Chapter 1: Trees Build Soil

Our favorite star, affectionately referred to as the Sun, generates energy that travels 93 million miles to Planet Earth. It makes this journey in 500 seconds! Trees and other plants have remarkable cells that use the Sun’s energy to change a mixture of water and carbon dioxide (called CO₂ for short) into oxygen and sugar. We call this process photosynthesis.

The sugar contains the carbon portion of the CO₂. Thanks to photosynthesis, carbon moves from floating freely in the atmosphere to being locked away in a stable state within the tree. This is an important feature called carbon sequestration, a topic we will revisit in more detail in Chapter 4.

Trees use this sugar for all sorts of things, including growth, reproduction, and defense. Much of the sugar is sent down to the root system and released as food for bacteria and fungi. Nematodes, protozoa, and others feed on this bacteria and fungi. Then earthworms and arthropods eat the nematodes and protozoa. These activities (and many others!) form a thriving, underground ecosystem. The majority of all life on land (known as terrestrial life) lives underground and participates in this ecosystem.

This living underground ecosystem is the very definition of healthy soil. In one teaspoon of soil, there can be billions of bacteria, protozoa, nematodes, and other tiny life! In the absence of this living underground ecosystem, we find dirt, not soil. To better understand this difference, let’s look at the history of how soil was formed from dirt.

We’ll begin our journey by zipping hundreds of millions of years back in time. We would find a planet composed mostly of rocks surrounded by oceans. There was no soil, no plants, and very little terrestrial life back then. If it rained, the water flowed over the rocks and quickly swept to the oceans. Over time, rain, wind, waves, and a variety of other processes weathered rocks into smaller particles, called sand, silt, and clay. These three are the definition of dirt.

But then something amazing started happening! Lichens began the process of turning dirt (sand, silt, and clay) into soil (dirt plus a living underground ecosystem). Lichens formed from a symbiotic relationship between fungi and algae or cyanobacteria. The algae and cyanobacteria could photosynthesize, which provided food for the lichen team. The other part of the lichen team—the fungi—used their teeny, tiny strands (called hyphae) to collect water in hard-to-reach places and to absorb nutrients by breaking down dirt with organic acids. Between the two organisms, the lichen team was able to generate food, stay hydrated, and absorb nutrients in what was an otherwise barren landscape of rocks and dirt.

Over time lichen multiplied and spread across the land. They dissolved rocks, freed nutrients, and soaked up carbon at an increasing rate. As all life does, each lichen eventually died and its remains provided an infusion of carbon and nutrients into what had previously only been dirt. This combination of dead lichens and dirt was better suited to hold rain for longer periods of time. As these pools of water became infused with carbon and nutrients became more common, the complexity of life increased. Lichens yielded to liverworts. Glassworts and club mosses emerged, followed by horsetails, ferns, and finally the trees, flowers, and grasses we know today.

In a few weeks on the Cosmic Calendar, photosynthesis played a key role in transforming land that was once dirt into a thriving ecosystem with deep soil. Where there are healthy plants and soil, there is also an abundance of fresh water.