Shades of Green: Earth's Forests -- Soil

Trees are extremely important in soil building. Their roots grow down and break up the bedrock into smaller soil particles and their fallen leaves contribute to the nutrient richness of the soil. Tree branches soften heavy rainfalls, and their roots provide a support structure within the soil, two factors that help prevent erosion.
Soil types are a major factor in determining what types of plants will grow in a certain area. Plants use inorganic elements from the soil, such as nitrogen, potassium and phosphorus, but the community of fungi, bacteria, and other microscopic creatures living within the soil are also vital. These living organisms help with the decomposition of dead plants and animals, breaking them down into soil.

Earthworms are just one of the many types of creatures that live in the soil. Photo by Maya Walters.

Soil is affected by climate and rainfall, geology and vegetation. The combination of sand, silt, gravel, and clay gives different soils different textures. Healthy, nutrient-rich soils which consist of a mixture of sand, silt, and clay are called "loam" soils. Different minerals give soils their colors.

Fungi, most of which produce mushrooms at certain times of the year, live in the soil and help to decompose dead leaves. Photo by Maya Walters.

At the end of each growing season in a temperate forest, over 70% of the biomass produced that year falls to the ground. This includes leaves, needles, twigs, and other organic material. It is then decomposed by fungi, and the nutrients return to the soil where they are re-used by other plants and trees.

Fallen leaves, and indeed entire fallen trees, are the soil for future forests. This organic matter is crucial because it contains the nutrients that will eventually be re-incorporated into the soil. It is also important in a partially decomposed state. Rotting leaves and wood are able to store moisture like a sponge, and help the forest soil retain rainfall. Without the organic matter from trees and other forest plants, the soil would become nothing but rocks and sand.

Tropical rainforests, surprisingly, contain some of the poorest soils of all. This is because of the torrential rains that fall regularly on these regions. The heavy rains dissolve nutrients in the soil, which then wash into streams and rivers and are carried away.

In coniferous forests, the litter layer is made up of tough, dry needles and fallen twigs. This layer doesn't decompose easily, and remains on the ground for many years. Usually small fires burn off the fallen needles before they can decompose.

After an area of forest is clear-cut, it is often burned. This destroys nutrients in the soil, as well as the "litter layer" of partially decomposed leaves and branches. Photo by Al Walters.

In temperate deciduous forests, soil is usually relatively rich. This is because every fall, the trees drop the leaves that they grew the previous spring. This vast amount of organic material contributes to the "litter layer" on forest soils. The fallen leaves are a great food source for the fungi and bacteria in the soil. These creatures slowly help the leaves to decompose, and they are eventually turned back into soil which the trees can use to grow new leaves in future seasons. The material at the top of the litter layer is newly fallen and recognizable. Towards the bottom, the older leaves are torn and usually covered with a slimy coating of microorganisms which feels gross, but it's vital to returning nutrients to the soil.

Sandy soils are relatively "newly" formed compared to other soil types. It is easy for both air and water to move between the large grains of sand, so this soil type stores water very poorly and is susceptible to drought. The opposite problem occurs with clay soils. They are often waterlogged because the tiny clay particles are packed tightly together, making it hard for air and water to move through. Clay soils can be dense enough to make it difficult for plant roots to spread through them.

Through their roots, trees bring water up from deep down in the subsoil. A large tree can release over 100 gallons of water into the air each day. This water vapor becomes part of the area's microclimate, which would be much drier without the moisture input from trees.

Too much precipitation results in the "leaching" of the soil. This happens when the water washes away most of the minerals and other nutrients. Water can also contribute to the soil's acidity, because as it moves through the soil, it removes minerals and leaves hydrogen atoms behind. The more hydrogen in the soil, the more acidic it becomes. This, however, only becomes a problem in areas with frequent, heavy downpours, and in these cases, plants have adapted to grow in poor soils. Leaching of the soil in these areas is natural, and in most other cases water moving through the soil is very beneficial to forests.

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