Types

What is Soil

Biome Processes Soil Science Europe

Taxonomy

Soil Orders

Fossil Soils

Paleopedological record

Mollisols

Horizons

During the next half billion, we get from nothing to complex soil profiles like the one on the right. Here you can see layers of ‘horizons’ each with their own characteristics, recognisable around the world.

The depth of soils vary enormously but have similar horizons. On the surface is the main organic matter (O horizon), that mixes with sand and silt as it goes down. Deeper down in the subsoil, soil from above has compacted but not enough to prevent roots and other growth. Below that there are usually rocks and bits of organic matter before you hit the bedrock. There are many different versions of this arrangement, which have been classified into various types, with details here.

These structures did not just happen but are the result of countless creatures and organisms in countless countries chewin’ and pooin’ to make the world go round. So how when and where do these soil profiles emerge – similar the world over?

However, all those horizons did not just appear overnight. How did they come from nothing to these deep horizons? As these structures were built, new soil functions - over two dozen, developed. That is the task we are embarking upon now, to find out what emerged first, when and how.

Horizons in various periods

300-250mya (Permian)
200-150mya (Jurassic)

Taxonomy

Soils are classified in a similar way to plants and animals. Soil taxonomy is an arrangement in a systematic manner; with six levels of classification. They are, from most general to specific: Order, suborder, great group, subgroup, family and series. Soil properties that can be measured quantitatively are used in this classification system. They include: depth, moisture, temperature, texture, structure, cation exchange capacity, base saturation, clay mineralogy, organic matter content and salt content.

There are 12 soil Orders (the top hierarchical level) in soil taxonomy according to US. Europe has different system. The names of the orders end with the suffix -sol. The criteria for the different soil orders include properties that reflect major differences in the genesis of soils.

The big question for us, is whether we can identify the first time each of these soils Orders appeared during the last half billion years. This may help us piece together soil evolution. eg we can identify histosols 400-360mya and Entisols, Incerptisols and vertisols during the Carboniferous 360-300mya.

It will be challenging, but let's look at what fossil soils may be able to tell us.

Soil Orders

in sequence of increasing degree of development are:

Entisols

Recently formed soils that lack well-developed horizons . Commonly found on unconsolidated river and beach sediments of sand and clay or volcanic ash, some have an A horizon on top of bedrock. They are 18% of soils worldwide.

1st appeared 450mya

Andisol

Volcanic ash soils. They are young soils. They cover 1% of the world's ice-free surface.

Mollisols

Soft, deep, dark soil formed in grasslands and some hardwood forests with very thick A horizons. They are 7% of soils worldwide.

Spodosols

Acid soils with organic colloid layer complexed with iron and aluminium leached from a layer above. They are typical soils of coniferous nd deciduous forests in cooler climates. They constitute 4% of soils worldwide.

1st appeared 350-300mya Early Soils

Ultisols

Acid soils in the humid tropics and subtropics, which are depleted in calcium, magnesium, and potassium (important plant nutrients). They are highly weathered, but not as weathered as Oxisols. They make up 8% of the soil worldwide.

Ist appeared = 400-360mya

Inceptisols

Young soils with subsurface horizon formation but show little eluviation and illuviation.
Eluviation is the removal of nutrients and organic material from superficial horizons, while illuviation is the deposition of these in deeper horizons, They constitute 15% of soils worldwide.
1st appeared 450mya

Aridisols

Dry soils forming under desert conditions which have fewer than 90 consecutive days of moisture during the growing season and are non-leached. They include nearly 12% of soils on Earth. They may have subsurface zones of caliche or duripan. Many aridisols have well-developed Bt horizons showing clay movement from past periods of greater moisture.

Alfisols

Soils with aluminium and iron. They have horizons of clay accumulation, and form where there is enough moisture and warmth for at least three months of plant growth. They constitute 10% of soils worldwide.

1st appeared 400-360mya

Oxisols

Heavily weathered and rich in iron and aluminum oxides (sesquioxides) or kaolin but low in silica. They have only trace nutrients due to heavy tropical rainfall and high temperatures and low CEC of the remaining clays. They are 8% of soils worldwide.
1st appeared 250mya

Histosol

Histosols

Histosol organic soils, formerly called bog soils, are 1% of soils worldwide.

Ist appeared = 400-360mya

Vertisols

inverted soils. They are clay-rich and tend to swell when wet and shrink upon drying, often forming deep cracks into which surface layers can fall. They are difficult to farm or to construct roads and buildings due to their high expansion rate. They constitute 2% of soils worldwide.

1st appeared 350-300mya Early Soils

Vertisol

Gelisol

permafrost soils have permafrost within two metres of the surface or gelic materials and permafrost within one metre. They constitute 9% of soils worldwide.

