Hi, my name is Leo Rivera. I’m the Director of Scientific Outreach here at METER Group, and in this course, we are going to talk about soil moisture release curves. We’re going to focus on understanding the fundamentals behind the measurements. So one of the first things I always like to ask is, well, what is a soil moisture release curve?

What I like to start with is really trying to understand the properties that we’re trying to measure. And I use this example often when we’re talking about extensive versus intensive properties. And it’s important to understand that there are two variables necessary to describe the state of matter or energy in the environment. This is true in many, many different cases.

We need to understand the extensive variable, which describes the extent or the amount of matter or energy that is there. Then you have the intensive variable which describes the intensity or the quality of matter or energy. And I think a good example that a lot of us can can understand is temperature versus heat content.

Temperature is the intensive variable, what we feel in the environment. Heat content is the actual amount of heat that is there in the environment. Heat content can change depending on different properties of a material, and it doesn’t always tell us a lot about how we’re actually going to feel in the environment.

We see the same thing when it comes to water content versus water potential. So to really focus on soil and what we’re trying to understand there, we’ll first focus on water content. We said earlier that water content is the extensive variable which tells us the amount of water in the environment.

Here you can see an example of how we measure volumetric water content, where we’re looking at the ratio of pore spaces that are filled with water versus filled with air. You can see some examples of soils with lower and higher water contents here. That’s our extensive variable. It tells us how much water is there.

Now we’re going to focus on this intensive variable, which is our water potential, or our matric potential. Often what we refer to which matric potential is the sum of the matric forces that bind water to the soil. So typically, there’s adhesive and cohesive forces that are doing that binding. Sometimes we have osmotic and gravitational forces that are also playing a role there, and we’ll dive a little bit deeper into that.

So as the water content decreases, the energy state of that water actually decreases it gets lower due to the binding of the water with the soil surface. You can see an example of what that looks like here. And because of that binding, that water actually becomes one less accessible to the plants, but also as that matric potential goes down, if there’s an area near there that has a higher matric potential, water is going to flow towards that area of low matric potential, because everything always wants to be in equilibrium ultimately. That’s kind of what drives some of these forces, the drive to reach equilibrium.

To dive a little bit deeper into the things that we need to understand about soil moisture release, what we really want to understand are the factors that can change the soil moisture release curve.

Components of water potential

So when we talk about water potential, it’s technically total water potential, which is the sum of several things.

• Matrix potential, which is the absorption to surfaces
• Gravitational potential, which is primarily due to a position within a gravitational field and the energy that we need to overcome that
• Osmotic potential, which is governed by the solutes within the water
• Pressure potential, which is the hydrostatic or pneumatic pressure from the actual size of the head of the column of water above it—something we normally only see in saturated conditions

Primarily we focus on matric potential because it is the main governing factor in most conditions. In some cases we do have to worry about osmotic potential in really saline environments, but typically that can be negligible.

Another key important part to understand about moisture release curves is hysteresis, and hysteresis can really complicate things. Oftentimes we may ignore it when we’re measuring moisture release curves. Hysteresis is a physical force that actually changes as we’re going through different wetting and drying cycles. So here you see an example of several scanning curves, where you may start at saturation, and then you dry the soil back down as it wefts back up.

It typically never, doesn’t reach that full saturation point and if we were to start drying again, then we didn’t wind up on a different curve. And so we call that these scanning curves. And so that can really complicate things when we’re trying to understand soil moisture release curves. We typically just focus on a desorption curve, and sometimes we may focus on a true adsorption curve as well.

When we’re talking about soil moisture release curves, one of our goals is really to identify what our water holding capacity is. And the reason that’s important is the water holding capacity can change with soil type. Water holding capacity is typicaly the upper end of the field capacity, which can be minus 10 or minus 33 kPa, depending on the soil. The lower end is the permanent wilting point. And so you can see here an example of that with three different soil types, and you can see how the available water shifts with these different soil types.

So you can see with the sandy soil, there’s less available water in that range than there is with the with the fine sandy loam, and even more so the silt loam, the silt loam has a lot more plant available water in that range than any of the other soils. So this is a factor that can change quite a bit with soil type. There are other things as well that can impact what that looks like, including capillary barriers in the soil and depth of ground or whatever, that can actually change what the water holding capacity of the soil actually looks like, but oftentimes we don’t measure those when we’re trying to govern or define what our water holding capacity is.

I’ve gone over a few basic fundamentals of matric potential and water content and what makes up the soil moisture release curve. In our next lesson, I’m going to talk about the tools behind making the measurements.