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Plant Ecology - Chapter 4
Soils & Minerals
Soil Structure & Texture
Soil structure - physical arrangement of soil particles into aggregatesControls soil porosity
Soil Structure & Texture
Soil texture - proportional distribution of different-sized particles
Soil Structure & Texture
Soil Structure & Texture
Loam soils have balance between sand, silt, clayEqual parts of sand and silt, less clayGenerally most desirable for agriculture
Soil Structure & Texture
Sandy soils - >50% sand particles - coarse textureHold water, minerals poorlyWarm quickly, cool quickly
Soil Structure & Texture
Clayey soils - >35% clayHold large volume of waterRetain water and minerals wellAlso retain pesticides, pollutants
Soil Structure & Texture
Poor infiltration = greater runoffPoor drainage, poor aerationSlow to warm and cool
Soil Structure & Texture
Silty soils - >50% siltIntermediate in characteristics between sandy and clayey soils
Soil Structure & Texture
Sand, silt particles irregular in shapeClay particles plate-like or rod-shapedClay particles have large surface-to-volume ratio
Soil Structure & Texture
Particle size, shape affects porosityPore space in sandy soils - 35-50%Clayey soils - 50-60% - smaller particle size & arrangement (cluster together)
Soil Structure & Texture
Clay particles usually bear strong negative electrochemical chargeAttract cations, such as important nutrients and water molecules
Soil Structure & Texture
H, Ca, Mg, K, Na ions most abundant in soils in humid regionsArid soils, H ions move to last on list, Na more importantAll attract, hold water molecules
Soil Structure & Texture
Cations attracted to clay particles partially available to plantsExchange between particles, soil waterIons can be taken up by plants from water, leached, or reattach to other particles
Soil pH
Soil pH in U.S. - 3.5 to 10Native vegetation adapted to all pHs, but most ag crops grow best in slightly acidic soilsChanging pH has strong effects on nutrient and toxin availability, soil biota (bacteria, fungi)
Soil pH
Forest trees especially tolerant of acidic soilsConifers tend to increase acidity of soil (lower pH) via decomposition of needles
Soil pH
Grasslands tend to grow on alkaline soilsLow rainfall results in soils with high pH
Soil Horizons & Profiles
Soils contain layers - horizonsSequence of horizons produces a soil profileO, A, B, C, R
Soil Horizons & Profiles
O horizon - organic matter
Soil Horizons & Profiles
A horizon - topsoilRegion of maximum leaching - eluviation
Soil Horizons & Profiles
B horizon - subsoilRegion of maximum deposition - illuviation
Soil Horizons & Profiles
C horizon - undeveloped mineral material
Soil Horizons & Profiles
R horizon - parent material (bedrock)
Soil Development
Residual soils - develop in place by breakdown of bedrockEcological succession100s to 1000s of years
Soil Development
Transported soils - carried from some other place - wind, water, glaciers
Soil Development
5 factors determine the kinds of soils that develop in an areaClimate, parent material, time, topography, living organisms
Soil order
Organic Matter, Organisms
Organic matter - humus - contributes, binds nutrients, retains water, acidifies soil (alters nutrient availability)Organism actions alter porosity, structure
Water in Soils
Water in Soils
Water in Soils
Water in Soils
Water in Soils
Hydraulic lift - deep-rooted plants pull water upwardMoves out into drier, shallow soilsAllows survival of shallow-rooted plants during drought, or in arid environments
Plant Nutrients
Macronutrients and micronutrientsEssential vs. beneficialTwo most important for plants: nitrogen and phosphorus
Nitrogen Fixers
Free-living nitrogen fixers - cyanobacteriaFlooded rice paddies, some surface soils
Nitrogen Fixers
Symbiotic nitrogen fixersRhizobium, Bradyrhizobium on legumes - root nodulesFrankia on non-legumesCyanobacteria external to some plant roots - loose associations
Phosphorus in Soils
Availability not directly related to amount present in soilPhosphorus bound in ways that make it unavailable to plantsBound to clay particles, metal ions, immobilized by microbes
Evergreen vs. Deciduous
Evergreens often characteristic of nutrient-poor soils, soils subject to droughtDeciduous leaves require higher investments in photosynthetic enzymes, other proteins
Mycorrhizae
Symbioses between various fungi, roots of terrestrial plantsExtremely common, widespreadApproach for dealing with phosphorus limitation, other nutrient shortages
Mycorrhizae
Two major groupsEndomycorrhizaeEctomycorrhizaeEndo- are most common, especially arbuscular mycorrhizae
Mycorrhizae
Arbuscular mycorrhizae - most abundant where phosphorus is limited, in warm, dry climatesImportant in tropical ecosystems, and for crop pants - woody and herbaceousFungal body grows inside root cells, with hyphae extending outward
Mycorrhizae
Arbuscular mycorrhizae - associations not highly specific - same fungus may have many plant host species, or one host may have several fungal associates
Mycorrhizae
Ectomycorrhizae - woody plants, especially temperate conifersHartig net between root cells, mantle network of hyphae outside root
Mycorrhizae
Fungal hyphae increase nutrient uptake from soilTransfer nutrients to root cells of host plants
Mycorrhizae
Increased uptake, especially of phosphorus, results from higher surface area, and production of enzymes that release phosphorus from clay
Mycorrhizae - more good
Improve metal uptakeImprove water uptakeBreak down soil proteinsProtect roots from toxinsProtect plant from fungal, bacterial diseases
Mutualism or Parasitism?
Fungi get carbon, energy from plant hostPlant gets nutrients, other benefitsEither “partner” can function as parasite at times - plant sheds fungus when times are good, or fungus gives little to plant
