How soils supply plant nutrients An Introduction to Soil Chemistry

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How soils supply plant nutrientsAn Introduction
to Soil Chemistry

• Prepared by
• Richard Stehouwer
• Department of Agronomy

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• Title the one-pager appropriately to reflect the
  content.
• Write two quotations form the reading. Use
  proper grammatical format.
• Use three visual images to create focus to your
  page.
• Place five essential vocabulary words around the
  images. Write the main idea of the reading.
• Write two questions and answer them.
• Put a symbolic border around the edges of the
  page.

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Our questions

1. What is in soil?
2. Where do soil nutrients come from?
3. How do plants get their nutrients?
4. Is there a difference in the adsorption process
   for different nutrients?
5. What about salinity?

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What makes up the soil?
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Soil ComponentsThe 4 parts of soil
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Lets go back to 9th grade

• Important cell processes
• Protein synthesis
• Photosynthesis
• Cell respiration
• What are the materials needed for these?
• Phosphate (ATP)
• Carbon
• Oxygen
• Nitrogen
• Carbon dioxide

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A. Supplying Plant Nutrients
Nutrients that plants obtain from the soil

• Macronutrients
• (needed in large amounts)
• Nitrogen (N)
• Phosphorus (P)
• Potassium (K)
• Calcium (Ca)
• Magnesium (Mg)
• Sulfur (S)

• Micronutrients
• (needed in small amounts)
• Chlorine (Cl)
• Cobalt (Co)
• Copper (Cu)
• Iron (Fe)
• Manganese (Mn)
• Molybdenum (Mo)
• Nickel (Ni)
• Zinc (Zn)

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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

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A. Recycling plant nutrients
N
P
K
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B. Breakdown of soil minerals
Zn
Ca
K
Ni
Cu
Mg
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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

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How do plants get their nutrients?
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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.

P
K
Ni
Ca
Mg
Zn
Cu
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Another word for this loss of nutrients is
LEACHING.
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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

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Is there a difference in the adsorption process?
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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

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1. Claya. 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

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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. Negative charges have to
be balanced by positive charges called cations.
1/20,000 in
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Cation Retention onSoil Clays
Calcium, 2
Magnesium, 2
Potassium, 1
Ammonium, 1
Sodium, 1
Copper, 2
Aluminum, 3
Hydrogen, 1
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2/3. Organic Matter and pH
Hydrogen
Nutrients
Increasing pH increases cation exchange capacity
of organic matter
Neutral pH, 7 (sweet soil)
Low pH, 4 - 5 (acidic soil)
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B. Negatively Charged Nutrients(Anions)

• Some very important plant nutrients are anions.
• Soils are able to retain some of these nutrient
  anions.
• Retention of nutrient anions varies from one
  anion to another

2-
1-
2-
1-
Nitrate
Phosphate
Sulfate
Chloride
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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
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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

NO3-
If nitrate is not taken up by plants it is very
likely to be lost from the soil
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H. Moving nutrients from soil to plants
Nutrients in soil solution
Nutrients on soil clay and organic matter
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Excessive Nutrient Loading
Nutrients in soil solution
X
Nutrients on soil clay and organic matter
Nutrient loss in drainage water
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What about salinity?

• Sometimes there is a build up of salt in the
  soil. (This is caused by our irrigation
  practices DONT 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.

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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

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