2 State Factor Model S = f(C,O,R,PM,T,…) Model has many shortcomings C – climateO – organismsR – reliefPM – parent materialT – time Model has many shortcomingsIt has never been solved mathematically and probably never will be.It oversimplifies the complexity of soil formation.It implies that four of the factors can be fixed and one varied to observe the effects of the one variable on rates or kinds of soil formation processes. Many studies have attempted to fix four of the factors to evaluate the influence of the fifth
3 “Sequence” Studies Toposequence: vary landscape position Climosequence: vary climateChronosequence: vary ageStream terracesBiosequence: vary vegetationLithosequence: vary parent materialThese approaches ignore interactions among the factorsThe state factor model helps understand differences among soilsParent material and relief are passive factorsClimate and organisms are active (or flux) factorsAdd materials to the soilDrive processesTime allows the other factors to act
4 Parent MaterialRock or sediment from which the soil develops has a strong influence on the properties of the soil that formsThe mineral components in the soil and its chemical and physical properties depend on:Mineral components in the parent materialIf precursor for a secondary mineral is not present in the parent material, the soil will never contain the secondary mineralPlagioclase feldspar smectiteK feldspar kaoliniteTime and environmental conditions of the weathering environmentPlagioclase feldspars smectite kaolinite
5 Parent Material – Soil Relationships Light colored crystalline rocks (granite and granitic metamorphic rocks: felsic)Common parent material in the Piedmont and Blue Ridge MountainsDominant mineralsK feldspar, quartz, mica (biotite or muscovite)Rock weathers to saprolite with low clay contentSoils derived from saprolite areSandy A / clayey Bkaolinitic,Moderately permeable,acidic, andhave low base saturation and nutrient reserves.
6 Parent Material – Soil Relationships Dark colored crystalline rocks (gabbro, basalt, and metamorphic counterparts: mafic)Dominant mineralsAmphibole, pyroxene, and plagioclase feldsparK feldspars, mica, and quartz are minor componentsSoils developed from these rocks or saproliteLoamy/silty A / v. clayey BAppreciable Fe-oxide minerals and are often dark redSmectite clays;Less acidic;Higher base saturation and higher productivity
7 Sedimentary Deposits Loess - windblown silts Common along rivers carrying meltwaters from glaciersProperties of loess and resulting soils depends on rocks passed over by the glacierProperties also vary with with distance from source areaSilty soils, or silty caps over underlying materialsThinner deposits with more clay as distance from the source increases
8 Sedimentary DepositsGlacial till - material deposited by glaciers and processes related to glaciationMost common parent material in the midwest of North America and over much of EuropeProperties reflect the properties of rock passed over by the glacierMidwest - limestone and shaleLoamy soils with high pH, high base saturation, and smectitic claysNortheast - granite and acid sandstoneAcid soils with sandy loam texture and low base saturation
9 Coastal Plain Sediments Associated with marine and near shore environmentsProperties depend on sediment source environment of depositionbeach and dunes - eolian sands; little silt and clayriverine deposits – texture varies depending on position in the floodplaindeltaic deposits - variable texture depending on depositional environmentshallow marine - carbonate minerals mixed with terrestrial materialsamount of terrestrial material depends on distance from shore and shelf positionExcept for limestone, sediments were derived from upland erosion of previously formed soilsPrevious weathering
10 Coastal Plain Sediments Limestone – rock with >50% carbonate minerals (calcite and dolomite)Carbonates dissolve during soil formationIn humid climates, all of the carbonates dissolve and are leachedIn semi-arid and arid climates, incomplete leaching results in the carbonates being re-distributed in the soil and concentrated in subsoil horizonsSilicate minerals composing the soil were impurities in the limestoneMay be as little as 2-5% of the rockShallow marine deposits - silicates are clay-sizedLimestone derived soils are often clayey
11 Coastal Plain Sediments Sandstone – rock composed of sand-sized mineralsQuartz is often the dominant mineralVarying amounts of more weatherable mineralsDepends on mineralogy of sand sourceProperties of soils derived from sandstone depend on the composition of the sandstoneShale – rock composed of clay-sized grainsComposed of clay minerals, quartz, and feldsparsShale derived soils are clayey.
