Planet Earth/7h. Soil: Living Dirt

Living Dirt

Earth is the only planet with soil. As truly defined, soil is the ecological biome that exists in the shallow subsurface on Earth and includes both living and dead organisms, as well as the natural mineral and rock resources that sustain the living biota within and above the shallow terrestrial subsurface of Earth. Soils are important for the terrestrial organisms that rely on the nutrients, moisture and substrate that provide attachment for plants, fungus, and bacteria through their extensive roots and distribution through this narrow horizon below Earth’s surface. Eroded rocks and minerals, lacking significant organic contain, like that found on other planets is called regolith. Regolith is the covering of unconsolidated dust, broken rocks, and various minerals that cover the deeper bedrock of a planet, and is found on Mars and the Moon. Soil on the other hand, which is unique to Earth, contains abundant organic material, both living and dead, such as decaying leaves, organic compost, bacteria, invertebrates, such as annelid worms, complex roots, fungal molds and burrows of animals. Hence, soil is part of Earth’s living biome.

Typical Soil Horizons

Typical Soil Profile

All soils exhibit a set of horizons which are identified by combination of leaching due to the precipitation of rain or snow melt moving into the soil subsurface and carrying dissolved ions deeper, while also the continued input of organic dead vegetation (leaves and other plant litter) and transported sediments onto the surface. Through these natural processes, soils develop a set of characteristic horizons. These soil horizons were codified by Russian soil scientist Vasily Dokuchaev. In the 1890s, Vaily Dokuchaev was commissioned to study soils around the Russian city of Nizhniy Novgorod. The city is located at the junction of the Oka River and Volga River, within the agricultural center of the Volga drainage basin that provides even today much of the world’s wheat. Soils and their study was important to continue the projection of crops in the region, and Vaily Dokuchaev codified soil horizons into a set of four major horizons.

O Horizon- The O horizon is the organic rich horizon at the top at the atmosphere-geosphere interface, between the gas filled part of Earth and the solid interior. It is called the O horizon because of the abundance of organic matter. Most of this organic matter is living and dead plants, but also includes living and dead bacterial, fungal and animal matter. The O horizon varies in thickness, sometimes its absence or paucity in certain biomes like desert and tundra environments. In other environments the O Horizon can be thick, especially when there is a supply of organic matter such as temperate deciduous biomes, where trees lose their leaves in the winters, and temperate grasslands, which go dormant during the winter months, adding organic matter to the surface of the soil, which gets buried over time. This horizon is nutrient-rich, and tends to be strongly humified with moisture retention.

A Horizon- The A horizon is a mixed horizon, with the cumulation of organic matter from above and the process of weathering, with the input of wind-blown dust that produces an organic and clay-rich zone. The A horizon is subjected to bioturbation, the process of burrowing and root growth that results in mixing or altering the subsurface by the acts of biological activity. This often includes burrowing, ingestion, and defecation within the sediment by subterranean animals, as well as the growth and movement of the soil to accommodate roots and tissue growth in plants. The A horizon is best identified for its dark brown or gray color, due to the rich organic content of carbon-based molecules, which are dark brown to black in color.

A trench to study the soil profile in Hilo Hawaii.

B Horizon- The B horizon is characterized by a red color, which stems from the process of oxidation, particularly of iron bearing minerals. As water moves through the soil, it transports dissolved ions into the deeper horizons. The B horizon is where iron minerals will oxidize with the oxygen-rich water, resulting in the deposition of red-colored rust, including minerals of hematite and limonite. Some of this oxidation might be mediated by bacteria. The B horizon is also affected by bioturbation, with roots of some plants extending into this layer of the soil, although less so than the above A horizon. The unique red color makes identification of the B horizon fairly easy in a dug hole, as it often stands out. Interestingly the B horizon tends to preserve subsequent soil deposition, as paleosol (fossilized soils) tend to lack the A horizon, but are often stacked B and C horizons, alternating between red colored sediments (B-Horizon) and the lower white colored sediments (C-Horizon).

C Horizon- The C horizon is characterized by its white color. This white band within the lower most part of the soil profile, contains deposits of calcium carbonate (CaCO₃), a white mineral. Calcium carbonate is a white chalky mineral that is formed from ions of Ca⁺² and the carbonate ion (CO₃^-2), which results from the dissolution of carbon dioxide (often coming from decomposition of organic matter in the higher A horizon) within the water. In wet environments C horizon is poorly formed as the water does not dry out to leave behind the white deposits of calcium carbonate, however in dry deserts, an abundance of calcium carbonate is precipitated out as the water evaporates in the subsurface, resulting in thick deposits called caliche. Caliche is white mineral deposits that acts as a cement or glue that binds other materials such as gravel, sand, clay, and silt together in a concrete looking hard matrix. It is common in arid environments like eastern Utah, where stones and other rocks near the surface can be coated in a white rind of calcium carbonate. The C horizon is the lowest of the soil horizons.

