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.
8. The position of the slope affects erosion and deposition.
9. Hilltops experience maximum erosion.
10. Lower slopes and valleys experience deposition of material from above.
11. Slope stability influences soil texture. Unstable slopes often produce coarse, poorly developed soils.

Geology / Parent Material

1. The parent material determines the mineral composition and texture of the soil.
2. Sandy parent materials form coarse soils, which are well drained but low in nutrients.
3. Clay-rich parent materials produce fine-textured soils, which retain nutrients but drain poorly.
4. Granite produces acidic, nutrient-poor soils, because its minerals weather slowly.
5. Basalt produces fertile soils, because it contains minerals rich in iron, magnesium, and calcium.
6. Calcareous (limestone-rich) parent materials produce alkaline soils, which contain abundant calcium carbonate.
7. Volcanic parent materials often produce extremely fertile soils, due to high levels of potassium, magnesium, and other nutrients.
8. In cold climates volcanic soils may remain undeveloped, as seen in Iceland where low biological activity prevents formation of thick soil layers.
9. Parent material may be transported, such as glacial till or river alluvium, meaning soil does not always form from underlying bedrock.

Time

1. Time allows soil-forming processes to operate, but it does not cause soil change by itself.
2. Older soils generally show:
   1. Greater horizon differentiation,
   2. More chemical weathering,
   3. More leaching,
   4. Thicker B horizons.
3. Young soils, especially after disturbances like glaciation or volcanic activity, may remain thin and poorly differentiated.
4. Polycyclic soils undergo frequent changes due to repeated erosion and deposition events.
5. Time needed varies by parent material:
   1. Sands form soil quickly.
   2. Granites and basalts weather slowly.

Soil Catena

1. A soil catena refers to a sequence of different soils along a slope, despite originating from the same parent material.
2. Differences arise due to variations in drainage, microclimate, erosion, and deposition.
3. Upper slopes show thin, well-drained soils.
4. Middle slopes show moderately developed soils.
5. Lower slopes show waterlogged or deeply developed soils because of deposition and a high water table.