Woody Plants BI237
Week 3
CHEMICAL PROPERTIES OF SOIL
MACRONUTRIENTS are required in relatively large amounts. They are:
Element
Form assimilated
Source
Important functions in brief
Carbon
CO2
Atmosphere
Basic building blocks of life.
Hydrogen
H2O
Water
Basic building blocks of life.
Atmosphere,
Water
Basic building blocks of life.
Oxygen
CO2,H2O
Nitrogen
NH4+, NO3-
Organic matter,
Atmosphere
DNA, RNA, protein, chlorophyll, etc.
Phosphorus
H2PO4-
Mineral soil,
Organic matter
DNA, RNA, membranes, etc.
Potassium
K+
Mineral soil,
Organic matter
Osmotic regulation, enzyme cofactor, etc.
Sulfur
SO42-
Mineral soil,
Organic matter,
Atmosphere
Proteins, etc.
Magnesium
Mg2+
Mineral soil
Chlorophyll, enzyme co-factor.
Calcium
Ca2+
Mineral soil
Cell walls, formation of nucleus and
mitochondria, etc.
Micronutrients (copper, zinc, iron, etc.) are required in small amounts.
Which nutrients can plants obtain directly from the atmosphere? Only C and O.
All other nutrients obtained from soil.
N often limits growth in temperate systems.
P limits growth in tropical forests.
Woody Plants BI237
Week 3
CATION EXCHANGE CAPACITY
Notice that many macronutrients have a positive charge (NH4+, K+, Mg2+, Ca2+)
(Positively charged ions are called cations, negatively charged ions are called anions.)
Clay micelles (phyllosilicates -- like layers of pages or leaves) are negatively charged.
Opposite charges attract, therefore soil will hold nutrient cations in solution.
The Cation Exchange Capacity (CEC) represents the total amount of cations (in
centimoles) that can be adsorbed by a kilogram of soil. For example, a moraine with
sugar maple and basswood may have a CEC of 18 centimoles/kg, while an outwash plain
with oaks and pines may have a CEC of 12 centimoles/kg.
When cations in the soil are adsorbed by plants, those held by clay particles are released.
A new equilibrium is established. Adsorbed cations are a "reservoir" of nutrients.
Some cations are held onto the clay very strongly. They are not so available to plants.
Some are less attracted to the clay. They are easier for plants to obtain but are also more
easily lost from the soil by leaching.
(With the exception of hydrogen, larger ions and those with a large positive charge are
most strongly held.)
Here they are in order, from most to least tightly held:
Al3+ > H++ > Ca2+ > Mg2+ > K+ = NH4+ > Na+
Cation exchange capacity rises as the proportion of clay rises.
However, highly weathered clays like those in old tropical soils have lower CEC.
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SOIL ACIDITY
pH = -log [H+]
pH 7 is neutral. Below that, each value on the scale has 10X more H+ ions in solution.
pH strongly influences nutrient availability.
If there are lots of H+ ions in the soil, they will tend to pre-empt the other cations from
sticking to the clay. (Remember that clay has a higher affinity for H+ than it does for
almost anything else.)
This can increase rates of mineralization and decrease activity levels of some soil
organisms.
An important source of H+ is respiration of roots and soil micro-organisms.
CO2 + H2O --> H2CO3
This weak acid (carbonic acid) can dissociate, releasing H+:
H2CO3 --> H+ + HCO3 -
Woody Plants BI237
Week 3
Conifer needles release even stronger organic acids that can hasten the acidifying of
forest soils.
Human industrial activity has also increased soil acidity (from acid rain). Soils that are
already acidic are especially vulnerable.
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SOIL ORGANIC MATTER
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Helps soil "hang together" (improves structure)
Holds water and nutrients
Supports microbes -- these are critically important in nutrient cycling
What is it made of?
Mostly cellulose, hemicellulose, and lignin -- materials in cell walls of plants
When broken down by micro-orgs, get: CO2 and humus
What is humus?
Chemically-resistant; dark in color, can remain for hundreds of years
Often has a negative charge -- can hold lots of nutrient cations
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HOW DO NUTRIENTS GET INTO THE SOIL IN THE FIRST PLACE?
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
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(Decomposition)
Weather from minerals (but not N)
Fall from sky (especially nitrates and sulfates from pollution)
Micro-organisms "fix" atmospheric N.
Remember that N is often a limiting factor for plants in temperate zones.
Even though N2 is the most common gas in the atmosphere, higher plants can't use it
from there.
Micro-orgs transform it into NH4+, a biologically useful form. Fixation requires energy.
Woody Plants BI237
Week 3
WHO FIXES NITROGEN?
1. Free-living N-fixing saprophytic bacteria
These organisms get the energy they need from decomposing organic matter.
They are restricted to ecosystems, like the Pacific Northwest and Northeastern US, where
there is a lot of organic matter lying around.
They don't really contribute much to the N budget.
2. Photosynthetic bacteria (aka cyanobacteria)
Cyanobacteria photosynthesize to get the E they need.
They need lots of light.
Sometimes they get together with fungi (the partnership is called a lichen).
Nostoc is a cyanobacteria inside lichens that grow on crowns of old-growth Douglas-fir.
3. Symbiotic bacteria live inside plant roots; induce formation of nodules, where N fixed.
Get lots of E from the plant; supply lots of N in return
A. Rhizobium -- bacterium that "infects" many members of the legume family
Includes many trees in the New World tropics. Black locust.
B. Frankia are actinomycetes (filamentous bacteria).
Live with Alnus, Comptonia, and bayberry among others.