🌱Intro to Soil Science Unit 3 – Soil Water: Retention and Movement

Key Concepts

• Soil water retention refers to the ability of soil to hold water against gravitational forces
• Soil water potential is a measure of the energy state of water in soil and drives water movement
• Water moves through soil via capillary action, gravity, and pressure gradients
  • Capillary action occurs in small pores and is driven by adhesive and cohesive forces
  • Gravity causes downward movement of water through larger pores
  • Pressure gradients result from differences in water potential between soil layers
• Field capacity is the amount of water retained in soil after excess water has drained away
• Permanent wilting point is the soil moisture content at which plants can no longer extract water
• Available water capacity is the amount of water held between field capacity and permanent wilting point
• Hydraulic conductivity measures the ease with which water moves through soil
• Soil texture, structure, organic matter content, and bulk density all influence soil water dynamics

Soil Water Basics

• Soil water content is the amount of water present in a given volume or mass of soil
  • Expressed as volumetric water content (volume of water per volume of soil) or gravimetric water content (mass of water per mass of dry soil)
• Soil water is held in pore spaces between soil particles
• Soil pores can be classified as macropores (larger than 0.08 mm) or micropores (smaller than 0.08 mm)
• Macropores are important for water infiltration and drainage, while micropores retain water for plant use
• Soil water can exist in three states: solid (ice), liquid (water), or gas (water vapor)
• The amount and distribution of soil water are influenced by climate, topography, soil properties, and vegetation
• Soil water plays a crucial role in plant growth, nutrient transport, and soil microbial activity

Water Retention Mechanisms

• Adhesion is the attraction between water molecules and soil particle surfaces
  • Responsible for the formation of thin water films around soil particles
• Cohesion is the attraction between water molecules themselves
  • Allows water to move through soil pores via capillary action
• Capillary forces are strongest in small pores, where the surface area to volume ratio is high
• Matric potential is the attractive force between water and soil particles, resulting from adhesion and cohesion
• Osmotic potential arises from the presence of solutes in soil water, which reduce water potential
• Gravitational potential is the energy of water due to its position in the soil profile
• Soil water retention curves describe the relationship between soil water content and soil water potential
  • Curves vary depending on soil texture, with clay soils retaining more water at a given potential than sandy soils

Soil Water Potential

• Soil water potential is a measure of the energy state of water in soil
  • Expressed in units of pressure (pascals, bars, or atmospheres)
• Total soil water potential is the sum of matric, osmotic, and gravitational potentials
• Matric potential is typically negative, as it represents the attractive forces between water and soil particles
• Osmotic potential is also negative, as the presence of solutes reduces water potential
• Gravitational potential is positive above the reference level (usually the water table) and negative below it
• Water moves from areas of high potential to areas of low potential
• Plant roots can extract water from soil when the root water potential is lower than the soil water potential
• The relationship between soil water content and potential is non-linear and hysteretic
  • Hysteresis occurs because the soil water retention curve differs during wetting and drying cycles

Water Movement in Soil

• Water enters the soil through infiltration, which is the downward movement of water from the surface
• Percolation is the continued downward movement of water through the soil profile
• Drainage occurs when water moves beyond the root zone and reaches the water table
• Capillary rise is the upward movement of water from the water table through small pores
• Evapotranspiration is the loss of water from the soil surface (evaporation) and plant leaves (transpiration)
• Darcy's law describes the flow of water through soil in response to a hydraulic gradient
  • Flow rate is proportional to the hydraulic conductivity and the hydraulic gradient
• Hydraulic conductivity is a measure of the ease with which water moves through soil
  • Depends on soil texture, structure, and water content
• Preferential flow occurs when water moves rapidly through macropores, bypassing the soil matrix

Measurement Techniques

• Gravimetric method involves weighing a soil sample before and after drying to determine water content
  • Requires soil sampling and is destructive, but is simple and accurate
• Volumetric water content can be measured using time-domain reflectometry (TDR) or frequency-domain reflectometry (FDR)
  • These methods rely on the dielectric properties of soil, which are influenced by water content
• Neutron probe measures the hydrogen content of soil, which is related to water content
  • Requires a radioactive source and is less commonly used due to safety concerns
• Tensiometers measure soil water potential by equilibrating with the surrounding soil
  • Consist of a porous cup filled with water and connected to a pressure gauge
• Resistance blocks or gypsum blocks measure soil water potential based on electrical resistance
  • Porous blocks are embedded in the soil and absorb or release water depending on soil water potential
• Remote sensing techniques, such as satellite imagery or ground-penetrating radar, can provide estimates of soil water content over large areas

Factors Affecting Soil Water

• Soil texture refers to the relative proportions of sand, silt, and clay particles
  • Sandy soils have large pores and low water retention, while clay soils have small pores and high water retention
• Soil structure describes the arrangement of soil particles into aggregates
  • Well-structured soils have a balance of large and small pores, promoting water retention and movement
• Organic matter content influences soil water by improving soil structure and increasing water-holding capacity
• Bulk density is the mass of dry soil per unit volume
  • High bulk density indicates soil compaction, which reduces pore space and water movement
• Soil depth determines the volume of soil available for water storage
  • Shallow soils have limited water-holding capacity compared to deep soils
• Slope and aspect affect soil water through their influence on runoff, infiltration, and evapotranspiration
  • Steep slopes promote runoff, while south-facing aspects (in the northern hemisphere) receive more solar radiation, increasing evapotranspiration
• Vegetation influences soil water through transpiration, interception of precipitation, and modification of soil properties
  • Plant roots create channels for water movement and contribute to soil organic matter

Practical Applications

• Irrigation scheduling relies on understanding soil water dynamics to optimize water application
  • Soil water sensors can be used to monitor soil moisture and guide irrigation decisions
• Drainage systems are designed to remove excess water from soil, improving aeration and root growth
  • Subsurface drainage tiles or ditches are commonly used in agricultural fields
• Soil water management is crucial for crop production, as both water excess and deficiency can reduce yields
  • Practices such as mulching, cover cropping, and conservation tillage can help optimize soil water
• Soil water information is used in hydrologic models to predict runoff, infiltration, and groundwater recharge
  • These models are important for water resource planning and flood forecasting
• Understanding soil water is essential for addressing environmental issues such as soil salinization, nutrient leaching, and groundwater contamination
  • Management practices that promote water use efficiency and reduce deep percolation can help mitigate these problems
• Soil water data is increasingly being used in precision agriculture to optimize inputs and maximize yields
  • Variable rate irrigation and fertilization can be based on spatial patterns of soil water availability
• Soil water monitoring is important for assessing the effectiveness of conservation practices, such as terracing or contour farming, in reducing erosion and improving water retention.