Millions of years of physical and biological weathering of solid rock produces soil. Soil consists of distinct layers, called horizons. The outer horizon, the A horizon, is called topsoil, which is a mixture of mineral particles, living organisms, and decaying organic matter. Plant roots are usually anchored in topsoil. The next horizon, the B horizon, is called subsoil. Minerals leached from the topsoil accumulate in the subsoil. The C horizon, the parent material, is slightly weathered. It contains almost no organic matter. The D horizon, the bedrock, is the rock from which the other horizons originated.

The texture of a soil is very important for plant growth and is determined by its composition of sand, silt, and clay. Sand is chemically inactive. It has little effect on soil chemistry and pH. Water and nutrients easily wash through sand. Silt is in between sand and clay. Clay has a very large surface area and small pores. It is chemically complex. It is negatively charged, so cations such as calcium and magnesium ions are attracted to clay particles. Clay is good at retaining nutrients.

Soil that contains a balanced mix of sand, silt, and clay is called loam. Loam is the most suitable soil for growing most crops. Not only can it hold enough water to keep plant roots moist, but it also has air spaces that allow oxygen to reach the roots.

Fertile soils also contain humus, which is partly decayed plant and animal matter. Humus is rich with nutrients and organic compounds derived from plants and animals.

Soil is filled with living things. There are more organisms in a gram of soil than there are humans on Earth! A spoonful of soil contains millions of organisms like earthworms, nematodes, arthropods, insects, protists, fungi, as well as about five billion bacteria. The activities of these small creatures affect both the physical and the chemical properties of soil. Animals like moles and rabbits dig tunnels through soil, which mixes it and helps circulate air. Earthworms also circulate air by burrowing through soil, and they secrete mucus that holds sandy particles together. Nematodes, or roundworms, circulate air and play an important role in decomposing organic matter. However, several species of nematode are agricultural pests that can damage crops.

Arthropods, such as millipedes, and insects, like beetles, break down leaves and other plant parts that become part of humus. Some species of insects are major agricultural pests when they eat plant roots. Bacteria, fungi, and soil algae perform the final decomposition of organic remains into inorganic nutrients. As we’ll see in the next section, certain bacteria also convert nitrogen gas into a form that’s usable by plants.

Just as the composition of soil can affect plants, plants can affect the composition of soil. Plant roots break up soil, allowing water, air, and animals to penetrate the ground. Roots also help protect soil from blowing or washing away. Finally, plants remove water and minerals from the soil, and when they die, they return organic matter back to the soil in the form of humus.

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Earthworms circulate air by burrowing through soil, and they secrete a mucus that holds sandy particles together. Plant roots break up soil and help protect soil from blowing or washing away. Plants also remove water and minerals from the soil, and return organic matter back to soil in the form of humus when they die. Decayed animal matter in the form of humus returns nutrients and organic compounds to soil. Rabbits dig tunnels through soil, which mixes it and helps circulate air. Insects, such as beetles, break down leaves and other plant parts that form humus. Bacteria, along with fungi and soil algae, perform the final decomposition of organic remains into inorganic nutrients. Bacteria also convert nitrogen gas into a form that’s usable by plants.

Now that we’ve learned about plants and soil, let’s learn about nitrogen fixation, a process essential for all forms of life on this planet.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education