Plant Ecology - Chapter 4 Soils & Minerals. Soil Structure & Texture Soil structure -...

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