3.2 Water retention curves and available water capacity
2 min read•july 24, 2024
Soil water retention is crucial for plant growth and soil health. It determines how much water a soil can hold and make available to plants. Understanding water retention curves helps us interpret soil moisture dynamics and calculate important parameters like and .
is the amount of water plants can access between field capacity and permanent wilting point. Factors like soil texture, , and structure influence water retention. Calculating AWC helps farmers and land managers make informed decisions about irrigation and crop selection.
Soil Water Retention and Available Water Capacity
Interpretation of water retention curves
Top images from around the web for Interpretation of water retention curves
HESS - Predicting the soil water retention curve from the particle size distribution based on a ... View original
Is this image relevant?
HESS - Sigmoidal water retention function with improved behaviour in dry and wet soils View original
Is this image relevant?
3.3 Soil Water Retention – Rain or Shine View original
Is this image relevant?
HESS - Predicting the soil water retention curve from the particle size distribution based on a ... View original
Is this image relevant?
HESS - Sigmoidal water retention function with improved behaviour in dry and wet soils View original
Is this image relevant?
1 of 3
Top images from around the web for Interpretation of water retention curves
HESS - Predicting the soil water retention curve from the particle size distribution based on a ... View original
Is this image relevant?
HESS - Sigmoidal water retention function with improved behaviour in dry and wet soils View original
Is this image relevant?
3.3 Soil Water Retention – Rain or Shine View original
Is this image relevant?
HESS - Predicting the soil water retention curve from the particle size distribution based on a ... View original
Is this image relevant?
HESS - Sigmoidal water retention function with improved behaviour in dry and wet soils View original
Is this image relevant?
1 of 3
- Water retention curve components show soil on X-axis (kPa or bar) and on Y-axis (%)
- Key features include at highest water content and inflection point where curve changes from steep to gradual
- Shape typically steep at low suction (near saturation) and gradual at high suction (drier conditions)
- demonstrates difference between wetting and drying curves due to pore geometry and air entrapment
- Curve interpretation reveals steeper curves indicate rapid water loss with increasing suction while gradual curves suggest slower water loss
Key soil water parameters
- Field capacity (FC) represents water content after excess drainage typically at -33 kPa soil water potential
- Permanent wilting point (PWP) indicates water content where plants can no longer extract water generally at -1500 kPa soil water potential
- Available water capacity (AWC) quantifies water available for plant use calculated as difference between FC and PWP expressed as depth of water or percentage by volume
Factors in soil water retention
- Soil texture affects retention with having large pores and low retention, medium pores and moderate retention, and small pores and high retention
- Organic matter increases water retention capacity improves and porosity (humus)
- Soil structure impacts pore size distribution influences water movement and retention (aggregates)
- inversely relates to water retention higher density reduces retention (compaction)
- affects retention properties ( vs )
- correlates with water holding capacity deeper soils retain more water (root zone)
Calculation of available water capacity
- Basic formula: AWC = FC - PWP measured in volumetric water content (%) or depth of water (mm or inches)
- Conversion: 1 mm of water equals 1 L/m²
- Calculation steps:
- Determine FC and PWP from water retention curve
- Subtract PWP from FC
- Multiply by soil depth for total AWC
- Consider different soil horizons use weighted averages for multiple layers
- Example: FC = 30%, PWP = 15%, Soil depth = 1 m AWC = (30% - 15%) × 1 m = 0.15 m or 150 mm of water
Key Terms to Review (23)
Available Water Capacity: Available water capacity refers to the amount of water that can be stored in soil and made available for plants, typically defined as the difference between field capacity and permanent wilting point. This concept is crucial for understanding how soil texture, particularly clay mineralogy, affects water retention, influencing the soil's ability to provide moisture to plants. It also connects to the dynamics of soil water content and potential, highlighting how various factors impact water availability in agricultural and natural ecosystems.
Brooks-Corey Model: The Brooks-Corey Model is a mathematical representation used to describe the relationship between soil moisture content and soil water potential. This model helps in understanding how water is retained in the soil and predicts how much water can be available for plants, making it crucial for evaluating water retention curves and available water capacity.
Bulk Density: Bulk density is the mass of soil per unit volume, including the spaces between soil particles, which directly influences soil porosity and its ability to retain water. It connects important features such as water retention, soil quality, and carbon storage by providing insight into soil structure and health.
Capillary action: Capillary action is the ability of water to move through small spaces or narrow tubes without the assistance of external forces, due to the cohesive and adhesive properties of water molecules. This phenomenon plays a critical role in the movement of water in soils, influencing water retention curves and available water capacity by affecting how water is held and distributed in the soil matrix.
Clay: Clay is a fine-grained soil material with particle sizes less than 0.002 mm, characterized by its plasticity when wet and hardness when dry. This unique structure plays a vital role in various soil functions, including nutrient retention, water availability, and the overall development of soil profiles and textures.
Clay mineralogy: Clay mineralogy is the study of the composition, structure, and properties of clay minerals, which are fine-grained natural rocks or soil materials composed primarily of clay-sized particles. Understanding clay mineralogy is essential for evaluating how soil retains water and influences its capacity to supply water to plants, directly affecting agricultural productivity and ecosystem health.
