A Horizon
A surface mineral layer. It will typically have humus mixed within the soil, have good biological activity, especially earthworm activity, being a good environment for micro-organism activity helping to decompose organic matter within this soil profile. This horizon is also typically relatively fertile, usually having a good range of available plant nutrients. The soil will often be fairly well drained, although still be able to retain adequate amounts of available water for plant use. An 'A' Horizon is typically darkish in appearance and is commonly referred to as topsoil.
Acid Soil
Acid Soil A soil that has a soil pH value of less than 7.0. An acid soil is often referred to as a soil with a low pH.
Aggregate
Either a soil aggregate which relates to a soil structural unit, or coarse material such as gravel used in drainage systems.
Air Filled Porosity
The percentage of pore spaces within a soil that are filled with air. A soil which has a low percentage of air-filled porosity will typically be poorly drained with poor root development. It is also called Air Porosity.
Alkaline Soil
A soil that has a soil pH value of greater than 7.0. A soil with a pH of greater than 8.5 is considered strongly alkaline.
Anaerobic Soil
A soil with an insufficient amount of oxygen for appropriate plant, or grass growth.
Anion Exchange Capacity
An expression or measure of how well a soil can retain or exchange anions. This takes place within a soil solution. Increasing levels of soil acidity increases a soils anion exchange capacity. Whilst clay particles and organic matter have predominantly negative charges they also, at the same time due to their complex structures, contain some areas of positive charges to which anions can be held and retained.
Available Water
Soil water that is available for uptake by the roots of a plant. Water is typically available for use by a plant if it is held within a soil between its field capacity and permanent wilting point.
Available Water Capacity
The relative proportion of water that is held on the outer surface of soil particles and is freely available to a plant. It is water that is held in a soil between what is called its field capacity (FC) and its permanent wilting point (PWP). A general available water capacity for a sandy loam type soil might be in the region of 12% (with its FC being 18% and PWP 6%); by contrast a clay loam might be in the region of 20% (with its FC being 43% and PWP 23%).
B Horizon
A mineral soil layer which is below an 'A' Horizon. It is commonly referred to as subsoil. It will typically be lighter in colour and less fertile than the 'A' Horizon. It may also be poorly drained due to finer, especially clay, particles having washed down from the higher layer. It is also called the zone of illuviation to indicate that material has accumulated in this layer. Some examples of elements that can collect in this layer are iron (potentially forming an iron pan) or calcium carbonate layer.