HOR #5

Soil Water Energy Concepts

Energy is Required for: 1. Retention & movement of water in soil 2. Water uptake & translocation in plants 3. Water loss to atmosphere Potential & Kinetic Energy - Kinetic energy - important factor in rapid, turbulent flow of water in a river (kinetic energy is negligible in mvmt of water in soil) Potential energy - most important in determining status & mvmt of soil water - All substances, including water, tend to move from HIGHER to a LOWER ENERGY STATE (if know energy states, we can predict mvmt of water wi/ a soil)

Pore Type & Shape

Packing pores (between primary soil particles) Interped pores (shape depends on ped/granules) Biopores (often long, narrow and branched; some are spherical) Created by earthworms, churn up soil

Structure & Related Properties of Water

Polarity - Charges NOT evenly distributed - Results in asymmetrical, V shape arrangement - 2 sides, want to balance! - Positive side attracted to: - Clay, OM, Humus - Negative side attracted to: - Other H+, Na+, K+, Ca++, Mg++

Height of Rise (HOR) in Soils

- Capillarity occurs in most moist soils - Movement rate & HOR is not = to straws: - Soil pores - NOT uniform shape/ size - Soil pores - NOT straight - Some pores may be filled with air

GRAVITATIONAL POTENTIAL

- Greater difference in height, the greater the gravitational potential. - Allows excess water to drain after irrigation or rainfall - Independent of soil properties

Surface Tension

- Liquid-air interface - Cohesion > water-air attraction - Results in water surface to act as elastic membrane

SOIL WATER POTENTIAL

- Soil's water energy status is always compared to pure water - Difference in energy at one location (or condition) compared to another WET SOIL > DRY SOIL High energy state Low energy state High water potential Low water potential

Hydrogen Bonding

A hydrogen atom of one water is attracted to the oxygen end of a neighboring water molecule - Accounts for the polymerization of water, high boiling point, specific heat and viscosity of water

Arrangement & Sorting

Arrangement: • Loose or tight Packing: • Sorted: particle of similar size (poorly graded) • Graded: many different size grains (poorly sorted)

Bulk Density vs. Porosity

BD commonly measures soil size BD effects health of plants in major way

2 Forces Responsible for BONDING

COHESION Water-water, weak bond ADHESION AKA adsorption, strong bond Water to solid surface

Capillarity Fundamentals & Soil Water

Capillary Mechanism - Controlled by adhesive and surface tension (cohesive) forces

Factors Affect Bulk Density

Compaction Soil texture Packing arrangement of particles Aggregation Organic matter Depth in profile

Soil Bulk Density

Db = Soil Mass Volume Solids + Pores Db - Soil including particles + pore spaces

PRESSURE POTENTIAL

Hydrostatic Potential - AKA as submergence potential - (+) - From weight of water - In saturated soils (in water table) Matric Potential - AKA suction or tension - (-) - From ADHESIVE forces, capillarity - In unsaturated soils (above water table)

Depth in Profile

Increased bulk density the deeper in the profile due to the weight of soil above Hardpan: Lower A, or B horizons Cemented particles with: OM Silica Sesquioxides CaCO

Soil Texture & Aggregation

Less total pore space in the sandy soils compared than in the clayey one because the clayey soil contains a large number of fine pores within each aggregate This is the reason why surface soils with coarse textures are typically more dense than finer textured soils

3 Forces Affecting Potential Energy

MATRIC - Adhesive/ attraction of water to soil solids (matrix) - Responsible for ADSORPTION & CAPILLARITY - Energy lowest particle surface OSMOTIC - Water attracted to ions or solutes GRAVITATIONAL - Pulls water downward

Pore Size Distribution

Macropores > 0.08 mm in diameter large, freely draining sands, inter-aggregate pores Water moves thru quickly H2O molecules - Freedom to move - nothing to hold onto! Micropores < 0.08 mm in diameter small, storage of water clays, intra-aggregate pores GREATER TOTAL POROSITY Store Water - LESS FREEDOM to move around & Macropores in clay - Greater influence water movement

MATRIC POTENTIAL & SOIL TEXTURE

Micropores Small pores Strong suction Strong capillarity Macropores Large pores Weak suction Weak capillarity

Effect of Cultivation on Aggregation

Short term: •Improvement in aggregation if done on moderately dry soil •Breaks up large clods, loosens soil, and increases porosity •Incorporates organic matter into the soil Long term: •Loss of aggregation (Destroy Soil Structure) •Enhanced oxidation of organic material reduces aggregation •Loss of macroporosity occurs if tillage is carried out in a wet soil (puddled) •Effect less pronounced where Fe & Al oxides plentiful

Relationship of Db Soil Strength & Root Growth

Soil Strength: Ability of soil to resist deformation HOWEVER; dependent on: Moisture: Wet soils are easier than dry soils Texture: Sand soils are easier than clayey soils Soil Strength IMPORTANT in building roads Wet soils COMPACT easier than dry soils

SOIL WATER POTENTIAL

TOTAL POTENTIAL Sum of gravitational, matric, hydrostatic & osmotic potentials

Soil Bulk Density

The mass of dry soil per unit of bulk volume, including air space (pores). Bulk Volume is determined before drying to constant weight at 105ºC. Density = Mass ( g ) Volume c㎥ Bulk Density - Organic Matter (soil) + Pore Space Particle Density (Dp) - Solid material ONLY, Not pore space

Porosity

The volume percentage of the total soil bulk not occupied by solid particles.

Influence of Human Practices on Db

Vehicular traffic & frequent pedestrian traffic Major impact on forest soils, which have low bulk density Tillage Loosens soil initially, but depletes organic matter, resulting in higher bulk density 9/19/2019

SOIL WATER: Characteristics & Behavior

Very SIMPLE COMPOUND, but a VITAL COMPONENT for every living cell! - Unique properties promote wide variety of physical, chemical & biological processes: - Weathering - Decomposition of organic matter - Plant growth - Pollution

OSMOTIC POTENTIAL

Water molecules CLUSTER around inorganic & organic solutes HIGH (Solute) >>>>>>>>>>>>>> Low (Solute) Low energy state High energy state Must have SEMIPERMEABLE MEMBRANE to allow water mvmt, BUT NOT solute mvmt THIS DOES NOT EXIST IN SOILS! Little effect on mass mvmt of soil water Does influence energy needed for plants to take up water

Pore Space of Mineral Soils

• 25% in compacted sub soils • 50% is ideal • 60% or more in well-aggregated,undisturbed soils with high organic matter content • 80%+ in organic soils (peat) • Cultivation can reduce pore space, organic matter content, and granulation Compacted subsoil 'B' Horizon Cultivate - deplete aggregation - NOW plow less

Micropores <0.08 mm

• Too small to permit air movement • Usually filled with water •A high porosity soil can still have slow gas and water movement if dominated by micropores. • Water generally unavailable to plants (held too tightly) • Reduces root growth and aerobic microbial activity

Macropores: > 0.08mm

• Where drainage of water occurs • Good for gas exchange (air movement) • Penetrable by roots and a multitude of organisms • Spaces between sand grains •This is why sandy soils have low total porosity but rapid drainage (hydraulic conductivity)

Db & Porosity in Soil Profile

•Macropores most prevalent near the surface •Micropores usually dominate at depth Db - Near surface - LOWER Db - More OM Db - Lower in profile - INCREASE Db - More weigh above & compacting; less organisms; lower - bedrock, less pore space