Soil Catena

Soil Catena is the sequence of soils along a slope.

Explain the effects of climate, relief and drainage on the formation of the three soil types shown. (10) The cold, wet climate that a Podzol develops in means that weathering and humification are both slow. This makes the soil shallow and acidic. The heavy rain also means leaching (and podsolisation) take place and this create a red-brown B horizon, where the iron has been deposited from the A horizon. The iron hard pan develops from this iron and this impedes drainage. The heavy rain therefore can make the soil waterlogged. However, as Podzols’ relief is normally at the top of a slope, drainage will be free in the A horizon (slope gradient and gravity). The milder temperate climate, typical of areas where Brown earth soil is found,, means more humification takes place and more biota exists in the soil. This makes a thick humus layer. The horizons are much less distinct as the biota mix the soil. The relief, often on a slope as shown in the diagram above means drainage is quite free and water can run-ff, but also causes some leaching in the soil, especially towards the lower end of the slope. For a Gley soil, these are typically found at the bottom of a slope (flat relief) and therefore there is poor drainage as there is nowhere for the rain water to run-off. This leads to the soil being waterlogged. This leads to slow weathering and humification, which leads to a thick organic later and acidic humus. Gleying is frequent, due to the flat relief, poor drainage and impermeable soil. To answer this question successfully you need to understand the 5 different soil processes found on page 5 of your summary notes. You must also have a good knowledge of three different types of soil (Pod, Brown, Gley) – their processes and characteristics.