SOIL

Copyright, 1996 © Dale Carnegie & Associates, Inc.

DEFINITION

Soil is the biologically active, structured porous medium that has developed below the continental land surface on our planet

The science of pedology is interested in the appearance of the soil, its mode of formation, its physical, chemical and biological composition, and its classification and distribution (Bridges, 1997).

The pedosphere is the envelope of the Earth where soils occur and soil forming factors are active.

The pedosphere only develops when there is a dynamic interaction between the atmosphere,

biosphere, lithosphere and the hydrosphere.

Civilizations depend on the quality of their soil to grow their food and to serve as a living filter that purifies the wastes they produce.

Only a thin layer of soil, called topsoil, can adequately support plant life.

While along great rivers the soil favorable to plant growth may be hundreds of feet thick, in most places it extends down only about six inches.

The replacement of such losses is very slow. Nature takes from 500 to 1,000 years to make 1 inch (2.5 centimeters) of topsoil and from 2,000 to 5,000 years to replace a loss of 5 to 10 inches (13 to 25 centimeters).

Therefore nations must learn to conserve their productive soil.

SOME IDEAS

HOW SOIL IS FORMED?

Soils are developed from mineral and organic matter and generally contain an active population of organisms.

Five major influences on soil formation include: the nature of the original parent material

transported, residual, and organic.

Weathering climate: temperature and precipitation. land surface features, the action of plants and animals.

These factors determine the physical and chemical properties of various kinds of soil.

COMPOSITION

Soils are composed of mineral matter and organic matter and contain pore spaces filled with water or air and soluble nutrients

Organic matter serves as a binder for mineral particles, contributing to good soil structure and tilth, which refers to the behavior of soil under cultivation. Organic matter content of mineral surface soils ranges

from less than 0.5 percent in highly weathered, sandy soils to more than 6 percent in poorly drained prairie soils.

Soil organic matter undergoes continual breakdown from fresh plant residue to relatively stable humus.

Soil water and gases fill the spaces between mineral and organic matter. The amount of space that is filled with water depend

on the degree of saturation.

The color, texture, and structure of different soils can reveal clues about soil development and the

presence of soil water. The type and amount of minerals in the soil and the

rate at which they dissolve into water help

determine the fertility of the soil.

SOIL COLORS

• Colors of organic matter range from the brown of undecomposed peat to the rich black of

humus. • Black colors are generally found in the surface layer of soil. When located deeper, the dark colors may represent soil buried by sedimentation or a

geologic deposit. •Red colors in soil, caused by the presence of iron oxide, are typically found in temperate region soils. Good drainage and warm temperatures encourage the formation of iron oxides. The process may require long periods of weathering and is

characteristic of older soils. •Brown and reddish brown represent a combination of organic

matter and oxidized iron. •Yellow colors are the result of oxidized iron with attached water molecules, indicating that yellow soils are slightly wetter

than red soils. •When iron is completely isolated from oxygen, as it would be in a saturated soil, the soil color turns bluish gray.

•Gray and white colors may reveal the sandy parent material or may indicate the removal of iron and organic matter from soil minerals through leaching. In arid regions, such as deserts and coastal areas, white indicates soluble salts or carbonate

accumulation in the soil.

TEXTURE AND STRUCTURE

Soil texture refers to the coarseness or fineness of mineral particles in the soil, while soil structure describes the physical arrangement of these particles. Texture and structure are important properties that help determine a soil's ability to supply water and nutrients to plants.

Three broad textural classes are used to describe soils: clays, sands, and loams.

Soil particles: Sand, silt, and clay, which comprise the soil particles

that are less than 2 millimeters in diameter, are often referred to as soil separates.

Stone, gravel, cobbles, and boulders may be part of a field soil, but, because they are larger than 2 millimeters, they are not included in the analysis of soil texture.

Sand particles range from 0.05 to 2 millimeters and are gritty to the touch.

Silt is as smooth as flour when dry and holds water well.

Clay particles are less than 0.002 millimeter in diameter and are the soil separates most involved in chemical reactions in the soil.

CHEMISTRY

The basic chemistry of soil includes colloidal structures, minerals, and macronutrients and micronutrients essential to plant life. Clay and organic matter are finely divided soil

particles called colloids, which provide the site for most of the chemical reactions in soils. Colloids, though small, possess large surface areas and electrical charges that attract nutrients and water. Soil colloids bind nutrients and prevent them from being leached out of the soil.

Clay minerals are silicates arranged in microscopic sheets of aluminum and silica. The specific arrangement of these sheets and the type of elements within them determine the clay type. Many of these clays possess negative electrical charges that help retain elements such as potassium, calcium, and magnesium for plant use. These nutrients can move into the soil solution and be absorbed by plant roots.

Essential plant elements are those required for plant survival and growth. Three of these elements --carbon, oxygen, and hydrogen--are supplied by water and air, while 14 others, categorized as either macro- or micronutrients, must be supplied by the soil. Macronutrients include primary elements (nitrogen, phosphorus and potassium) and secondary elements (calcium, magnesium, and sulfur).