Field Capacity: Field capacity is the maximum amount of soil moisture that the soil can retain after excess water has drained away and the rate of downward movement has decreased. This concept is crucial as it determines how much water is available for plants, influencing their growth and productivity, and it relates closely to soil characteristics like texture and structure, which affect water retention, movement, and availability.
Hydraulic Conductivity: Hydraulic conductivity is a measure of a soil's ability to transmit water when subjected to a hydraulic gradient. This property is essential for understanding how water moves through different soil types and plays a crucial role in water retention, drainage, and the interaction between soil and plant systems. Factors like soil texture, structure, and mineralogy significantly influence hydraulic conductivity, making it vital for assessing water availability and movement in agricultural and natural ecosystems.
Hysteresis effect: The hysteresis effect refers to the phenomenon where the moisture retention characteristics of soil differ depending on whether the soil is being wetted or dried. This results in two distinct water retention curves: one for water being added to the soil and another for water being removed. This effect is significant because it influences the amount of available water capacity, which directly impacts plant growth and soil management practices.
Kaolinite: Kaolinite is a clay mineral that is part of the kaolin group, primarily composed of aluminum silicate and known for its fine particle size and distinct white color. This mineral plays a critical role in soil structure, water retention, and nutrient availability, linking it closely to clay mineralogy, the classification of soil minerals, and the dynamics of soil moisture retention.
Montmorillonite: Montmorillonite is a type of clay mineral that is part of the smectite group, known for its unique ability to expand and contract when wet or dry. This property makes montmorillonite highly effective in retaining water and nutrients, playing a critical role in soil fertility and structure. Its high surface area and cation exchange capacity allow it to hold onto essential ions, which are vital for plant growth, while its swelling behavior affects the overall dynamics of water movement in soil.
Organic matter: Organic matter refers to the decomposed remains of plants, animals, and microorganisms in the soil, playing a crucial role in enhancing soil health and fertility. It connects various aspects of soil function, including nutrient availability, water retention, and overall soil structure.
Pressure Plate Extractor: A pressure plate extractor is a device used to measure the water retention properties of soil by applying controlled pressure to a saturated soil sample. This apparatus allows researchers to determine how much water remains in the soil at different tension levels, contributing to understanding water retention curves and available water capacity. It helps in characterizing the soil's ability to hold water and informs agricultural practices and soil management strategies.
Sand: Sand is a granular material composed of finely divided rock and mineral particles, specifically classified as having a particle size between 0.05 mm and 2 mm in diameter. This unique size range makes sand a crucial component of soil texture, influencing how soil retains water, its structure, and its overall fertility. The presence of sand in soil affects its drainage capabilities, aeration, and the availability of nutrients to plants.
Saturation Point: The saturation point refers to the maximum amount of water that soil can hold in its pores, where all pore spaces are filled with water. At this stage, the soil cannot absorb any more water, and excess water begins to pool on the surface. Understanding the saturation point is crucial for evaluating water retention curves and determining the available water capacity in soils, which affects plant growth and water management.
Silt: Silt is a fine-grained soil particle that falls between sand and clay in terms of size, typically measuring 0.002 to 0.05 mm in diameter. It plays a crucial role in soil properties, affecting water retention, nutrient availability, and overall soil texture. Silt particles have unique physical characteristics that influence how soil interacts with water and air, making it an essential component in understanding soil behavior.
Soil depth: Soil depth refers to the vertical measurement from the soil surface to the underlying bedrock or parent material. This measurement is crucial as it influences water retention, root development, and nutrient availability for plants, directly affecting soil health and productivity.
Soil Structure: Soil structure refers to the arrangement of soil particles and the spaces between them, which influences how water, air, and nutrients move through the soil. The configuration of these particles forms aggregates, affecting various soil properties and their functions in ecosystems.
Tensiometer: A tensiometer is an instrument used to measure the moisture tension or soil water potential in soil. It helps understand how water moves through the soil and how plants access this water, which is crucial for maintaining healthy growth. By providing insight into the soil-water relationship, tensiometers play a vital role in irrigation management, ensuring plants receive the right amount of water without over-irrigation.
Van Genuchten Model: The van Genuchten model is a mathematical representation used to describe the relationship between soil water content and soil water potential. This model is crucial for understanding how water moves through soil, as it helps predict soil moisture retention and availability to plants. By characterizing the water retention curves of various soils, it provides insights into soil-plant-water interactions, available water capacity, and how different soils respond to changes in moisture levels.
Volumetric water content: Volumetric water content is a measure of the amount of water present in a given volume of soil, expressed as a percentage or fraction of the total soil volume. This concept is crucial for understanding water retention curves, which illustrate how soil holds water at various tension levels, and helps determine the available water capacity that can support plant growth.
Water potential: Water potential is a measure of the potential energy of water in a system compared to pure water, which serves as a reference point. It indicates the direction in which water will move, from areas of higher potential to areas of lower potential, and is crucial for understanding how water interacts with soil and plant systems, especially in relation to water retention curves and available water capacity.
Wilting Point: Wilting point is the soil moisture level at which plants can no longer extract water from the soil, leading to a state where they begin to wilt and suffer stress. This crucial threshold relates closely to how various soil types, including clay, interact with water, impacting plant health, water movement in soil, and overall water retention capacity.