Classification of Soils

Soil Classification concerns the grouping of soils with a similar range of properties (chemical, physical and biological) into units that can be geo-referenced and mapped. Soils are a very complex natural resource, much more so than air and water.

Soils contain all naturally occurring chemical elements and combine simultaneously solid, liquid and gaseous states. Moreover, the number of physical, chemical and biological characteristics and their combinations are nearly endless. Many different approaches have been proposed to come to a sensible grouping of different soils.

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.

Sandy Soil

Classification of Soils

  • 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).
  • It is most extensively used construction material.
  • It consists of particles of rock and hard minerals, such as silicon dioxide.
  • They are the largest type of soil particles, where each particle is visible to naked eye.
  • The large, relatively stable sand-particle size increases soil aeration, improves drainage in tight soils and creates plant-growth supporting qualities, or tilt.
  • The particle size of course sand ranges from 2 – 4.75mm, Medium sand ranges from 0.425 – 2 mm and fine sand ranges from 0.075 – 0.425 mm.
  • The bigger particle size of the sand gives wet or dry sandy soil a grainy texture when you rub it between your fingers, and it makes the soil light and crumbly even when you try to stick it together in your hand.
  • The particle shape is angular, sub angular, rounded, flat or elongated.
  • The texture is rough, smooth, or polished.
  • These soils have quick water drainage and are easy to work with.
  • 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.

Clay Soil

  • 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.
  • Clay particles are the finest of all the soil particles, measuring fewer than 0.002 mm in size.
  • It consists of microscopic and sub-microscopic particles derived from the chemical decomposition of rocks.
  • Clay is a fine grained cohesive soil.
  • They stick together readily and form a sticky or gluey texture when they are wet or dry.
  • 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.
  • Clay expand when in contact with water and shrink when getting dry.
  • Compared to sand particles, which are generally round, clay particles are thin, flat and covered with tiny plates.
  • Organic clay is highly compressible and its strength is very high when dry, which is why it is used in construction as mud mortar.

Silt Soil

Silt Soil

  • 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.
  • Silt is a sediment material with an intermediate size between sand and clay.
  • Carried by water during flood it forms a fertile deposit on valleys floor.
  • The particle size of silt ranges from 0.002 and 0.06 mm.
  • Due to its fineness, when wet it becomes a smooth mud that you can form easily into balls or other shapes in your hand and when silt soil is very wet, it blends seamlessly with water to form fine, runny puddles of mud.
  • By adding organic matter, the silt particles can be bound into more stable clumps.

Peat Soil

Peat Soil

  • 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.

Chalk Soil

Chalk Soil

  • Chalk soil can be either light or heavy but always highly alkaline due to the calcium carbonate (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.

Loam Soil

Loam Soil

  • 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.