Soil hydrologic properties and processes

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Soil hydrologic (hydraulic) properties include infiltration and water retention. These properties significantly affect plant growth, pollutant transport, and subsurface water transport (e.g. groundwater recharge).

Soil water and water retention

Schematic showing soil water holding properties. Click on image to enlarge. Image: Oklahoma State University Extension

Water retention curve for a sand(Ss), either silt or clay-loam(Uu), either loam-silt or clay(Lu), and either clay or peat(Tt).

Soil properties related to water content and water retention are critical to plant establishment and growth. Understanding some basic terms related to soil water is important to understanding water storage and transport. These terms are illustrated in the adjacent schematic.

• Soil water (moisture) content can be expressed on a mass or volume basis. This video describes the two measurements and how they are related.
  • Gravimetric water content: the ratio of the mass of water held in the soil to the dry soil.
  • Volumetric water content: the ratio of the volume of water to the unit volume of soil. Volumetric water content can be expressed as a ratio (e.g. cm³/cm³), percentage (e.g. 50% of the soil volume), or depth of water per depth of soil (e.g. centimeters per meter of soil).

Example: assume a wet soil weighs 50 grams, a dry soil 40 grams, and soil bulk density is 1.3. Gravimetric water content is ((0.5-0.40)/0.40) = 0.10/0.40 = 0.25 g/g. To derive volumetric water content, multiply the gravimetric water content by the bulk density and divide by density of water (1.0 g/cm³). Volumetric water content is therefore 0.25*1.3/1.0 = 0.325 cm³/cm³, or 32.5%.

• Water holding capacity: the total amount of water a soil can hold at field capacity.
• Field capacity: the amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has decreased. Physically it is the water content retained in soil at −33 kPa (or −0.33 bar) of hydraulic head or suction pressure.
• Wilting point: the volumetric water content in the soil is too low for the plant's roots to extract water. When the water content of a soil is below the permanent wilting point, water is still present in the soil, but plant roots are unable to access it. Physically, it is the water content at −1,500 kPa (−15 bar) of suction pressure.
• Moisture retention curve: the relationship between the water content (θ), and the soil water potential (ψ), typically expressed as a curve. The curve is characteristic for different types of soil and is therefore also called the soil moisture characteristic.

Water infiltrates a dry soil through both gravitational forces and suction forces exerted by soil materials. Water is held in soil by suction forces exerted by materials such as clay. Water not held under suction drains freely due to gravitational forces. The time for a soil to drain due to gravity varies depending on soil texture and structure. Sands drain more quickly than clays because of more direct, well-connected drainage pathways. A soil's field capacity, or the amount of water remaining after gravity drainage, varies with soil and is not a exact value. Typical field capacity for sandy soils is 0.10-0.15 cm³/cm³ and for clays is about 0.3 cm³/cm³. Water subject to gravity drainage is available to plants, but because drainage occurs rapidly in most soils (less than 48 hours), it is generally not an important source of water to plants. Below field capacity, plant roots must exert force to extract water held to soil. As a soil dries, the amount of suction required increases until a plant can no longer extract water, typically at about-15 bars. This is called the wilting point. Water between the wilting point and field capacity is plant available water. Typical plant available water is about 0.10 cm³/cm³ for sand and clay and about 0.15-0.20 for loams. The lower values for sand is due to low water content of the soil, while the low value for clays is due to clays tightly holding water.

Water content can be measured in the lab or in the field. For point-in-time measurements, lab methods are satisfactory, being easy to take and inexpensive. Continuous measurements of soil water content require in-field methods that are often expensive and sometimes subject to inaccuracies. For information on sampling for water content, link here.

For more reading

• Soil water status: content and potential
• Understanding Soil Water Content and Thresholds for Irrigation Management
• Water content
• Soil and water

• Video: Water content calculations
• Video: Gravimetric and volumetric water content