On August 1, 2013, the National Aeronautics and Space Administration [NASA] released images from the Mars rover Curiosity. Mars has a more Earth-like environment than any other planet in our solar system, so it is dubbed the second Earth and is considered the best candidate for human settlement. However, the images of Mars just showed a barren wasteland covered with vast expanses of sand, dust and rock, unlike the blue planet Earth.
Birth of living soil
One of the reasons Earth can become a beautiful planet that has life is that soil covers most of its land surface. Topsoil is the upper, outermost layer of soil. It is usually the top 30 cm [12 in]. On Mars, however, there is no topsoil that covers its surface. The entire surface of Mars is covered by a thick layer of dust.
When it comes to soil, most people think first of rock particles such as sand and gravel. However, solid material such as sand and gravel takes up only 45% of the soil—less than half of it. Water and air take up 25% each; organic matter which provides nutrition to plants takes up 5%; and other microorganisms live in the space filled with air. Earth’s soil, containing weathered rock particles, organic matter, water, and air, is alive with numerous organisms living in it.
Soil is created as rocks are broken down from larger to smaller fragments under the effects of rain, wind, water, temperature changes, and organisms over a long period of time. When water seeps into cracks of rocks and freezes, it causes the rocks to expand and break; the roots of plants growing into cracks of rocks can also break rocks apart as the roots grow; wind, too, wears away solid rocks over a long period of time. The transformation process of a solid rock into soil is called weathering. When the soil is mixed with water, air, and organic material made from fallen leaves, branches, and dead bodies of various organisms, Earth’s own soil is born.
On average, it takes about 200 years to form 1 cm [0.4 in] of rich soil, and 1,000 to 10,000 years to form a 30-centimeter [12-inch] layer. It needs a long time to form healthy soil, as it is a result of the physical and chemical breakdown of rocks and the biological decomposition of fallen leaves and dead bodies of animals.
Soil, the base of life
Living soil provides habitat and food for a variety of organisms. It also maintains the ecosystems through its purifying function. The soil interacts with a vast array of large and small organisms—plants that have roots in the ground, visible animals such as moles, earthworms, and ants, and invisible microorganisms.
It is admirable to see how a small seed planted in soil grows, puts forth leaves and flowers, and becomes a big tree that bears fruit. The moist and airy soil contains water and nourishment that are necessary for plants, and maintains temperatures, so that seeds can germinate.
There are sixteen essential elements that plants must have in order to grow properly: water, carbon dioxide, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, etc. Plants take carbon dioxide and oxygen directly from air and absorb all other nutrients from soil through their roots. Soil also supports and protects the roots of the plants so that they can stay upright.
Nitrogen is one of the essential elements that help plants grow. However, plants cannot absorb nitrogen directly from air. They can only take it from soil through their roots as ammonium ions, nitrate ions, nitrite ions, etc. Bacteria living in soil play an important role of converting nitrogen into forms that plants can absorb. The best known of the symbiotic nitrogen-fixing bacteria are root nodule bacteria. Mycorrhizal fungi1 attach to plant roots and help plants absorb nutrients.
1. Mycorrhizal fungi are a group of organisms that inhabit in the roots of plants, provide plants with phosphorus, nitrogen, etc., and gain organic matter
There are also many other microorganisms that serve as janitors; they purify and fertilize soil by decomposing dead bodies of animals and plants, excreta, floating matter, etc. Tens of millions to hundreds of millions of microbes live in one gram of soil and make the soil healthy, though they are invisible to our eyes.
Earthworms, which we can easily see after rains, make up 80% of all organisms living underground by weight. Their vertical burrowing carries and mixes material up and down through soil, which results in good ventilation. It is just like plowing the fields at farming. Earthworms produce worm castings by eating organic matter like fallen leaves, breaking them down, and discharging feces. The castings improve soil fertility and provide an optimal environment for microbes to live.
About 95% of the insects live in soil for a certain period of time to avoid their natural enemies and grow into imagoes safely. Cicadas live in the damp soil under the tree roots for four to five years and even up to seventeen years, and they live above ground for about ten days and die. Nearly five hundred million insects, including eggs, larvae, and imagoes, live in approximately 3,300 m² [35,521 ft²] of soil2.
2. Reference: EBS TV Documentary about Soil Production Team, Soil: Joy, Anger, Sorrow, and Pleasure of Lives That Breathe Together (in Korean, 흙 함께 숨쉬는 생명들의 희로애락), Low Hills, 2008
The organisms that fly in the sky, too, use soil to build their nests or live in soil. Female solitary wasps build rounded walls by lumping soil together with their chins and legs, so they can lay their eggs there. Black-capped kingfishers hide their eggs in soil in the cliff which is the safest place for them. Swallows that come in the spring build nests under the roofs, using soil, straw, and grass roots to raise their young. People have relied on soil, too. Agriculture, the basis of human civilization, started from soil, and around 1.5 billion people—22% of the world population—are still living in houses built with soil. Basically, the soil is the base of life to people.
Earth protects ecosystems
Organisms release contaminants and leave their dead bodies behind. Microorganisms in soil silently decompose these materials; they purify the contaminated materials to keep the environment suitable for other organisms to live. Earth’s soil performs unique and various cleansing functions to maintain strong and healthy ecosystems by interacting organically with the atmosphere, the hydrosphere, and the biosphere.
Underground water and soil moisture, which take up about 30% of the world’s fresh water, are mostly formed as rain, snow, and hail are absorbed to the ground. Underground water is also used for drinking water as foreign substances are filtered while going through soil deposits. Soil preserves water resources necessary for ecosystems as it stores water, which in turn prevents floods. Evaporation of water from soil also helps control the temperature of the atmosphere.
Carbon is a basic element for organisms. Plants convert carbon dioxide which is in the atmosphere into organic material through photosynthesis, and animals obtain carbon by absorbing organic material produced by the plants. When the carbon is used as the source of energy, it returns to the atmosphere through breathing; and when it constitutes the body, it becomes soil organic matter after death by organisms in soil. The organisms born in soil return to soil. The carbon that is stored in soil in the form of organic matter is twice as much as the carbon that exists in the atmosphere. Recently, there have been many studies to increase the amount of carbon storage by restoring soil to prevent global warming.
Soil is the basis of material cycles, and plays an important role in preventing global warming by purifying nature and by storing water and carbon. It is estimated that soil and underground water in Korea have a value of about 26.4 trillion KRW (approx. 23 billion USD) and 40.8 trillion KRW (approx. 35 billion USD) respectively; in actuality, Earth’s soil has a very significant meaning that it cannot be converted into currency.
Topsoil, the living soil, wraps Earth in a very thin layer, so it is described as Earth’s skin. Soil is very thin; it is only a little thicker than 1/20,000,000 of the radius of Earth (approx. 6,380 km or 3,960 mi). We can understand how thin soil is, when we think about the human skin which is less than 2 mm thick and is a little less than 1/1,000 of the person’s height. However, soil—Earth’s skin—is the base for organisms and helps them maintain their life.
The first and foremost condition for organisms to live on a foreign planet is that liquid water must exist on the surface made of soil and rocks, like Earth. However, Earth is the only planet where water has been found and living soil exists. It means that Earth is the only planet that has the environments for organisms to live.
For this is what the LORD says—he who created the heavens, he is God; he who fashioned and made the earth, he founded it; he did not create it to be empty, but formed it to be inhabited—he says: “I am the LORD, and there is no other.” Isaiah 45:18