Soil Makeup

Soils have two main factors which determine their type, the soil structure and soil texture. The soil texture refers to the size and shape of the soil particles/granules. The soil structure refers to the way the soil granules fit together to form a ped (this why study of soils is known as Pedology). A ped in a large bit of soil that can withstand a few cycles of being wet then dry and is separated from other peds by lines of weakness, often cracks. The soil texture is therefore directly linked to the soil structure. Smaller granules can fit together more tightly than large ones, so you get less air spaces. There are three types of granule in most soils, sand, clay and silt. The percentage of each determines the properties of the soil. For example a beach is almost 100% sand, this means it fits together loosely allowing water to drain straight through it. Unlike a clay soil that fit's tightly together and has few air spaces (pores), and therefore is often water logged as water can not drain through it. We often display these relationships as a pyramid as shown below:

The term "Loam", occurs a few times in the diagram above. We use it to describe a fertile soil. You will notice that the loams always have a good mix of all three components. This is because a fertile soil must be porous to allow rainwater to infiltrate the ground, but not become water logged or dry. It must be held together well in order to provide support to plants. And finally it must be able to hold and release nutrients. Well sand is good for allowing water to pass through it, look at a beach for proof. Silt is a term that describes very small particles, these help bind the soil together to provide good anchorage for plant roots, which means plant will not be blown away by the wind, or knocked over by animals. Clay is actually able to provide nutrients! It is often thought of as a bad soil type due to it's ability to bind the soil as well, but it's absence would render a soil deficient. Clay is able to form chemical bonds with the nutrient ions (* nutrients that plants can use MUST be ions as the process that allows the plant to absorb nutrients is ion exchange) due to it's polar properties. Water is absorbed by osmosis as a result of ion exchange (see the Biology Zone).

So far this has all been about the soil texture i.e. the granules that make up the soil. But earlier I said soil structure is a key factor as well. As I said earlier the structure is directly linked to the texture, but over factors can influence the structure of the soil's peds. Trampling is the best example of an anthropogenic (human) effect on soil structure. Things such a tractors often compress the soil leaving tracts (maybe why they call them tractors) that no plants will grow in! The soil in the same as the rest of the field, so why can't plants grow? The answer is that compression of the soil changes the shape and position of the peds. This means that the soil may no longer be porous and so it will become water logged, and plants will not be able to break the soil and take root. As a result the soil is unfertile. Other processes can cause the same problems and it is not always humans that are the cause.

There are a few types of ped that are considered standard, to name a few:- Crumb, Columnar, Granular, Blocky, Platy and Prismatic. However a soil is more than just a topsoil! To study soils properly we must look at the whole picture, the Horizon...