Simply put, all life on land depends upon the soil, from the plants growing in the soil, to animals that eat the plants, and animals that eat other animals, without soil there would be no life on land.
SOIL FORMATION
It’s no co-incidence that life on land evolved at the same time as the first soils appeared. The explosion of life on land around 450 million years ago is directly linked to the relationship between plants and symbiotic bacteria and fungi: it’s a little-known fact that plants work in tandem with fungi, with this mutually beneficial relationship aiding soil formation, breaking down the rocks and releasing essential nutrients.
There are many types of soils, but all soils are formed by three different types of processes, working together to produce very many different types of soils from the parent material in the bedrock.
PHYSICAL WEATHERING
Physical weathering is caused simply by wind, water and changes in temperature. In the tropics, factors include the intense heat of bush fires and lightning which can split rocks.
In temperate lands, frost is important as a weathering agent. Water gets into cracks within the rock and freezes. This expansion opens up the cracks and splits the rock.
In the tropics where there is a rapid warming and cooling in the atmosphere, rocks may crumble. When the wind carries particles of sand and grit these have an abrasive effect on exposed rocks.
Any process which allows water and air to get inside the rock structure contributes to the weathering. The impact of heavy rain may also cause particles of rock to break away.
CHEMICAL WEATHERING
As rain falls it dissolves some carbon dioxide from the atmosphere to form a very weak acid called carbonic acid. Over a period of time this acid reacts with the mineral particles of the rock, particularly calcium carbonate causing decomposition.
This type of weathering happens most where the bedrock is limestone.
BIOLOGICAL WEATHERING
Most importantly, trees or other plants can cause rocks to split and crack as their roots penetrate and expand. Animals digging and burrowing in the ground also cause some breakdown. Even minute plants like lichens can break up the particles of a rock.
SOIL STRUCTURE
During the process of soil formation, the soil builds up in layers called horizons. It is important to understand soil horizons if soils are to be properly managed.
The first horizon above the bedrock is soil horizon C, which essentially is weathered rock. Soil horizon B is the subsoil, which is unsuitable for growing things. Soil horizon A is the topsoil – this is where the organic matter is found, and ideally a well-structured soil with lots of organic matter. However, there is huge variation in the quality of soils, depending upon the type of bedrock, climate, and agricultural practices. It’s worth taking a look at the difference between the top soil structure of a well-managed soil compared to one which has been intensively farmed using methods such as ploughing.
In the diagram below,1 the soil on the left has been well managed, with a good, loose- fitting crumb structure and a high percentage of organic matter, both of which allow roots to penetrate easily. On the right the soil has been intensively ploughed, giving rise to a hard, compacted top and subsoil, which is difficult for soils to penetrate.
1 Magdoff F. Van Es H. & Sustainable Agriculture Research & Education (Program).
(2009). Building soils for better crops : sustainable soil management (3rd ed.). SARE. P61.
There are many soil management tools available to the farmer which will promote good, healthy soils structure and top quality crops.
The first step for any farmer is to find out more about their soils: in the next article I will introduce you to some basic soil tests that any farmer can do.
An article by Maureen Odendaal
If you would like to delve deeper into your farm soils and land management practices please get in touch with AGREN to assess your farm.
