Ecosystems
CITE
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Soil. (2022). In Q-files Encyclopedia, Life, Ecosystems. Retrieved from
https://www.q-files.com/life/ecosystems/soil
"Soil." Life, Ecosystems, Q-files Encyclopedia, 10 May. 2022.
https://www.q-files.com/life/ecosystems/soil.
Accessed 19 Mar. 2024.
Soil 2022. Life, Ecosystems. Retrieved 19 March 2024, from
https://www.q-files.com/life/ecosystems/soil
Life, Ecosystems, s.v. "Soil," accessed March 19, 2024.
https://www.q-files.com/life/ecosystems/soil
Soil
Soil is a vital part of the natural world. It consists of fragments of rock, such as sand grains, mixed with humus, the rotting remains of leaves, animal droppings and other plant and animal matter. Between the particles are large air spaces. These are important because they allow water to drain through the soil. They also allow oxygen to reach both the plant roots and the organisms (living things) that inhabit the soil. An average soil will consist of about 45% rock fragments, 25% water, 25% air and 5% organic material. Nutrients that are essential to plant life, such as nitrogen (in the form of nitrates) and phosphorus (phosphates), are found in the soil.
Soil layers
A slice through the ground reveals the different soil layers, called horizons. On top is leaf litter with old leaves, twigs and feathers (the O horizon). Below is topsoil (A horizon), rich in dead and decaying remains of plants and animals, the humus, and home to small soil creatures. It also contains the thin roots of small plants. Next is the subsoil, the B horizon. This has less organic matter, and more and larger rock fragments. The roots of bushes and trees grow into the subsoil for firm anchorage. Below, in the C horizon, the rock fragments dominate until the bedrock itself is reached.
Water seeps down through the topsoil, a process known as percolation. As it does so, it takes with it soluble substances, such as mineral salts, which it deposits in the subsoil. This is known as leaching.
Soil life
A handful of soil is like a factory where a mass of tiny organisms, most of which are too tiny to see with the naked eye, are working away recycling the remains of dead animals and plants into nutrients. These are, in turn, taken up by the roots of living plants. The tiniest of the “workers” are called bacteria. That handful of soil will contain over 10 billion of them—more than there are people living in the world.
Microscopic organisms made of only one cell, called protozoans, feed on bacteria, while they are themselves eaten by larger animals such as worms. Earthworms also have a vital role to play tunnelling through the soil, letting air in, helping water drain through and mixing the different soil layers. Insects, spiders, mites and centipedes hunt in the soil, and larger animals like moles and voles make their burrows there.
Types of soil
Soils vary enormously in their thickness, particle size and the main minerals and nutrients they contain. The climate, the kinds of rocks that lie beneath and the main types of plants that grow in soil all affect its character and its nourishing ability, or fertility.
Deep soil that has plenty of rotting plant and animal matter is very fertile and many plants grow in it. But specialist plants like cacti can grow even in thin, dry, nutrient-poor, sandy, desert soils. Acid rain caused by pollution has made large areas of soil in some areas too acid to support much life.
Soil nutrients
All life on land depends on soil. Plants use it to anchor their roots. They take up water, nutrients and minerals from the soil and transport them via the stem to the leaves where food is made by a process called photosynthesis—a chemical reaction using sunlight. Most land animals feed on organisms that grow or live in the soil, either directly, or when they eat other animals that do so.
The nutrients pass on to the herbivore or meat-eating animal, and return to the soil through waste or when an animal dies. Fungi, microbes and soil animals all help the rotting process by which living things decay and release the nutrients into the soil, to be taken up by plants once more.
Rhizosphere
The soil immediately surrounding a plant's roots makes up what is known as the rhizosphere. This important region of the soil is inhabited by a vast and hugely diverse concentration of micro-organisms. They feed off the proteins and sugars (together known as root exudates) released by the roots, along with old cells that have flaked off. In return, the rhizospheric micro-organisms break down organic material in the soil into simple compounds, which the plant absorbs. The micro-organisms also produce growth hormones and other chemicals that help the plant grow. Scientists describe this partnership—one which benefits both plants and micro-organisms—as symbiosis.
Consultant: Chris Jarvis