NUTRIENTS

Micronutrients are also known as trace elements because plants need them in such small quantities. These elements include iron, manganese, copper, zinc, boron, chlorine, cobalt, and molybdenum. The availability of micronutrients is determined by how acidic or alkaline the soil is.

Plant nutrients such as nitrogen and sulfur exist mostly as negatively charged ions in the soil and therefore are not held by soil colloids. As a result, these elements are subject to leaching by water moving through the soil, especially in sandy soils. Leaching of nitrogen into the groundwater is a serious environmental problem

in sandy irrigated regions.

SOIL WATER

Water is held in the soil by cohesion to other water molecules and by adhesion to colloid surfaces. Soils with smaller pores are more effective at holding water against the forces of gravity. Large pores are best at conducting water through the soil when water content is high.

When the ground is saturated, all the pores in a soil are filled with water. After about two days, water drains from the large pores, and the soil is in a condition known as field capacity. This is the maximum amount of water a soil can hold against the forces of gravity. Some water is held so tightly by adhesion that plants cannot pull it away. When plants cannot remove any more water from the soil, the field has reached what is called the wilting point. The amount of water between field capacity and the wilting point indicates the amount that is available to plants.

SOIL PROFILES

Soil profiles show master horizons and evidence of dominant processes that led to their development. Generally, A, B, and C are master mineral horizons and O or LFH are master organic horizons. While some pedons may not have all these horizons, all pedons have some of them.

Soil landscapes evident around us are the product of soil forming factors acting on surface geological deposits over time. Factors influencing soil formation (or soil development) include kind of parent material, topography (or surface form), climate, vegetation, and activities of man acting over a period of time. The surficial deposit which comprises the parent material of soil is characterized by its physical and chemical properties such as soil texture (or mix of sand, silt, clay), reaction and salinity. Over time, this parent material is transformed to yield unique

soils and soilscapes.

SOIL PROFILES

A horizon: A mineral horizon formed at or near the surface in the zone of removal of materials in solution and suspension, or maximum in situ accumulation of organic carbon, or both.

B horizon: A mineral horizon characterized by one or more of the following: 1. An enrichment in silicate clay, iron,

aluminum, or humus. 2. A prismatic or columnar structure that

exhibits pronounced coatings or stainings associated with significant amounts of exchangeable sodium.

3. An alteration by hydrolysis, reduction, or oxidation to give a change in color or structure from the horizons above or below, or both.

C horizon: A mineral horizon comparatively unaffected by the pedogenic processes operative in A and B, except gleying, and the accumulation of carbonates and more soluble salts.

SOIL CLASSIFICATION Soils are classified on the basis of soil depth,

color, texture, structure, chemical composition, and the presence of certain diagnostic horizons. Diagnostic horizons are based on combinations of thickness, color, chemistry, or texture. Entisols are soils in recent geologic deposits that have little

or no horizon development. Inceptisol are soils in which soil development is just

beginning is known as. Aridisols are found in desert regions and are characterized

by light color, low organic matter, and accumulations of carbonates, gypsum, and salts.

Vertisols are clay soils that develop cracks in the dry season, which allow surface material to fall in, inverting the normal layers of soil.

Mollisols are dark-colored prairie soils with thick topsoils; these are among the most productive agricultural soils in the world.

Alfisols are slightly more moist and acidic than Mollisols and border the wetter regions of grasslands.

Ultisols exhibit clay accumulations in the subsoil and are named according to the weathering they undergo. These soils develop in warm regions with heavy rainfall.

Oxisols are highly oxidized soils of the humid tropics; most minerals have been leached from these soils, leaving primarily iron and aluminum oxides. When limed and fertilized, however, these soils can be made extremely fertile.

Histosols, or organic soils, contain at least 20 percent organic matter and can be productive for certain kinds of crops. The soils of the Florida Everglades are largely Histosols.

SOIL MANAGEMENT AND RESEARCH The study of soil is carried out by several types of

scientists. Pedologists study the soil as a natural body without necessarily focusing on its use. Soil chemists, physicists, mineralogists, and microbiologists conduct research on soil properties and behavior. Edaphologists study the soil as a medium for the production of crops.

Soil scientists attempt to find ways of managing the soil so that it will provide maximum crop yields without depleting this valuable resource. They recognize that the soil--as a living, dynamic medium--has many vital uses.

Soil scientists are also concerned with finding ways to minimize or prevent soil erosion and to increase the buildup of organic matter in soil. Research in farming methods has resulted in new ways to use contour ploughing, terraced farming, rotation of crops, fertilization, and ground-cover plants to protect and enrich the soil in many parts of the world. The study of soils--their fertility and their capacity to filter waste products--will become increasingly important as the world demand for food rises and levels of pollution increase. Protection of the thin layer of the Earth 's surface

called soil is vital to survival.