Soils are complex in structure

Soils are complex systems made up of living and nonliving components. The living components include plant roots as well as populations of bacteria, fungi, protists, and animals such as earthworms and insects (Figure 37.2). The nonliving portion of the soil includes rock fragments ranging in size from large rocks through sand and silt and finally to tiny particles called clay that are 2 |im or less in diameter. Soil also contains water and dissolved mineral nutrients, air spaces, and dead organic matter. The air spaces are crucial sources of oxygen (in the form of O2) for plant roots. The characteristics of soils are not static. Soils change constantly through natural phenomena such as rain, temperature extremes, and the activities of plants and animals, as well as human activities—agriculture in particular.

The structure of many soils changes with depth, revealing a soil profile. Although soils differ greatly, almost all soils consist of two or more recognizable horizontal layers, called

Organic matter (from plants, animals, and fungi)

Organic matter (from plants, animals, and fungi)

Crumb Soil Structure

Air and water

Aggregates of clay particles

37.2 The Complexity of Soil Even a tiny crumb of soil has both organic and inorganic components.

Air and water

Aggregates of clay particles

A horizon Topsoil

B horizon Subsoil

C horizon Weathering parent rock (bedrock)

A horizon Topsoil

B horizon Subsoil

C horizon Weathering parent rock (bedrock)

Soil Profile Horizons

37.3 A Soil Profile The A, B, and C horizons can sometimes be seen in road cuts such as this one in Australia.The dark upper layer (the A horizon) is home to most of the living organisms in the soil.

37.2 The Complexity of Soil Even a tiny crumb of soil has both organic and inorganic components.

37.3 A Soil Profile The A, B, and C horizons can sometimes be seen in road cuts such as this one in Australia.The dark upper layer (the A horizon) is home to most of the living organisms in the soil.

horizons, lying on top of one another. Mineral nutrients tend to be leached from the upper horizons—dissolved in rain or irrigation water and carried to deeper horizons, where they are unavailable to plant roots.

Soil scientists recognize three major horizons (A, B, and C) in the profile of a typical soil (Figure 37.3). Topsoil is the A horizon, from which mineral nutrients may be depleted by leaching. Most of the dead and decaying organic matter in the soil is in the A horizon, as are most plant roots, earthworms, insects, nematodes, and microorganisms. Successful agriculture depends on the presence of a suitable A horizon.

Topsoils are composed of different proportions of sand, silt, and clay. In pure sand there are abundant air spaces between the relatively large particles, but sand is low in water and mineral nutrients. Clay contains many mineral nutrients and more water than sand does, but the tiny clay particles pack tightly together, leaving little space to trap air. A little bit of clay goes a long way in affecting soil properties. A loam is a soil that has significant amounts of sand, silt, and clay, and thus has sufficient levels of air, water, and nutrients for plants. Loams also contain organic matter. Most of the best topsoils for agriculture are loams.

Below the A horizon is the B horizon, or subsoil, which is the zone of infiltration and accumulation of materials leached from above. Farther down, the C horizon is the parent rock that is breaking down to form soil. Some deep-growing roots extend into the B horizon to obtain water and nutrients, but roots rarely enter the C horizon.

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