A Definition of Soil

Soil makes up the pedosphere layer on Earth and is easy enough to recognise. Unless you’ve studied it though, it’s actually quite hard to articulate what it is you’re recognising. So what exactly is ‘soil’?

Soils differ markedly in colour and texture, but all contain the same five components of:

• inorganic* (mineral) particles, derived mainly from weathering of rocks
• organic* materials derived from dead and decaying plants, animals and animal products
• water, also known as the ’soil solution’ in which plant nutrients are dissolved
• air, which fills the spaces between soil particles not filled by soil solution
• living organisms, such as microorganisms, insects, earthworms, and even small burrowing animals

* Please note that to a chemist, ‘organic’ means ‘contains carbon’ and ‘inorganic’ means ‘doesn’t contain carbon’. The presence of carbon in soil implies an origin from living and once-living organisms. Even carbon-rich coals and charcoals originated from once living organisms.

How Soils Form

Soil formation occurs over thousands of years of ‘weathering’. Weathering is the breaking up of rocks and sediments by physical, chemical and biological activities.

Physical weathering is the break up of rocks without physical change. Rocks fragment into smaller and smaller particles until they eventually become the mineral component of soil. Wind, temperature, water and pressure are behind this type of weathering. Wind is abrasive and has a sand-blasting effect. Temperature fluctuations over days, seasons and years cause rock to expand and contract until it cracks. Water continually freezing and thawing in holes or cracks will also break rocks with time. Plant roots in cracks can exert pressure and push rocks apart as they grow and thicken.

Chemical weathering is the break down of rock through chemical changes. Water and oxygen are the major forces here. One example is water acting on granite. The water reacts with feldspar crystals in the granite to create clay minerals. The clay minerals weaken the rock and increase the likelihood of it fragmenting. Another example is oxygen reacting with iron to create iron oxides. These oxides are more fragile than iron and compromise the rock’s structure, leading to it eventually breaking away.

Biological weathering is a form of chemical weathering, in that some plants and microorganisms release acids that break down rocks and mineral compounds. Lichens (which are not single organisms but rather fungi and algae or fungi and cyanobacteria living together in beneficial relationships) produce a weak acid capable of dissolving rock.

The extent to which each of these occurs is a function of the climate, environment, organisms and rock type specific to a region, and the reason soils vary so much globally. Rocks rich in quartz, such as sandstone, will form sandy soils. Rocks poor in quartz, such as shale or basalt, will not. Rocks in hot, dry deserts will break down differently to rocks on a wet, temperate coast and form different soils as a result.

At the end of the day we have the soil we have, and must work with it and its characteristics accordingly. The next post will look at the properties of soils: their colour, texture, mineral content, structure, and chemistry.