Soil Ecosystems

Mineral Matter

• Soil's mineral matter is the product of rock's gradual erosion by wind and water into minute particles. These particles come in three forms: clay, silt and sand -- with clay particles being the smallest and sand particles the largest. Mineral content determines a soil's texture and affects other characteristics, such as fertility. For instance, sandy soil is coarse and does not hold water well. Loam soils, on the other hand, have a balanced mixture of clay, silt and sand; this balance allows for good water retention and aeration, making loam excellent for growing crops.

Organic Matter

• Whereas soil's mineral matter derives from rock, its organic matter derives from living things. Decomposer organisms in the soil break down the wastes and remains of plants, animals and other organisms, a process that enriches the soil with nutrients, such as nitrogen and phosphorus, and creates humus. Not only is humus a key component of soil structure, it also retains water and nutrients that plants and other organisms in the soil need. A soil ecosystem's health depends on having adequate organic matter, so an ongoing supply of fresh organic matter is needed to replace what the living things in soil consume.

Soil Biota

• A soil ecosystem teems with living things, including plant roots, bacteria, protozoa, fungi, nematodes, arthropods and earthworms. These soil dwellers form a complex food web. Fungi and bacteria decompose organic material, and some kinds work symbiotically with plants, providing them with nitrogen or enabling them to absorb more water and nutrients. Nematodes, protozoa and arthropods feed on the fungi and bacteria. Earthworms improve soil quality in multiple ways, such as enhancing its aeration, disseminating nutrients and microbes and intermingling soils from different layers.

Role in Ecological Cycles

• Soil ecosystems also figure in the nitrogen, phosphorus, carbon and other ecological cycles. For instance, microorganisms in the soil contribute to the nitrogen cycle by creating nitrates and ammonium, through a process called nitrogen fixing, that plants and other living things need. By absorbing, retaining and releasing water, soil plays a part in the planet's water cycle. In addition to providing food and other resources for humans and animals, plants are critical to the oxygen and carbon cycles, and many land plants rely on soil ecosystems for anchorage, water and nutrients.