Soil and Managements espyr1 SSA s4

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1 Soils in Urban Singapore (SSA2215 S(4)) ©Lim Fang Jeng

Soils are basic and yet valuable material for flora & fauna Soils can ensure healthy plant growth

Unfortunately, soils are not well managed in Singapore

Soils – Basic Concept

Soil: loose surface layer of earth which supports the growth of vegetation Regolith: Loose and unconsolidated material found under the soil profile Bedrock: Original parental rock from which soils are formed

Soil Profiles – Soil Horizon

Soil Formation

- Weathering creates regolith which will then form soils and sediments which will then form sedimentary rocks

- Three types of weathering:

o Physical ( disintegration of rocks)

 Break up to smaller pieces due to alternating temperature, wetting and drying, plant roots, etc

o Chemical (decomposition of rocks)

 Alteration of chemical & mineralogical composition of weathered materials

 Temperature affects the rate of soil formation (Arrhenius equation) Rate = e^−∆

E_a RT

 A= constant; ∆Ea= activation energy; R=Universal Gas’s constant; T=absolute temperature

 This leads to a doubling rate of reaction for every 10^oC rise in temperature o Biological

Organic (O) horizons:

- Accumulation of organic matters o Organic debris (Oj⁾

o Partly decomposed form, or humus (Oa⁾

A horizon (Dark coloured)

- Rich in vegetation roots and mixed with humus E horizon (pale in color)

- Loss of clay(leaching) materials, oxides which will be accumulated to B layer

B horizon

- Gain of material from A & E (Zone of illuviation) - Illuviation – Accumulation of materials to the lower

horizon

- Elluviation – Downward transport of particles C horizon

- Layer of parent materials - Relatively unweathered materials - Less biological activity

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 Disintegration of rocks & minerals by bacteria/plants/animals

 Organisms can influence the pH value of the soil

 Chelation: a biological process where organism produces organic substances to decompose minerals and rocks

- Factors determining soil formation o Dokuchaiev (1882-1900)

 S(soil)=f(climate, vegetation) o Jenny (1941,1946)

 S(soil)=f(cl, o, r, p, t)

 cl= climate; o=organisms; r=relief; p=parent material; t=time

 Now! Human activities also come into account!!! - Temperature and precipitation

o Humid & warm favours intensive chemical & biological weathering o However, it is a slow process

- Geology & rocks

o Least resistant rocks to soil formation are mainly mafic materials

 Disintegration to form various oxides, OH^- & hydrous oxides of Fe & Al o Quartz has high resistant to soil forming

 Hard to break down

 Forms unaltered sand in soil - Biological processes

o_Plants

 Absorb nutrients and return dead plant tissues to the O horizon o_Animals

 Insects – ingest large amount of decaying leaf matter

Soil substances

Solid Inorganic mineral and organic materials Liquid Water but with various ions (soil solution)

Gas Atmospheric gases and gases liberated through biological activity and chemical reaction within soil

- Primary minerals: Original silicate minerals of igneous rock - Secondary minerals: Clay materials in soils

- Pores: Soils which have less pores are said to be low porosity - Relative mobility of major constitutes released during weathering

Ca²⁺> Mg²⁺> Na⁺> K⁺> Fe²⁺> Si⁴⁺> Fe³⁺> Al³⁺ - Soil gases:

o Nitrogen (79%) o Oxygen (20%)

o Carbon dioxide (0.25%)

H₂O + CO₂_{↔ H}₂CO₃_{↔ H}⁻+ HCO₃⁻

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Physical properties

- Soil colour (determined by Munsell chart)

- Bulk density : density of dry soil (determine the compactness of soil) o Reflects the infiltration rate of soil

- Soil texture : determined by percentage of sand, silt and clay

- Soil structure : Presence of aggregations (lumps or clusters) of soil particles

Chemical properties

- Soil colloids: very fine clay

o Chemically active due to their great surface area

- Cation exchange: Substitution of one cation for another of a different element in a mineral structure - Cation exchange capacity (CEC): capacity of a given quantity of soil to HOLD and EXCHANGE cation

- Base cations are important for plant nutrients (in Alkaline condition) - Acid-generating cations (in acidic condition)

Soil Classification

- UNESCO system and USDA Soil Taxonomy - In Singapore, the common soil types are

o Oxisols – highly weathered o Ultisols – Tropical

o Inceptisols – Weekly developed horizons o Entisols – Soil lacking horizons

0 0.5 1 1.5 2 2.5 3

-4 1 6 11 16

Bad Soil Good Soil

Jurong Formation

Old Alluvium

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Role of Soils in Singapore

- Support vegetation growth, though less agriculture activities are found in Singapore - Horticulture – landscape gardening, revegetation

- Catchment protection

o Support plants in Nature Reserves

o Allows rainfall to be intercepted and infiltrate through soils before water ends up in reservoirs - Land reclamation

- Singapore has nice environment for healthy soil formation o Humid tropical climate

o Deeply weathered mantle

o Rapid decomposition of organic matters o Active biological activities

- Natural soils in Singapore are (NOT OF A GOOD QUALITY!):

o Highly weathered – deep soil profiles, lower organic matters and pH values o_{Low CEC}

- Urban soils in Singapore: o Modified

o Designed by engineers - Conditions of soils in Singapore:

o Poor quality with low fertility

o Heavily leached – Low organic matter o Heavily disturbed due to Human Activities

Heavy Compaction

- Stepping on soils

o Impact on aeration and plant growth

o Results in unhealthy plant growt, tree falling, root exposure, etc

o Impact on urban hydrology (soil infiltration rate reduced) and catchment protection - Clayey soil (Unhealthy soils which are common in Sinagpore)

Examples of Unhealthy soils (Hong Kong) and Strategies

- Natural soil forming processes almost non-existent - High compaction

- Little organic matter (Lower CEC) - Lots of stones

- High bulk density (Less pores)

- High pH values (Alkaline – waste materials from construction) - Contrasting soil layers if they are present

o Impact on hydrological processes in terms of movement of H²O within soils Management strategies

- Detailed soil studies should be carried out to study weather the soils are suitable for plant growth - Preventive measures should be implemented before and during construction period

- Plow soils occasionally, Put in trampling resistant soil (coarser particles), build concrete pathways, etc - Use waste materials to improve soil quality by using sewage wastes + horticultural waste to form

materials which has provide good condition for plant growth