11 Functions of Soils Medium for plant growth Regulator of water supplies Recycler of raw materialsHabitat for soil organismsEngineering medium
12 Soil as Medium for plant growth Physical supportAirWaterTemperature moderationNutrient elementsProtection
13 Soil as Regulator of Water Supplies Soil regulates both the quality and quantity of water in rivers, lakes, and underground aquifersQuantity of water suppliesSome of the water may be stored in the soil and used by trees and other plantsQuality of water suppliesWater is purified and cleansed as it soaks through the upper layers of soil.
14 Soil as Recycler of Raw Materials Soils play a role in geochemical cyclesAssimilate organic wasteTurn it into beneficial humusConvert the mineral nutrients into plant and animal usable formsReturning carbon to the atmosphere to be used for photosynthesis again
15 Soil as Habitat for soil organisms Handful of soil is home to billions of organisms in thousands of speciesHow does this happen?Micro-environment differencesPore spacesMoistureTemperatureOrganic matter
16 Soil as Engineering Medium Soil is firm and solidGood base to build structuresSoils differ in stabilityDesigns for structures are different for soilsPhysical properties influence engineering uses
17 Composition of SoilsSoil consists of three major phases OR four components:Solid phase (Soil Minerals)Liquid phase (Soil Water)Gas phase (Soil Air)
19 Soil Solid Phase (two components) Inorganic mineralsSoil minerals are either primary or secondary mineralsPrimary minerals were formed in the original igneous rocks.Secondary minerals formed in soil by weathering of the primary minerals.Organic matter (humus).Humus is the product of the decay of organic residues such as wood, leaves, and other biological materials.
20 Soil water (soil solution) Water is vital to the ecological functioning of the soil.Soil solution which contains water, dissolved ions, molecules and gases.Soil water is different from drinking water in 2 ways:Soil water is held by many types of forces within the pores of the soil.Soil water is never pure but contains hundreds of dissolved organic and inorganic compounds.
21 Soil Atmospherecontains similar gases as found in the atmosphere above the soilBut often in very different proportions.Usually higher in carbon dioxide and lower in oxygen than the atmosphere.
22 Interaction of the Components The components interact to determine the nature of a soil e.g.,soil moisture controls air and nutrient supplymineral particles control water movementOrganic matter controls arrangement of minerals which influence pores that determine water and air relationships
24 Soil Formation Five factors of soil formation Weathering of rocks and mineralsProcesses of soil formation
25 Five factors of soil formation s = f(cl, o, r, p, t…)Where:s = any soil propertycl = climate (rainfall & temperature)o = organisms (biota)p = parent materialr = relief (slope aspect and position)t = time (relative age of soil formation)
26 1. Parent MaterialNature of parent material influences the characteristics of soilsE.g., texture of sandy soils is determined by parent materialmovement of water is controlled by texture of the parent materialInfluences the chemistry of the soilInfluences the type of clay minerals present in soil.
27 Weathering of Rocks and Minerals What is weathering?.. the modification or breakdown and destruction of the physical and chemical characteristics of rocks and minerals and carrying away the soluble products...the nature of the breakdown depends on the type of material
28 Types of Rocks Igneous rocks Sedimentary rocks Metamorphic rocks Form from molten magmaGranite and dioriteSedimentary rocksCompacted or cemented weathering products from preexisting rocksSandstone and shaleMetamorphic rocksFormed by change in the form of other rocksGneiss, marble, and slate
29 Two main types of weathering a) Physical (Mechanical) weathering.. Causes rocks to disintegrate into smaller pieces without affecting their compositionFactors:TemperatureAbrasion by water, ice and windPlants and animals
30 b) Chemical weathering .. Degradation of rocks and minerals by the chemical activities of water, oxygen, and microbial actionFactors of Chemical weathering:HydrationHydrolysisDissolutionCarbonationOxidation-reductionComplexation
31 Hydration Process of binding of water molecules to a mineral HydrolysisSplitting of water molecules into its components. Split components in turn attack the minerals.DissolutionProcess of hydrating of ions until they become dissociated
32 Carbonation Oxidation-Reduction Complexation Carbon dioxide dissolves in water to form carbonic acid which accelerates chemical breakdown of materialsOxidation-ReductionMinerals that contain Fe, Mn,or sulfur are susceptible to this reaction when exposed to environments different from the ones in which they formed. This destabilizes the mineral.ComplexationBiological processes produce organic acids that can form complexes with elements within the structure of a mineral thereby pooling the element from the mineral and destabilizing it.
