Comparing soils

Type of Soil Example of Test Jar

Sandy soils are typically comprised of approximately 80 - 100% sand, 0 - 10% silt and 0 - 10% clay by volume. Sandy soils are light and typically very free draining, usually holding water very poorly due to very low organic content.

                                                           

Loam soils are typically comprised of approximately 25 - 50% sand, 30 - 50% silt and 10 - 30% clay by volume.  Loam soils are somewhat heavier than sandy soils, but also tend to be fairly free draining, again, due to typically low organic content.                                                            

  Clay soils are typically comprised of approximately 0 - 45% sand, 0 - 45% silt and 50 - 100% clay by volume.  Clay soils are not typically free draining, and water tends to take a long time to infiltrate. When wet, such soils tend to allow virtually all water to run-off. Clay soils tend to be heavy and difficult to work when dry.                                                               

 

Soil profiles

Soil profiles for different soils

General soil profile

• The topsoil is the richest part of the soil in terms of minerals and humus.

• When hedges are removed the topsoil may be removed by high winds and can create `dust bowls` - plants and crops cannot grow in the soil if this happens.

An experiment to show capillarity according to the diameter of the glass tube

Capillarity in soil

• Two forces operate in capillarity ( how water moves up through soil):

• Adhesive forces – forces of attraction between unlike molecules such as water and soil particles

• Cohesive forces – forces of attraction between like molecules such as water and water

• The diagram opposite shows how the diameter of the tube affects the height that the dye will reach and this is linked to the two forces of adhesion and cohesion.

Experiment to show capillarity in 4 soils

• The water would travel further in column D because this contains silt particles which have the fewest air spaces and there are stronger adhesive forces ( between soil particles and water) than in other soils where the air spaces are larger.

Soil drainage experiments

• Sand drains easily because the particles have large air spaces in between and the water can drain easily

• Clay soils are compact with very little air in the soil. They do not drain very well at all.

• Mixed soils such as LOAM (garden soil) are a good mixture of sand, clay, humus, stones etc and it drains quite well and, generally, does not become water logged unless there is excessive rain.