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Physical Science Applications in Agriculture

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How? Learning Objectives Define pH and discuss its role in plant nutrition. Explain how soils become acidic. Explain how soil pH is measured. – PowerPoint PPT presentation

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Title: Physical Science Applications in Agriculture


1
Physical Science Applications in Agriculture
  • Unit
  • Physical Science Systems

2
Problem Area
  • Agricultural Production Systems

3
Testing Common Substances for pH
  • Lesson

4
Ever see limestone being applied on a growers
field?
  • What is so important about this powdery
    substance?
  • What does it do for the soil?
  • Does the composition or type of limestone
    (liquid, dry, palletized) make a difference in
    its effectiveness?

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5
Which plant looks healthier?
  • Examine the pictures or actual plants that have
    received different levels of fertilizer.
  • Which plant looks healthier?
  • Which plant would produce better and more
    product?
  • How much fertilizer do they apply to get optimum
    growth of our vegetable plants?
  • Is there a way to determine this? How?

6
Learning Objectives
  1. Define pH and discuss its role in plant
    nutrition.
  2. Explain how soils become acidic.
  3. Explain how soil pH is measured.
  4. Explain why lime is applied to acid soils.
  5. Discuss the effectiveness of lime on acidic soils.

7
Terms
  • Acid
  • Calcium carbonate equivalent
  • Cation
  • Cation exchange capacity
  • Lime requirement
  • Percent base saturation
  • pH scale
  • Soil pH

8
What is pH?
  • Soil pH is the measure of acidity or alkalinity
    of the soil.

9
What is the pH scale?
  • The pH scale is a fourteen point scale used to
    measure pH.
  • A neutral pH is 7.0.
  • A solution with a pH between zero and 6.9 is
    considered acid.
  • A solution with a pH between 7.1 and 14.0 is
    considered alkaline or base.
  • The scale is expressed in logarithmic terms.
  • Each unit change in pH corresponds to a tenfold
    change in acidity or alkalinity.
  • A pH of 6.0 is 10 times more acidic than a pH or
    7.0.

10
How does pH affect plant nutrition?
  • The pH value of soil is important to
    agriculturalists because certain nutrients become
    unavailable to plants if the pH value is too high
    or too low.
  • The amount of nutrients that are available is
    dependent upon soil pH.

11
How does pH affect plant nutrition?
12
How do soils become acidic?
  • pH is determined by the concentration of hydrogen
    (H) ions and hydroxyl ions (OH-) in the soil
    solution.

13
How do soils become acidic?
  • A sample of pure water has an equal number of H
    and OH- and is neutral.
  • An acid is a substance that releases hydrogen
    ions.
  • When saturated with H, a soil behaves as a weak
    acid.
  • The more H held on the exchange complex, the
    greater the soils acidity.

14
Several factors influence soil pH.
  • Soil organic matter is continuously being
    decomposed by micro organisms into organic acids,
    carbon dioxide, and water, forming carbonic acid.
  • Carbonic acid reacts with Ca and Mg carbonates in
    the soil to form more soluble bicarbonates, which
    are leached away, leaving the soil more acid.

15
Several factors influence soil pH.
  • As water from rainfall passes through the soil,
    basic nutrients such as calcium and magnesium are
    leached.
  • They are replaced by acidic element including
    aluminum, hydrogen, and manganese.

www.goeverett.com/misticbev/assets/images
16
Several factors influence soil pH.
  • Soils formed under forest vegetation tend to be
    more acidic than those developed under
    grasslands.

www.itc.nl/personal/.../ t1765e0s.htm
17
Several factors influence soil pH.
  • Soils often become more acidic when crops are
    harvested because bases are
  • removed.
  • Legumes generally contain higher levels of bases
    than grasses.
  • Legumes also release H ions into their
    rhizosphere when actively fixing atmospheric N.

18
Several factors influence soil pH.
  • Nitrogen from fertilizer, organic matter, manure,
    and legume N fixation produces acidity.
  • Nitrogen fertilization speeds up the rate at
    which acidity develops.
  • At lower N rates, acidification rate is slow, but
    is accelerated as N fertilizer rates increase.

19
ACID AND ALKALINE SOILS
20
How is soil pH measured?
  • The two most commonly accepted methods of
    measuring soil pH are indicator dyes and the pH
    meter.

ltpwww.gsfc.nasa.gov... globe/pvg/19-5ph.jpg
21
How is soil pH measured?
  • Indicators are frequently used in the field to
    make a rapid pH determination and must be used by
    a trained hand to avoid major error.
  • The more accurate and widely used method is the
    pH meter used in soil testing laboratories.

www.bbc.net.uk/garde... asics/soil_test7.jpg
22
Why is lime applied to acidic soils?
  • Since various plants require different pH levels
    for optimum growth, growers must attempt to
    adjust soil pH to suit the crop or plant being
    grown.
  • This involves the use of limestone to raise pH or
    the use of alum to lower pH.
  • Lime requirement is the amount of agricultural
    limestone needed to establish the desired pH
    range.

23
Why is lime applied to acidic soils?
  • Soil pH is an excellent single indicator of soil
    acidity, it does not determine lime requirement.
  • Lime requirement of a soil is not only related to
    the pH but also to its buffer capacity or cation
    exchange capacity.
  • Cation exchange capacity is the total number of
    exchangeable cations, an ion with a positive
    charge, a soil can adsorb.
  • The relative amount of the cation exchange
    capacity filled with basic cations is called
    percent base saturation.
  • Soil pH is a measure of the percent base
    saturation.

24
Why is lime applied to acidic soils?
  • Lime replaces hydrogen and aluminum on the cation
    exchange sites with calcium and changes hydrogen
    ions to water.

www.imdexminerals.com.au/ag.asp
25
Factors to figure how much lime is required
  • the present pH
  • the desired pH
  • the cation exchange capacity of the soil
  • the liming material to be used

www.hort.wisc.edu/.../ soiltest/soiltest.htm
26
The effectiveness of lime on soil.
  • The effectiveness of lime depends on
  • Purity
  • Fineness
  • Rate it dissolves
  • Measured as the calcium carbonate equivalent

27
What determines the effectiveness of lime on acid
soils?
  • The neutralizing power of lime depends upon its
    purity, measured as the calcium carbonate
    equivalent.
  • Neutralizing values of all liming materials are
    determined by comparing them to the neutralizing
    value of pure claim carbonate.
  • Setting the neutralizing value of calcium
    carbonate at 100, a value for other materials can
    be assigned.

28
What determines the effectiveness of lime on acid
soils?
  • When a given quantity of lime is mixed with the
    soil, its reaction rate and degree of reactivity
    are affected by particle size.
  • Coarse live particles react more slowly and less
    fully.
  • Fine lime particles react more rapidly and much
    more completely.
  • Cost of lime increases with the fineness of
    grind.
  • The goal is a material that requires a minimum of
    grinding, yet contains enough fine material to
    cause a rapid pH change.

29
Other important factors determining the
effectiveness of lime
  • Placement for maximum contact with the soil in
    the tilled layer is essential.
  • Even when properly mixed with the soil, lime will
    have little effect on pH if the soil is dry.
  • Moisture is essential for lime-soil reaction to
    occur.
  • The full benefit of lime is not seen until the
    second or third year after application.
  • Lime does not react with acidic soil very far
    from the lime particle.

30
Review/Summary
  • What is pH and how does it affect plant
    nutrition?
  • How do soils become acidic?
  • How is soil pH measured?
  • Why is lime applied to acidic soils?
  • What determines the effectiveness of lime on acid
    soils?
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