Transcript LECTURE 10 - Rhodes University

LECTURE 10
Introduction to some
chemical properties
of soils :
Factors affecting plant growth
(2)
What chemical properties affect
nutrient availability?
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Amount of clay and humus
Cation Exchange Capacity (CEC)
Soil pH
Amount of moisture
Temperature
Why is the amount of clay and
humus important?
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The exchange of nutrient ions between soil particles
and roots is one of the most important processes in
nature (others are photosynthesis and respiration).
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Ion: “An atom or group of atoms that are positively charged
(cations) because of the loss of electrons, or negatively charged
(anions) because of a gain of electrons.” – Soil Science Society of SA
Ion exchange occurs mostly on the surfaces of the finer
or colloidal fractions of both inorganic matter (clay) as
well as organic matter (humus).
Properties of soil colloids…
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Extremely small (too small to be seen with an ordinary
light microscope).
Large surface area (surface area of 1g of colloidal clay
is at least 1000x that of 1g of coarse sand).
Surface charges
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Most soil colloids have negative charges, but some mineral
colloids in very acid soils have a net positive charge.
Influences both physical and chemical properties.
Adsorption of cations and water.
Why are most colloids negatively
charged?
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Ionization of hydrogen from hydroxyl ions on
clay surfaces.
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Higher pH = more ionization of hydrogen.
Isomorphous substitution.
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Definition: “The replacement of one atom by another of
similar size but not necessarily the same valence in a
crystal structure without disrupting or seriously changing
the structure.” – Soil Science Society of SA
Colloids and cation exchange…
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Cation exchange occurs at the surfaces of
colloidal particles.
Cation exchange = “The interchange between
a cation in solution and another cation
adsorbed on the surface of any surface-active
material such as a clay colloid or organic
colloid.” – Soil Science Society of SA
Why is cation exchange important?
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Exchangeable K, Mg and Ca are important sources of
plant K.
The amount of lime required to neutralize a soil inc. as
CEC inc.
Cation exchange sites slow losses by leaching.
Cation exchange sites hold fertilizers and so reduce
their mobility.
Cation exchange sites adsorb many metals from
wastewater, preventing them from entering the ground
water.
Cation Exchange Capacity (CEC)…
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Definition: “The sum total of exchangeable cations that
a soil can adsorb. This soil property is due to the
negative electrical charge of the colloidal (both organic
and inorganic) fraction of most soils. The negative
charge is balanced by adsorbed cations so that the soil
system as a whole is electrically neutral. The balancing
cations represent a definite quantity referred to as the
cation exchange capacity (CEC).” – Soil Science Society of
SA
Soil pH…
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This gives a measure of the acidity or basicity of a
soil.
0-7 = acidic; 7-14 = basic.
Acidity is measured by determining the
concentration of Hydrogen (H+) ions in the soil.
Higher concentration of H+ ions = high acidity,
higher concentration of OH- ions = high basicity.
In general, the ideal pH for plant growth is about
5.5 in organic soils and about 6.5 in mineral soils.
Where do H+ and OH- ions come
from?
Why is soil pH important?
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Affects solubility of minerals.
Affects type, numbers and activity of
microorganisms.
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Fungi tolerate acidity better than bacteria. Bacteria often
negatively affected by high acidity (i.e. low pH).
Indirectly affects aggregate stability.
Determines what happens to many soil
pollutants.
CEC increases with soil pH.
Soil Buffering Capacity…
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The tendency of soils is to resist changes of
the pH of the soil solution.
This resistance is termed “buffering”.
Soils have different buffering capacities.
Generally, higher CEC = greater buffering
capacity.
Buffering capacity indicates dynamic
equilibrium of soil solution.
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Changes of all types tend to be resisted by the system.