How soils supply plant
nutrients
An Introduction to Soil Chemistry
Prepared by:
Richard Stehouwer
Department of Agronomy
 Title the one-pager appropriately to reflect the
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 Write two quotations form the reading. Use
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 Use three visual images to create focus to your
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 Place five essential vocabulary words around
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 Write two questions and answer them.
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page.
Our questions
1.
2.
3.
4.
What is in soil?
Where do soil nutrients come from?
How do plants get their nutrients?
Is there a difference in the
adsorption process for different
nutrients?
5. What about salinity?
What makes up the soil?
Soil Components
The 4 parts of soil
About ½ of the
soil volume is
solid particles
Mineral
Matter
45%
Organic
Matter
5%
Soil
Air
25%
Soil
Water
25%
About ½ of the
soil volume is
pore space
Lets go back to
th
9
grade…
• Important cell processes:
– Protein synthesis
– Photosynthesis
– Cell respiration
• What are the materials needed for these?
– Phosphate (ATP)
– Carbon
– Oxygen
– Nitrogen
– Carbon dioxide
A. Supplying Plant Nutrients
Nutrients that plants obtain from the soil
Macronutrients:
Micronutrients:
(needed in large amounts)
(needed in small amounts)
• Nitrogen (N)
• Chlorine (Cl)
• Phosphorus (P)
• Cobalt (Co)
• Potassium (K)
• Copper (Cu)
• Calcium (Ca)
• Iron (Fe)
• Magnesium (Mg)
• Manganese (Mn)
• Sulfur (S)
• Molybdenum (Mo)
• Nickel (Ni)
• Zinc (Zn)
2. Where do plant nutrients
come from?
A. Decaying plant litter
B. Breakdown of soil minerals
C. Addition by humans
– Commercial fertilizer
– Manure
– Lime
– Other
A. Recycling plant
nutrients
B. Breakdown of soil minerals
Acid
Water
Zn
Ca
Ni
K
Mg Cu
C. Nutrient additions by
humans
• Commercial fertilizers
– Nutrients are in a form that is available to plants
– Dissolve quickly and nutrients go into soil water
• Lime
– Dissolves slowly as it neutralizes soil acidity
– Releases calcium and magnesium
• Organic nutrient sources
– Manure, compost, sewage sludge
– Decay and nutrient release is similar to crop litter
How do plants get their
nutrients?
A. The soil solution
• Soil water is a complex
solution that contains
– Many types of nutrients
– Other trace elements
– Complex organic molecules
• Nutrients in the soil
solution can be readily
taken up by plant roots
• If nutrients remained in
solution they could all be
quickly lost from the soil.
N
Zn
Ni
P
Ca
K
Mg
Cu
Another word for this loss of
nutrients is LEACHING.
B. Adsorption
• Adsorption refers to the ability
of an object to attract and hold
particles on its surface.
+
-
• Solid particles in soil have the
ability to adsorb
– Water (WHC)
– Nutrients and other chemicals
• The most important adsorbers in
soil are
– Clays
– Organic matter
Is there a difference in the
adsorption process?
A. Cation Exchange Capacity
• Cation exchange capacity
(CEC) is the total amount of
cations that a soil can retain
• The higher the soil CEC the
greater ability it has to
store plant nutrients
• Soil CEC increases as
1. The amount of clay increases
2. The amount of organic
matter increases
3. The soil pH increases
1. Clay
a. Surface Area
¼ cup
¼ cup of clay has more surface area
than a football field
The large surface area of
clay allows it to
• Adsorb a lot of water
• Retain nutrients
• Stick to other soil particles
b. Clay has a negative
charge.
Clay particles are stacked
in layers like sheets of
paper.
Each clay sheet is slightly
separated from those on
either side.
Each sheet has negative
charges on it.
1/20,000 in
Negative charges have to
be balanced by positive
charges called cations.
Cation Retention on
Soil Clays
Calcium, +2
Magnesium, +2
Potassium, +1
Ammonium, +1
Sodium, +1
Copper, +2
Aluminum, +3
Hydrogen, +1
2/3. Organic Matter and
pH
Hydrogen
Nutrients
Increasing pH
increases cation
exchange capacity
of organic matter
Low pH, 4 - 5
(acidic soil)
Neutral pH, 7
(“sweet” soil)
B. Negatively Charged
Nutrients
(Anions)
• Some very important plant nutrients are anions.
Nitrate
Phosphate
Sulfate
Chloride
• Soils are able to retain some of these nutrient
anions.
• Retention of nutrient anions varies from one anion
to another
Phosphate retention in soil
3. Adsorption on oxide surfaces
Phosphate anions Each held by two
chemical bonds to the
iron oxide surface
Iron oxide surface
Nitrate (NO3-)
retention in soils
Unlike phosphate, nitrate
is very weakly held by soils
• Nitrate does not react to
form new solids
• Nitrate is not held by oxide
surfaces
If nitrate is not taken
up by plants it is very
likely to be lost from
the soil
NO3-
H. Moving nutrients from soil to plants
Nutrients in
soil solution
Plant
Root
Nutrients on soil clay
and organic matter
Excessive Nutrient Loading
Nutrients in
soil solution
Plant
Root
Nutrients on soil clay
and organic matter
Nutrient loss in
drainage water
What about salinity?
• Sometimes there is a build up of salt in the soil. (This
is caused by our irrigation practices… DON’T worry
we will look at it more next week)
• When this happens osmosis occurs and draws the
water out of the roots. This causes the plant to die.
• Only specially adapted plants can survive such
environments.
The black box is open
• Soil consists of mineral and organic matter,
air and water
• Soils are able to adsorb nutrients and other
chemicals
• The most important adsorbers are clay and
organic matter
• Adsorbed nutrients are available
to plants
• Adsorbed nutrients are not prone
to loss in drainage water
• Soil adsorption capacity can be
exceeded leading to greater
nutrient loss