It Takes a Fungus to Make a Soil

Fortunately for the thirsty flowers, they get help from strange fungi waiting in the top two inches of these desert soils. In wet, fertile areas, the topsoil is constantly enriched by the carbon-containing debris of plants, including leaves, roots, and twigs. In the riotously productive rain forest, the mass of plant debris on the ground supports such a population of bacteria, insects, fungus, and other plants that few of these nutrients remain in the soil. In most other areas, the nutrients provided by decay of plant matter build up in the topsoil. Unfortunately, desert soils get very few nutrients from decaying plants. Termites gobble the bits and pieces of wood, harvester ants and rodents gather up every scrap of plant remains, and the dry, harsh climate limits the ability of soil bacteria to break down the remaining organic debris.

Desert soils do have unique species of fungi. Most desert soils are graced by a fungus that grows in the top few inches of soil that engulf the sand grains. These fungal hyphae form a vital, symbiotic relationship with the usually short, struggling roots of desert plants. The fungus colonizes the roots of the plant, and the plant and the fungus then cooperate to grow little tendrils called mycorrhizae that envelop the root. This fungal add-on increases the root’s ability to absorb moisture tenfold, maximizing the yield for the plant in the dry desert soil. In return, the plant creates energy from sunlight in its leaves and feeds the buried fungus. As a sort of final tip, the fungus also helps the roots absorb essential minerals like phosphorus, zinc, and copper, normally scarce in the impoverished desert soils. This interdependent soil chemistry helps account for the extraordinary delicacy of many desert soils. For instance, during World War II, General George Patton’s troops practiced tank maneuvers in many areas of the Mojave Desert. Now some 60 years later, the tank tracks remain clearly visible in the slow-healing desert soil because the tanks destroyed the fungal networks and prevented any other plants from taking root.