II.G._ TREE AUTECOLOGY: PLANT NUTRIENTS
Note: in Kimmins Chapter 11, review general soil properties but focus on plant nutrients; in Chapter 5
focus on plant nutrition and nutrient uptake.
Principal questions to be addressed:
1. What are the required elements?
2. How are plant nutrients stored in the soil?
3. How are nutrients obtained by plants?
CHEMICAL ELEMENTS REQUIRED BY HIGHER PLANTS
A. Nutrients absorbed from the soil solution
Macronutrients:
Nitrogen, Phosphorus, Sulfur, Potassium, Magnesium, Calcium
Micronutrients:
Iron, Molybdenum, Manganese, Copper, Boron, Zinc, Clorine, Cobalt
B. From CO2 and H2O: Carbon, Oxygen, Hydrogen
NUTRIENT STORAGE IN THE SOIL
1. In forms that are not immediately available to plants:
a. in minerals
b. in organic compounds
2. In relatively available forms as ions (cations are positively charged groups of atoms, and anions
are negatively charged groups of atoms). Nutrient cations and anions can be
a. adsorbed (attached) to soil colloids, or
b. be in soil solution
Examples of relatively available forms:
ammonium salts
NH4+
nitrate salts
NO3phosphate of Ca, K, Mg
HPO4-soluble organic phosphate
H2PO4potassium salts (with sulfates and carbonates)
potassium ions on colloids
K+
calcium on colloids
C++
calcium salts
C++
magnesium on colloids
Mg++
magnesium salts
sulfites
SO3--
sulfates of Ca, K, etc.
SO4--
INFLUENCES ON CATION EXCHANGE CAPACITY
Cations are mainly adsorbed to negatively charged colloidal (< .001 mm in diameter) clay
and colloidal organic matter (i.e. humus); in much smaller quantities they are in the soil solution.
Micelle = a minute silicate clay colloid particle; large surface area is normally negatively
charged.
Cation exchange is the exchange of adsorbed cations on a micelle (or colloid) with those in
solution
A. Soil texture -- increased surface area of finer soils result in a greater CEC
B. Type of soil colloid
The surface area and electrical charge of micelles largely determine the capacity of the soil
to store cations.
1. Humus (colloidal substances that are decomposed from the original organic tissues and
synthesized by various soil organisms)
Very high CEC
Humus is rapidly decomposed (unstable)
2. Mineral colloids
a. Silicate clays and aluminosilicate clays
Sheet-like structures of aluminum, silicon, oxygen, etc. give the clay crystal both an external
surface and an internal surface (i.e. in between layers).
i. non-expanding silicate clays -- rigid, fixed structure; less internal surface area and much
less CEC (e.g. kaolinite typical of humid tropics, halloysite, illite, etc.)
ii. expanding silicate clays -- shrinking and swelling clays; much greater CEC (e.g.
montmorillonite and vermiculite groups)
b. Hydrous oxide clays (iron and aluminum oxides with water)
Non-expansive; smaller CEC
c. Non-crystalline mineral colloids:
e.g. allophane (high CEC)
ANION NUTRIENT EXCHANGE
Sulfites, sulfate, nitrates and phosphates are negatively charged (anions) and are mostly in
the soil solution. Consequently, they are easily leached and lost from the soil.
Soils have an anion exchange capacity (AEC) that is usually much less than CEC. Some
clay and organic colloids are amphoteric (i.e. carry both positive and negative charges), and oxides
of iron or aluminum also provide positive charges to which anions are attracted. AEC generally is
greater in more acid soils.
1. positive charges on part of the humus
2. positive charges on exposed edges of crystals in silicate clays where H+ groups are
attached to OH-Anions are also slowly released by the decomposition of organic compounds.
MEASURES OF SOIL FERTILITY
Determination of “availability” of nutrients varies with the analytical method, relationship to
tree growth is often not clear, and the static assessment of nutrient content neglects the dynamic
aspects of nutrient cycling. It is also difficult to determine the appropriate soil volume from which
roots absorb nutrients.
1. Cation exchange capacity (but includes non-nutrient cations too).
2. Soil reaction = acidity or alkalinity of the soil.
pH = the logarithm of the reciprocal of H+
acid if,
H+ > OH-alkaline or basic, if
H+ < OH-Sources of soil acidity:
a. CO2 in solution (carbonic acid)
b. decay of organic matter (organic acids)
c. weathering of granitic rocks rich in feldspars (K and Na)
d. abundance of precipitation
3. Percent base saturation = proportion of CEC occupied by bases rather than Al or H (i.e. sum of
the “exchangeable” cations other than hydrogen or aluminum).
NUTRIENT UPTAKE
Uptake of nutrients from soil solution is not the only mechanism of uptake. There can be
direct uptake from soil minerals (i.e. lithoponics in which roots absorb nutrients from rocks), and
uptake of nutrients by leaves when in contact with a nutrient solution.
Nutrient uptake from the soil solution is affected mainly by:
1. Diffusion transfer – rate at which nutrients diffuse from the surrounding soil to the root (rate of
flow of ions from areas of high to low concentration varies for different ions).
2. Mass transfer (or mass flow) – rate at which water containing nutrients moves from the
surrounding soil to the root.
3. Root interception – rate at which new roots grow into unoccupied soil.
Biotic influences on nutrient uptake include:
1. Root exudates alter the solubility of cations.
2. Exudates of microorganisms (bacteria) in the rhizosphere alter solubility of cations.
3. Biological activity within root hairs (organic molecules) can attract some cations.
4. Mycotrophy—association of roots with mychorrhiza is highly important to uptake, especially of
nitrogen, phosphorous, and potassium.