The shape of the land surface, also referred to as relief or topography, is a key soil-forming factor as it has an important influence on local climate, vegetation and the movement of water. Mountains can affect the amount and intensity of precipitation and vegetation growth on a large scale; whereas locally, the angle or slope of the ground controls drainage and movement of materials. Even small variations in elevation can be important in flat lands. River terraces or small depressions can lead to localised improved drainage or waterlogging, respectively. Micro-topography can be particularly important if saline groundwater occurs close to the surface as it will affect evaporation rates. The position of soil in the landscape is very important.
Generally, soils found at the top of a slope tend to be freely draining, while those at the foot of a slope or on the floor of a valley are often poorly drained. In some cases, the water table may be near to or at the surface. In this case, different soils may form on the same parent material, under the same climate and even vegetation type (e.g. a grass-covered slope). Soils occurring on the middle of slopes receive sediment and solutions from higher up but, at the same time, lose material to soils below. In these cases, the actual shape of the slope is important as smooth, irregular, convex or concave slopes will result in different soil characteristics.
Climate – Temperature
In addition to drainage, rainfall and solar radiation, another key factor for soil formation is temperature. While modified by latitude, proximity to the sea and some meteorological conditions known as inversions, ambient temperature generally drops with increasing elevation. Soil formation depends enormously on the climate as temperature and moisture levels affect weathering processes and biological activity. Where precipitation exceeds evapotranspiration, leaching or saturated soils can occur. When the opposite is true, salts can rise to the surface. Chemical weathering is very active in areas with high temperatures and high humidity, while physical weathering dominates in hot, dry desert regions. The broad climatic patterns are driven by ocean currents, weather systems, distance from the sea and topography (mountain chains often act as climatic barriers). The following 4 main climatic groups can be distinguished:
- Dry, (arid and semi arid)
- Temperate and
Climate – Precipitation
Rainfall is distributed very unevenly across the planet. Many areas receive either too much rain or too little. In relation to soil formation, humid conditions lead to more chemical weathering, higher levels of organic matter and leaching of minerals and organic matter. Heavy or prolonged rain can lead to soil erosion and saturated soils. A lack of rain will give rise to the development of crusts and accumulation of salts
- The main forms of precipitation include drizzle, rain, sleet, snow, graupel (soft hail or snow pellets) and hail.
- Precipitation occurs when a portion of the atmosphere becomes saturated with water vapour, so that the water condenses and ‘precipitates’.
- Fog, mist and dew are not considered as precipitation. While low compared to rain, their contribution can be significant ecologically, especially in arid climates.