Dirt is the living matrix of all terrestrial life on Earth; as it nourishes all forms of life it is also full of life, a handful containing tens of millions of microorganisms. The central question of the film is: how are we caring for this precious resource? Experts point
out that, as we draw upon the soil for our needs, we must also replenish what we take. Yet, there are numerous examples of humans’ mistreatment of the soil: the blasting away of mountain tops in our search for coal, cutting down the rainforest to clear land for planting, using chemical fertilizers and pesticides in growing crops, and paving over large areas of our cities and suburbs, to name just a few. All of these human activities are detrimental to maintaining healthy, living soil and have led to serious environmental degradation including climate change.
What is dirt?
Dirt, or soil, is the loose top layer of Earth’s crust, a mixture of mineral and organic materials plus air and water. Soil is formed slowly as rock (the parent material)
erodes into tiny pieces near the Earth’s surface. As organic matter decays, it mixes with inorganic material such as rock particles, minerals and water to form soil. Natural processes can take more than 500 years to form one inch of topsoil. Soil is made up of distinct horizontal layers called horizons. They range from rich, organic upper layers (humus and topsoil) to underlying rocky layers (subsoil, regolith and bedrock).
The contents of soil vary in different locations and are constantly changing.
There are many different kinds and types of soils; these are the basic ones:
- Sandy soil is usually made up of granules of mineral and rock, is quite gritty, with large spaces between particles, allowing for easy flow of water and minerals.
- Clay soil has extremely small particles with little space between individual particles, allowing for virtually no drainage. Clay soil is not a good growing medium because water tends to not be able to escape, and it is difficult for root systems to break through the clay layer. Clay soils tend to be much older than sandy soils, since it takes many, many years for rock particles to break down to be small enough to form clay.
- Silty soil is one of the most fertile types of soil, with rich nutrients and good drainage. Silty soil is very similar in composition to sandy soil, albeit with more nutrients and minerals. It is generally quite dark and pungent, and is excellent for planting almost anything.
- Loamy soil is made up of a few different types of soil, with varying amounts of clay, silt, and sand mixed together. Loamy soil holds water well because of the heavy grittiness provided by the sand, has exceptional drainage so that the water doesn’t build up too much and rot plant roots, and is nutrient rich. Loamy soil is the ideal soil for gardening.
What does dirt do?
- supports the growth of plants, which can be used for clothing, shelter and food • for humans and animals
- pottery and dishes, made from clay soil
- bricks and other building material, made from mixtures of soil and animal excrement
- cooling medium (mud) for animals, especially pigs
- protection from insects (dirt “showers” used by elephants)
- facials and mud pack treatments in spas
- pigments derived from certain types of soil, used for painting and body decoration
- (in development) energy for operating lights and other equipment, created by microscopic organisms in soil
Soil and the cycles of life
The Water Cycle (or hydrologic cycle) describes the circulation of water from the land to the sky and back again. The Sun’s heat causes water to evaporate from the Earth’s land masses and bodies of water. Plants also lose water through transpiration. The water vapor forms clouds, and when the clouds meet cool air over land, the water vapor condenses, resulting in precipitation. Of the water that falls, some soaks into the ground and some of that gets trapped between rock or clay layers, forming groundwater. But most of the water flows downhill into streams and rivers, eventually returning to the seas as slightly salty water.