Soil Moisture: Understanding VWC vs. Degree of Saturation

Two common terms are:

• VWC (Volumetric Water Content)
• Degree of Saturation

Both describe soil moisture, but they do not mean the same thing. The easiest way to think about the difference is:

• VWC tells you how much of the total soil volume is water.
• Degree of saturation tells you how full the empty spaces in the soil are with water.

Soil may look solid, but it is actually made up of many small particles with spaces between them. Those spaces can hold:

• Water
• Air

Plant roots grow through these spaces and take up water from the soil. VWC and degree of saturation both describe soil water, but they use a different reference point.

VWC stands for Volumetric Water Content. In simple terms:

Imagine you have exactly 1 liter of soil. If that soil contains 200 milliliters of water, then VWC = 20%. That means:

This makes VWC a useful way to track how much water is actually stored in the soil.

Degree of saturation looks at the empty spaces between soil particles. It tells you:

Imagine 1 liter of soil contains:

• Solid soil particles: 600 mL
• Pore space: 400 mL

Now imagine that 200 mL of that pore space is filled with water. The soil would have VWC = 20%, because 200 mL of the full 1,000 mL is water. But the degree of saturation would be 50%, because 200 mL of the 400 mL of pore space is filled with water.

Using the example above, the total soil volume of 1,000 mL is made up of:

• Soil particles: 600 mL
• Water: 200 mL
• Air: 200 mL

The result is:

• VWC: 20%
• Degree of saturation: 50%

Why? Because VWC looks at the entire soil volume. Degree of saturation looks only at the spaces between the soil particles.

A sponge is an easy way to picture the difference. A sponge has solid material and lots of small holes that can hold water.

• VWC: how much of the entire sponge's volume is water?
• Degree of saturation: how full are the sponge's holes with water?

If every hole in the sponge is filled with water, the sponge is 100% saturated. But the sponge itself has not become 100% water. So:

The same idea applies to soil.

If soil is 100% saturated, it means:

This can happen after heavy rain, over-irrigation, flooding, or in areas with poor drainage. When soil stays highly saturated for too long, plant roots may have trouble getting enough oxygen.

So degree of saturation can be useful for identifying:

• Waterlogged soil
• Poor drainage
• Over-irrigation
• Areas that stay too wet for too long

Different soils have different amounts of pore space. For example, these all hold water differently:

• Sandy soil
• Clay soil
• Loam
• Organic-rich soil
• Potting mix

Two soils may both show VWC = 30%, but they may not be in the same moisture condition. One soil may be relatively wet, while another may still have a lot of room to hold more water.

Neither measurement is always better. They answer different questions.

• How much water is in the soil? → VWC
• How full are the soil's pore spaces? → Degree of saturation
• How much did soil moisture increase after irrigation? → VWC
• Is the soil becoming waterlogged? → Degree of saturation
• How is soil moisture changing throughout the day? → VWC
• Is drainage poor or is the soil staying overly wet? → Degree of saturation

For everyday irrigation management, VWC is usually the easier and more useful measurement. It gives a direct way to track how much water is being stored in the soil over time.

Degree of saturation is especially useful when you want to understand how close the soil is to being completely filled with water.

Imagine a soil moisture sensor shows the following values through the day:

• Morning: VWC 28%
• Midday: VWC 25%
• Late afternoon: VWC 22%
• After irrigation: VWC 31%

This makes it easy to see what is happening. The soil is gradually drying as plants use water and moisture is lost. After irrigation, the VWC rises again.

This makes VWC very useful for seeing:

• When the soil is drying
• How much irrigation increased soil moisture
• How quickly the soil dries afterward
• Differences between parts of a field or greenhouse

Now imagine you have had several days of heavy rain. If the soil stays close to full saturation for a long period, that can indicate:

This can be useful when looking for:

• Poor drainage
• Standing water
• Root-zone oxygen problems
• Over-irrigation

So while VWC tells you how much water is in the soil, saturation helps describe how full the soil's available space has become.

One important thing to remember is that neither VWC nor degree of saturation tells you everything about plant water availability. A high VWC does not automatically mean the plant has ideal growing conditions.

Different soils hold onto water differently. For example, a clay soil can hold a large amount of water very tightly, while sandy soil drains much more quickly. That means the same VWC value can have a different meaning depending on the soil.

It is best to use soil moisture measurements together with:

• Soil type
• Crop type
• Weather
• Irrigation amount
• Plant condition

If you remember only two things, remember these:

• VWC = How much of the total soil volume is water
• Degree of saturation = How full the soil's pore spaces are with water

For example, soil can have VWC = 20% while at the same time having degree of saturation = 50%. And:

For most day-to-day irrigation monitoring, VWC is the easier measurement to use because it shows how much water is stored in the soil and how that amount changes over time.

Degree of saturation is especially useful for understanding when soil is becoming excessively wet, poorly drained, or waterlogged.

Together, the two measurements give a clearer picture of what is happening below the surface.