See pages
261 274, 279 287, and 291 297 of the text for discussion of reactions
that affect plant availability of N, P, and K in soil.
A.
General mechanisms that affect availability of various elements and
relevant terms.
Adsorption-desorption
or ion exchange; Note: adsorption refers to an attraction to
a surface while absorption refers to being incorporated into something.
See diagram on page 144 of the text.
Mineralization:
organic ---> inorganic
Precipitation-dissolution: when dissolved substances form
solids (solid phases) and drop out of solution, the process is called
"precipitation"; it is not the same as adsorption; dissolution
is the opposite process whereby solids go into the solution phase
and become "solutes".
Fixation:
elements or certain ions such as NH4 become physically and chemically
bound in a nonexchangeable form. An example is the entrapment of
potassium between silica layers of clay minerals. Some clays also
fix ammonium in the same way. Phosphorus can also become "fixed"
in unavailable forms but the mechanism is different than ammonium
or potassium fixation. Phosphorus is fixed by being bound chemically
to iron and aluminum compounds.
Denitrification:
biochemical reduction of nitrate or nitrite to gaseous nitrogen
Mineralization:
conversion of an organic form of an element into an inorganic form.
B.
Nitrogen
Most
of the nitrogen in soil is present as soil organic matter and is
unavailable to plants. Organic matter is about 50% carbon and 5%
nitrogen, thus it has a carbon/nitrogen ratio of about 10. Nitrogen
is released very slowly from soil organic matter as a result of
soil microbial activity. This process is affected by moisture, temperature,
tillage or any physical disturbance such as soil wetting and drying.
The amount of nitrogen released by this process is also dependent
on the soil organic matter content. A soil with 1% organic matter
will supply much less plant available nitrogen than one that is
5% organic matter, under similar conditions.
The
decomposition of crop residues is usually limited by the C/N ratio
of the residue as well as environmental conditions. Soil microorganisms
have first choice of available nitrogen. Higher plants get what
is left over. Alfalfa will decompose more quickly in soil than corn
stalks because of its higher nitrogen content (lower C/N ratio).
Nitrate,
being an anion, is the most mobile form of N in soil; it moves within
the water as water percolates through soil and is easily lost through
this leaching process. Ammonium and ammonia forms are cationic and
held by cation exchange sites. However, ammonium and ammonia forms
are converted to nitrate by soil microorganisms. The reaction is
fast in warm moist soils. The reaction rate becomes slower as the
soil becomes more acidic. From spring through fall most ammonium
fertilizer is converted to nitrate within a few days to a week or
two. Urea nitrogen is hydrolyzed to ammonium nitrogen by a soil
enzyme and then quickly converted to nitrate provided conditions
are right.
C.
Phosphorus
Phosphorus
is strongly held by soil clays and iron and aluminum compounds associated
with soil clays; also, phosphorus forms very sparingly soluble compounds,
precipitates, in soil. These reactions proceed rapidly after fertilizer
is applied. Most phosphorus in soil is precipitated, fixed, or adsorbed.
These solid phases form phosphate reserves that can replenish the
soil solution when phosphorus is taken up by plants, but the reaction
is slow. Only small amounts of phosphorus are present in the soil
solution. Because of these reactions with soil, P is very immobile,
not subject to significant leaching losses. However, P is readily
lost through erosion of surface soil and the associated P.
D.
Potassium
Potassium
exists as exchangeable K and in K-bearing minerals. In soils that
contain mica-type clay minerals and vermiculate some K exists as
"fixed" K within the clay mineral structure. Fixed K is
not exchangeable.