Soils: Physical Properties & Water Relations

What is Soil?

• Soil is a mixture of living and non-living components:
  • Mineral particles: derived from rocks
  • Organic matter
  • Dead & decaying matter
  • Root exudates
  • Bacteria & fungi
  • Water: in pores
  • Gases: e.g., O2, CO2, N_2
  • Anaerobic vs. aerobic soils: Roots and microbial respiration
• Soil is a living habitat for plants and organisms.
• Soils are darker in color.

Ecological Importance of Soil

• Site of decomposition of organic matter
  • Habitat of the decomposers (fungi and bacteria)
  • Return of organic nutrients to inorganic form (mineralization)
• Source of water and nutrients for plants
• Vegetation-Soil (Edaphic) Relationships
  • Often strongly correlated
  • Basis for soil classification

Soil-Vegetation Relationships on the Edwards Plateau, Texas

• Includes maps illustrating soil types, woody cover, and grassy cover distribution across the Edwards Plateau.

Soil Development

• Involves two major processes:
  • Weathering of rocks: chemical & physical
  • Incorporation of organic matter (O.M.)
• Factors influencing soil development
  • Climate: primarily precipitation, temperature, evaporation
  • Vegetation: amount and type of O.M.
  • Parent material: underlying substrate
  • Topography: gravity effects
  • Age: young vs. old soils
• Type of vegetation is influenced by climate.
• Soil builds on limestone, resulting in buffering effects and high pH in the soil.
• Minerals affect the type of mineral particles.

Soil Profiles

• Soil profiles reflect the history of soil development.
  • A Horizon = topsoil
  • B Horizon = subsoil
  • Humus = partially decomposed organic matter
  • Eluviation = leaching of material by water
  • Illuviation = deposition of materials

Soil Development in Semi-arid Climates: Grasslands

• Loess = fine, wind-blown clay
• Mollisol = soft soils of grasslands; thick fertile, dark organic horizon

Soil Development in Humid Climates: Forests

• Ultisol = intensely weathered soils of humid climates; acidic with substantial clay translocation

Soil Taxonomy & Classification

• Soil Order
  • Most inclusive, broadest category
  • 12 soil orders worldwide
  • Soil orders are frequently defined by a single dominant characteristic affecting soils in that location, e.g., the prevalent vegetation (Alfisols, Mollisols), the type of parent material (Andisols, Vertisols), or the climate variables such as lack of precipitation (Aridisols) or the presence of permafrost (Gelisols).
  • Amount of physical and chemical weathering present (Oxisols, Ultisols), and/or the relative amount of Soil Profile Development that has taken place (Entisols) are also significant in several soil orders
  • The 12 soil orders are Entisols, Inceptisols, Andisols, Mollisols, Alfisols, Spodosols, Ultisols, Oxisols, Gelisols, Histosols, Aridisols, and Vertisols.

Major Soil Orders in the United States

• Lists the 12 orders of soil taxonomy: Alfisols, Andisols, Aridisols, Entisols, Gelisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, Vertisols

Soil Texture

• Soil texture determines many of the properties of soils that affect plants
  • Particle size classes:
  • Gravel = >2 mm dia.
  • Sand = 0.05-2 mm dia.
  • Silt = 0.002-0.05 mm dia.
  • Clay = <0.002 mm dia.
  • Texture = particle size distribution

Soil Texture Triangle

• Illustrates the relationship between sand, silt, and clay content in determining soil texture (e.g., sandy loam, silty clay, loam).
• Fine textured vs. coarse textured soils; Loams

Soil Texture & Water Relations

• Soil moisture content
  • Gravimetric moisture = mass H_2O/mass dry soil
  • Volumetric moisture = vol H_2O/vol soil
• Bulk density = mass soil/volume soil
• Field Capacity (FC)
  • water content after drainage by gravity
  • maximum water holding capacity
• Permanent Wilting Point (PWP)
  • Water left in soil when plants wilt (permanently)
  • Lower limit of available water
• Available Water = FC - PWP
• Plants can’t access all water

Soil Water Availability

• Shows the relationship between saturation, field capacity, wilting point, and hygroscopic coefficient.

Available Soil Water and Texture

• Graph showing the relationship between soil texture (sand, loam, silt loam, clay loam, clay) and available water, unavailable water, field capacity, and permanent wilting point.

Soil Water Potential

• A direct measure of available water
• Water potential components in soil:
  • Matric potential (\Psi_m)
  • Osmotic potential (\Psi_{\pi})
  • Pressure potential (\Psi_p)
• Total soil water potential:
  • \Psi{soil} = \Psim + \Psi{\pi} + \Psip
  • \Psi_{\pi} only important in saline soils
  • \Psi_p usually minimal in well-drained soils

Measurement of Soil Water Potential

• Thermocouple psychrometers
• Measure humidity (dew point) in soil ~ water potential
• Interface with data loggers

Relationships between soil water potential and soil water content (Soil texture)

• Graph illustrating the relationship between soil water potential and soil water content for different soil textures (sand, sandy loam, clay).
• PWP = -1.5 MPa
• FC = -0.01 MPa

Season changes in soil water potential in a California blue oak woodland

• Graphs showing seasonal changes in soil water potential with depth for a dry year (1994) and wet year (1995).

Soil Texture and Water Movement in Soils

• Flux of water movement (J_w):
  • Jw = -K \frac{\Delta \Psi{soil}}{\Delta z}
  • Where,
  • K = hydraulic conductivity
  • \Delta \Psi_{soil} = water potential gradient
  • \Delta z = distance

Hydraulic redistribution of soil water

• Wet soil to dry soil

Hydraulic Redistribution

• Night-time increases in available soil water indicates water is leaving plants and entering the soil when transpiration is suppressed

Circumstances and ecological significance of hydraulic redistribution

• Conditions necessary for HR
  • Increase available pool of water for transpiration and gas exchange
  • Enhance nutrient uptake in organic, surface soils
  • Influence on plant-plant interactions and ecohydrology

Hydraulic redistribution and species interactions

• Predawn water potentials of three understory shrub species with and without a Prosopis glandulosa overstory showing facilitation, competition and neutral effects and soil during a fall dry-down period showing hydraulic lift in control clusters