Fossil Soils

Present-day classifications are not that useful when trying to workout what fossil soils represent. This is because many techniques used to classify present day soils just would not have been preserved - like moisture content and bulk density. Fossil soils or Paleosols can be classified (Mack, James and Monger, 1993) based on the evaluation of the relative prominence in a paleosol of six pedogenic features or processes: organic matter content, horizonation, redox conditions, in situ mineral alteration, illuviation of insoluble minerals/compounds, and accumulation of soluble minerals.

Paleopedological record

We are going to use what is now called the paleopedological record, the fossil record of soils (Retallack, 2001). The paleopedological record consists chiefly of paleosols buried by flood sediments, or preserved at geological unconformities, especially plateau escarpments or sides of river valleys. Other fossil soils occur in areas where volcanic activity has covered the ancient soils.

An important difference between the paleopedological record and the fossil record of plants and animals is that very few of the soils found are extinct types. Despite the difficulties of identification mentioned earlier, this makes paleopedology (the study of fossil soils) potentially very useful to understanding the ecological relationships in past ecosystems - not to mention soil evolution. Extracted from ''Soils of the Past' An Introduction to Paleopedology G J Retallack

450 mya

In Early Silurian (445-420mya) Entisols and Inceptisols, (which we now call 'young'soils) developed with the growth of land vegetation under a protective ozone layer.

Entisol

Inceptisol

400mya

Several new soil orders emerged.

The first, histosols, emerged in the Devonian (420-360mya) but are rare as fossils because most of their mass consists of organic materials that tend to decay quickly.

Histosol

Alfisols and ultisols emerged in the late Devonian (400-360mya) and early Carboniferous (360-320mya), and have a continuous, though not rich, fossil record in eras since then.

Alfisol

Ultisol

350 mya

Spodosols are known only from the Carboniferous (360-300mya) and from a few periods since that time - though less acidic soils otherwise similar to spodosols are known from the Mesozoic (250-66mya) and Tertiary (66-2.6mya) and may constitute an extinct suborder.

Spodosol

250 mya

During the Mesozoic (250-66) records poor. Most surface soils were very old. Oxisols and orthents (entisols that lack horizon development) are the dominant groups

Oxisol

66mya

Paleocene (66-56mya) abundant oxisols and ultisols in.

Eocene (56-44mya) and Miocene, (23-5mya) as grasslands evolved.

Mollisols, the major agricultural soils of the present, are unique in their geological youth,

Mollisol

Mollisols

While all orders of soil have been around for the last 2.5my (Quaternary period), Mollisols are what we generally call 'soil' today - the classic deep fertile soil good for growing crops.

They are also called Chernozems and black soils in other classifications. Yet they have been around only about 50my, whereas Histosols have been around for hundreds of millions of years. In many ways this site is about how the Earth evolved Mollisols

The differences between the two soil orders are:

Formation:
- Mollisols: These soils are formed under grassland vegetation, especially in the world's major grassland areas. They are typically associated with the Great Plains in North America, the Pampas in South America, and the steppes in Eurasia.
- Histosols: Histosols, on the other hand, are formed in water-saturated conditions with the accumulation of organic matter. They are commonly found in wetland areas, such as bogs, swamps, and peatlands.
Horizon Composition:
- Mollisols: Mollisols are characterized by a thick, dark, and fertile A horizon (topsoil) rich in organic matter. This layer is often referred to as the mollic epipedon.
- Histosols: Histosols have a distinctive O horizon (organic horizon) at the surface, consisting mainly of partially decomposed plant material, which can accumulate to form peat.
Organic Content:
- Mollisols: While Mollisols have a significant amount of organic matter in their A horizon, it is not as highly decomposed as the organic material found in Histosols.
- Histosols: Histosols are characterized by a high content of organic material, primarily from the accumulation of dead plant remains in waterlogged conditions. This organic material can accumulate to the point of forming peat.
Productivity:
- Mollisols: Mollisols are highly fertile and productive soils, supporting the growth of diverse crops. They are often referred to as "prairie soils" and are well-suited for agriculture.
- Histosols: Histosols, due to their waterlogged conditions and the accumulation of peat, are generally less suited for agriculture. They are often associated with wetland ecosystems.
Drainage:
- Mollisols: Mollisols typically have good natural drainage, which contributes to their suitability for agriculture.
- Histosols: Histosols, being formed in water-saturated conditions, often have poor drainage, making them less suitable for traditional agriculture without proper management.

Mollisols are associated with grasslands and are fertile, well-drained soils suitable for agriculture, Histosols are formed in waterlogged conditions, accumulate organic material to form peat, and are typically found in wetland areas. Mollisols (ie present day farm soils), have the ability to mix organic matter with minerals, provide drainage and enable the soil to breathe. They constitute about 7% of the world's ice-free land. (More details from Liu et al., 2012)

Page updated

Google Sites

Report abuse