Soil basics: texture

‘Soil’ is composed of the mineral particles, organic matter, and living organisms of a sample that pass through a 2 mm sieve. I’m not going to give a precise definition of ‘soil’ because, to a certain degree, it depends on what you want it to be. Some might argue that beach sand is (or is not) soil. Others might argue that lake sediments can be considered (or not) to be of the soil order ‘hydrosol’. I don’t personally have strong feelings on the matter one way or the other… But anyway, let’s talk about ‘texture’.

Soil particles can be anywhere from single atoms to 2 mm across; soil ‘texture’ is a description of the distribution of particle sizes in a particular soil sample. A high proportion of large particles (such as sand grains) causes a soil to have a ‘sandy’, ‘coarse’, or ‘light’ texture. These terms are largely interchangeable. A moderate to high proportion of small particles (such as clay particles) causes a soil to have a ‘clayey’, ‘fine’, or ‘heavy’ texture. Again, these terms are fairly interchangeable.

Soils with particle distributions that fall in the middle are generally called a ‘loamy’ texture.

Why should anyone care about texture?

Texture is one soil property that is very difficult to change. You can generally make a ‘coarse’ textured soil finer by adding a little clay and a lot of money, but you’d be hard pressed to ever have enough sand to make a ‘fine’ textured soil coarse.

Texture is almost as important as climate when it comes to what can grow where. Texture controls how much water gets into the soil, how much stays in the soil, and how hard plants have to work to get their roots through the soil. Texture also affects nutrient content of the soil by providing (or not) charged surfaces that catch, hold, and release nutrients over time. Soils with large particles have large spaces between those particles. This means water penetrates quickly and also drains quickly. These soils cannot hold onto large amounts of water and are generally quite limited when it comes to surfaces capable of holding nutrients. In contrast, soils with small particles usually* have small spaces and good nutrient holding capacity. Water moves very slowly though these soils and they may have poor infiltration (water sits on top of the soil or runs off) and poor drainage (leading to waterlogging).

Soil texture in urban environments

Soil texture is an important environmental variable that construction companies and engineers deal with regularly. Clay soils can be strongly acid or alkali (corrosion), they can shrink and swell (cracking), and they can be a real bugger to clean off machinery (contamination). You might notice an increase in the number of burst water mains during the change of season (from dry to wet or wet to dry). This is caused by clay soils absorbing water and swelling and then drying out and shrinking**. Even if it doesn’t move far, soil is stronger than pipes and eventually nature will win.

 

* this can be improved by having good soil ‘structure’.

** think of the difference between a wet and dry sponge.