Soil and topographic factors
Edaphic factors are the properties of soil that act on organisms: chemical composition, pH, salinity, structure, moisture, air and heat regime, organic matter (humus). Soil forms from the weathering of rock together with the activity of organisms; V. V. Dokuchaev named five factors: parent rock, climate, relief, living organisms and time (today human economic activity is added). Improper use leads to soil erosion, salinisation and waterlogging – a pressing problem also for irrigated land in Uzbekistan. Topographic (relief) factors – altitude above sea level, steepness and exposure of a slope – change climate and soil. In mountains temperature falls by about 6 °C per 1000 m, air pressure drops and ultraviolet radiation rises, so plants are arranged in altitude belts (altitudinal zonation). In the Northern Hemisphere a south-facing slope receives more sun: it is warmer and drier, while a north-facing slope is cooler and moister; on steep slopes soil is washed away and mainly xerophytes grow. Mountains act as geographic isolation in the formation of new species and as a barrier to migration.
In the schoolyard or garden compare soil moisture (checked by hand) in two places, one open to the wind and one sheltered, and record the result. Soil may contain wire or broken glass – wear gloves or tell your teacher; wash hands afterwards.