Measuring forest soils: from simple probes to satellites

This article is based on our podcast episode released on 13 November 2024. [Listen to the full episode here]

In Danish forests, measuring soil moisture often comes down to a forester's skilled touch. Remove the top layer of soil, feel the dampness with your fingers – it's a practice as old as forestry itself. 

But as droughts threaten forests worldwide and climate patterns shift, this ancient art is meeting modern science in unexpected ways. From simple electrical probes to NASA's eye in the sky, the race is on to improve how we monitor forest soils. That’s exactly what we’ll look into today. 

From garage tech to satellites

The simplest technological approach involves basic sensors with two probes. "These sensors are quite simple," says Jens, "and to some extent, also quite cheap." However, these simple tools reveal a complex reality of soil measurement.

The most accurate method involves taking a soil sample, weighing it, drying it completely in an oven, and weighing it again. As Rasmus explains, "You can't just poke something into the soil and get it. That's how it is with a lot of things you measure. You can't actually measure things directly."

Instead, modern sensors use what Rasmus describes as "a fork with only two probes" that send electrical signals through the soil. These can measure various properties like electrical conductivity and resistance, but there's a catch: you need to understand the soil type to get accurate readings.

NASA joins the moisture hunt

Perhaps the most surprising development comes from space. NASA launched a dedicated soil moisture satellite in 2015, which covers the entire Earth every three days.

"Soil emits signals that can be detected by satellites. By testing these signals on the ground, researchers make sure the satellite data is accurate."

The satellite project includes ground testing in US forests to calibrate the measurements. The satellite doesn’t provide exact moisture levels. However, it can track changes over time. This might be exactly what forest managers need.

Global vs local solutions

The choice of measurement method often comes down to scale. For smaller Danish forests, manual measurements or simple sensors might suffice. A network of basic probes could run for up to ten years on a single battery, providing consistent data about soil conditions across a few hundred hectares.

But as Rasmus notes, "If you take that setup and say, okay, let's do this in one of the large forests in South America, you can't. That's why when you have to deal with large areas, satellites are such a good thing."

Even drones, with their limited flight time, struggle to cover vast forest areas efficiently. Satellites, despite offering lower resolution, provide something unique: consistent, large-scale monitoring without the need for ground equipment. It's a trade-off between precision and practicality that every forest manager must consider.

The key question isn't always about getting the most precise measurements, but rather about getting data that's "good enough" to make informed decisions. As Rasmus and Jens discuss, sometimes understanding the trend – whether soil moisture is increasing or decreasing over time – can be more valuable than knowing the exact percentage at any given moment.

Making smarter forest decisions

While measuring soil moisture might seem like a technical detail, it represents a broader shift in forest management. As climate patterns change and droughts become more common, understanding soil conditions could become crucial.

As Rasmus points out, "If you string together drought years and you actually can measure now, okay, this is the third year where our trees are not getting enough water, it might give you an idea to make a management decision. Maybe saying okay, this species of trees, they can't survive this. We need to remove these and plant some trees that fit better with the current circumstances."

Do you want to learn more about soil moisture monitoring? Here are the resources we used the this weeks podcast:

• Measuring moisture in forest soils from space
• How soil moisture sensors work