4-6 Soils
What Is Soil and Why Is It Important? The Base of Life on Land
Soil is a thin covering over most land that is a complex mixture of eroded rock, mineral nutrients, decaying organic matter, water, air, and billions of living organisms, most of them microscopic decomposers. It is the base of life on land because it provides most of the nutrients needed for plant growth.
Soil is also the Earth’s primary filter that cleanses water as it passes through. It is also a major component of the Earth’s water recycling and water storage processes.
What Major Layers Are Found in Mature Soils? Layers Count
Mature soils, or soils that have developed over a long time, are arranged in a series of horizontal layers called soil horizons, each with a distinct texture and composition that varies with different types of soils. A cross-sectional view of the horizons in a soil is called a soil profile.
The top layer is the surface litter layer, or O horizon. It consists mostly of freshly fallen undecomposed or partially decomposed leaves, twigs, crop wastes, animal wastes, fungi, and other organic materials. Normally, it is brown or black.
The topsoil layer, or A horizon, is a porous mixture of partially decomposed organic matter, called humus, and some inorganic mineral particles. It is usually darker and looser than deeper layers. A fertile soil that produces high crop yields has a thick topsoil layer with lots of humus. This helps topsoil hold water and nutrients taken up by plant roots.
The two top layers of most well-developed soils teem with bacteria, fungi, earthworms, and small insects that interact in complex food webs. Bacteria and other decomposer microorganisms found by the billions in every handful of topsoil break down some of its complex organic compounds into simpler inorganic compounds soluble in water.
The colour of its topsoil tells us a lot about how useful a soil is for growing crops. Dark-brown or black topsoil is nitrogen-rich and high in organic matter. Grey, bright yellow, or red topsoils are low in organic matter and need nitrogen enrichment to support most crops.
Some of the precipitation that reaches the soil percolates through the soil layers and occupies many of the soil’s open spaces or pores. This downward movement of water through soil is called infiltration. As the water seeps down, it dissolves various minerals and organic matter in upper layers and carries them to lower layers in a process called leaching.
How Do Soils Differ in Texture and Porosity? Composition and Spaces Are Important
Soils vary in their content of clay (very fine particles), silt (fine particles), sand (medium-sized particles), and gravel (coarse to very coarse particles). The relative amounts of the different sizes and types of these mineral particles determine soil texture.
Soil texture helps determine soil porosity, a measure of the volume of pores or spaces per volume of soil and of the average distances between those spaces. The average size of the spaces or pores in a soil deter- mines soil permeability: the rate at which water and air move from upper to lower soil layers.