Bedrock – Below the C horizon is the bed rock, or the parent rock layers. These layers are weathered through the process of water transport through the soil horizons (chemical weather), as well as the action of biological activity to split or break these rocks in the subsurface. The amount of uplift and subsidence in an area can determine the thickness of each of the soil horizons as well. An area of active uplift and erosion will have shallow soil horizons, while areas of active subsidence and deposition will have thicker and often multiple bands of various soil horizons, as older soil horizons get buried by newer soils.

Other soil horizons – There are other horizons, as well as ways to subcategorize the O, A, B and C horizons, but nearly all soils on Earth will have some combination of these four soil horizons.

Types of Soils

Soils can be characterized by five major influences, 1) climate (such as the amount of rain), 2) the terrain or topography (including water drainage), 3) the biota, the plants and animals that live within and on the soil, and 4) the rocks and minerals in the bedrock, and sediments transported into the area. The final 5th major influence is the amount of time between ground disturbances, or soil maturity.

The United States Department of Agriculture defines twelve major soil types resulting from the five major influences, each ending with the suffix -sol.

Soils in wet climates

Histosol – organic soils found in poorly drained wet marshes and swamps, sometimes called bog soils, these soils are characterized by wet dark black soils, in a reducing oxygen environment that preserves thick deposits of carbon rich molecules in the subsurface.

Vertisol – clay-rich wet dark soils that dry out periodically leaving deep forming cracks and slip surfaces due to the swelling of the clay material and form vertical slip marks within the mud. These soils are typical found near lakes, ponds and wet basin centers that are frequently flooded, such along flood plains. They are organic rich, with abundant bioturbation.

Ultisol – soils found in the humid hot tropics and subtropics, heavily leached of ions of calcium, magnesium and potassium due to intense rain, but exist in well drained terrain. They are heavily weathered by transitory nature of the rain water moving through these soils. They are highly weathered, forming thick B horizons, and the soil is a dark reddish color.

Oxisol – are heavily weathered reddish soils found in tropical rainforests. They are enriched in red iron oxides and weathered white clays like kaolinite. Like ultisols, they are heavily leached of many other ions, due to the frequency of rain and warm temperatures of their tropical climate. They tend to be organic poor soils, with thin O and A horizons. The reddish clay found in these soils is called laterite and is mined as a source of aluminum in places like India, which is subjected to seasonal monsoonal rains.

• 
  Histosol
• 
  Vertisol
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  Ultisol
• 
  Oxisol

Soils in dry and cold climates

Aridisol – are dry soils found in desert regions which have limited rain fall. They have little organic matter, and form weak O and A horizons. They are characterized by extensive C horizons, with thick deposits of calcium carbonate including deposits of white caliche in some regions. The thickness of the red B horizon is related to the local climatic history of the area, and can be fairly thick. Soils in cold climates

Gelisol – are permafrost soils formed within permafrost and found within frozen regions of Earth near the poles, where ice plays a major part in the formation of these soils. The movement of sediments is a result of the freeze/thaw cycle of weathering, and the limited amount of organic matter and rainfall.

Spodosol – are found across boreal forests in the northern hemisphere. They are fairly acidic soils with iron and aluminum leached layers that form a white band below or within the A horizon. This unique horizon is called the E horizon (Eluvium) which is caused by the leaching of minerals within this zone, leaving behind a layer of white clay. This is a result of the amount of precipitation, from rain and snow, exceeding the amount of evaporation, leaving a white band in the upper part of the soil profile. These soils are typical restricted to coniferous and deciduous forests in cold climates.

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  Aridisol
• 
  Gelisol
• 
  Spodosol

Soils in disturbed environments

Inceptisol – are young, immature soils, with little formation of soil profile horizons in the subsurface, with little amounts of leaching or weathering. They tend to contain a fair amount of rock fragments, and formed in steep terrains with close proximity to the undelaying parent bedrock or in areas with frequent ground disturbance.

