Soil under the microscope
Just as we humans have parents, so all soils are formed from materials called parent materials. Most soils are formed from hard rock or sediments, many of which date back millions of years. The main exception to this is the peat soils which are derived mainly from buried plant remains. Those soils that have formed from the breakdown of the underlying rock, mineral sediments laid down perhaps by wind and water, or the extensive deposits of the last Ice Age are dominated by mineral particles which have become free after the breakdown of larger rock fragments over many centuries. The mineral particles in soils come in a large variety of shapes and sizes.
These particles have been classified into four main size fractions:
In the case of we humans, our bones form our skeleton. In the case of soils this mixture of mineral grains of different sizes forms the skeleton of the soil.
Soils vary greatly, both laterally and vertically, in terms of the proportions of these particles. Sandy soils have at least 70 percent of sand-sized grains whereas clay soils are dominantly (>60 per cent) of clay particles. Because of the size and nature of the particles that make up these soils they behave very differently. Sandy soils have a coarse feel to them, clay soils have a smooth, velvety feel, particularly when moist. Sandy soils tend to hold little moisture and tend to be droughty whereas clay soils hold lots of water some of which is available to plants.