How Do You Measure Soil Water?

This article was adapted from “Assessing Soil Health: Soil Water Cycling” first published on September 7, 2020 in Crops & Soils magazine. You can find the original article here.

Back it up: Measuring soil water gives you a better understanding of the water cycling happening in your field. Soil water cycling includes infiltration, redistribution, and the supply of plant available water, according to the Soil Health Institute.

• During extreme weather events—like drought or intense rainfall—the soil’s ability to both let water in and distribute that water is tied closely to the health of the soil.
• Soil water cycling impacts a field’s resilience to drought, whether or not you can drive equipment over the soil without causing ruts or compaction, erosion risk, and the amount of surface and groundwater that results from a weather event.
• And where water goes, nutrients go! Understanding how water cycles through soil gives insights into whether or not applied nutrients will make it to the right place at the right time.

Break it down: Four factors impact soil water cycling. These factors include:

1. Compaction. Measuring soil bulk density or penetration resistance can tell you a lot about soil compaction.
   • Compaction prevents air, water, and roots from getting into the soil.
   • The more compact a soil is, the more likely that water is lost through surface runoff, rather than entering the soil and becoming available to plant roots.
   • Plus, soil runoff carries away sediment and valuable nutrients (one of a farmer’s highest input costs) to nearby waterways. Excess nutrients like nitrogen and phosphorus in waterways contributes to eutrophication, which increases the amount of algae and harmful algal blooms in waterways.
2. Infiltration rate. How fast does water move into the soil? There are two ways to measure infiltration rate.
   • Surface infiltration: You can measure this using a six-inch diameter ring on the soil surface and timing how long it takes for 444 mL, or one inch, of water to seep into the soil.
   • Hydraulic conductivity (Ksat): Taking Ksat measurements can be complex and time consuming, and measurements tend to vary a lot. This means you will likely need to take more measurements to understand whether or not a management change impacted Ksat. There are several methods to measure Ksat, including a recently developed automated method that uses a single ring infiltrometer.
3. Plant-available water (PAW). This is the amount of water that is held by the soil between field capacity (maximum) and permanent wilting point (minimum). Both of these are a function of soil texture and organic carbon content, according to the Soil Health Institute.
   • Taking PAW measurements directly requires collecting an intact soil core.
   • But there’s good news! You don’t have to measure PAW directly—if you know your soil organic carbon (SOC) and soil texture, you can use this carbon-sensitive pedotransfer function to calculate PAW without taking additional measurements.
4. Wet aggregate stability. This is a measure of a dry soil’s ability to resist dispersion when rewetted. A stable aggregate indicates higher soil porosity, more plant available water, and greater infiltration compared to an aggregate that quickly breaks up.
   • You can measure wet aggregate stability in one of four ways: using the Cornell Sprinkle Infiltrometer, the wet sieve, the SLAKES test, or the soil stability index.
   • One thing to note: the different wet aggregate stability measure are not interchangeable—once you pick a method to use, stick with it!
   • If you want to simplify the process, check out the smartphone app for the SLAKES test. It’s free and doesn’t require specialized lab equipment.

In short, this is a brief overview of the four methods of measuring soil water—head over to the original Crops & Soils article for more in-depth information. And when it comes to picking a measurement, make sure that it’s one that will provide useful information for all the folks involved. After all, measuring soil water is an important part of understanding your overall soil health.

Photo by Steve Harvey on Unsplash.