Essay - Soil Characteristics

There are six main factors to take into account when describing the characteristics of a soil:

  • Colour - Soils exist in a wide range of colours including black, grey, white, brown, yellow, red and green. The colour depends largely on the parent rock/bedrock and the processes involved in its formation. It is often described using Munsell sol colour charts, which identify colours using the different components, namely hue (basic colour), value (lightness/darkness) and chroma (colour intensity/paleness or strength)

  • Texture - This refers to the feel of the soil (smoothness or coarseness). It is largely influenced by the size of the grains and particles that make up the soil, which naturally affects the size of the pore spaces between the individual grains and particles. The differing size of pore spaces means that soils have different aeration levels (amount of air travelling through) and drainage levels (amount of water travelling through). Texture determines the soils ability to contain and retain water and nutrients and how easily roots will be able to penetrate it. There are four main textures:

  1. Sandy - Coarse with big grains and particles and small pore spaces. Such soils rarely suffer from waterlogging but are susceptible to leaching and so are often found to be infertile.

  2. Clay - Smooth with tiny grains and particles and small pore spaces. Such soils are easily waterlogged, particularly in winter when there are periods of prolonged or intense rainfall. At such times, they are rather heavy and sticky. In contrast, during the summer months, their surfaces tend to become dry and cracked. They are usually high in nutrients as they do not facilitate leaching.

  3. Silty - Powdery with grains and particles which are bigger than clay but smaller than sandy. They are very sticky when wet and while they do tend to be badly drained, they rarely get waterlogged.

  4. Loam - These are part sandy, part clay and part silty, therefore, they have characteristics of all three. They have medium sized pore spaces and as a result, rarely become waterlogged or dry and cracked but do crumble quite easily. They are high in nutrients and are the ideal soil for agriculture.

  • Structure - Refers to the shape of soil grains and particles and the way in which they are assembled and cemented together by humus and/or water. When a plant is pulled from the ground, soil grains and particles remain lumped in its roots; these lumps are known as peds and can be easily viewed under a microscope. The shape of these peds indicates the structure of the soil. The pore spaces between these peds either allow air and water to pass through easily or overcropping/overcultivation damage and the structure of a soil, reducing its ability to support vegetation. There are four main structures:

  1. Crumb/granular - Small and round in shape, similar to breadcrumbs. Air and water can easily travel through

  2. Platy - Thin, flat and overlapping peds prevent both air and water from passing through, often resulting in waterlogging

  3. Blocky - Cube-shaped and tight-fitting

  4. Prismatic - Column-shaped

  • Humus content - The humus content of a soil is important due to its ability to support plant life. It also helps to bind soil together and keep it moist. A it is washed into the ground by precipitation, it releases key nutrients into the soil.

  • pH - This is the measure of how acidic or alkaline a soil is. The acidity or alkalinity level of a soil determines what sort of vegetation and animal life it can support. The majority of vegetation favours slightly acidic soils (Those with a pH of about 6.5)

  • Water content and retention - The amount of water a soil contains or can retain depends on its:

  1. Texture - Coarse soils, with their large pore spaces, allow water to pass through and, so, there is little retention. On the other hand, smooth soils with their small pore spaces do not allow water to move through so they retain large amounts of water

  2. Structure - Crumb/granular structures allow water to easily travel through the soil meaning that they can dry out quickly. In contrast, platy structures restrict water movement and, so, they retain large amounts of water.

  3. Humus content - Soils rich in humus tend to retain more water than those with a low humus content.