12 Relief Primary effect is its influence on hydrology Water moves downhill, often laterallyIn humid climates, lower landscape positions generally have seasonal water tablesConvex positions have more runoff and erosion than planar or concave positionsMore runoff = less water infiltration = less soil developmentEnhanced erosion also removes surficial soil and retards development Concave positions accumulate water and sedimentOver-thickened A and E horizonsThick A horizons due to slower organic matter decompositionShallow subsurface water movement may carry mobile constituents to lower topographic positions.
13 Climate Solar radiation Temperature Precipitation Water is the driving force for soil formationClimate effects are primarily related to the intensity of leaching and the amount of biomass production.Across a precipitation gradient ( mm) in the north-central U.S., as amount of precipitation increasedpH decreaseddepth to carbonates increasedN content of soil increasedclay content increased.
14 Climate Rainfall amount greatly affects weathering, leaching Low rainfall (< 20”/yr): low rate of weathering, limited clay formationModerate rainfall (20-30”/yr): 2:1 clays stable (rate of base cation leaching low)Higher rainfall (40-50”/yr): 1:1 (kaolinite) favored due to loss (over time) of basic cations2:1’s may persist if low Ksat (due to swelling clays) limits leaching …Very high rainfall (70-100”/yr): intense weathering, leachingKaolinite weather to gibbsite (clay destruction)—loss of SiFe oxides accumulateThis is all affected by time over which rainfall occurs…
15 ClimatePolar climates - freeze-thaw cycles produce ice wedges and frost heaving in polar climatesCold temperatures can also slow weathering reactionsSoils on very old landscapes in Antarctica do not have Bt horizons because weathering reactions are slowTemperature influences the type and quantity of vegetation in an areaAmount and quality of organic matterWater balance controls amount of water available to drive soil formation and the depth to which leaching occursNet precipitation or rainfall surplus = precipitation - evapotranspiration“Average" or extreme events
18 BiotaPrimary impact on soil development is vegetation (native, not present)Soils developed under grasslands have thick dark surface horizonsfibrous root systema greater proportion of the biomass of grasses is in rootsAs roots die, the organic matter is in the soil.higher lignin content and are more resistant to decompositionSoils developed under hardwoods have thinner A horizonstap root systemOrganic matter concentrated in a limited areaLeaves fall to the surfaceOther mechanisms are needed to incorporate decomposing leaves into the soil
20 BiotaConifers/high-tannin plants: soluble humic compounds lead to formation of Bh horizonsForm in sandy deposits with high seasonal water tableRedox and chelation combine to form bleached albic E horizonsHumics and Fe precipitate to form Bh, Bhs horizonsOccurs at surface of high water table (re-oxidation)In GA: live oak/palmetto on v. sandy marine deposits (Flatwoods)Humans have also had an appreciable impact on soil development through agriculture, mining, and other soil disturbing activities.
22 Time Passive factor Over time, the possible fates for the soil are: Only impact is to allow the two active factors, climate and biota, to express themselvesOver time, the possible fates for the soil are:continue indefinitely in its current formrate of erosion = rate of soil formationbecome more developedrate of erosion < rate of soil formationbecome the parent material for another soilexisting soil modified by a new set of processes in a new environmentbecome buried by a new parent materialdisappear - be eroded to become parent material for a new soil
23 Time What is the rate of soil formation? “it depends”“it is a combination of factors”Rate depends on the interaction of the other four state factors“Rapid Processes” (a few decades to a few hundred years):A horizon formationstructure formationleaching of water soluble components in humid climates“Intermediate Processes” (a few thousand years):subsoil organic matter accumulation (Bh horizon formation)subsoil carbonate accumulation (Bk horizon formation)“Slow Processes” (a few 10’s of thousand years)clay translocation (better considered to be many thousands, i.e. 7-10)induration of subsoil by carbonates, Fe oxides, and other mobile components