1. Biotic and Abiotic factors that control soil development
Soil Forming Factors
Biotic and Abiotic factors that control soil development

2. Soil is a function of
G +
C +
O +
R + T

3. PARENT MATERIAL
From the rocks
Geology
BIOTIC or ABIOTIC ?
Soil gradually develops from weathered rock called regolith
Minerals / nutrients in the soil come largely from the rock below.

4. Different Minerals weather at different rates and in different ways.
e.g. Granite
Quartz – hard – physical weathering – sands
Feldspar and Mica – ‘softer’ – chemical weathering - clays

5. Sand
Coarser texture
Good drainage (‘dries out’)
Shallower soil
Fewer nutrients (‘hungry’ soil)
Clay
Finer texture
Poor drainage (‘heavy soil’)
Deeper soil
More nutrients

6. Parent Rock affects …
Depth
Texture
Drainage / permeability
Quality / nutrients
Colour
Some rock types are the dominant soil forming factor in the U.K. …
Limestone soils –
Thin
stoney
Alkaline
Much calcium carbonate

7. CLIMATE – most important soil forming factor at the world scale
CLIMATE – most important soil forming factor at the world scale. BIOTIC or ABIOTIC ?
Wet climate
Much vegetation
More leaf litter
More humus

8. If Precipitation is heavy
Main soil water movement will be downwards
Leaching nutrients downwards (K, Ca, Mg etc.)
Eluviation of clay minerals

9. Where evapotranspiration
Exceeds precipitation
Main soil moisture movement is upwards
By capilliary action
Taking nutrients up towards the surface
Giving darker surface layers

10. RELIEF or TOPOGRAPHY BIOTIC or ABIOTIC?
Higher land is wetter
Higher land is cooler with shorter growing season
Aspect is important – south facing slopes are sunnier and drier in the northern hemisphere

11. Leaching on well drained slope
Peaty gley – on flat summit
Poor drainage on flat ground
At bottom of slope
= waterlogging and gleying

12. The catena concept – nature of soil varies with position on a slope

13. ORGANISMS ( BIOTA) BIOTIC or ABIOTIC
Plants, bacteria, fungi and animals all interact in the nutrient cycle.
Plants take up nutrients from soil water
Plants return nutrients in leaf litter
Decomposers rot the litter and make humus. Others mix it into the soil.

14. Mull Humus
Deciduous leaves Not too acidic Encourage bacteria and worms Mix quickly into the soil
Crumbly , black, nutrient rich soil e.g Brown Earths
Mor Humus
Acidic pine needles slow to decompose cold, wet upland areas
Fibrous, acidic and nutrient deficient surface horizon called mor e.g. in Podsols.

15. TIME BIOTIC or ABIOTIC?
Soils form slowly
400 years for 10 mms.
Upland Northern Britain soils are under 10,000 years old
Parent material and climate influence rate of development
Sand or clay
Wet and hot or cold and dry?

16. Originally the soil is like its parent material
Over time the soil is less of a regolith
With more organic matter and organisms
Horizons develop as the soil reaches a state of equilibrium with the environment.

17. A mature soil has four main components Biotic = …% Abiotic = …%
40%
45% interchangeable
10% + 5%