Entisol – are sandy soils, that lack distinct soil horizons, which often form from windblown sediments, and little organic input. They are often uncharacteristic of scrub lands, and difficult to classify, given their uniform profile. Found in beach sediments, lake short sediments, or sand dunes, the input of sediments outpaces the input of organic matter, to produce a more uniform color. They are one of the most common type of soils on Earth.

Andisol – are soils formed on volcanic ash. These soils are fertile, as they have yet to leach many nutrients, and often will generate significant plant growth over time. Depending on the volcanic history, these soils will produce a banding of organic rich dark lays and red layers, with frequent bands of volcanic ash interspersed within the soil profile.

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  Inceptisol
• 
  Entisol
• 
  Andisol

Soils due to the living biota

Mollisol – are dark black organic rich soils found in grassland and some hardwood forests that are highly sought after because of their importance in agriculture. These soils have thick A horizon (top soil), which are rich in organic matter, and other nutrients. Mollisol soils form on grasslands where organic matter builds up from the annual death and re-growth of grasses, adding new organic material to the thick A horizon. These important soils account for only 7% of the world’s soils.

Alfisol – are soils that frequently form under deciduous hardwood forests, within humid semi-wet regions. They exhibit significant accumulation of clay minerals, with little leaching of aluminum and iron, leaving a fairly rich soil, found in many temperate forests and agricultural lands. They are not as organically rich as mollisol.

• 
  Mollisol
• 
  Alfisol

Soil Maps

Global map of soil regions using the above classification.

Surveys of soils are frequently carried out by governments to better understand the availability of lands to different types of crops. The type of soil often is an important determining factor in what will grow in the soil. In the United States geologists survey lands as part of the Soil Conservation Service, currently named the Natural Resources Conservation Service to map out the various soil types in a region of interests. Soils change with land use, climate conditions and the amount of disturbance on the land. Soils increase their organic content over time, but much of this organic content (O and A horizons) is lost due to continuous farming, soil erosion and changes in climate. The increase use of fertilizers is a consequence of the loss of these soil nutrients, which add nitrogen, phosphate and potassium (potash) to the soil. These are required for plants to grow, which normally gain them through the organic decaying matter of dark organic rich soils. When these materials are absence or eroded, then artificial nitrogen, phosphate and potassium (potash) is added to the soil to increase crop yields. This results in high levels of chemicals of nitrogen, phosphate and potassium, with low organic matter entering the soil, with each crop harvest. Overtime the soils become nutrient poor as little organic matter is inputted into the soils, which are turned over with each plowing of the field. Mapping soils become a major way to track changes in the soil profiles over time, and to access the health of the landscape toward agricultural practices.

Palmer Soil Moisture Index

An example of the Palmer Drought Index from 2013, showing drought in California.

Palmer drought index was developed in by Wayne Palmer in the 1960s to access the changing soil moisture levels using weather data to predict the amount of soil moisture in a region in a given month or week. Soil moisture is of vital importance to overall health of crops and for industrial agriculture. These indexes are able to recognize regions with abnormal drought-like conditions, and access the severity of those droughts. Palmer index maps of soil moisture factor in the duration of the dry conditions and the amount of moisture that is retained in the soil for plants to use. As a short term assessment they can be used to determine when and where droughts occur, and the severity of the drought compared to previous years.

Soil Erosion

Soil erosion

Most arable land on the surface of the Earth is disturbed by the rotation of crops, from planting to harvesting, tilling to seeding. Each year a farm is utilized this way, there is a risk of soil erosion, a result of exposed dirt and soil transported due to exposure to rain and wind. This erosion of the soil is particularly dangerous to the agricultural industry, because it rids the soil of the light organic rich matter that provides nutrients to plants and crops grown on the land. Alternating or laying a field fallow, as well as crop rotation can help maintain the soil, and re-generate new organic matter, but often this costs money in having a field not produce crops during a growing season. These trade-offs between planting or not planting can result in more barren crop lands, or longer termed returns on crops. Understanding the over-all health of the land and its soils is particularly challenging given the economic incentives to plant and harvest each year. With the risk of droughts and other weather changes, these farming can be a risky venture, even in the best of times. Protection of soil erosion is enormously important to maintain the yields of crops available for the long term, and help keep food prices from rising. Soils are a renewable resource, but it takes time for soils to reach organic levels necessary to produce high yields, often this is circumvented with the use of fertilizers. With the lack of organic matter, the soils are more prone to drought, due to increased evaporation, as well as erosion from wind and water.

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g. Earth’s Biomes and Communities.	h. Soil: Living Dirt.	i. Earth’s Ecology: Food Webs and Populations.