Ecosystems and Biomes
Ecosystem
1.1 Introduction
1.2 Structure of Ecosystems
1.3 Functions of Ecosystems
1.4 Benefits of Ecosystem
Biomes
2.1 – Biome – Introduction
2.2 – Classification and Distribution of Biomes
1.1 Ecosystem – Introduction
An ecosystem, then, is fundamentally an association of plants and animals along with the surrounding nonliving environment and all the interactions in which the organisms take part. The concept is built around the flow of energy among the various components of the ecosystem, which is the essential determinant of how a biological community functions.
1.2 Structure of Ecosystems
Ecosystems have two basic components:
The abiotic (non-living) components
The biotic (living) components
The abiotic components:
The physical and chemical components of an ecosystem constitute its abiotic structure. It includes climatic factors, edaphic (soil) factors, geographical factors, energy, nutrients and toxic subsistence.
Physical Factors: The sunlight and shade, intensity of solar flux, duration of sun hours, average temperature, maximum-minimum temperature, annual rainfall, wind, latitude and altitude, soil type, water availability, water current etc. are some of the important physical features which have a strong influence on the ecosystem.
Chemical factors: Availability of major essential nutrients like carbon, nitrogen, phosphorus, potassium, hydrogen, oxygen and sulphur, level of toxic subsistence, salt causing salinity and various organic subsistence present in the soil or water largely influence the functioning of the ecosystem.
All the components of the ecosystem are influenced by the abiotic components and vice versa, and they are linked together through energy flow and matter cycling.
The biotic components:
The living components of an ecosystem constitute its biotic structure as following:
Producers (autotrophs): They utilize energy from the sun and nutrients from the abiotic environment (carbon dioxide from the air or water, other nutrients from the soil or water) to perform photosynthesis and grow. Producers are generally green plants.
Consumers. All the organisms that can not make their own food (and need producers) are called consumers or heterotrophic components. They obtain food by eating other organisms. There are different levels of consumers.
On the basis of their size, they are categorized as following:
Macro consumers
Micro consumers
Macro consumers
Macro Consumers are also classified depending on what they eat.
Herbivores are those that eat only plants or plant products. Examples are grasshoppers, mice, rabbits, deer, beavers, moose, cows, sheep, goats and groundhogs.
Carnivores, on the other hand, are those that eat only other animals. Examples of carnivores are foxes, frogs, snakes, hawks, and spiders.
Omnivores are the last type and eat both plants (acting primary consumers) and meat (acting as secondary or tertiary consumers).
Micro consumers
Micro consumers decompose the dead and decaying organism is known as decomposers e.g. fungi, bacteria etc. Decomposers are essential for the long term survival of ecosystem. Without them, enormous wastes of plant litter, dead animal bodies, animal excreta and garbage will be collect on the mother planet. Also important elements like nitrogen, phosphorous, and potassium would be remain indefinitely in dead matter. Then the producers would not get them and this would make life impossible.
Micro consumers are of two types:
Parasites: Parasites derive their food from the living organism. The parasites may be microscopic bacteria, fungi, viruses, or macroscopic like angiosperms, nematodes, tapeworm, flatworms, and roundworms. They check the population of the host.
Detritivores and scanvengers: The dead organic matter of an organism (both plants and animals) is known as detritus. Organisms depend upon detritus are known as Detrivorous e.g. protozoans, insects, and snails. They are known as reducers. They clean the environment by eating up dead organisms. Hence they are also called scavengers.
1.3 Functions of Ecosystems :
Every ecosystem performs under natural conditions in a systematic way. It reserves energy from the sun and passes it on through various biotic components and in fact, all life depends upon this flow of energy. Besides energy, various nutrients and water are also required for life processes which are exchanged by the biotic components within themselves and with their abiotic components within or outside the ecosystem. The biotic components also regulate themselves in a very systematic manner and show mechanism to encounter some degree of environmental stress.
The major functional attributes of an ecosystem are as follows:
Trophic levels, Food chain, and food webs
Energy flow
Cycling of nutrients (Biogeochemical cycle)
Primary and secondary production
Ecosystem development and regulation
Trophic levels
In ecology, the trophic level is the position that an organism occupies in a food chain – what it eats, and what eats it.
In the ocean, phytoplankton is the primary producer (the first level in the food chain or the first trophic level).Phytoplankton converts inorganic carbon into protoplasm. Phytoplankton is consumed by microscopic animals called zooplankton (these are the second level in the food chain). Zooplankton is consumed by Crustaceans (the third level in the food chain).Fish that eat crustaceans could constitute the fourth trophic level, while seals consuming the fishes are the fifth.
Trophic levels are very similar on land, with plants being the first trophic level, goats eating the grass being the second, and humans eating the goats being the third.
The amount of biomass produced for a given amount of solar energy is highest at the first level. Less biomass is produced at the second level, for some energy is lost during the conversion. The more trophic levels there are, the more energy is lost through conversion.
Food chain
A food chain describes a single pathway that energy and nutrients may follow in an ecosystem. On average 10% of the organism’s energy is passed on to its predator. Trophic levels are different steps in the passage of food.
Types of Food chains
Predator or Grazing food chain: It always starts with green plants and culminates in top carnivorous. It may of two types as follows:
Terrestrial grazing food chain
Aquatic grazing food chain
Terrestrial grazing food chain: It begins from producer (terrestrial green plants e.g. grasses) and then proceeds to herbivores, carnivores and then to top carnivores as given in the following figure.
Aquatic grazing food chain : It begins from aquatic green plants e.g. phytoplankton, then proceed to herbivores, carnivores and then to top carnivores.
Parasitic or Auxillary food chain: This food chain starts from producers which prepare organic food through the process of photosynthesis. From here, the food goes to herbivores and then to parasites and finally to hyperparasites. Thus here food proceeds from larger to smaller organisms. This chain is terminated by by a parasite. Parasite is an organism that feeds on another living organism called host e.g. Green Plants > Sheep > Liver fluke.
Food web
Interlinking of the food chains form a food web. The concept of food chain looks very simple, but in reality it is more complex. Think about it. How many different animals eat grass? And how many different foods does the hawk eat? One doesn’t find simple independent food chains in an ecosystem, but many interdependent and complex food chains that look more like a web and are therefore called food webs.
Energy flow
In ecology, energy flow (calorific flow) refers to the flow of energy through a food chain. The sole source of the energy in an ecosystem is the light received from the sun. According to Geiger, 42% of incoming solar radiation are reflected,(33%by clouds and 9% by dust particles), about 10% are absorbed by Ozone, oxygen and water vapour.Finally only 48% reach the surface of the earth. A portion of this radiant energy is used by producers the green plants and converted into chemical energy. This chemical energy is partially transferred to consumers through food. The concept of the energy flow in the ecosystem is governed by the first law of of thermodynamics.i.e. Energy can neither be created nor destroyed and every transfer of the energy is accompanies some loss of energy in the form of heat.
Nutrient / Biogeochemical cycle
Biogeochemical cycle is a circuit or pathway by which a chemical element or molecule moves through both biotic (“bio-“) and abiotic (“geo-“) compartments of an ecosystem.
