Soil Colors Shining Through

By Laura C. Rubio Lebrón

In our arduous, laborious, and now-ended quest to install minirhizotron tubes at the El Verde field station for the Drought Experiment, we saw all different kinds of soil colors and textures. Our goal was to create holes the tubes could slide into. We used an auger to drill into the ground and collected the soil in a bucket. After digging into the ground three to four times, you could see the colors start to change. While in undergrad, I always found soil science to be particularly interesting. To me, it was amazing to see what I had learned in the classroom while out on the field!

Figure 1. Top 10 cm of soil at COFFEE 1 plot

The top layer of the soil was a dark brown, which is typically the color one associates with it. This color is largely due to the large amount of organic matter present on the surface. However, once we started digging into the soil, things got interesting.

Figure 2. First auger soil removal

Once we reached below 20 cm, we could clearly see yellow soil. This is typically due to the presence of ferrous iron (Fe3+).

Figure 3. Last auger soil removal

The lower layers of the soil profile had a much more reddish tint. Though also due to iron oxides, this color can mostly be attributed to ferric iron (Fe2+). This red color also tends to be associated with older soils with good drainage.

Figure 4. Midway auger soil removal

This was my favorite picture by far. We found the point at which the soil horizons change. The gradient between them is clearly visible and so visually appealing.

A closer look

Figure 5. Yellow soil from upper levels

Figure 6. Red soil from deep in the soil profile (1.5m)

Did you know that soil colors can be codified? The Munsell color system was created to attribute a value to a soil color. Using the value, hue and chroma of a soil color, it can be easily replicated and referenced. Having a Munsell color chart present in the field is necessary to properly catch those subtle nuances between the shades and evaluate which color the soil is. We didn’t have one with us but maybe next time!

Red soil gives away a lot of its characteristics through its color. This color is usually symbolic of older, more weathered, and more acidic soil. In this sample we can see some white spots we hypothesized to be calcium carbonate, which was likely added to raise the pH of the soil.

Figure 7. Red soil with white spots

In this sample we can see how deep red soil has mixed with yellow soil from higher in the profile. This could indicate a landslide happened at one point and caused these layers to meet and mix. The black streaks in the sample come from ash that was used add nutrients to the soil from when the site was a coffee plantation. The white dots, once again, are most likely calcium carbonate used to lime the soils.

Figure 8. Yellow soil with red and black

streaks and white spots

As was particularly evident in the sample from Figure 8, soil holds a wealth of information. Soil tells a story, it shows its history. Though when walking through a forest one can get distracted by what lies above us, let’s not forget that soil is where it all starts.