Soil Systems (5.1)

Select from the list of words at the end to allow you to type out the words that are blanked out from the text. There are four main horizons – O, A, B and C. These can be further divided into many more sub-horizons but for the purposes of this course – four is enough. Soils will not always have all the horizons.The Organic horizon (referred to as the O horizon) is on the top of the soil and includes all the dead organic matter that accumulates on top of the soil, such as leaves. This is gradually decomposed.The A horizon is the top soil layer. This layer is usually dark in colour due to the high proportion of humus (decomposed organic matter). The high organic content means it is a zone of highest biological activity. This layer has often lost clay and dissolved substances as it has been eluviated or washed out.The B horizon is the sub-soil and tends to be the zone of illuviation or accumulation - minerals and particles are washed into this horizon from the ones above. Plant roots are likely to be found in this layer but very little humus is found here.The C horizon is the decomposed parent material. In most cases this layer is not really affected by soil processes but it has weathered. This layer may contain large lumps of parent material. For each item, click and select if it is an input or an output. Inputs to soil Minerals from weathering of parent material, Organic matter from the living organisms that are on and in the soil Nitrogen gas by nitrogen fixing bacteria Water by precipitation leading to infiltration, or by throughflow. Outputs from soil Minerals by leaching Clay particles washed by water Nitrates through uptake by plants Clay by ingestion by animals (animals eat the soil to neutralise toxins) For each item, click and select if it is a storage or a flow. Storages in soil Organic matter Organisms Water Minerals Air Nutrients e.g. nitrates, phosphates Flows in soil Leaching Evaporation Decomposition Weathering