There’s a significant difference between the way that soil is formed and develops naturally, and the way that commercial topsoil is made.
In this guide we outline the differences between the two in terms of how they are formed or made, and what they each contain.
Summary – Commercial Topsoil vs Natural Topsoil
Commercial topsoil is manufactured by humans, and not formed in nature
It can be manufactured a lot quicker than natural topsoil is formed
Soils are either mined from one location under specific conditions, or sourced from different locations, and brought to one central topsoil manufacturing site
After the soil has been processed, cleared and screened, it can be pre mixed with other types of soil (to give it particle properties and traits), and also organic matter (to give it nutrients)
Other nutrients, minerals and other additives may also be added to the soil mix
Essentially, commercial topsoil allows custom control over certain features of the topsoil, and this allows different types of topsoil to be made too
Natural topsoil on the other hand tends to form/develop over hundreds and thousands of years
Soil comes from the parent rock beneath it, and forms over time with other factors like the climate and local conditions and factors acting against it
These local factors are what make soils different to each other across the world
Some natural soils are naturally fertile, whilst others are not
Some natural soils can be made more fertile with amendment or modification by humans, and some cannot
How Natural Topsoil Is Formed/How It Develops
Below we outline what natural soil is made of, how it is formed, and how it develops over time.
What Natural Soil Is Made Of
A collection of different descriptions of what natural soil is made of:
– All soils are all made of mineral particles, organic matter, air and water (sciencelearn.org.nz).
– Soil is largely made up of grains of weathered rock and the remains of dead, decayed plants [but] Soil [can also be] a living system bursting with microbes, fungi, insects, worms and other invertebrates (bbc.com)
– Soil is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life [and has a] three state system of solids, liquids, and gases (wikipedia.org)
– Soils.org has a good diagram that shows the different layers of soil, and what each layer of soil is made of.
– Another way to describe soil might be that it has physical, chemical and biological characteristics, such as the physical particles it’s made of, the chemicals it contains (like nutrients), and the biological life contained with in it (like microorganisms)
How Natural Soil Develops, & Factors That Determine How Natural Soil Forms
Soils are the product of a) Climate, organisms and topography, acting on b) parent (geologic) material over time.
Said another way ‘Soil is formed over long periods of time from localized materials [mainly the parent rock material underneath the soil], which are broken down by temperature changes, rainfall, drought and wind. Biological factors, such as microorganisms, animals, humans and plants also affect the development of soil.’
Thus the great diversity of geologic materials, geomorphic processes, climatic conditions, biotic assemblages and land surface ages [are] responsible for the presence of an enormous variety of mineral and organic soils.
There’s six key factors that determine how soil is formed and how it develops:
– Parent Rock Material
Parent rock is the material lying underneath the soil.
The minerals and properties of the parent rock are inherited by the soil, so the soil often has characteristics that it gets from the parent rock.
Parent rock material varies in different places around the world, hence why we get different soils in different places.
Some are hard, some are soft, some have a lot of clay, whilst others have more sand, and so on.
Some parent material stays in the same place, whilst others are transported (washed, blown etc.) from one place to another.
In terms of soil order types, Mollisols are one of the most naturally fertile soils.
Mollisols have a ‘parent material [that] is typically base-rich, calcareous and includes limestone, loess, or wind-blown sand. The main processes that lead to the formation of grassland Mollisols are melanisation, decomposition, humification and pedoturbation. (wikipedia.org)
The other key factor to how soil is formed.
Climate – rainfall and temperature in particular – are responsible for breaking down parent material over time.
Rain, snow, heat and cold (through expansion and contraction) all contribute to breaking down of the parent material.
Once the soil is formed, climate and the weather is responsible for the yearly development of the soil, such as how well minerals and salts can move through the soil.
– Landscape (also called Relief)
Landscape involves where the soil lays.
Soil on the top or in the middle of hills and inclines for example tend to erode and be shallower over time than soil at the bottom of inclines and hills.
This is due to wind and soil erosion washing and blowing the soil down the incline.
– Living Organisms & Plant Life
A big part of the long term health and fertility of the soil as well as the development.
When living things die, they decompose and turn into organic matter and humus.
Microorganisms in the soil then break down or mix up that organic matter and make it available as nutrients for plant life.
The roots of trees, plants and crops also get into the cracks in the soil and help make nutrients available.
