Agriculture Reference
In-Depth Information
Except for nitrogen that is captured directly from the
air by symbiotic microorganisms that live in the roots of
most members of the Fabaceae and a few other plant
families and passed on to the host plants in an available
form, most plants obtain their nitrogen from ion exchange
with the soil solution as NO 3 - or from NH + adsorbed to
humus or clay minerals. Available forms of nitrogen in
the soil are generally kept at low levels by rapid uptake
of nitrogen when it is available coupled with nitrogen's
high potential for leaching loss with rainfall or irrigation
percolation.
for example, it appears that potassium activates certain
enzymes that are responsible for peptide bond synthesis
and the incorporation of amino acids into protein. Potas-
sium needs to be present for the formation of starches and
sugars, as well as for their later transport throughout the
plant. This nutrient has been shown to be needed for cell
division and growth, and is in some way linked to cell
permeability and hydration. Plants show better resistance
to disease and environmental stress when potassium sup-
plies are adequate.
Plants obtain potassium in the form of the cation K + ,
taking it in through the roots as exchangeable ions from
adsorption sites in the soil matrix or from a dissolved form
in the soil solution. When potassium is deficient, plants
primarily show disruptions in water balance; these include
drying tips or curled leaf edges, and sometimes a higher
predominance of root rot. Potassium is usually quite abun-
dant in soils, with plant tissues being made up of 1 to 2%
potassium by dry weight under optimum conditions, but
excessive removal through harvest or soil leaching can
lead to potassium deficiency.
Phosphorus
Phosphorus is an important component of nucleic acids,
nucleoproteins, phytin, phospholipids, ATP, and several
other types of phosphorylated compounds including some
sugars. Phosphorus is built into the DNA of chromosomes
and the RNA of the nucleus and ribosomes. Cell mem-
branes depend on phospholipids for the regulation of
movement of materials in and out of the cells and
organelles. Phosphorus in the form of phosphates (PO 4 +)
occurs in certain enzymes that catalyze metabolic reac-
tions. Sugar metabolism in plants, for example, depends
on phosphoglucomutase. Phosphorus also occurs in pri-
mary cell walls in the form of enzymes that affect cell
wall permeability. The initial reactions of photosynthesis
also involve phosphorus; it is found in the five-carbon
sugar with which CO 2 initially reacts.
Phosphorus is absorbed as phosphates from the soil
solution through plant roots. Phosphates in solution are
readily available and taken up by plants, but except in soils
that are derived from parent materials high in phosphorus
or where phosphorus levels have built up over time in
response to many years of fertilization, available phospho-
rus in most soils is quite low. Plants will opportunistically
take up large amounts of this nutrient when it is available,
accumulating about 0.25% of dry weight, but are quick to
show signs of deficiency when it is lacking. Leaves take
on a bluish cast or remain dark green, and purple pigments
(anthocyanins) become prominent on the underside of the
leaves and along the veins or near the leaf tip. Root and
fruit development are severely restricted when phosphorus
is limiting.
Other Macronutrients
Three other nutrients — calcium (Ca), magnesium (Mg),
and sulfur (S) — are also considered to be macronutrients,
but this classification is more a function of the relatively
high levels in which they accumulate in plant tissue and
less because of their importance in different plant struc-
tures or processes. This is not to say that they do not play
valuable roles, because when any of these nutrients are
deficient in the soil, plant development suffers and symp-
toms of deficiency show up quickly. Calcium and magne-
sium are readily absorbed by plant roots through cation
exchange (as Ca 2+ and Mg 2+ ), but sulfur is taken up spar-
ingly as an anion (SO 4 2- ) from organically bound sites in
the soil or upon dissociation of sulfates of Ca, Mg, or Na.
Micronutrients
Iron (Fe), copper (Cu), zinc (Zn), manganese (Mn),
molybdenum (Mo), boron (B), and chlorine (Cl) make up
what are called the micronutrients or the trace elements.
Each one plays some vital role in plants, but usually in
extremely small quantities. In fact, most of these elements
are toxic to plants when they occur in the soil in large
quantities. All are taken up from the soil solution through
ion exchange at the root surface.
The role that each of the micronutrients plays in
plants' life processes is outlined in Table 3.3. As one
would imagine, any of the important physiological pro-
cesses listed could be inhibited or altered by a deficiency
of the micronutrient concerned. Many inorganic fertilizers
carry small quantities of these elements as contaminants,
and mixtures of trace elements are now commonly added
Potassium
Potassium is not a structural component of the plant, nor
a component in enzymes or proteins. Its function seems
to be primarily regulatory: it is involved, for example, in
osmoregulation (stomatal movement) and as a cofactor for
many enzyme systems. We know a lot about where potas-
sium occurs in the plant, but much less about what it
actually does. Most metabolic processes that have been
studied are affected by potassium. In protein metabolism,
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