All chemical elements occurring in organisms are part of biogeochemical cycles. In addition to being a part of living organisms, these chemical elements also cycle through abiotic factors of ecosystems such as water (hydrosphere), land (lithosphere), and the air (atmosphere); the living factors of the planet can be referred to collectively as the biosphere. All the chemicals, nutrients, or elements—such as carbon, nitrogen, oxygen, phosphorus—used in ecosystems by living organisms operate on a closed system, which refers to the fact that these chemicals are recycled instead of being lost and replenished constantly such as in an open system. The energy of an ecosystem occurs on an open system; the sun constantly gives the planet energy in the form of light while it is eventually used and lost in the form of heat throughout the trophic levels of a food web.
Balance in Ecosystem
An ecosystem develops over a long period. It reaches a state of delicate balance. All the species in the food webs get enough to eat. The food is just enough for them to multiply at the correct rate to keep the ecosystem going. Abiotic conditions like the climate, water availability, and sunlight are just right.
How do so many species live together in an ecosystem without fierce competition taking place? Organism do compete for food, but this does not lead generally to examination of species. The reason is that each species in an ecosystem has found its habitat.
The balance in an ecosystem can be easily upset. Some times it is due to natural causes like fires or earthquakes. In recent times, however, human activities have been disturbing the balance of many ecosystems.
The interconnections in nature are so complex that we can not predict all the consequences of degradation of ecosystems. Scientists agree, however, on effects like the following:
•Many species are becoming extinct, affecting many food webs.
•Global warming is taking place and natural disasters are increasing in frequency and severity.
•Natural resources like water are becoming scare.
Ecological Succession / Adaption
Ecological succession, a fundamental concept in ecology, refers to more-or-less predictable and orderly changes in the composition or structure of an ecological community. Succession may be initiated either by formation of new, unoccupied habitat (e.g., a lava flow or a severe landslide) or by some form of disturbance (e.g. fire, severe windthrow, Water logging etc.) of an existing community. The former case is often referred to as primary succession, the latter as secondary succession.
Process of Succession
The process of succession takes place in a systematic order of sequential steps as follows:
Nudation: It is the development of the bare area without any life form.
Invasion: It is the successful establishment of one or more species on a bare area through dispersal or migration.
Competition and coaction: As the number of individuals grows there is competition, both interspecific (between different species) and intra specific ( within the same species), for space, water and nutrition.They influence each other in a no. of ways, known as coaction.
Reaction: The living organism grow, use water and nutrients from the substratum, in turn, they have a strong influence on the environment which is modified to a large extent and this known as reaction.
Stabilization: The succession ultimately culminates in a more or less stable community called climax which is the equilibrium with the environment.
Ecological Adoption: All living organism adapt themselves against the unfavorable conditions of the atmosphere. Any such feature that the organism develops for coping up with the atmospheric condition is known as adoption. Plants being manufacturer of food are termed as food producers.
Plants have been divided into the following major ecological groups on the basis of their water relation.
Hydrophytes – (hydro- water; Phyta-Plants) Plants of water bodies.
Xerophytes – (Xero – drying up; phytes-plants) Plants of Dry region.
Mesophytes – (Meso–middle, Intermediate; phyton–plants) Plants of moderate atmospheric conditions.
Halophytes – (Halo- salt; phytes-plants) – Plants of Salt region.
Epiphytes – They grow on the plants but unlike parasites they manufacture their own food.
1.5 Benefits of Ecosystem
The benefits that humans derive from ecosystems can be direct or indirect.
Direct benefits are harvested largely from the plants and animals in an ecosystem in the form of food and raw materials. These are the most familiar “products” an ecosystem yields as crops, livestock, fish, game, lumber, fuel wood, and fodder. Genetic resources that flow from the biodiversity of the world’s ecosystems also provide direct benefits by contributing genes for improving the yield and disease resistance of crops, and for developing medicines and other products.
Indirect benefits arise from interactions and feedback among the organisms living in an ecosystem. Many of them take the form of services, like the erosion control and water purification and storage that plants and soil microorganisms provide in a watershed, or the pollination and seed dispersal that many insects, birds, and mammals provide. Other benefits are less tangible, but nonetheless highly valued: the scenic enjoyment of a sunset, for example, or the spiritual significance of a sacred mountain or forest grove. Every year, millions of people make pilgrimages to outdoor holy places, vacation in scenic regions, or simply pause in a park or their gardens to reflect or relax. As the manifestation of nature, ecosystems are the psychological and spiritual backdrop for our lives.
2.1 – Biome – Introduction
Among terrestrial ecosystems, the type that provides the most appropriate scale for understanding world distribution patterns is called a biome, defined as any large, recognizable assemblage of plants and animals in functional interaction with its environment—in other words, a collection of plants and animals over a large area that have broadly similar adaptations and relationships with the environment and climate. A biome is usually identified and named on the basis of its dominant vegetation, which normally constitutes the bulk of the biomass (the total weight of all organisms—plant and animal) in the biome, as well as being the most obvious and conspicuous visible component of the landscape.
2.2 – Classification and Distribution of Biomes
There is no universally recognized classification system of the world’s terrestrial biomes, but scholars commonly accept 10 major types:
1.Tropical rainforest
2.Tropical deciduous forest
3.Tropical scrub
4.Tropical savanna
5.Desert
6.Mediterranean woodland and shrub
7.Midlatitude grassland
8.Mid Latitude deciduous forest
9.Boreal forest
10.Tundra (Arctic and alpine)
Tropical Rainforest
Tropical rainforests occur in a broad zone outside the equator. Annual rainfall, which exceeds 2000 to 2250 millimeters, is generally evenly distributed throughout the year. Temperature and humidity are relatively high through the year. Flora is highly diverse: a square kilometer may contain as many as 100 different tree species as compared to 3 or 4 in the temperate zone. The various trees of the tropical rain forests are closely spaced together and form a thick continuous canopy some 25 to 35 meters. Every so often this canopy is interrupted by the presence of very tall trees (up to 40 meters) that have wide buttressed bases for support. Epiphytic orchids and bromeliads, as well as vines (lianas), are very characteristic of the tropical rainforest biome. Some other common plants include ferns and palms. Most plants are evergreen with large, dark green, leathery leaves.
The tropical rainforest is also home to a great variety of animals. Some scientists believe that 30 to 50% of all of the Earth’s animal species may be found in this biome.
Decomposition is rapid in the tropics because high temperatures and an abundance of moisture. Because of the frequent and heavy rains, tropical soils are subject to extreme chemical weathering and leaching. These environmental conditions also make tropical soils acidic and nutrient poor.
Undergrowth is relatively sparse in the tropical rainforest because the lack of light precludes the survival of most green plants. Only where there are gaps in the canopy, as alongside a river, does light reach the ground, resulting in
the dense undergrowth associated with a jungle. Epiphytes like orchids and bromeliads hang from or perch on tree trunks and branches. Vines and lianas often dangle from the arching limbs.
The interior of the rainforest, then, is a region of heavy shade, high humidity, windless air, continuous warmth, and an aroma of mold and decomposition. As plant litter accumulates on the forest floor, it is acted on very rapidly by plant and animal decomposers. The upper layers of the forest are areas of high productivity, and there is a much greater concentration of nutrients in the vegetation than in the soil. Indeed, most selva soil is surprisingly infertile.
