Introductory Soil Science

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Introductory Soil Science

• Lecture Week 7
• Nutrients

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Plant Nutrients

• Sixteen chemical elements are known to be
  important to a plant's growth and survival.
• Two main groups
• non-mineral and
• mineral.

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Non Mineral Nutrients

• Hydrogen (H), Oxygen (O), Carbon (C).
• found in the air and water. 
• Converted by photosynthesis into starches and
  sugars
• Plants food

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Mineral Nutrients

• Macro nutrients (used in large quantities)
• Primary
• Nitrogen (N), phosphorus (P) potassium (K)
• Secondary Minerals
• Calcium (Ca), Magnesium (Mg), Sulfur (S)
• Micronutrients (used in small quantities)
• boron (B), copper (Cu), iron (Fe), chloride (Cl),
  manganese (Mn), molybdenum (Mo) and zinc (Zn).

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Importance of Nutrients in Natural Systems

• Nutrient cycling is an important ecosystem
  process.
• Land degradation may adversely affect cycles -
  restoration may have to address this.
• Nutrient status influences the type of plant
  community

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Essential Plant Nutrients

• C, H and O
• Macronutrients - N, P, K, Ca, Mg and S.
• Micronutrients - Fe, Mn, Cu, Zn, B, Mo, Cl
  and Co.

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Generalised Nutrient Cycle (cont.)

• Reserves are mainly in the soil.
• Different nutrients behave differently.
• Annual cycles may involve little new addition
  from the soil reservoir.

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Nutrient Availability

• Majority is not immediately available.
• Nutrients exist in four ways in the soil
• in soil water- readily available (ionic forms),
• on colloidal particles- less available, cation
  exchange,
• in weathering parent material- not immediately
  available,
• in organic matter - not immediately available.

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The Nitrogen Cycle

• A distinctive cycle because N does not occur in
  minerals.
• One of the most important ( often limiting) for
  plant growth.
• The cycle can be considered in four parts
• ammonification,
• nitrification,
• nitrogen fixation,
• denitrification.

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The Nitrogen Cycle (cont.)
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Ammonification NitrificationProteins, etc.
? NH4 NH4 ?NO2_ ?NO3_

• Heterotrophs breakdown polymers (partic.
  proteins) to produce ammonium ions.

• Chemoautotrophs convert ammonium ions to nitrite
  ions, and then others convert them to nitrate
  ions.
• These are aerobic bacteria.

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Nitrogen FixationN2 ? NO3_

• Converted by cyanobacteria and bacteria
• free-living in the soil, or
• in mutualistic relationships with higher plants.
• Best known is the association between legumes
  Rhizobium.
• Also Casuarinas actinomycetes cycads
  cyanobacteria the latter also occur in lichens.

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Denitrification NO3_ ? N2

• Free-living, soil bacteria do the conversion.
• N gas diffuses back into the atmosphere - lost to
  the cycle.

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Nitrogen Cycle - Other Aspects

• Uptake of N
• as NO3- by higher plants
• as NH4 by microorganisms (more competitive)
• Losses of NO3-
• leaching
• denitrification
• fire
• Gains of NO3-
• fixation in leguminous understorey spp. thought
  to be imp. In Aust. Forests.
• thunderstorm activity.

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Nutrient Status of Aust. Soils

• old age and poverty
• Salinity (see OH)
• Paradox of high species richness on very
  infertile soils.

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pH and Nutrient Availability

• Soil pH usually falls in the range 4 - 10.
• pH has marked effects on the chemical form in
  which nutrients occur.
• Nutrient availability can therefore be altered
  (see figure).

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Example - P availability

• pH lt 6.5 Al, Fe /or Ca insoluble phosphates
  begin to form
• 6.6 - 7.5 H2PO4- ions - soluble
• 7.5 - 8.5 Ca insoluble phosphate
• gt 8.5 HPO4-- ions - soluble

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Consequences of High/low pH

• High pH
• a number of micronutrients less available
• example of lime-induced chlorosis
• difficult/expensive to correct - add S
• Low pH
• more serious
• more nutrients are less available
• greater availability of Mn Al - toxicity
• bacterial activity declines
• easier to correct by liming (CaCO3)

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Major Ideas

• Most essential nutrients derive from weathering
  of pm.
• Nutrient cycling is a fundamental ecosystem
  process.
• Most nutrients are held in reserves that are not
  immediately available.
• N is not derived from pm - has an atypical cycle.
• Aust. soils have poor nutrient status -
  evolutionary adaptations of our plants (include.
  mutualistic relationships).
• pH has considerable influence on nutrient
  availability.