Soil water potential and soil moisture characteristic curve

Soil water potential

Soil water potential Soil water potential characterizes the "energy state" of soil pore water relative to a reference point (usually pure water). Specifically, it can be defined as the work required to free a unit volume or mass of soil pore water from the constraints of soil particles and soil salt solution and reach the state of free water (pure water) under constant temperature. Based on this definition, the unit of soil water potential can be expressed in J/m3 or J/kg.

Common units of soil water potential

Common units of soil water potential are: J/kg, kPa, MPa, Bar. The conversion relationship between them is

1J/kg=1kPa=0.001Mpa=0.01Bar.

In addition, the soil water potential can also be expressed by the height of the water column, with the unit being cm, 1cm=0.981hPa. After converting the soil water potential into the height of the water column, multiply it by -1 and take the logarithm to the base 10. This calculated value is represented by the symbol pF. Soil pore water will move from high to low along the water potential gradient. The water absorption of plant roots can be explained by the water potential gradient: the soil water potential is high and the plant root water potential is low. Under this water potential gradient, the roots can obtain water from the soil pores; when drought occurs, as the soil and plant root water potentials gradually become equal, the water potential gradient disappears and water movement no longer occurs; when the drought intensifies and the soil water potential drops further, when it is lower than the plant root water potential, the water potential gradient reverses, the plant roots lose water, and the plants face the risk of death from water loss. Therefore, soil water potential is of great significance in understanding the movement of water in the "soil-plant-atmosphere continuum."

Soil Water Potential Sensor

Image: https://ecdn6.globalso.com/upload/p/1459/image_news/2025-05/51e65db404d6d6c159f3c82c24233eb9.png

The soil water potential sensor consists of a humidity sensor and a porous material with a known moisture release curve. When the porous material reaches moisture equilibrium with the surrounding soil, the humidity sensor measures the moisture content of the porous material and converts the moisture content into water potential based on the moisture release curve.

Soil moisture characteristic curve

The matrix potential of soil water (or soil water suction) changes with the change of soil moisture content, and the relationship curve is called the soil moisture characteristic curve.

Generally, the curve uses soil water content Q (expressed as volume percentage) as the horizontal coordinate and soil water suction S (expressed as atmospheric pressure) as the vertical coordinate. The effectiveness of soil water to plants is ultimately determined by the soil water potential, not the water content itself. If the soil water content is measured, the matrix potential value can be found according to the soil water characteristic curve, so as to determine the effectiveness of the soil water content to plants. The soil water characteristic curve can reflect the water holding and water release characteristics of different soils, and can also be used to understand some soil water constants and characteristic indicators of a given soil type. The inverse of the slope of the curve is called the specific water capacity, which is an important parameter when using diffusion theory to solve water movement. The inflection point of the curve can reflect the soil moisture state under the corresponding water content. For example, when the suction tends to 0, the soil is close to saturation, and the water state is mainly capillary gravity water; when the suction increases slightly and the water content decreases sharply, the suction value expressed by the negative pressure head is approximately equivalent to the rising height of the capillary water; when the suction increases and the water content decreases slightly, the capillary suspended water in the soil is the main water content, and the water content is close to the field water holding capacity; the difference between the saturated water content and the field water holding capacity can reflect the soil water supply, etc. Therefore, the soil moisture characteristic curve is the most important and basic tool for studying soil water movement, regulating and utilizing soil water, and improving soil.

The soil moisture characteristic curve has the following applications:

Convert the matrix potential and water content. Soil moisture can be converted into soil matrix potential according to the soil moisture characteristic curve, and the effectiveness of soil moisture for crops can be determined based on the matrix potential. The matrix potential can also be converted into water content, and the field water holding capacity, wilting humidity and the corresponding effective water range can be found according to the soil moisture characteristic curve. The reciprocal of the slope of the soil moisture characteristic curve, that is, the change in water content caused by a unit matrix potential change, is called specific water density, which is an important indicator for measuring the effectiveness of soil moisture for plants and reflecting the water holding capacity of soil.

Indicates specific water density. The reciprocal of the slope of the soil moisture characteristic curve, that is, the change in water content caused by a unit matrix potential change, is called specific water density, which is an important indicator for measuring the effectiveness of soil moisture for plants and reflecting the water holding capacity of soil.

It can indirectly reflect the distribution of soil pores. If the pores in the soil are imagined as circular capillaries of various pore sizes, the relationship between S and the capillary diameter d can be simply expressed as S=4d. Where is the surface tension coefficient of water, which is generally 75 times 105N/cm at room temperature. When applying mathematical and physical methods to quantitatively analyze water movement in soil, the moisture characteristic curve is an indispensable and important parameter.

It can judge the soil texture and the distribution of soil moisture in the suction section. The inflection point of the curve can reflect the soil moisture state under the corresponding water content. For example, when the suction tends to 0, the soil is close to saturation, and the moisture state is mainly capillary gravity water; when the suction increases slightly and the water content decreases sharply, the suction value expressed by the negative pressure head is approximately equivalent to the rising height of the capillary water; when the suction increases and the water content decreases slightly, the capillary suspended water in the soil is mainly, and the water content is close to the field water holding capacity; the difference between the saturated water content and the field water holding capacity can reflect the soil water supply, etc. Therefore, the soil moisture characteristic curve is the most important and basic tool for studying soil moisture movement, regulating and utilizing soil water, and improving soil.

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