34 Classification of Parent Materials Organic DepositsWeathering of rocks in placeDeposition of weathered rock materials from elsewhereParent materials are classified by their mode of placement at their current location.
37 2. ClimateMay be the most influential of the four factors acting on the parent materialDetermines the nature and intensity of weathering (precipitation and temperature)Both affect the physical, chemical and biological processesClimate also exerts influence indirectly through a second soil forming factor, the living organisms (natural vegetation).Climate is so important in soil formation that certain evidence of climatic change could be found in the soil
38 PrecipitationWater is essential for all the major chemical weathering reactions.The deeper water penetrates the parent material, the more effective it is in soil development.Water percolating through the soil profile transports soluble and suspended material from the upper to the lower layers.Thus percolating water stimulates weathering reactions and helps differentiate soil horizons.
39 Temperature Every 10 deg C, the rate of chemical reaction doubles If warm temperatures and abundant water are present in soil at the same time, the processes of weathering, leaching, and plant growth will be maximum and lead to deep soil profiles.Compare this to very modest soil profile development processes that are obtained in cold areas
40 3. Organisms (Biota) a) Role of natural vegetation Soil organisms, both the animals (fauna) and the plants (flora) physically churn the soil and help stabilize the soil structurea) Role of natural vegetationOrganic matter accumulationCation cycling by treesHeterogeneous rangelands
42 b) Role of animalsAnimals such as gophers, moles, prairie dogs bore into lower soil horizons and bring materials to the surface –tunnels.Earthworms and termitesBring about considerable soil mixingHuman influencedestruction of natural vegetationSoil tillage for crop productionIrrigationFertilizer application
43 4. Topography Relates to the configuration of the land surface It is described in terms of differences in elevation, slope and landscape positionSteep slopes encourage soil loss by erosion and allow less rainfall to enter the soilThus prevents formation of soils from getting ahead of soil destructionIn the depressions where runoff tends to concentrate, the soil is usually more deep
45 5. TimeTime that materials have been subjected to weathering is important because soil forming processes take time to show their effects.Clock of soil formation starts when e.g.Landslide exposes a new rockFlooding river deposits sediment on floodplainGlacier melts and dumps load of debrisBulldozer cuts and fills landscape, etc.Rates of weathering and soil developmentThis is a function of the interaction of time and the other factors of soil formation.
47 Processes of Soil formation Processes that are involved in soil formation can be placed in four main groupsAdditionsTransformationsTransfers or TranslocationsLosses
48 AdditionsAdditions entail the inputs of materials to the developing soil profile from outside sources. E.g.Addition of organic matter from Plant leaves and sloughed-off rootsAddition of water by precipitationAddition of dust particles that fall on the soil surfaceAddition of salts and silica dissolved in groundwater and deposited near or at soil surface
49 TransformationsTransformations entail disintegration and altering of composition and form of primary mineralsPhysical weathering to smaller particlesDecomposition of organic residuesRecombination of decomposition products to form new minerals such as silicate clays and oxidesSynthesis of organic acids, humus and other productsAggregation of mineral particles
50 TranslocationsTranslocations involve the movement of organic and inorganic materials laterally within a horizon or vertically from one horizon to the another.Movement of waterMovement of dispersed fine clay particlesMovement of dissolved organic substancesThe most common translocation agents are water and soil organisms.
51 Losses Materials are lost from the soil by the following: Drainage and leaching to groundwaterErosion of surface materialsEvaporationPlant uptakeMicrobial decompositionAnimals and humans
52 Combination of Processes LossesTranslocationsAdditionsTransformations
53 Example of the soil forming processes in action Addition of litter and root residuesTransformation of plant substances into humusAccumulation of humus enhances water and nutrient capacity (feedback for plant growth)Microorganisms attracted by humus buildup accelerate transformations and cause translocations
54 Soil Horizon Development A-Horizon developmentAccumulation of organic matterClumping of individual soil particlesDistinct from parent material and other layersB and C horizon developmentCarbonic and organic acids are carried by water into soil where dissolve various minerals (transformations)Soluble materials (ions –Ca2+, CO32-, SO42-, etc) are carried by water and precipitate in the soil from upper to lower horizons (translocation)Weathering of primary minerals into secondary mineralsWetting and drying cracks soils and makes structures.
56 Soil-Landscape-Ecosystem Relationships The above processes of soil genesis, operating under the influence of the five environmental factors discussed previously gives us a framework for understanding the relationships between particular soils and the landscapes and ecosystems in which the soils exist.