Rainforest fauna is largely arboreal (tree dwelling) because the principal food sources are in the canopy rather than on the ground. Large animals are generally scarce on.
Tropical Deciduous Forest
The locational correlation of this biome with specific climatic types is irregular and fragmented, indicating complex environmental relationships, although many tropical deciduous forests are found in the transition zone between tropical wet (Af) and tropical savanna (Aw) climates. Such regions have high temperatures all year, but generally less—or more seasonal—rainfall patterns than in the tropical rainforest biome. There is structural similarity between tropical rainforest and tropical deciduous forest, but several important differences are usually obvious. In the tropical deciduous forest, the canopy is less dense, the trees are somewhat shorter, and there are fewer layers, all these details being a response to either less total precipitation or less periodic precipitation. As a result of a pronounced dry period that lasts for several weeks or months, many of the trees shed their leaves at the same time, allowing light to penetrate to the forest floor.
This light produces an understory
of lesser plants that often grows in such density as
to produce classic jungle conditions. The diversity of tree
species is not as great in this biome as in tropical rainforest,
but there is a greater variety of shrubs and other lesser
plants.
The faunal assemblage of the tropical deciduous forest
is generally similar to that of the rainforest. Although there
are more ground-level vertebrates than in the selva, arboreal
species such as monkeys, birds, bats, and lizards are
particularly conspicuous in both biomes.
Tropical Scrub
The tropical scrub biome is widespread in drier portions of climate and in some areas of subtropical steppe (BSh) climate . It is dominated by low-growing, scraggly trees and tall bushes, usually with an extensive understory of grasses. The trees range from 3 to 9 meters (10 to 30 feet) in height. Their density is quite variable, with the trees sometimes growing in close proximity to one another but often spaced much more openly. Biodiversity is much less than in the tropical rainforest and tropical deciduous forest biomes; frequently just a few species comprise the bulk of the taller growth over vast areas. In the more tropical and wetter portions of the tropical scrub biome, most of the trees and shrubs are evergreen; elsewhere most species are deciduous. In some areas, a high proportion of the shrubs are thorny.
The fauna of tropical scrub regions is notably different from that of the two biomes previously discussed. There is a moderately rich assemblage of ground-dwelling mammals and reptiles, and of birds and insects.
Tropical Savanna
There is an incomplete correlation between the distribution of the tropical savanna biome (also called the tropical grassland biome) and that of the tropical savanna (Aw) climate. The correlation tends to be most noticeable where seasonal rainfall contrasts are greatest,
a condition particularly associated with the broadscale annual shifting of the intertropical convergence zone (ITCZ).
Savanna lands are dominated by tall grasses . Sometimes the grasses form a complete ground cover, but sometimes there is bare ground among dispersed tufts of grass in what is called a bunchgrass pattern.
The name “savanna” without any modifier usually refers to areas that are virtually without shrubs or trees, but this type of savanna is not the most common. In most cases, a wide scattering of both types dots the grass-covered terrain, and this mixture of plant forms is often referred to as parkland or park savanna.
In much of the savanna, the vegetation has been altered
by human interference with natural processes. A considerable
area of tropical deciduous forest and tropical scrub,
and perhaps some tropical rainforest, has been converted
to savanna over thousands of years through fires set by
humans and through the grazing and browsing of domestic
animals.
The savanna biome has a very pronounced seasonal
rhythm. During the wet season, the grass grows tall, green,
and luxuriant. At the onset of the dry season, the grass
begins to wither, and before long the above-ground portion
is dead and brown. At this time, too, many of the trees and
shrubs shed their leaves. The third “season” is the time of
wildfires. The accumulation of dry grass provides abundant
fuel, and most parts of the savannas experience natural
burning every year or so.
The recurrent grass fires are stimulating
for the ecosystem, as they burn away the unpalatable
portion of the grass without causing significant damage to
shrubs and trees. When the rains of the next wet season
arrive, the grasses spring into growth with renewed vigor.
Savanna fauna varies from continent to continent. The
African savannas are the premier “big game” lands of the
world, with an unmatched richness of large animals, particularly
ungulates and carnivores, but also including a
remarkable diversity of other fauna. The Latin American
savannas, on the other hand, have only a sparse population
of large wildlife, with Asian and Australian areas intermediate
between these two extremes.
Desert
In previous chapters, we noted a general decrease in precipitation
as one moves away from the equator in the low latitudes. This progression is matched by a gradation from the tropical rainforest biome of the equator to the desert
biome of the subtropics.
The desert biome also occurs extensively in midlatitude locations in Eurasia, North America, and South America with a fairly close correlation to subtropical desert (BWh) and midlatitude desert (BWk) Climates Desert vegetation is surprisingly variable.
It consists largely of xerophytic plants such as succulents and other drought-resisting plants with structural modifications that allow them to conserve moisture and drought evading plants capable of hasty reproduction during brief rainy times. The plant cover is usually sparse, with considerable bare ground dotted by a scattering of individual plants. Typically the plants are shrubs, which occur in considerable variety, each with its own mechanisms to combat the stress of limited moisture. Succulents are common in the drier parts of most desert areas, and many desert plants have either tiny leaves or no leaves at all as a moisture conserving strategy. Grasses and other herbaceous plants are widespread but sparse in desert areas. Despite the dryness, trees can be found sporadically in the desert, especially in Australia.
Animal life is inconspicuous in most desert areas, leading to the erroneous idea that animals are nonexistent. Actually, most deserts have a moderately diverse faunal assemblage, although the variety of large mammals is limited. A large proportion of desert animals avoid the principal periods of desiccating heat (daylight in general and the hot season in particular) by resting in burrows or crevices during the day and prowling at night.
Animal life is inconspicuous in most desert areas, leading to the erroneous idea that animals are nonexistent. Actually, most deserts have a moderately diverse faunal assemblage, although the variety of large mammals is limited. A large proportion of desert animals avoid the principal periods of desiccating heat (daylight in general and the hot season in particular) by resting in burrows or crevices during the day and prowling at night.
Generally speaking, life in the desert biome is characterized by an appearance of stillness. In favorable times and in favored places (around water holes and oases), however, there is a great increase in biotic activities, and sometimes the total biomass is of remarkable proportions. Favorable times are at night, and particularly after rains. For example, a heavy rain might trigger the germination of wildflower seeds that had remained dormant for decades.
Mediterranean Woodland and Shrub
As shows, the mediterranean woodland and shrub biome is found in six widely scattered and relatively small areas of the midlatitudes, all of which experience the pronounced dry summer–wet winter precipitation typical of mediterranean (Cs) climates. In this biome, the
dominant vegetation associations are physically very similar to each other but taxonomically quite varied. The biome is dominated mostly by a dense growth of woody shrubs known as a chaparral in North America (Figure 11-33a), but having other names in other areas; chaparral includes many species of sclerophyllous plants, adapted to the prominent summer dry season by the presence of
small, hard leaves that inhibit moisture loss. A second significant plant association of mediterranean regions is an open grassy woodland, in which the ground is almost completely grass covered but has a considerable scattering of trees as well.
