Types of soil – Soils come in many textures, colours, mineral and organic compositions, and more. Find out more on sandy soils, loamy solis, chalky soils, peaty soils, clay and silty soils here.
Soil can be categorized into sand, clay, silt, peat, chalk and loam types of soil based on the dominating size of the particles within a soil.
Soil is a natural resource that can be categorised into different soil types, each with distinct characteristics that provide growing benefits and limitations.
Identifying the type of soil you require for a project is paramount to support the healthy growth of plant life.
Soil can be categorised into sand, clay, silt, peat, chalk and loam types of soil based on the dominating size of the particles within a soil.
In this article we will also learn, What determine the type of soil ?
Types of Soil
Just like plants and people, soils have different characteristics that make them unique. Knowing the kind of soil you have helps you determine its strengths and weaknesses. While soil is composed of many elements, the place to begin is with your soil type. You just have to observe the composition of the soil’s particles.
You can use following image to draw the line diagram to identify types of soils.
The soil is basically classified into four types:
- Sandy soil.
- Silt Soil.
- Clay Soil.
- Loamy Soil.
Sandy Soil is light, warm, dry and tend to be acidic and low in nutrients. Sandy soils are often known as light soils due to their high proportion of sand and little clay (clay weighs more than sand).
These soils have quick water drainage and are easy to work with. They are quicker to warm up in spring than clay soils but tend to dry out in summer and suffer from low nutrients that are washed away by rain.
The addition of organic matter can help give plants an additional boost of nutrients by improving the nutrient and water holding capacity of the soil.
Sandy soil consists of small particles of weathered rock. Sandy soils are one of the poorest types of soil for growing plants because it has very low nutrients and poor in holding water, which makes it hard for the plant’s roots to absorb water. This type of soil is very good for the drainage system. Sandy soil is usually formed by the breakdown or fragmentation of rocks like granite, limestone, and quartz.
Clay Soil is a heavy soil type that benefits from high nutrients. Clay soils remain wet and cold in winter and dry out in summer.
These soils are made of over 25 percent clay, and because of the spaces found between clay particles, clay soils hold a high amount of water.
Because these soils drain slowly and take longer to warm up in summer, combined with drying out and cracking in summer, they can often test gardeners.
Clay is the smallest particles amongst the other two types of soil. The particles in this soil are tightly packed together with each other with very little or no airspace. This soil has very good water storage qualities and making hard for moisture and air to penetrate into it. It is very sticky to the touch when wet, but smooth when dried. Clay is the densest and heaviest type of soil which do not drain well or provide space for plant roots to flourish.
Silt Soil is a light and moisture retentive soil type with a high fertility rating.
As silt soils compromise of medium sized particles they are well drained and hold moisture well.
As the particles are fine, they can be easily compacted and are prone to washing away with rain.
By adding organic matter, the silt particles can be bound into more stable clumps.
Silt, which is known to have much smaller particles compared to the sandy soil and is made up of rock and other mineral particles which are smaller than sand and larger than clay. It is the smooth and quite fine quality of the soil that holds water better than sand. Silt is easily transported by moving currents and it is mainly found near the river, lake, and other water bodies. The silt soil is more fertile compared to the other three types of soil. Therefore it is also used in agricultural practices to improve soil fertility.
Peat soil is high in organic matter and retains a large amount of moisture.
This type of soil is very rarely found in a garden and often imported into a garden to provide an optimum soil base for planting.
Peaty soils are acidic and as a result, does not support decomposition very well. It is dark in color, rich in organic material, although contains less nutrients than loamy soils. It retains water very well.
Chalk soil can be either light or heavy but always highly alkaline due to the calcium carbonate or lime within its structure.
As these soils are alkaline they will not support the growth of ericaceous plants that require acidic soils to grow.
If a chalky soil shows signs of visible white lumps then they can’t be acidified and gardeners should be resigned to only choose plants that prefer an alkaline soil.
Chalky soils are alkaline with a pH of about 7.5. It is not acidic and often stony with chalk or limestone bedrock. It is free draining because of its coarse and stony nature. Not the best for crops to grow in as they lack manganese and iron.
Loam soil is a mixture of sand, silt and clay that are combined to avoid the negative effects of each type.
