Plant Ecology - Chapter 4 Soils & Minerals

Soil Structure & Texture Soil structure - physical arrangement of soil particles into aggregates Controls soil porosity

Soil Structure & Texture Soil texture - proportional distribution of different-sized particles

Soil Structure & Texture

Loam soils have balance between sand, silt, clay Equal parts of sand and silt, less clay Generally most desirable for agriculture

Soil Structure & Texture Sandy soils - >50% sand particles - coarse texture Hold water, minerals poorly Warm quickly, cool quickly

Soil Structure & Texture Clayey soils - >35% clay Hold large volume of water Retain water and minerals well Also retain pesticides, pollutants

Soil Structure & Texture Poor infiltration = greater runoff Poor drainage, poor aeration Slow to warm and cool

Soil Structure & Texture Silty soils - >50% silt Intermediate in characteristics between sandy and clayey soils

Soil Structure & Texture Sand, silt particles irregular in shape Clay particles plate- like or rod-shaped Clay particles have large surface-to- volume ratio

Soil Structure & Texture Particle size, shape affects porosity Pore space in sandy soils % Clayey soils % - smaller particle size & arrangement (cluster together)

Soil Structure & Texture Clay particles usually bear strong negative electrochemical charge Attract cations, such as important nutrients and water molecules

Soil Structure & Texture H, Ca, Mg, K, Na ions most abundant in soils in humid regions Arid soils, H ions move to last on list, Na more important All attract, hold water molecules

Soil Structure & Texture Cations attracted to clay particles partially available to plants Exchange between particles, soil water Ions can be taken up by plants from water, leached, or reattach to other particles

Soil pH Soil pH in U.S to 10 Native vegetation adapted to all pHs, but most ag crops grow best in slightly acidic soils Changing pH has strong effects on nutrient and toxin availability, soil biota (bacteria, fungi)

Soil pH Forest trees especially tolerant of acidic soils Conifers tend to increase acidity of soil (lower pH) via decomposition of needles

Soil pH Grasslands tend to grow on alkaline soils Low rainfall results in soils with high pH

Soil Horizons & Profiles Soils contain layers - horizons Sequence of horizons produces a soil profile O, A, B, C, R

Soil Horizons & Profiles O horizon - organic matter

Soil Horizons & Profiles A horizon - topsoil Region of maximum leaching - eluviation

Soil Horizons & Profiles B horizon - subsoil Region 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 bedrock Ecological succession 100s 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 area Climate, 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

Hydraulic lift - deep- rooted plants pull water upward Moves out into drier, shallow soils Allows survival of shallow-rooted plants during drought, or in arid environments

Plant Nutrients Macronutrients and micronutrients Essential vs. beneficial Two most important for plants: nitrogen and phosphorus

Nitrogen Fixers Free-living nitrogen fixers - cyanobacteria Flooded rice paddies, some surface soils

Nitrogen Fixers Symbiotic nitrogen fixers Rhizobium, Bradyrhizobium on legumes - root nodules Frankia on non- legumes Cyanobacteria external to some plant roots - loose associations

Phosphorus in Soils Availability not directly related to amount present in soil Phosphorus bound in ways that make it unavailable to plants Bound to clay particles, metal ions, immobilized by microbes

Evergreen vs. Deciduous Evergreens often characteristic of nutrient-poor soils, soils subject to drought Deciduous leaves require higher investments in photosynthetic enzymes, other proteins

Mycorrhizae Symbioses between various fungi, roots of terrestrial plants Extremely common, widespread Approach for dealing with phosphorus limitation, other nutrient shortages

Mycorrhizae Two major groups Endomycorrhizae Ectomycorrhizae Endo- are most common, especially arbuscular mycorrhizae

Mycorrhizae Arbuscular mycorrhizae - most abundant where phosphorus is limited, in warm, dry climates Important in tropical ecosystems, and for crop pants - woody and herbaceous Fungal 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 conifers Hartig net between root cells, mantle network of hyphae outside root

Mycorrhizae Fungal hyphae increase nutrient uptake from soil Transfer 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 uptake Improve water uptake Break down soil proteins Protect roots from toxins Protect plant from fungal, bacterial diseases

Mutualism or Parasitism? Fungi get carbon, energy from plant host Plant gets nutrients, other benefits Either “partner” can function as parasite at times - plant sheds fungus when times are good, or fungus gives little to plant