Today, as part of the series of posts on soils, we are going to look at ‘soil texture’. Soil forms the basis for all life but it’s important to know about its mineral constitution as well as its biological profile.
Texture refers to the ‘feel’ of the soil. This is affected by the constituent materials found within it, specifically sand, silt and clay particles. A coarse sand will feel gritty but a wet clay will feel heavy and sticky. The texture of a soil has a direct impact on the way the soil reacts to certain environmental conditions – for example, towards drought or heavy rain (with sandy soils more freely draining).
There is a big difference in the size of the different particles.
Coarse sand = diameter 2-0.2mm
Fine sand = diameter 0.2-0.02mm
Silt = diameter 0.02-0.002mm
Clay = diameter less than 0.002mm
Note how the clay particles are much smaller than the sand particles – this is important as it means the total surface area of a clay soil is much greater and so the capacity to hold water is also much greater.
Between the sand, silt and clay particles there are lots of pores. In fact a soil as a whole is generally 45% mineral, 5% organic matter (depending on the soil) and 50% pore space through which air and water can pass.
- Made up of weathered primary rock minerals.
- The particles are irregular in outline.
- They are large and so do not pack together easily.
- Large pore spaces in between.
- Air gets in very easily and water flows rapidly through it.
- Particles are smaller than sand.
- Smaller pore spaces.
- More difficult to drain.
- Unlike clay particles, silt does not flocculate.
- Very small indeed.
- Secondary minerals (chemically altered by weathering) – chemical weathering (hydration, hydrolysis and dissolution) causes the original mineral matter to degrade into its chemical constituents – mainly silicon, aluminium, iron, magnesium, potassium and calcium. Once released, they recrystallize to form ‘secondary minerals’.
- Large surface area (they can hold more water) although the pore spaces are tiny.
- Negatively charged (which means they attract positively charged cations – such as calcium, magnesium, hydrogen, aluminium, potassium).
- Clay has a capacity for water retention and will expand on being wetted and shrinking on drying. Anyone familiar with claylands will have seen deep and wide cracks in grass fields during times of drought and probably got clay stuck on their boots.
Classifying a soil texture
Before you can describe the soil texture you need to know the relative proportions of sand, silt and clay within a soil sample. Once you have done this (burn off the organic matter first with dilute hydrogen peroxide then pass the material through a series of sieves – remember how sand, silt and clay are all different sizes) you can label the soil using the soil texture triangle.
Have a go at describing a soil. Go along each side, marking the percentage of each constituent material, then draw lines down the tringle and see where the three cross over.
If, for example, your soil sample has 50% sand, 20% silt and 30% clay, it would be described as a ‘sandy clay loam’. If it was 70% clay, 20% silt and 10% sand it would be described as a ‘clay’. The term ‘loam’ is used to describe soils that have a broadly similar concentration of sand, silt and clay particles.