The plant species vary from region to region. Oaks of various kinds are by far the most significant genus in the Northern Hemisphere mediterranean lands, sometimes occurring as prominent medium-sized trees but also appearing as a more stunted, shrubby growth. In all areas, the trees and shrubs are primarily broadleaf evergreens.
Their leaves are mostly small and have a leathery texture or waxy coating, which inhibits water loss during the long dry season. Moreover, most plants have deep roots. Summer is a virtually rainless season in mediterranean climates, and so summer fires are relatively common.
Many of the plants are adapted to rapid recovery after a wildfire has swept over the area. Some species have seeds that are released for germination only after the heat of a fire has caused their seedpods to open. Part of the seasonal rhythm of this biome is that winter floods sometimes follow summer fires, as slopes left unprotected by the burning away of grass and lower shrubs are susceptible to abrupt erosive runoff if the winter rains arrive before the vegetation has a chance to resprout. The fauna of this biome is not particularly distinctive. Seed-eating, burrowing rodents are common, as are some bird and reptile groups. There is a general overlap of animals between this biome and adjacent ones.
Sometimes a continuous ground cover is missing, and the grasses grow in discrete tufts as bunchgrass or tussock grass.
Most of the grass species are perennials, lying dormant during the winter and sprouting anew the following summer. Trees are mostly restricted to riparian locations, whereas shrubs and bushes occur sporadically on rocky sites. Grass fires are fairly common in summer, which helps to explain the relative scarcity of shrubs. The woody plants cannot tolerate fires and can generally survive only on dry slopes where there is little grass cover to fuel a fire. Grasslands provide extensive pastures for grazing animals, and before encroachment by humans drastically changed population sizes, the grassland fauna comprised large numbers of relatively few species. The larger herbivores were often migratory prior to human settlement. Many of the smaller animals spend all or part of their lives underground, where they find some protection from heat, cold, and fire.
Mid latitude Deciduous Forest
Extensive areas on all Northern Hemisphere continents, as
well as more limited tracts in the Southern Hemisphere,
were originally covered with a forest of largely broadleaf
deciduous trees. Except in hilly country,
a large proportion of this midlatitude deciduous forest
has been cleared for agriculture and other types of human
use, so that very little of the original natural vegetation
remains.
The forest is characterized by a fairly dense growth of
tall broadleaf trees with interwoven branches that provide
a complete canopy in summer. Some smaller trees and
shrubs exist at lower levels, but for the most part, the forest
floor is relatively barren of undergrowth. In winter, the
appearance of the forest changes dramatically, owing to the
seasonal fall of leaves.
Tree species vary considerably from region to region,
although most are broadleaf and deciduous. The principal
exception is in eastern Australia, where the forest is composed almost entirely of varieties of eucalyptus, which are broadleaf evergreens. Northern Hemisphere regions have a northward gradational mixture with needleleaf evergreen species. An unusual situation in the southeastern United States finds extensive stands of pines (needle leaf evergreens) rather than deciduous species occupying most of the well-drained sites above the valley bottoms. In the Pacific Northwest of the United States, the forest association is primarily evergreen coniferous rather than broadleaf deciduous.
This biome generally has the richest assemblage of fauna to be found in the midlatitudes, although it does not
have the diversity to match that of most tropical biomes.
It has (or had) a considerable variety of birds and mammals, and in some areas reptiles and amphibians are well represented. Summer brings a diverse and active population of insects and other arthropods. All animal life is less numerous (partly due to migrations and hibernation) and less conspicuous in winter.
Boreal Forest
One of the most extensive biomes is the boreal forest, sometimes called taiga after the Russian word for the northern fringe of the boreal forest in that country. The boreal forest occupies a vast expanse of northern North America and Eurasia. There is very close correlation between the location of the boreal forest biome and the subarctic (Dfc) climatic type, with a similar correlation between the locations of the tundra climate and the tundra biome.
This great northern forest contains perhaps the simplest assemblage of plants of any biome.
Most of the trees are conifers, nearly all needleleaf evergreens, with the important exception of the tamarack or larch which drops its needles in winter. The variety of species is limited to mostly pines, firs, and spruces extending broadly in homogeneous stands. In some places, the coniferous cover is interrupted by areas of deciduous trees.
These deciduous stands are also of limited variety (mostly birch, poplar, and aspen) and often represent a seral situation following a forest fire.
The trees grow taller and more densely near the southern
margins of this biome, where the summer growing season
is longer and warmer. Near the northern margins, the
trees are spindly, short, and more openly spaced. Undergrowth is normally not dense beneath the forest canopy, but a layer of deciduous shrubs sometimes grows in profusion.
The ground is usually covered with a complete growth of
mosses and lichens, with some grasses in the south and a
considerable accumulation of decaying needles overall.
Poor drainage is typical in summer, due partially to permanently frozen subsoil, which prevents downward percolation of water, and partially to the derangement of normal surface drainage by the action of glaciers during the recent Pleistocene ice age. Thus, bogs and swamps are numerous, and the ground is generally spongy in summer. During the long winters, of course, all is frozen.
The immensity of the boreal forest gives an impression of biotic productivity, but such is not the case. Harsh climate, floristic homogeneity, and slow plant growth produce only a limited food supply for animals. Faunal species diversity is limited, although the number of individuals of some species is astounding. With relatively few animal species in such a vast biome, populations sometimes fluctuate enormously within the space of only a year or so. Mammals are represented prominently by species that have been traditionally hunted for their fur and by a few species of ungulates. Birds are numerous and fairly diverse in summer, but nearly all migrate to milder latitudes in winter. Insects are totally absent in winter but superabundant during the brief summer.
Tundra
The tundra is essentially a cold desert or grassland in which moisture is scarce and summers so short and cool that trees are unable to survive. This biome is distributed along the northern edge of the Northern Hemisphere continents, correlating closely to the distribution of tundra (ET) climate. The plant cover consists of a considerable mixture of species, many of them in dwarf forms. Included are grasses, mosses, lichens, flowering herbs, and a scattering of low shrubs. These plants often occur in a dense, ground-hugging arrangement, although some places have a more sporadic over with considerable bare ground interspersed. The plants complete their annual cycles hastily during the brief summer, when the ground is often moist and waterlogged because of inadequate surface drainage and particularly inadequate subsurface drainage—often as a consequence of permafrost just below the surface.
Animal life is dominated by birds and insects during
the summer. Extraordinary numbers of birds flock to the
tundra for summer nesting, migrating southward as winter
approaches. Mosquitoes, flies, and other insects proliferate astoundingly during the short warm season, laying eggs that can survive the bitter winter. Other forms
of animal life are scarcer—a few species of mammals and
freshwater fishes but almost no reptiles or amphibians.
Alpine Tundra: An alpine version of tundra is found in
many high-elevation areas. Many mountain areas above the timberline exhibit areas with a sparse cover of vegetation consisting of herbaceous plants, grasses, and low shrubs.
.
