IB Environmental systems and societies 5.1F Soil Formation Notes

Five Interacting Factors Control How Soils Form

1. Soil development is driven by five factors: climate, organisms, relief, parent material, and time.
2. Together they determine a soil's texture, structure, fertility, depth, and horizons.

Climate: the Dominant Factor

1. Climate has the strongest influence, controlling weathering, organism activity, humus production, and leaching.
2. High temperatures speed up chemical weathering, especially in the tropics.
3. Freeze and thaw cycles in cold or temperate climates drive mechanical weathering that shatters rock.

1. High rainfall causes intense leaching, producing deep but nutrient-poor soils.
2. Low rainfall limits leaching and can leave salts accumulating near the surface, as in desert soils.
3. Cold climates slow decomposition, so thick layers of undecomposed litter build peaty soils.

Organisms

1. Soil organisms drive nutrient recycling, humus formation, and structure.
2. Plants add organic matter through litter, root decay, and exudates.
3. Vegetation also sets soil chemistry: forests tend toward acidic soils, while grasslands build nutrient-rich upper horizons.
4. Earthworms and burrowing animals cause bioturbation, improving aeration, drainage, and mixing.
5. Microbes decompose organic matter and release acids that help break down minerals.

Relief and the Soil Catena

1. Relief is the slope angle, length, aspect, and drainage of the land.
2. Steep slopes carry thin soils because runoff and gravity remove material faster than it accumulates.
3. Gentle slopes and valley bottoms collect deep or waterlogged soils.
4. Aspect matters too: sun-facing slopes are warmer and drier, while shaded slopes stay cool and moist with thicker organic layers.

1. A soil catena is the sequence of different soils down a single slope, all from the same parent material.
2. Upper slopes hold thin, well-drained soils, middle slopes moderately developed soils, and lower slopes deep or waterlogged soils.

Parent Material

1. Parent material sets the soil's mineral composition and starting texture.
2. Sandy parent material gives coarse, well-drained but nutrient-poor soils.
3. Clay-rich parent material gives fine soils that hold nutrients but drain poorly.
4. Granite weathers slowly into acidic, nutrient-poor soils, while basalt and volcanic ash weather into fertile soils rich in iron, magnesium, and calcium.
5. Parent material can also be transported, for example as glacial till or river alluvium, so soil does not always form from the rock directly beneath it.

Time

1. Time lets the other factors act, but it does not change soil on its own.
2. Older soils usually show greater horizon differentiation, more weathering, and more leaching.
3. Young soils, such as those after glaciation or volcanic activity, are often thin and poorly developed.

Summary

1. Climate, organisms, relief, parent material, and time interact to produce every soil.
2. Their balance, captured by CLORPT, explains why soils differ so much from place to place.