5.1F Soil formation (HL) Notes

1. Soil development is influenced by five fundamental factors: climate, organisms, geomorphology (relief/landscape), parent material (geology) and time.
2. These factors interact to determine soil texture, structure, fertility, water-holding capacity, mineral composition, and the depth of horizons.

Climate as a Factor in Soil Formation

1. Climate is the most influential soil-forming factor, strongly controlling weathering rates, organism activity, humus production, and leaching.
2. Temperature influences weathering speed:
   1. High temperatures increase chemical weathering, especially in tropical biomes.
   2. Low temperatures slow down decomposition, leading to thick organic layers in cold biomes.
3. Temperature contrast within a biome affects freeze–thaw cycles, which enhance mechanical weathering and break down rock.
4. Precipitation controls leaching:
   1. Very high rainfall leads to intense leaching, often forming nutrient-poor soils, such as oxisols in rainforests.
   2. Low rainfall leads to limited leaching and accumulation of salts, common in desert soils.
5. Humus formation is climate-dependent:
   1. Temperate biomes have moderate decomposition, producing moderately thick humus.
   2. Cold biomes accumulate thick layers of undecomposed litter, leading to peaty soils.

Organisms and Their Influence on Soil Formation

1. Soil organisms influence nutrient recycling, humus formation, and soil structure.
2. Plants contribute organic matter through leaf litter, root decay, and root exudates.
3. Vegetation type controls soil acidity and nutrient status:
   1. Forests often develop more acidic soils due to acidic litter.
   2. Grasslands have nutrient-rich upper horizons, due to dense root networks.
4. Animals such as earthworms, insects, and burrowing mammals promote bioturbation, improving aeration, drainage, and mixing of organic matter.
5. Microorganisms (fungi, bacteria) drive decomposition and chemical changes, releasing humic acids that help break down minerals.
6. The presence or absence of vegetation affects evaporation, precipitation interception, and microclimate, altering moisture conditions.

Geomorphology (Relief and Landscape)

1. Relief refers to slope angle, slope length, aspect, and drainage characteristics of the landscape.
2. Steep slopes have thin soils because gravity and runoff remove material faster than it can accumulate.
3. Gentle slopes allow deeper soils to develop, as weathered material remains in place.
4. Valley bottoms often experience waterlogging, producing gley soils due to poor drainage.
5. Aspect refers to slope orientation.
6. Slopes facing the sun are warmer and drier, encouraging faster decomposition but also greater evaporation.
7. Shaded slopes remain cooler and moister, slowing decomposition and resulting in thicker organic layers.