Soil
SOIL SOIL – INTRODUCTION
SOIL – FORMING FACTORS
SOIL COMPONENTS
SOIL PROPERTIES PEDOGENIC REGIMES
SOIL CLASSIFICATION
GLOBAL DISTRIBUTION OF MAJOR SOILS
SOIL – INTRODUCTION
Soil is the essential medium in which most terrestrial life is nurtured. Almost all land plants sprout from this precious medium, spread so thinly across the continental surfaces that it has an average worldwide depth of only about 15 centimeters (6 inches).
It is a nearly infinitely varying mixture of weathered mineral particles, decaying.
organic matter, living organisms, gases, and liquid solutions.
Soil is a relatively thin surface layer of mineral matter that normally contains a considerable amount of organic material and is capable of supporting living plants. It occupies that part of the outer skin of Earth that extends from the surface down to the maximum depth to which living organisms penetrate, which means basically the area occupied by plant roots.
SOIL-FORMING FACTORS
Soil is an ever-evolving material. Metaphorically, soil acts like a sponge—taking in inputs and being acted upon by the local environment—changing over time and when the inputs or local environment change. Five principal soil forming factors are responsible for soil development: geology, climate, topography, biology, and time.
The Geologic Factor
The source of the rock fragments that make up soil is parent material, which may be either bedrock or loose sediments transported from elsewhere by water, wind, or ice. Whatever the parent material, it is sooner or later disintegrated and decomposed at and near Earth’s surface, providing the raw material for soil formation.
The Climatic Factor
Temperature and moisture are the climatic variables of greatest significance to soil formation. As a basic generalization, both the chemical and biological processes in soil are usually accelerated by high temperatures and abundant moisture and are slowed by low temperatures and lack of moisture. One predictable result is that soils tend to be deepest in warm, humid regions and shallowest in cold, dry regions.
The Topographic Factor
Slope and drainage are the two main features of topography that influence soil characteristics. Wherever soil develops, its vertical extent undergoes continuous, if usually very slow, change. This change comes about through a lowering of both the bottom and top of the soil layer. The bottom slowly gets deeper as weathering penetrates into the regolith and parent material and as plant roots
extend to greater depths. At the same time, the soil surface is being lowered by sporadic removal of its uppermost layer through normal erosion, which is the removal of individual soil particles by running water, wind, and gravity.
The Biological Factor
From a volume standpoint, soil is about half mineral matter and about half air and water, with only a small fraction of organic matter. However, the organic fraction, consisting of both living and dead plants and animals, is of utmost importance. The biological factor in particular gives life to the soil and makes it more than just “dirt.” Every soil contains a quantity (sometimes an enormous quantity) of living organisms, and every soil incorporates some (sometimes a vast amount of) dead and decaying organic matter.
Earthworms: The cultivating and mixing activities of
earthworms are of great value in improving the structure,
increasing the fertility, lessening the danger of accelerated
erosion, and deepening the profile of the soil. The distinctive
evidence of this value is that the presence of many well nourished
earthworms is almost always a sign of productive,
or potentially productive, soil.
Microorganisms in the Soil: Another important component of the biological factor is microorganisms, both plant and animal, that occur in uncountable billions. An estimated three-quarters of a soil’s metabolic activity is
generated by microorganisms. These microbes help release nutrients from dead organisms for use by live ones by decomposing organic matter and by converting nutrients to forms usable by plants.
The Time Factor
For soil to develop on a newly exposed land surface requires time, with the length of time needed varying according to the nature of the exposed parent material and the characteristics of the environment. Soil-forming processes are generally very slow, and many centuries may be required for a thin layer of soil to form on a newly exposed surface. A warm, moist environment is conducive to soil development. Normally of much greater importance, however, are the attributes of the parent material. For example, soil develops from sediments relatively quickly and from bedrock relatively slowly.
Soil Erosion: Most soil develops with geologic slowness—
so slowly that changes are almost imperceptible within a
human life span. It is possible, however, for a soil to be degraded,
either through the physical removal associated with
accelerated erosion or through depletion of nutrients, in
only a few years.
SOIL COMPONENTS
Soil is made up of a variety of natural components existing together in myriad combinations. All these components can be classified, however, into just a few main groups:
inorganics, organics, air, and water.
Inorganic Materials
As mentioned earlier, the bulk of most soils is mineral matter, mostly in the form of small but macroscopic particles.
Inorganic material also occurs as microscopic clay particles and as dissolved minerals in solution. About half the volume of an average soil is small, granular mineral matter called sand and silt. These particles may consist of a great variety of minerals, depending on the nature of the parent material from which they were derived, and are simply fragments of the wasting rock.
The smallest particles in the soil are clay, which is usually a combination of silica and oxides of aluminum and iron found only in the soil and not in the parent material. Clay has properties significantly different from those of larger (sand and silt) fragments.
Organic Matter
Although organic matter generally constitutes less than 5
percent of total soil volume, it has an enormous influence
on soil characteristics and plays a fundamental role in the
biochemical processes that make soil an effective medium
of plant growth.
Litter: Leaves, twigs, stalks, and other dead plant parts
accumulate at the soil surface, where they are referred to
collectively as litter.
Humus: After most of the residues have been decomposed,
a brown or black, gelatinous, chemically stable organic
matter remains; this is referred to as humus.
Soil Air
Nearly half the volume of an average soil is made up of
pore spaces. These spaces provide a labyrinth
of interconnecting passage ways, called interstices,
among the soil particles. This labyrinth lets air and water
penetrate into the soil. On the average, the pore spaces
are about half filled with air and half with water, but at
any given time and place, the amounts of air and water are quite variable, the quantity of one varying inversely with that of the other.
Soil Water
Water comes into the soil largely by percolation of rainfall
and snowmelt, but some is also added from below
when groundwater is pulled up above the water table by
capillary action. Once water has penetrated the soil,
it envelops in a film of water each solid particle that it
contacts, and it either wholly or partially fills the pore
spaces. Water can be lost from the soil by percolation
down into the groundwater, by upward capillary movement
to the surface followed by evaporation, or by plant
use (transpiration).
SOIL PROPERTIES
As one looks at, feels, smells, tastes, and otherwise examines soils, various physical and chemical characteristics appear useful in describing, differentiating, and classifying them. Some soil properties are easily recognized, but most can be ascertained only by precise measurement.
Color
The most conspicuous property of a soil is usually its color, but color is by no means the most definitive property. Soil color can provide clues about the nature and capabilities of the soil, but the clues are sometimes misleading. Soil scientists recognize 175 gradations of color. The standard colors are generally shades of black, brown, red, yellow, gray, and white. Soil color occasionally reflects the color of the unstained mineral grains, but in most cases, color is imparted by stains on the surface of the particles; these stains are caused by either metallic oxides or organic matter. Black or dark brown usually indicates a considerable humus content; the blacker the soil, the more humus it contains. Color gives a strong hint about fertility, therefore, because humus is an important catalyst in releasing nutrients to plants.
Dark color is not invariably a sign of fertility, however, because it may be due to other factors, such as poor drainage or high carbonate content.
Reddish and yellowish colors generally indicate iron oxide stains on the outside of soil particles. These colors are most common in tropical and subtropical regions, where many minerals are leached away by water moving under the pull of gravity, leaving insoluble iron compounds behind. In such situations, a red color bespeaks good drainage, and a yellowish hue suggests imperfect drainage. Red soils are also common in desert and semi desert environments, where the color is carried over intact from reddish parent materials rather than representing a surface stain.
