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Chapter 15 Global Soils

Visualizing Physical Geography by Timothy Foresman & Alan Strahler

Chapter Overview

Soil Formation

Soil Properties

Soil Chemistry

The Global

Distribution of Soils

Soil Formation

Soil = Natural terrestrial surface layer that contains minerals and organic matter and that supports or is capable of supporting plants, animals, and microbial communities

Components of Soil

Often 50% mineral and organic material

50% open pore spaces

Every land surface on Earth is covered by soil.

True

False

Soil Formation

Components of Soil

Chemical processes alter the mineral composition of the parent material.

Parent material = The inorganic material base from which soil is formed.

Weathering is the physical, chemical, and biological processes that break down the parent material.

Soil Formation

Components of Soil

Regolith: broken down bedrock; layer of rocks and minerals lying above bedrock

Humus: finely divided, partially decomposed organic matter found in soils

Soil Formation

Soil Horizons

Soil horizons = A distinctive layer of soil, more or less horizontal, set apart from other layers by differences in physical or chemical composition

Soil horizon labels: O, A, E, B, and C

Soil Formation

Soil Horizons

Soil profile = A display of soil horizons on the face of a freshly cut vertical exposure through the soil.

Soil Formation

Soil-Forming Processes

Enrichment and removal

Enrichment: material (organic and inorganic) is added to the soil

Removal: material eroded from top of soil or carried out of soil by seeping water and taken to groundwater below

Soil Formation

Soil-Forming Processes

Transformation: soil materials altered within horizons (e.g., decomposition of organic matter to form humus)

Translocation: soil materials moved vertically

Eluviation = The downward transport of dissolved or suspended materials to lower soil horizons or out of the soil completely

Illuviation = The accumulation in a lower soil horizon of materials brought down from upper horizons

Soil Formation

Soil-Forming Processes

Translocation

In desert climates, translocation can lead to salinization, the upward wicking of salt-laden water from ground water or from irrigation; pure water evaporates from soil surface and leaves salt behind

Ancient Sumerian civilization: decline in agricultural productivity due to salinization

Soil Formation

Factors in Soil Formation

Arable soils = Land suitable for vegetation, including forest or crops

Biological organisms: live in soil and affect soil properties

Physical movement

Digestive processing

Soil Formation

Factors in Soil Formation

Nature’s soil engineers (beavers)

Beaver dams create ponds that affect hydrology, ecology and soil formation processes

Soil Formation

Factors in Soil Formation

Climate

Above or below freezing

Most plants germinate above 5ºC (41ºF)

Time

Topography

Parent material

Human activity: building cities on the best places to grow food?

Soil Properties

Soil Color

Black soil

Red soil

White soil

Texture

Structure

Moisture

Soil Chemistry

Acidity and Alkalinity

Soil pH

Optimal range for many crops

The Global Distribution of Soils

Soil orders and global agriculture

Soil Is the Base of Life on Land

Soil composition: eroded rock, minerals, decaying organic matter, water, air, billions of living organisms

Soil formation: topsoil is replaced by nature but at very slow rates—1 inch of soil can take hundreds of years to form

Soil erosion: by wind or water; substantial human effect

The Global Distribution of Soils

Degrading cropland with water mismanagement

Desertification: productive potential of soil falls by 10% or more due to drought or human activities that reduce or degrade topsoil

Salinization: making topsoil saltier by repeated applications of water in dry areas

Waterlogging: accumulation of water underground from repeated irrigation

Human Effects on Soils

Desertification

Human Effects on Soils

Natural capital degradation: desertification of arid and semiarid lands. It is caused by a combination of prolonged drought and human activities that expose soil to erosion. Question: Can you see any geographical pattern associated with this problem? (Data from U.N. Environment Programme and Harold E. Drengue)

Desertification in West Africa

Human Effects on Soils

Severe desertification. Sand dunes threaten to engulf an oasis in the Sahel region of West Africa. Such severe desertification is the result of prolonged drought from natural climate change and destruction of natural vegetation from human activities such as farming and overgrazing.

Soil Problems on Heavily Irrigated Land

Human Effects on Soils

Salt accumulation in topsoil in Colorado

Human Effects on Soils

Natural capital degradation: Because of high evaporation, poor drainage, and severe salinization, white alkaline salts have displaced crops that once grew on this heavily irrigated land in the U.S. state of Colorado.

Reducing Soil Erosion

Soil conservation, some methods

Terracing

Contour planting

Strip cropping with cover crop

Alley cropping, agroforestry

Windbreaks or shelterbeds

Conservation-tillage farming

No-till

Minimum tillage

Identify erosion hotspots

Human Effects on Soils

Fig. 7-14, p. 153

Soil conservation methods include (a) terracing, (b) contour planting and strip cropping, (c) alley cropping, and (d) windbreaks.

Restore Soil Fertility

Modern agriculture “mines” select soil nutrients & replaces them with commercial fertilizer (nitrogen, phosphorus, potassium)

Trace elements not replaced, living systems not maintained

Use organic fertilizers to restore soil (manure, compost)

Rotate crops; let soil rest (fallow)

Make the soil the end, not the means

Human Effects on Soils