These soils are fertile, easy to work with and provide good drainage. Depending on their predominant composition they can be either sandy or clay loam.
As the soils are a perfect balance of soil particles, they are considered to be a gardeners best friend, but still benefit from topping up with additional organic matter.
The ideal soil is 40 percent sand, 40 percent silt, and 20 percent clay. You’ll hear this mixture referred to as loam. It takes the best from each soil particle type. It has good water drainage and allows air to infiltrate the soil like sand, but it also holds moisture well and is fertile like silt and clay.
|Intake Rate||Field Capacity|
|Silty Clay Loam||0.15||3.81||0.2365|
|Sandy Clay Loam||0.25||6.35||0.306|
Types of Soils Based on Grain Size
- Cobbles : These are larger than pebbles and smaller than boulders. The size is between 64 to 256mm.
- Gravels: The diameter is between 4.75 and 76.2mm
- Clay: Soil grains finer than 0.002mm
- Silt: Soil grain with size ranging from 0.075 to 0.002mm
- Sand: Particles seen by naked eye and size less than 4.75mm
Types of Soils Based on Origin
The soils can be classified into two based on origin as:
Residual Soils: These types of soils will remain at the place of their formations. They are located near the weathered parent rock. The two factors governing the depth of the residual soils are the exposure time and climatic conditions. These are stiff and stable in temperature zones. The size of residual soil is indefinite.
Transported Soils: As the name say, these are soils that are transported from the area of weathering to some other location by means of transportation agents like wind, water, ice or gravity. Based on which they are classified as shown in below table.
Different Transported Soils and their Transporting Agent
|Sl. No||Soil Type||Transporting Agent||Properties|
|1||Alluvial Soil||Running Water||Coarser and Finer Particles|
|2||Lacustrine Soil||Water – Deposited at quiet lakes||Coarser and Finer Particles|
|3||Marine Soils||Sea water||Coarser and Finer Particles|
|4||Aeolian Soils||Wind||Silt – low density and high compressibility|
|5||Colluvial Soils||Gravitational Force|
|6||Glacial Soil||Glaciers||Finer Particles, boulders|
The soils formed at a place may be transported to other places by agents of transportation, such as water, ice, wind and gravity.
Organic Soils & Inorganic Soils: The soils that have organic origin comes under the category of organic soils. These are formed by the growth and decomposition of the plants, shells of organisms, inorganic skeletons etc. Sometimes inorganic content by some source can make the soil inorganic.
Different Soils Used in Practice
- Bentonite : The clay formed by the decomposition of volcanic ash. Properties of clay are exhibited highly. This contain high amount of montmorillonite.
- Kaolin: This is also called as china clay which is pure form of white clay. The main application are in clay industries
- Boulder Clay: This is acombination of sedimented deposited clay (glacial clay) with unsorted rocks and pulverised clay.
- Varved Clays: Consist of layers of silt and flat clays.
- Shale : This material have a state between clay and slate
- Marl : This material is a combination of clay, loam and calcareous sands
- Peat : Fibrous aggregate with decomposed fragments of vegetable matter. Very compressible in nature and used for supporting of structures.
- Loam : This material is a combination of silt, clay and sand
Further reading about soil types
Gelisols are soils that are permanently frozen (contain “permafrost”) or contain evidence of permafrost near the soil surface. Gelisols are found in the Arctic and Antarctic, as well as at extremely high elevations. Permafrost influences land use through its effect on the downward movement of water and freeze-thaw activity (cryoturbation) such as frost heaves. Permafrost can also restrict the rooting depth of plants. Gelisols make up about 9% of the world’s glacier-free land surface.
Spodosols are among the most attractive soils. They often have a dark surface underlain by an ashy, gray layer, which is subsequently underlain by a reddish, rusty, coffee-colored, or black subsoil horizon. These soils form as rainfall interacts with acidic vegetative litter, such as the needles of conifers, to form organic acids. These acids dissolve iron, aluminum, and organic matter in the topsoil and ashy gray horizons. The dissolved materials then move to the colorful subsoil horizons. Spodosols most often develop in coarsely textured soils (sands and loamy sands) under coniferous vegetation in humid regions of the world. They tend to be acidic and have low fertility and low clay content. Spodosols occupy about 4% of the world’s glacier-free land surface.
