Measuring the Soil Water Retention Curve with the Stevens pF Matric Potential Sensor and Hydra Probe II Soil Moisture Sensor

The recently-released pF Sensor Matric Potential Probe from Stevens Water allows the measurement of a soil’s matric potential (Ψ_m), which quantifies the effect of capillary forces and adsorption of water on solid soil particles under unsaturated conditions. This represents the energy that is binding the water molecules to the soil particles.

It's these capillary forces that pull the water into the smaller pore spaces of the soil, which can holds the water in the soil against the force of gravity and a plant’s ability to uptake water. The term “matric potential” was derived from the word “matrix” referring to the soil/air/water matrix that makes up most commonly found soils.

An analogy for this would be to think of soil like a sponge. If a sponge is dry, it will quickly draw water in and hold it in it pore spaces. Once the sponge is wet and all of the pore spaces are filled with water, any additional water will drip from the bottom. Soil will function in a similar manner when saturated with water.

Stevens pF Sensor Soil Matric Potential Probe

Figure 1: The Stevens pF Sensor Matric Potential Probe.

The drier the soil the higher matric potential of the soil, with more pressure needed to overcome this bond and release any remaining water within the soil matrix. As a soil absorbs more water and moves closer to being fully saturated, the pressure needed to overcome this bond becomes much lower.

Knowing the specific retention of different soils is extremely important to several areas of science:

Planet science:	the amount of water available to plants in a soil has an obvious impact on how well a plant will survive in a given environment. All plants have a specific amount of water they need, which falls between the permanent wilting point (how dry a soil can become before the plant starts to die) and field capacity (the maximum amount of water that can be retained in that type of soil before gravity pulls it downward).
Soil monitoring:	monitoring soil moisture and matric potential together help hydrologists understand the general movement patterns of water through the soil. Knowing the water flow and the direction of this flow through soil is very important for determining factors such as flooding potential, reservoir recharge, river discharge, and the general water budget of a soil or region.
Water table recharge studies:	understanding how much water is retained by the soil and how much continues to pass through can be used to help determine groundwater recharge rates. This is particularly useful because millions of people get their drinking water and irrigation water from groundwater wells.
Climate modeling:	moisture retained in soil can have an impact on climate and weather, and understanding how much water is being held in the soil matrix can add valuable data points to these studies.
Erosion and land slide studies:	if the soil on a cliff, steep hill, or other vertical geographic feature becomes too heavily saturated with water, the soil can become unstable and result in a landslide or other geologic movement. The soil water retention curve can help determine the infiltration rate of the water which can help in the understanding of runoff patterns and indentify erosion and landslide risks.

To better understand soil water retention, the Stevens pF Sensor can be used in conjunction with the Stevens Hydra Probe to calculate a very accurate soil water retention curve for any type of soil.

Using the Stevens pF Sensor to measure the soil matric potential (pF), and the Stevens Hydra Probe to measure the soil moisture content of a soil (cm³/cm³), a very complete picture of soil water retention is given:

Figure 2: The soil water retention curves for common sand, silt, and clay soils.

The graph above shows the soil water retention curve for three typical soil types: sand, silt, and clay. The graph indicates that as the soil water content decreases, the amount of pressure needed to extract remaining water from the soil matrix increases.

The pF Sensor returns measurements in units also known as “pF”, which are the logarithm of the pressure it takes to pull water out of soil in units of hectopascals (hPa). Converting the measurements from hPa to pF makes the measurements quickly understandable in more easily managed numbers.

Measuring soil matric potential in dry soils can be problematic for typical tensiometers, as they can only measure wet soil and only have a range of approximately 0 – 3 pF (0 – 850 hPa). However measuring the soil matric potential in dry soils can be extremely important for many applications.

The Stevens pF Sensor utilizes a patented heat-based measurement technology that can continue to measure soil matric potential far beyond the range of standard tensiometer, typically offering over one thousand times the measurement range. The pF sensor is also unaffected by dry soil or high-saline soils, conditions which will prevent typical tensiometers from operating correctly.

Figure 3: Highlighted is the measurement range of the Stevens pF Sensor versus a standard tensiometer. Image adapted from Scheffer & Schachtschabel, Lehrbuch der Bodenkunde.

The Stevens Hydra Probe paired with the Stevens pF Sensor represents the most accurate soil measurement sensors available, featuring SDI-12 communication for easy connection to data loggers, radios, PLCs, or other equipment.

For more information about soil moisture measurement or help with your application, contact Stevens today at (800) 452-5272 or email at info@stevenswater.com.