The Five Factors of Soil Formation

If you have ever looked closely at soils in different areas you will know that they all look, feel and smell different. They also play host to different plants and some contain more visible organisms than others. Why is this? How have they come to be so different? What factors should be considered?

These were the very questions that engaged the early soil scientists in the nineteenth and early twentieth centuries. The history of soil science is a fascinating one and is truly global in its tale. Figures such as Vasily Dokuchaev (pictured above with the incredible beard) made the case for studying soil science as a subject in its own right and, in the nineteenth century, thinking moved from seeing soil as a purely mineral bank, formed as a result of its underlying geology, to a much more complex and dynamic entity.

In 1941, a professor at the University of California at Berkeley, Hans Jenny (pictured above), a Swiss born pedologist, pooled previous knowledge and outlined an equation that would account for the formation of soils. He published his thinking in a monograph called Factors of Soil Formation: A System of Quantitative Pedology (New York, 1941). He termed this soil formation ‘pedogenesis’.

S = f(cl, o, r, p, t, …)

f = a function of

S= soil formation

cl = climate

o = organisms

r = relief (or topography)

p = parent material

t = time

He left the ‘…’ in case there were other factors that he had not considered at the time.

Time is perhaps the most important factor when it comes to soil formation. Everything takes a very long time indeed. In fact, it takes around 500 years to create just under an inch of topsoil. On the other hand, it can be destroyed in next to no time, through erosion, degradation or cultivation.

Let’s take these factors in turn:

Climate

Here we are mostly talking about temperature, moisture and wind. Different levels of temperature and moisture cause different patterns of weathering and leaching.

Temperatures change through the seasons but they also change daily. This can have a correlation with moisture, biological activity and the rates of chemical reactions. This therefore all impacts on weathering.

Climate impacts on the kinds of vegetation that will grow and therefore the amount of organic matter that will build up on the surface, once this vegetation has died and is rotting away.

In wetter regions the rain often washes calcium and other bases out of the soil to leave it acid, which changes its composition.

Wind will redistribute sand and other particles, often carrying them vast distances. the ‘loess’ soils of central Asia are as a result of wind (or aeolian) transportation.

Ice is also a factor and can move huge amounts of soil (and parent materials) resulting in moraines.

Organisms

Plants, animals, micro-organisms, and humans all affect soil formation.

Certain bacteria (cyanobacteria that can fix their own nitrogen) and lichens (semi algae, semi fungi) have a key role in building up early ‘layers’ of organic matter and generating soil formation in the first place. Over time, as organisms die and are broken down by other organisms – bacteria, fungi, protists – and organic matter builds up and is transported within soils by other organisms.

Certain organisms (such as earthworms) will mix soil (known as bioturbation) and form burrows and channels, improving drainage and the ability of air to pentrate soils.

Relief

Material will be forced into valleys by gravity. Valley soils will therefore be deeper and richer with organic matter and soils on slopes or at higher altitudes tend to be poorer and thinner. Relief also affects temperature and precipitation (directly impacting climate).

Parent Material

The type of parent material determines which minerals will predominate in the soil. E.g. limestone soil – higher in calcium. As the parent material weathers, nutrients are released into soil solution, which subsequently can be taken up by plants and other organisms or leached from the soil.

When rocks weather (this can happen physically or chemically) they are broken into materials such as sands or clays (clays are known as ‘secondary minerals’ given that in formation the particles go through a process of chemical change). We will look at clays, sand and silt (texture) later this week.

Time

As already suggested, nothing happens quickly when it comes to soil formation (apart from the ability to destroy soils!).

That was a very brief introduction to the matter of how soils form. There is of course a lot more detail to the process but I hope I have encouraged you to look into the matter more closely. Tomorrow we will look at the constituent materials of the soils themselves – the sand, silt and clay.

In the mean time – here is a link to a citizen science project being organised by OPAL (Open Air Laboratories) who are asking people to survey earthworms.