Gray and bluish colors typically indicate poor drainage, whereas mottling indicates saturated conditions for part of the year.
Texture
All soils are composed of myriad particles of various sizes,
as, although smaller particles usually
predominate. Rolling a sample of soil between the fingers
can provide a feel for the principal particle sizes. The gravel, sand, and silt separates are fragments of the weathered parent material and are mostly the grains of minerals found commonly in rocks, especially quartz, feldspars, and micas. These coarser particles are the inert materials of the soil mass, its skeletal framework. As noted previously, only the clay particles take part in the intricate
chemical activities that occur in the soil.
The texture triangle shows the standard classification scheme for soil texture; this scheme is based on the percentage of each separate by weight. Near the center of the triangle is loam, the name given to a texture in which none of the three principal separates dominates the other two.
This fairly even-textured mix is generally the most productive for plants.
TABLE Standard U.S. Classification of Soil Particle Size Separate Diameter
Gravel Greater than 2 mm (0.08 in.)
Very coarse sand 1–2 mm (0.04–0.08 in.)
Coarse sand 0.5–1 mm (0.02–0.04 in.)
Medium sand 0.25–0.5 mm (0.01–0.02 in.)
Fine sand 0.1–0.25 mm (0.004–0.01 in.)
Very fine sand 0.05–0.1 mm (0.002–0.004 in.)
Coarse silt 0.02–0.05 mm (0.0008–0.002 in.)
Medium silt 0.006–0.02 mm (0.00024–0.0008 in.)
Fine silt 0.002–0.006 mm (0.00008–0.00024 in.)
Clay 0.002 mm (less than 0.00008 in.)
Structure
The individual particles of most soils tend to aggregate into clumps called peds, and it is these clumps that determine soil structure. The size, shape, and stability of peds have a marked influence on how easily water, air, and organisms (including plant roots) move through the soil, and consequently on soil fertility. Peds are classified on the basis of shape as spheroidal, platy, blocky, or prismatic, with these four shapes giving rise to seven generally recognized soil structure types.
Aeration and drainage are usually facilitated by peds of intermediate size; both massive and fine structures tend to
inhibit these processes.
SOIL CHEMISTRY
The effectiveness of soil as a growth medium for plants is based largely on the presence and availability of nutrients, which are determined by an intricate series of chemical reactions. Soil chemistry involves the study of microscopic soil particles and electrically charged atoms or groups of atoms called ions.
Colloids
Soil particles smaller than about 0.1 micrometer in diameter
are called colloids. Inorganic colloids consist of clay
in thin, crystalline, platelike forms created by the chemical
alteration of larger particles; organic colloids represent
decomposed organic matter in the form of humus; and
both types are the chemically active soil particles.
Cation Exchange
As we saw earlier, cations are positively charged ions. Elements that form them include calcium, potassium, and magnesium, which are all essential for soil fertility and plant growth. Colloids carry mostly negative electrical charges on their surfaces, and these charges attract swarms of nutrient cations that would be leached from the soil if their ions were not retained by the colloids.
Acidity/Alkalinity
Chemical solutions—including those in soil—can be characterized
on the basis of acidity or alkalinity. An acid is a chemical compound that produces hydrogen ions (H+) or hydronium ions (H3O+) when dissolved in water, whereas a base is a chemical compound that produces hydroxide ions (OH−) when dissolved in water. An acid reacts with a base to form a salt. Solutions that contain dissolved acids are described as being acidic. Those that contain dissolved bases are called either basic or alkaline solutions. Any chemical solution can be characterized on the basis of its acidity or alkalinity.
Soil Profiles
The development of any soil is expressed in two dimensions:
depth and time. There is no straight-line relationship
between depth and age, however; some soils deepen
and develop much more rapidly than others.
Four processes deepen and age soils: addition (ingredients
added to the soil), loss (ingredients lost from the soil),
translocation (ingredients moved within the soil), and transformation
(ingredients altered within the soil).
The five soil-forming factors discussed earlier—geologic,
climatic, topographic, biological, and time—influence the
rate of these four processes, the result being the development
of various soil horizons and the soil profile.
Soil Horizons
The vertical variation of soil properties is not random but
rather an ordered layering with depth. Soil tends to have
more or less distinctly recognizable layers, called horizons,
each with different characteristics. The horizons are
positioned approximately parallel with the land surface,
one above the other, normally, but not always, separated
by a transition zone rather than a sharp line. A vertical
cross section (as might be seen in a road cut or the side
of a trench dug in a field) from the Earth’s surface down
through the soil layers and into the parent material is
referred to as a soil profile.
Soil Profiles
The development of any soil is expressed in two dimensions:
depth and time. There is no straight-line relationship
between depth and age, however; some soils deepen
and develop much more rapidly than others.
Four processes deepen and age soils: addition (ingredients
added to the soil), loss (ingredients lost from the soil),
translocation (ingredients moved within the soil), and transformation
(ingredients altered within the soil).
The five soil-forming factors discussed earlier—geologic,
climatic, topographic, biological, and time—influence the
rate of these four processes, the result being the development
of various soil horizons and the soil profile.
Figure presents an idealized sketch of a well-developed soil profile, in which six horizons are differentiated:
The O horizon is sometimes the surface layer, and in it organic matter, both fresh and decaying, makes up most of the volume. This horizon results essentially from litter derived from dead plants and animals. It is common in forests and generally absent in grasslands. It is actually
more typical for soils not to possess an O horizon; the surface horizon of most soils is the A horizon.
The A horizon, colloquially referred to as topsoil, is a mineral horizon that also contains considerable organic matter. It is formed either at the surface or immediately below an O horizon. A horizons generally contain enough partially decomposed organic matter
to give the soil a darker color than underlying horizons. They are also normally coarser in texture, having lost some of the finer materials by erosion and eluviation. Seeds germinate mostly in the A horizon.
The E horizon is normally lighter in color than either the overlying A or the underlying B horizon. It is essentially an eluvial layer from which clay, iron, and aluminum have been removed, leaving a concentration of abrasion-resistant sand or silt particles.
The B horizon, usually called subsoil, is a mineral horizon of illuviation where most of the materials removed from above have been deposited. A collecting zone for clay, iron, and aluminum, this horizon is usually of heavier texture, greater density, and relatively greater clay content than the A horizon.
The C horizon is unconsolidated parent material (regolith) beyond the reach of plant roots and most soil-forming processes except weathering. It is lacking in organic matter.
The R horizon is bedrock, with little evidence of weathering.
Principal Pedogenic Processes
A large number of processes are responsible for the formation of soils. This fact is evident by the large number of different types of soils that have been classified by soil scientists. However, at the macro-scale we can suggest that there are five main principal pedogenic processes acting on soils. These processes are laterization, podzolization, calcification, salinization, and gleization.
