5.1 - soil, inputs, outputs, flows, formation, conservation, horizons

soil definition

outermost layer of Earth’s surface consisting of complex mixture of:

• 45% weathered rock

• 25% air

• 25% water

• 5% organic matter

gravitational water

free water that drains through large soil pores due to gravity

• not available for plants

capillary water

held in smaller pores by surface tension against gravity

• available to plants

hygroscopic water

tightly bound to soil particles by adsorption

• unavailable to plants

ecosystem functions of a soil

• water, nutrients, anchorage

• medium for plant growth

• nutrient cycling

• habitat

• buffer for temperature change

• buffer for water flow

• buffer for pH and nutrient retention

soil profile

combination of soil horizons

• vary in organic contents, minerals and composition

soil horizons definition

distinct horizontal layers in soil

• develop as a result of the transfer processes that occur in the open system of a soil

soil horizons levels:

• O → organic

• A → surface/ top soil

• B → subsoil

• C → substratum/ parent material

• R → bedrock

O horizon composition

• dead and decaying organic matter

  • leaf litter

  • dead organisms

• humus

* very vulnerable to erosion

A horizon composition

• fully decomposed organic matter

* most fertile because its nutrient rich

* has lots of air spaces where plants can root

* vulnerable to erosion

B horizon composition

• deposited minerals

• metal salts

C horizon composition

• weathered rock

R horizon composition

• unweathered rock

humus definition

dark substance of decomposed organic matter in soil that results from the breakdown of plant and animal residues by microorganisms

humus properties

• nutrient rich/ nutrient retention capacity

• improves soil texture

• high water retention capacity

• aeration capacity

litter definition

decomposing plant and animal matter

soil as carbon sink

stores atmospheric carbon dioxide in the form of soil organic matter and other organic compounds

tropical vs other soils as carbon sink

• tropical soils store little carbon because wet and warm conditions

  • sped up decomposition

    • most carbon is recycled back into atmosphere rather than soil

    • nutrients are rapidly taken up by plants

properties of tropical soils

• fast soil formation

  • due to quick weathering → release of minerals → soil horizons develop fast

• very fast decomposition → quick recycling into biomass → deep but nutrient poor horizons

soil inputs

• plant/ leaf litter

• dead animal biomass and manure

• nutrients from weathered rock

• solar energy and heat

• water

• anthropogenic

• gas

anthropogenic soil inputs

• compost → improve physical/ biological/ chemical

• fertilisers

• agrochemicals → crop protection/ disease control

• irrigation

• salinisation

soil flows; transfers

• biological mixing of nutrients and organic matter

• infiltration

• percolation

• leaching

• uptake of nutrients by plants

• soil erosion

infiltration definition

precipitation seeps into soil

percolation definition

water moving through the soil

soil flows; transformations

• decomposing of organic matter

• chemical weathering → inorganic matter ↑ → rock breaks down forming soil and releasing minerals

• nutrient cycling

• salinisation

soil storages

• weathered materials

• organic matter

• air and moisture

soil outputs

• decomposition 

• erosion

• water and nutrient uptake by roots

• leaching

• diffusion

• evaporation

• heat loss

podzolization

leaching under acidic conditions in which iron and aluminium are leached from upper horizons and deposited in lower

fertile soil formation process

1. mechanical and chemical weathering

   • produces small particles and releases minerals

2. deposition of sediment

   • increasing soil depth

3. dissolving of particle minerals

   • release soluble minerals

4. colonisation of parental rock by pioneer species (bacteria and fungi)

5. nitrifying bacteria fix atmospheric nitrogen

   • add available nitrogen to soil

6. introduction of saprothrops/ decomposers

   • incorporate organic matter into the soil

7. growth of mosses

   • stabilise soils

8. earthworms churn

   • spread soil particles and open soil pores

9. precipitation adds water to soil

factors affecting soil formation

• C → climate (temperature, precipitation → warm and wet → fast development)

• O → organisms

• R → relief/ topography (slope, sun exposure, drainage)

• P → parent material

• T → time

soil conservation techniques

• crop rotation → minerals from rakes up from soils by crops need replacement

• compost → humus content/ decomposer communities need to be maintained

• avoid over-irrigation → reduce leaching of minerals

• terracing → prevents erosion of soil particles

• avoid over-grazing → maintain soil texture