases with depth due to CaCO3precipation at higher pH. No significant DOC complexation</p><p>Mg: Relatively constant, somewhat more DOC-complexed at depth. Mg enters soil waters fairly easily; there are few reactions for its removal (e.g., incorporation in CaCO3), so it only diminishes slightly with depth.</p><p>Zn: Analogous to Ca. At high pe ZnCO3 (pH 8-9) or Zn(OH)2 (pH 9-12) forms. No significant DOC complexation.</p><p>Fe: Similar to Al except its peak concentration is somewhat higher up in the profile because of significant DOC complexation above the saprolites.</p><p>Cr: Generally soluble but even more so in the presence of DOC. It's profile looks similar to Ca except that in the upper layers, it is almost all DOC-complexed.</p><p>Cu: Moderately soluble but more so in the presence of DOC. Similar to Cr but found only in A zone.</p><p>GG425/625 Wk 7 L13, S2018</p><p>Organic Solids in Soils.</p><p>Organic solids typically make up </p></li><li><p>12</p><p>GG425/625 Wk 7 L13, S2018</p><p>An example of the effect of soil DOC on silica dissolution rate, </p><p>and a possible mechanism, are given below. </p><p>GG425/625 Wk 7 L13, S2018</p><p>Table 16.1. Major Classes of Organic Compounds in Soil from Manahan Ch16</p><p>Compound Type</p><p>Composition Significance</p><p>Humus Degredation-resistent residue from </p><p>plant decay, largely C, H, and O</p><p>Most abundant organic component, improves soil </p><p>physical properties, exchanges nutrients, reservoir of </p><p>fixed N</p><p>Fats, resins, and </p><p>waxes</p><p>Lipids extractable by organic </p><p>solvents</p><p>Generally, only several percent of soil organic matter may </p><p>adversely affect soil properties by repelling water, </p><p>perhaps phytotoxic</p><p>Saccharides Cellulose, starches, hemi-</p><p>cellulose, gums</p><p>Major food source for soil microorganisms, help to </p><p>stabilize soil aggregates </p><p>N-containing </p><p>organics</p><p>Nitrogen bound to humus, amino </p><p>acids, amino sugars, other </p><p>compounds</p><p>Provide nitrogen for soil fertility </p><p>Phosphorous </p><p>compounds</p><p>Phosphate esters, inositol </p><p>phosphates (phytic acids), </p><p>phospholipids </p><p>Sources of plant phosphate</p></li><li><p>13</p><p>GG425/625 Wk 7 L13, S2018</p><p>A Soil Primer:</p><p>Soils are the combined products of rock </p><p>breakdown and biological processes.</p><p>Soils are basically a stratified gradient </p><p>between mostly organic, biological and </p><p>resistive inorganic materials on the top </p><p>and rock weathering products below.</p><p>Ground water flow through soils is </p><p>mostly vertical (top down), leading </p><p>to distinctive layering.</p><p>Soil horizons generally build from the </p><p>bottom up; the further down one goes </p><p>toward bedrock, the more similar the </p><p>material gets to bedrock composition. </p><p>Notice the relationship between the </p><p>zones and tree roots.</p><p>GG425/625 Wk 7 L13, S2018</p><p>Soil zone nomenclature derives from physical and chemical properties that occur more or less in stratified horizons in the </p><p>soil column: </p><p> The A-zone is the least like the rock from which it was originally produced.</p><p> The C-zone is the most like the rock from which it was originally produced.</p><p> The B-zone is intermediate. It contains solid residues of sparingly soluble materials mobilized and redeposited</p><p>from the A-zone.</p></li><li><p>14</p><p>GG425/625 Wk 7 L13, S2018</p><p>y Organic matter and porosity generally decrease with depth in a soil.</p><p>y Mineral grains in the very upper reaches of a soil are very resistive to weathering.</p><p>y Saprolite occurs at the base of the soil zone, so far removed from the organic zones of </p><p>the soil that DOC plays little role in its formation.</p><p>GG425/625 Wk 7 L13, S2018</p><p>Many soils, such as this one, </p><p>show a classic topsoil </p><p>horizon but this is not always </p><p>the case.</p><p>O Horizon - decomposing </p><p>organic matter</p><p>A1 Horizon - brown humic-rich, </p><p>some mineral matter.</p><p>A2 Horizon - light grey, </p><p>intensely leached; including loss </p><p>of Fe &amp; Al; mostly residual SiO2.</p><p>B horizon -brown horizon, </p><p>accumulation of clays &amp; Fe-</p><p>oxides</p><p>Soil images from: http://soils.usda.gov/</p></li><li><p>15</p><p>GG425/625 Wk 7 L13, S2018</p><p>Soils of tropical and </p><p>subtropical regions tend to </p><p>be deeply weathered. </p><p>They are mixtures of </p><p>quartz, kaolin, free oxides, </p><p>and some organic matter. </p><p>For the most part they lack </p><p>well defined soil horizons.</p><p>GG425/625 Wk 7 L13, S2018</p><p>In humid temperature </p><p>regions relatively</p><p>organic-rich </p><p>and </p><p>clay-rich zones commonly</p></li><li><p>16</p><p>GG425/625 Wk 7 L13, S2018</p><p>Organic matter dominated </p><p>soils tend to form in wet </p><p>boggy areas. </p><p>Wet conditions favor plant </p><p>growth and thus greater </p><p>organic matter production. </p><p>Water logged soils quickly </p><p>become very reducing. </p><p>Why?</p><p>Cool to temperate </p><p>conditions and reducing </p><p>conditions both slow </p><p>hetereotrophic organic </p><p>matter degradation. </p><p>GG425/625 Wk 7 L13, S2018</p><p>Very Organic or peat soils </p><p>(&gt;25%) are wet throughout </p><p>the year. Plant debris </p><p>decomposes slowly and </p><p>thus builds up. </p><p>In this profile there is a 1m </p><p>thick layer of organic matter </p><p>over the B-zone. </p><p>Cultivation of these soils </p><p>often require draining first to </p><p>lower the water table. </p></li><li><p>17</p><p>GG425/625 Wk 7 L13, S2018</p><p>Soils from very arid </p><p>environments support </p><p>limited plant growth. </p><p>Precipitation of minerals </p><p>from simple salts are </p><p>characteristic: </p><p>calcium carbonate, </p><p>gypsum. </p><p>These soils tend to have </p><p>low organic content. </p><p>GG425/625 Wk 7 L13, S2018</p><p>Caliche a layer. </p><p>This common at </p><p>shallow levels in </p><p>soils from arid </p><p>regions. It is </p><p>common in </p><p>leeward Hawaii </p><p>locales. </p><p>Caliche (CaCO3)is a precipitate mineral that forms near the base of the B-zone </p><p>of many soils. </p><p>Ca2+ and...</p></li></ul>