Histosols are mainly composed of organic material in their upper portion. The Histosol order mostly contains soils commonly called bogs, moors, peatlands, muskegs, fens, or peats and mucks. These soils form when organic matter, such as leaves, mosses, or grasses, decomposes more slowly than it accumulates due to a decrease in microbial decay rates. This most often occurs in extremely wet areas or underwater; thus, most of these soils are saturated year-round. Histosols can be highly productive farmland when drained; however, draining these soils can cause them to decompose rapidly and subside dramatically. They are also not stable for foundations or roadways and may be highly acidic. Histosols make up about 1% of the world’s glacier-free land surface.
Andisols typically form from the weathering of volcanic materials such as ash, resulting in minerals in the soil with poor crystal structure. These minerals have an unusually high capacity to hold both nutrients and water, making these soils very productive and fertile. Andisols include weakly weathered soils with much volcanic glass, as well as more strongly weathered soils. They typically occur in areas with moderate to high rainfall and cool temperatures. They also tend to be highly erodible when on slopes. These soils make up about 1% of the glacier-free land surface.
Vertisols are clay-rich soils that contain a type of “expansive” clay that shrinks and swells dramatically. These soils therefore shrink as they dry and swell when they become wet. When dry, vertisols form large cracks that may be more than one meter (three feet) deep and several centimeters, or inches, wide. The movement of these soils can crack building foundations and buckle roads. Vertisols are highly fertile due to their high clay content; however, water tends to pool on their surfaces when they become wet. Vertisols are located in areas where the underlying parent materials allow for the formation of expansive clay minerals. They occupy about 2% of the glacier-free land surface.
Oxisols are soils of tropical and subtropical regions, which are dominated by iron oxides, quartz, and highly weathered clay minerals such as kaolinite. These soils are typically found on gently sloping land surfaces of great age that have been stable for a long time. For the most part, they are nearly featureless soils without clearly marked layers, or horizons. Because they are highly weathered, they have low natural fertility, but can be made productive through wise use of fertilizers and lime. Oxisols are found over about 8% of the glacier-free land surface.
Aridisols are soils that occur in climates that are too dry for “mesophytic” plants—plants adapted to neither a too wet nor too dry environments—to survive. The climate in which Aridisols occur also restricts soil weathering processes. Aridisols often contain accumulations of salt, gypsum, or carbonates, and are found in hot and cold deserts worldwide. They occupy about 12% of the Earth’s glacier-free land area, including some of the dry valleys of Antarctica.
Ultisols are soils that have formed in humid areas and are intensely weathered. They typically contain a subsoil horizon that has an appreciable amount of translocated clay, and are relatively acidic. Most nutrients are held in the upper centimeters of Ultisol soils, and these soils are generally of low fertility although they can become productive with additions of fertilizer and lime. Ultisols make up about 8% of the glacier-free land surface.
Mollisols are prairie or grassland soils that have a dark-colored surface horizon. They are highly fertile and rich in chemical “bases” such as calcium and magnesium. The dark surface horizon comes from the yearly addition of organic matter to the soil from the deep roots of prairie plants. Mollisols are often found in climates with pronounced dry seasons. They make up approximately 7% of the glacier-free land surface.
Alfisols are similar to Ultisols but are less intensively weathered and less acidic. They tend to be more inherently fertile than Ultisols and are located in similar climatic regions, typically under forest vegetation. They are also more common than Ultisols, occupying about 10% of the glacier-free land surface.
Inceptisols exhibit a moderate degree of soil development and lack significant clay accumulation in the subsoil. They occur over a wide range of parent materials and climatic conditions, and thus have a wide range of characteristics. They are extensive, occupying approximately 17% of the earth’s glacier-free surface.
Entisols are the last order in soil taxonomy and exhibit little to no soil development other than the presence of an identifiable topsoil horizon. These soils occur in areas of recently deposited sediments, often in places where deposition is faster than the rate of soil development. Some typical landforms where Entisols are located include: active flood plains, dunes, landslide areas, and behind retreating glaciers. They are common in all environments. Entisols make up the second largest group of soils after Inceptisols, occupying about 16% of the Earth’s surface.
Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.