It takes time for bedrock and parent material to break down and help soil form.
Further to that, it takes time for topsoil to develop from the break down of organic matter (500 to 1000 years for 1-2 cms in some places).
Older soils are typically more fertile and deeper than younger soils.
– The Way The Soil Is Managed By Humans
The human factor that impact soil development is how it is managed.
Certain human activities, particularly agricultural and gardening activities, can be responsible for things like erosion, contamination, adding water or nutrients, adding and removing cover crops, and so on – and, all of these things can impact how the soil develops over time.
What About The Formation Of Natural Topsoil Specifically?
Top soil is the top most layer of soil – it is usually the more fertile layer of soil (with decomposed organic matter, nutrients & minerals) that can be used for plant and crop production.
soils.org has a good resource that shows the different layers of soil, from top to bottom, including humus (mostly organic matter), topsoil (mostly mineral from parent material, and some organic matter), eluviated horizon, subsoil, parents material, and finally at the deepest layer – bedrock.
Natural topsoil is formed from natural processes involving the breakdown of organic matter, and take a very long time to form on its own:
… it takes time for topsoil to develop from the break down of organic matter (500 to 1000 years for 1-2 cms in some places).
Something to note about naturally formed topsoil is that it is different, and can’t be replaced as easily as commercial topsoil can be made. fao.org outlines this well:
[there’s a difference between replacing nutrients and soil organic matter vs replacing the actual topsoil material itself]
The effects of water and wind erosion are largely irreversible
Although plant nutrients and soil organic matter may be replaced, to replace the actual loss of soil material would require taking the soil out of use for many thousands of years, an impractical course of action.
In other cases, land degradation is reversible: soils with reduced organic matter can be restored by additions of plant residues, degraded pastures may recover under improved range management.
Why Natural Soil Differs From Place To Place
The obvious answer is that the six factors listed above vary from place to place.
For example, parent rock material varies all around the world, and climate differs from location to location – just as two examples.
As the factors vary, the soils also vary.
Soils are the product of climate, organisms and topography, acting on parent (geologic) material over time.
Thus the great diversity of geologic materials, geomorphic processes, climatic conditions, biotic assemblages and land surface ages in the United States is responsible for the presence of an enormous variety of mineral and organic soils.
The Different Soils Around The World
How Is Commercial Topsoil Made?
Commercial topsoil is natural soil that goes through a man made process to produce a finished commercial topsoil product.
It can be purchased from the garden or home/outdoors shop, and is widely available to individuals.
It can even be used on the industrial scale.
There’s many different types and quality of topsoil available.
For example, there might be a difference between an economy, and a premium topsoil mix.
There might also be a difference in topsoils intended for landscaping vs gardening vs farming applications.
Different topsoils might also contain different mixes of soil types – such as sand, silt and clay.
They might also contain different balances of macro nutrients and micro nutrients (nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur are some of the most important macro nutrients).
Some soil is mined from one site with controlled or specific conditions, where as others might have soil sourced from several different sites with varying conditions.
Depending on the brand and product of topsoil, it can be made in different ways.
Despite the above variables and factors that make each commercial topsoil product different, they generally follow the same process in being made:
– Soil Is Sourced
Different types of soil are sourced from one, or several soil sites.
Different sites might have different conditions for the soil
– Clearing & Grubbing
Soils are cleared of vegetation, and roots below the surface are removed
The soil is pushed into manageable piles
– Pre Mixing Of Other Soils
The right quantities of sand, silt and clay can be mixed together to get the right topsoil texture.
Adding more sand can help the soil drain better
– Pre Mixing Of Organic Matter
Organic matter such as compost is mixed with soil.
Organic matter is used to add nutrients to the soil mix
The soil is put into a topsoil screening plant which screens out materials bigger than a certain size – for example, stones, rocks, pieces of wood and plastic, metal or other types of manufactured waste (note, not all topsoils are screened)
The same plant tumbles the soil.
This helps the soil become more uniform and gives it a better consistency
– Bagging & Delivery
The soil is now ready to be bagged and delivered to a shop, or straight to the customer
– Other Notes About Commercial Topsoil
Topsoil should meet quality standards in your country.
Companies should also list on their website or packaging how they source and make their topsoil, and what it’s best for (different topsoil blends and products might be suitable for different applications).
So, research the topsoil you need, and also what you are buying, before you buy it.