Laterization
Laterization is a pedogenic process common to soils found in tropical and subtropical environments. High temperatures and heavy precipitation result in the rapid weathering of rocks and minerals. Movements of large amounts of water through the soil cause eluviation and leaching to occur. Almost all of the by products of weathering, very simple small compounds or nutrient ions, are translocated out of the soil profile by leaching if not taken up by plants for nutrition. The two exceptions to this process are iron and aluminum compounds. Iron oxides give tropical soils their unique reddish coloring. Heavy leaching also causes these soils to have an acidic pH because of the net loss of base cations.
Podzolization
Podzolization is associated with humid cold mid-latitude climates and coniferous vegetation. Decomposition of coniferous litter and heavy summer precipitation create a soil solution that is strongly acidic. This acidic soil solution enhances the processes of eluviation and leaching causing the removal of soluble base cations and aluminum and iron compounds from the A horizon. This process creates a sub-layer in the A horizon that is white to gray in color and composed of silica sand.
Soil Calcification
Calcification occurs when evapotranspiration exceeds precipitation causing the upward movement of dissolved alkaline salts from the groundwater. At the same time, the movement of rain water causes a downward movement of the salts. The net result is the deposition of the translocated cations in the B horizon. In some cases, these deposits can form a hard layer called caliche. The most common substance involved in this process is calcium carbonate. Calcification is common in the prairie grasslands.
Salinization is a process that functions in the similar way to calcification. It differs from calcification in that the salt deposits occur at or very near the soil surface. Salinization also takes place in much drier climates.
Gleization is a pedogenic process associated with poor drainage. This process involves the accumulations of organic matter in the upper layers of the soil. In lower horizons, mineral layers are stained blue-gray because of the chemical reduction of iron.
SOIL CLASSIFICATION
Some of the most significant products of scholarly studies
are classification systems. If phenomena can be classified
meaningfully, it becomes easier to remember them
and to understand the relationships among them. Our
consideration thus far has included various classifications
(for example, climate and biomes). In no other
subdiscipline of physical geography, however, is the matter
of classification more complicated than with soil.
The Soil Taxonomy
Over the past century, various soil classifications have been devised in the United States and other countries. As the knowledge of soil characteristics and processes has become greater, so have the efforts at soil classification become more sophisticated. Several different systems have been developed in other countries, particularly in Canada, the United Kingdom, Russia, France, and Australia. Moreover, United Nations agencies have their own classification schemes. The system that is presently in use in the United States is called simply Soil Taxonomy.
TABLE Name Derivations of Soil Orders
Order Derivation
Alfisols “al” for aluminum, “f” for iron (chemical symbol Fe), two prominent elements in these soils
Andisols andesite, rock formed from type of magma in Andes Mountains volcanoes; soils high in volcanic ash
Aridisols Latin aridus, “dry”; dry soils Entisols last three letters in “recent”; these are recently formed soils
Gelisols Latin gelatio, “freezing”; soils in areas of permafrost
Histosols Greek histos, “living tissue”; these soils contain mostly organic matter
Inceptisols Latin inceptum, “beginning”; young soils at the beginning of their “life”
Mollisols Latin mollis, “soft”; soft soils
Oxisols soils with large amounts of oxygencontaining compounds
Spodosols Greek spodos, “wood ash”; ashy soils
Ultisols Latin ultimus, “last”; soils that have had the last of their nutrient bases leached out
Vertisols Latin verto, “turn”; soils in which material from O and A horizons falls through surface cracks and ends up below deeper horizons; the usual horizon order is inverted
Soil Order
At the highest level of the Soil Taxonomy
is soil order, of which only 12 are recognized worldwide.
The soil orders, and many of the lower-level
categories as well, are distinguished from one another
largely on the basis of certain diagnostic properties, which
are often expressed in combination to form diagnostic horizons.
Soil orders are subdivided into suborders, of which
about 50 are recognized in the United States. The third
level consists of great groups, which number about
250 in the United States. Successively lower levels in
the classification are subgroups, families, and series.
About 19,000 soil series have been identified in the
United States to date, and the list will undoubtedly
be expanded in the future. For the purpose of comprehending
general world distribution patterns, however,
we need to concern ourselves only with orders principal type
of soil in each region and is useful in portraying the general
distribution of the major soil types.
GLOBAL DISTRIBUTION OF MAJOR SOILS
There are 12 orders of soils, which are distinguished largely on the basis of properties that reflect a major course of development, with considerable emphasis on the presence or absence of notable diagnostic horizons.
Entisols (Very Little Profile Development)
The least well developed of all soils, Entisols have
experienced little mineral alteration and are virtually
without pedogenic horizons. Their undeveloped state is
usually a function of time (the very name of the order
connotes recency); most Entisols are surface deposits
that have not been in place long enough for pedogenetic
processes to have had much effect. Some, however, are
very old, and in these soils the lack of horizon development
is due to a mineral content that does not alter readily,
to a very cold climate, or to some other factor totally
unrelated to time.
The distribution of Entisols is therefore very widespread
and cannot be specifically correlated with particular
moisture or temperature conditions or with certain
types of vegetation or parent materials
Inceptisols (Few Diagnostic Features)
Another immature order of soils is the Inceptisols. Their
distinctive characteristics are relatively faint, not yet prominent
enough to produce diagnostic horizons.
If the Entisols can be called “youthful,” the Inceptisols might
be classified “adolescent.” They are primarily eluvial soils
and lack illuvial layers.
Like Entisols, Inceptisols are widespread over the world
in various environments. Also like Entisols, they include
a variety of fairly dissimilar soils whose common characteristic
is lack of maturity. They are most common in
tundra and mountain areas but are also notable in older
valley floodplains. Their world distribution pattern is very
irregular. This is also true in the United States, where they
are most typical of the Appalachian Mountains, the Pacific
Northwest, and the lower Mississippi Valley.
Andisols (Volcanic Ash Soils)
Having developed from volcanic ash, Andisols have been
deposited in relatively recent geological time. They are not
highly weathered, therefore, and there has been little downward translocation of their colloids. There is minimum profile development, and the upper layers are dark.
Their inherent fertility is relatively high.
Andisols are found primarily in volcanic regions of
Japan, Indonesia, and South America, as well as in the very
productive wheat lands of Washington, Oregon, and Idaho.
Gelisols (Cold Soils with Permafrost)
Gelisols are young soils with minimal profile development.
They develop only slowly because of cold
temperatures and frozen conditions. These soils typically
have a permafrost layer that is a defining characteristic. Also
commonly found in Gelisols is cryoturbation or frost churning,
which is the physical disruption and displacement of soil
material by freeze–thaw action in the soil. Most of the soilforming
processes in Gelisols take place above the permafrost
in the active layer that thaws every year or so.
Gelisols are the dominant soils of Arctic and subarctic
regions. They occur in association with boreal forest and
tundra vegetation; thus, they are primarily found in Russia,
Canada, and Alaska and are prominent in the Himalaya
Mountain country of central Asia. Altogether nearly
9 percent of Earth’s land area has a Gelisol soil cover.
