LECTURE 3Factors Influencing Soil Formation Biota & Time
Processes of soil formation
Truncated soil(A.I. Bennett)
The 5 factors influencing soil formation Parent materialsClimateBiotaTopographyTime
Sequences that develop when one of these factors is dominant in determining differencesParent material lithosequenceClimate climosequenceBiota biosequenceTopography toposequenceTime - chronosequence
Biota (living organisms)Contribute towards:Organic matter accumulationBiochemical weatheringProfile mixingNutrient cyclingAggregate stabilityCan be divided intoNatural vegetationAnimals
Natural VegetationReduces natural soil erosion rates.Slows down rate of mineral surface soil removal.Certain leaves produce organic acids that bring iron and aluminium into solution.Accelerate the downward movement and accumulation of these metals in the B-horizon.
Development of the A horizon:Comparison of soils under grassland and forest vegetation.Thickness of A-horizonStructural stability of mineral soil aggregates
Cation cycling by trees:Ability of vegetation to take up mineral elements from the soil influences the characteristics of soils that developNB acidity!Comparison between coniferous forest and deciduous forest
Heterogeneity in rangelandsIn arid and semi-arid landscapes, there is competition for limited soil waterDoes not permit vegetation dense enough to cover soil surfaceThis alters soil properties in several ways:
Trapping of windblown dustScavenging of nutrients form interplant areasDeposition of nutrients under plant canopiesAddition of organic acidsDevelopment of islands of enhanced fertility, thicker A-horizons etc.
AnimalsRole of large animals, e.g. moles, rabbits, gophersBring materials from deep in the profile to the surfaceOpen the profile up to air and waterRole of smaller creatures, e.g. earthworms, ants, termitesSoil mixing (bioturbation)Aeration of soilIncrease in stability of soil aggregates (assuring infiltration of water)Activities of animals decrease the differences among soil horizons.
Human influences?It is believed that Native Americans regularly set fires to maintain prairie grasslands in Indiana and Michigan.Human destruction of natural vegetation.Tillage of soils.Irrigation, addition of fertilizers.Mining, urbanization.
TimeStopwatch starts ticking when parent materials are deposited or exposed.Rates of weatheringyoung vs. mature soils?Time spans generally in the range of hundreds to hundreds of thousands of years.Very rare that soil formation can be observed in a human lifetime.
Chronosequences:Methods such as carbon dating, analysis of fossils and human artifacts etc. used as evidence regarding the time spans involved in soil formation, but another tool is to analyse a chronosequence.E.g. on alluvial terraces of differing age.
Interaction with parent materials:Residual parent materials vs. transported parent materials.Interaction with other soil forming factors.
S = (c, p, r, v, o, t)
Processes of soil formation
TransformationsMineral weathering.Organic matter breakdown.Some soil constituents are modified or destroyed, others are synthesized.
TranslocationsMovements of organic and inorganic materials from one horizon up or down to another.Material moved by water and soil organisms.
AdditionsOrganic matter from leavesDust from atmosphereMinerals from groundwaterOthers?
LossesLeaching to groundwater.Erosion.Others?