Soil – it’s not just dirt!

Harris dirt 2

Pedology – this is a crucial field of science when it comes to growing plants on the land, but what does it actually mean? A popular guess would be that the root ‘ped’ is derived from the Latin word for foot, such as pedestrian, biped, or pedestal. On the other hand, the ‘science of feet’ makes absolutely no sense when talking about agriculture! According to the dictionary, pedology is soil science. In this case, ‘ped’ comes from the Greek (not Latin) pedon meaning ground or earth. To expand upon that a bit, pedology is the study of soil’s physical properties, chemical properties, texture, contributions to an ecosystem, and how it moves. It’s impossible to photo 12imagine a society where there was no soil, no ground, no basis for life. I hope that after reading this blog you give a little more thought to what it is we walk and live on every day.

Physical properties

Did you know the ideal soil for farming is only composed of about 50% solids? This percent can be further broken down into about 5% organic matter, with the remaining 45% being mineral content. But now this begs the question – what is the other 50% of soil made of? The remaining space is split equally between available water, unavailable water, and pore space. Simply put, pore space is the tiny pockets of air that microorganisms live in and plant roots use for gas exchange. Available water is soil water that is held a pressure that is easily taken up by plant roots. Using common sense this means that unavailable water is held at too high of a pressure for plant roots to take up, basically stuck to the soil particles and probably won’t move anytime soon.chart (1)


Chemical properties

Believe it or not, soil has chemistry too! Lots of farmers complete soil tests on their land, which will measure many varying characteristics within the soil. To start off with, soil pH is very important when considering nutrient uptake availability! Even if a nutrient is abundant within the upper portion of a soil profile, a plant cannot use it unless the soil’s pH is ideal for that specific element. The most common way of raising the pH of soil is by adding agricultural lime, also known as calcium carbonate. Another important quality of soil is its cation exchange capacity, also called CEC. Although it may sound complicated, CEC refers to the ability of the soil to hold and exchange positive charges. Some common cations are calcium, magnesium, potassium, sodium, and hydrogen. Farmers and researchers alike are able to calculate their soil’s CEC, which then translates to how many nutrients the soil can hold at a given time.


Soil is composed of three main types of particles: sand, silt, and clay. Each differs from the next in terms of shape, size, and chemical properties.

  • Sand is the largest particle with a size from 0.05 mm – 2.0 mm. Fields with high quantities of sand have good aeration but poor water holding capacity.
  • Silt is unique with it being smaller than sand but larger than clay. Its size ranges from 0.002 mm – 0.05 mm, and it comes with a high available water holding capacity.
  • The final size is clay, which is 0.002 mm or smaller. This is obviously the smallest soil particle, has a high water holding capacity, and exhibits very poor aeration.

When defining a soil texture, a loam is a mixture of all three textures and is ideal for growing crops in the Midwest!

soil triangle

This is a texture triangle, useful when determining your soil’s texture! Photo from FAO


Role in the ecosystem

Soil is found everywhere around the world, from agricultural fields, to big cities, to forests, and everything in-between! Generally speaking, the ground in metropolitan areas will be very compacted and likely not supporting any biota beneath the surface. In contrast, once outside of urban areas the biota dependent upon the soil vastly changes. Some soil microorganisms include bacteria, fungi, nematodes, algae, and many more! Did you know that a fungi called mycorrhiza has a symbiotic relationship with plant roots?

Where will it go next?

You might think it’s uncommon to hear about soil moving, but it has two main mechanisms for relocating to other regions. Erosion can be caused by wind and water, both having potentially detrimental effects on the soil. Water erosion occurs in three steps: 1) sheet erosion, 2) rill erosion, and 3) gully erosion. Sheet erosion is the film of soil moving from the impact of a rain drop or in a film of water. This is the most difficult to spot and occurs over almost all bare soil during a rain storm. Rill erosion occurs once small channels are formed from the movement of water. If the situation becomes too dire, then gullies will form. This is when the big channels are too deep for field equipment to cross. Wind erosion also occurs in three main steps. The first step is called saltation, and this occurs when fine sand particles are bouncing across a landscape. If the wind picks up, the following step is when particles are becoming suspended in the air. The final step of wind erosion is called creep, which is the rolling and sliding of particles that are too big for the air column.

So the next time you’re out driving along a road, walking through a park, or tending to your garden, I hope you’re thinking about more than solely what’s on the top of the soil!



P.S. Yes I am a new name to these blogs, and I’m here to stay for a while! I recently started as the new intern with Iowa Agriculture Literacy Foundation and I’m thrilled for what this year has in store. So a little bit about me – I’m currently a student at Iowa State University double majoring in Agronomy and Agriculture Communications. I love growing plants of all types, and that might show a little in future blogs! I look forward to creating some more intriguing and informative posts that you all can enjoy!

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