Histosols (Organic Soils on Very Wet Sites)
Least important among the soil orders are the Histosols,
which occupy only a small fraction of Earth’s land surface,
a much smaller area than any other order. These are
organic rather than mineral soils, and they are invariably
saturated with water all or most of the time. They may
occur in any waterlogged environment but are most characteristic in mid- and high-latitude regions that experienced Pleistocene glaciation. Some Histosols are composed largely of undecayed or only partly decayed plant material, whereas others consist of a thoroughly decomposed mass of muck. The lack of oxygen in the waterlogged soil slows down the rate of bacterial action, and the soil becomes deeper mostly by growing upward, that is, by more organic material being added from above.
Histosols are usually black, acidic, and fertile only for
water-tolerant plants. If drained, they can be very productive
agriculturally for a short while. Before long, however, they are likely to dry out, shrink, and oxidize, a series of steps that leads to compaction, susceptibility to wind erosion.
Aridisols (Soils of Dry Climates)
Nearly one-eighth of Earth’s land surface is covered with
Aridisols, one of the most extensive spreads of any soil
order. They are preeminently soils of thedry lands, occupying environments that do not have enough water to remove soluble minerals from the soil.
Thus, their distribution pattern is largely correlated
with that of desert and semidesert climates.
Aridisols are typified by a thin profile that is sandy and
lacking in organic matter, characteristics clearly associated
with a dry climate and a scarcity of penetrating moisture.
The epipedon is almost invariably light in color. There are
various kinds of diagnostic subsurface horizons, nearly all
distinctly alkaline. Most Aridisols are unproductive, particularly
because of lack of moisture; if irrigated, however,
some display remarkable fertility. The threat of salt accumulation is nonetheless ever present.
Vertisols (Swelling and Cracking Clays)
Vertisols contain a large quantity of clay that becomes a dominant factor in the soil’s development. The clay of Vertisols is described as “swelling” or “cracking” clay. This clay-type soil has an exceptional capacity for absorbing water: when moistened, it swells and expands; as it dries, deep, wide cracks form, sometimes 2.5 centimeters (an
inch) wide and as much as 1 meter deep. Some surface material falls into the cracks, and more is washed in when it rains. When the soil is wetted again, more swelling takes place and the cracks close. This alternation of wetting and drying and expansion and contraction produces a churning effect that mixes the soil constituents (the name Vertisol connotes an inverted condition), inhibits the development of horizons, and may even cause minor irregularities in the land surface. An alternating wet and dry climate is needed for Vertisol formation because the sequence of swelling and contraction is necessary. Thus, the wet–dry climate of tropical and subtropical savannas is ideal, but there must also be the proper parent material to yield the clay minerals. Consequently, Vertisols are widespread in distribution but are very limited in extent. The principal occurrences are in eastern Australia, India, and a small part of East Africa.
Mollisols (Dark, Soft Soils of Grasslands)
The distinctive characteristic of Mollisols is the presence of a mollic epipedon, which is a mineral surface horizon that is dark and thick, contains abundant humus and basic cations, and retains a soft character (rather than becoming hard and crusty) when it dries out.
Mollisols can be thought of as transition soils that evolve in regions not dominated by either humid or arid conditions. They are typical of the midlatitude grasslands and are thus most common in central Eurasia, the North American Great Plains, and the pampas of Argentina. The grassland environment generally maintains a rich clay–humus content in a Mollisol soil. The dense, fibrous mass of grass roots permeates uniformly through the epipedon and to a lesser extent into the subsurface layers. There is almost continuous decay of plant parts to produce a nutrient-rich humus for the living grass. Mollisols on the whole are probably the most productive soil order. They are generally derived from loose parent material rather than from bedrock and tend to have favorable structure and texture for cultivation. Because they are not overly leached, nutrients are generally retained within
reach of plant roots. Moreover, Mollisols provide a favored habitat for earthworms, which contribute to softening and mixing of the soil.
Alfisols (Clay-Rich B Horizons, High Base Status)
The most wide ranging of the mature soils, Alfisols occur
extensively in low and middle latitudes, as shows. They are found in a variety of temperature and moisture conditions and under diverse vegetation associations. By and large, they tend to be associated with transitional environments and are less characteristic of regions that are particularly hot or cold or wet or dry. Their global distribution is extremely varied. They are also widespread in the United States, with particular concentrations in the Midwest.
Alfisols are distinguished by a subsurface clay horizon
and a medium to generous supply of basic cations, plant
nutrients, and water. The epipedon is ochric (light-colored),
, but beyond that, it has no characteristics that are particularly diagnostic and can be considered an ordinary eluviated horizon. The relatively moderate conditions under which Alfisols develop tend to produce balanced soils that are reasonably fertile.
Ultisols (Clay-Rich B Horizons, Low Base Status)
Ultisols are roughly similar to Alfisols except that Ultisols are more thoroughly weathered and more completely leached of nutrient bases. They have experienced greater mineral alteration than any other soil in the midlatitudes, although they also occur in the low latitudes. Many pedologists believe that the ultimate fate of Alfisols is to degenerate into Ultisols.
Typically, Ultisols are reddish as a result of the significant proportion of iron and aluminum in the A horizon. They usually have a fairly distinct layer of subsurface clay accumulation. The principal properties of Ultisols
have been imparted by a great deal of weathering and leaching. Indeed, the connotation of the name (derived from the Latin ultimos) is that these soils represent the ultimate stage of weathering. The result is a fairly deep soil that is acidic, lacks humus, and has a
relatively low fertility due to the lack of bases.
Ultisols have a fairly simple world distribution pattern. They are mostly confined to humid subtropical climates and to some relatively youthful tropical land surfaces.
Spodosols (Soils of Cool, Forested Zones)
The key diagnostic feature of a Spodosol is a spodic
subsurface horizon, an illuvial dark or reddish layer
where organic matter, iron, and aluminum accumulate.
The upper layers are light-colored and heavily leached
. At the top of the profile is usually an
O horizon of organic litter. Such a soil is a typical result of
podzolization.
Spodosols are notoriously infertile. They have been
leached of useful nutrients and are acidic throughout.
They do not retain moisture well and are lacking in humus
and often in clay. Spodosols are most widespread in areas
of coniferous forest where there is a subarctic climate. Alfisols,
Histosols, and Inceptisols also occupy these regions,
however, and Spodosols are sometimes found in other
environments, such as poorly drained portions of Florida.
Oxisols (Highly Weathered and Leached)
The most thoroughly weathered and leached of all soils are the Oxisols, which invariably display a high degree of mineral alteration and profile development. They occur mostly on ancient landscapes in the humid tropics, particularly in Brazil and equatorial Africa, and to a lesser extent in Southeast Asia. The distribution pattern is often spotty, with Oxisols mixed with less developed Entisols, Vertisols, and Ultisols. Oxisols’ are totally absent
from the United States, except for Hawai‘i, where they are common.
Oxisols are essentially the products of laterization (and in fact were called Latosols in older classification systems).
They have evolved in warm, moist climates, although some are now found in drier regions, an indication of climatic change since the soils developed. The diagnostic horizon for Oxisols is a subsurface dominated by oxides
of iron and aluminum and with a minimal supply of nutrient bases (this is called an oxic horizon). These are deep soils but not inherently fertile. The natural vegetation is efficient in cycling the limited nutrient supply, but if the flora is cleared (to attempt agriculture, for example), the nutrients are rapidly leached out, and the soil becomes impoverished.
.