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Chemical Analyses: Solids: Macronutrients
Essential elements used by plants in relatively large amounts for
plant growth are called macronutrients. The major macronutrients
are nitrogen (N), phosphorous (P), and potassium (K). Calcium (Ca),
magnesium (Mg), and sulfur (S) are also macronutrients. All six
nutrients are important constituents in soil that promote plant
growth. Concentrations of these macronutrients in the soil are generally
determined before the site is disturbed in order to complete a site
reclamation plan. Knowing the initial macronutrient concentrations
of the soil before disturbance can allow reclamationists to ensure
that the same concentrations of macronutrients are in the soil after
reclamation for revegetation purposes. When a site is revegetated,
will fertilizer application be necessary? How much fertilizer will
be necessary? Can the fertilizer simply be applied to the surface
or will it need to be tilled into the soil? Which plants will grow
in the area? These are some of the important questions that must
be answered prior to mineland restortation. For more information
on revegetation practices for reclamation, click
here.
In addition to macronutrients, there are various trace elements
that are necessary for plant growth. These trace elements are needed
in smaller quantities than macronutrients. If the trace element
is required for plant growth it is called a micronutrient. These
include aluminum, arsenic, boron, cadmium, chlorine, copper, iron
(sometimes thought of as a macronutrient), lead, manganese, sodium,
zinc, and others. For more information on trace elements, see Munshower
(1994).
Nitrogen
Nitrogen is important for growth because it is a major part of
all amino acids, which are the building blocks of all proteins,
including the enzymes, which control virtually all biological processes.
A good supply of nitrogen stimulates root growth and development,
as well as the uptake of other nutrients. Plants deficient in nitrogen
tend to have a pale yellowish green color (chlorosis), have a stunted
appearance, and develop thin, spindly stems (Brady
and Weil, 1999).
Much of the nitrogen reserve is stored in the soil as organic matter
and most of this organic fraction is found in the upper soil horizons.
At surface mines, the upper soil horizons are usually removed and
stockpiled prior to disturbance. The storage of topsoil allows for
relatively rapid conversion of organic nitrogen to soluble nitrate
(NO3-) and is subject to leaching or conversion to nitrogen gas
(denitrification)
which volatilizes out of solution into the atmosphere. Thus, when
stored topsoil is spread on a disturbed landscape, nitrogen reserves
may be depleted or altered by several chemical and biological phenomena
and the healthy cycling of nitrogen through the ecosystem inhibited
or prevented (Munshower,
1994). Commonly, nitrogen fertilizer is land applied to reclaimed
mine sites where revegetation is desired. Tilling is generally not
necessary to incorporate the nitrogen into the soil because of the
leaching ability of nitrogen. Although, it is often tilled in since
nitrogen fertilizer is often incorporated with other macronutrient
fertilizer which do need tillage.
Nitrogen in soils can be in various different forms. Nitrogen is
very dynamic and is constantly changing chemical species and concentrations.
In most soils, nitrate is the common ionic form of plant-available
nitrogen, but this element may also exist as ammonium (NH4+) or
nitrite (NO2-) as well as other ions. Nitrogen is also incorporated
in organic matter and microbes. When organic matter decomposes by
microbial processes or when the microbes themselves die and decompose,
nitrogen is released in various forms into the soil solution (Brady
and Weil, 1999). There are various different methods that are
used to measure total nitrogen (organic and inorganic) as well as
methods that measure organic nitrogen, NH4+, NO3-, and NO2- separately.
The Kjeldahl method is the most commonly used method for measuring
total nitrogen (Page
et al., 1982). Methods for measuring organic and inorganic nitrogen,
separately, are also given in Page
et al., 1982.
Phosphorous
Phosphorous enhances many aspects of plant physiology, including
the fundamental processes of photosynthesis, nitrogen fixation,
flowering, fruiting (including seed production), and maturation.
Root growth, particularly development of lateral roots and fibrous
rootlets, is encouraged by phosphorous. Phosphorous uptake by plants
is about one-tenth that of nitrogen and one-twentieth that of potassium.
Its deficiency is generally not as easy to recognize in plants as
are deficiencies in many other nutrients. A phosphorous-deficient
plant is usually stunted, thin-stemmed, and spindly, but its foliage
is often dark, almost bluish, green. Thus, unless much larger, healthy
plants are present to make a comparison, phosphorous-deficient plants
often seem quite normal in appearance. In severe cases, phosphorous
deficiency can cause yellowing and senescence of leaves (Brady
and Weil, 1999).
Phosphorous is usually plant-available in soil as inorganic phosphate
ions (HPO42- and H2PO42-) and sometimes as soluble organic phosphorous.
The HPO42- anion dominates in strongly acidic soils while the H2PO4-
anion dominates in alkaline soils. Both anions are important in
near-neutral soils. The major portion of the total soil phosphorous
- 96% to 99% - is not plant-available. The bulk of the soil phosphorous
exists in three general groups of compounds - namely, organic phosphorous,
calcium-bound inorganic phosphorous, and iron- or aluminum-bound
inorganic phosphorous. Most of these phosphorous groups have very
low solubility and are not readily available for plant uptake. When
soluble sources of phophorous, such as fertilizers and manures,
are added to soils, they are fixed and, in time, form highly insoluble
compounds that are not plant available. Fixation reactions in soils
may allow only small fractions (10% to 15%) of the phosphorous in
fertilizers and manures to be taken up by plants in the year of
application. Consequently, when budget allows, application of two
to four times as much phosphorous than expected for plant uptake
is common (Brady
and Weil, 1999). Unlike nitrogen, phosphorous has a low solubility
and, therefore, it must be incorporated into the soil with a plow,
disk, or chisel to ensure good soil-root-phosphorus contact (Munshower
(1994)). Methods for measuring total and organic phosphorous
are given in Page
et al., 1982.
Potassium
Of all the essential elements, potassium is the third most likely,
after nitrogen and phosphorous, to limit plant productivity. For
this reason, it is commonly applied to soils as fertilizer and is
a component of most mixed fertilizers. Potassium is known to activate
80 different enzymes responsible for such plant and animal processes
as energy metabolism, starch synthesis, nitrate reduction, photosynthesis,
and sugar degradation. Potassium plays a critical role in reducing
the loss of water from leaves and increases the ability of the roots
to take up water from the soil. It also helps plants adapt to environmental
stresses. Good potassium nutrition is linked to improved drought
tolerance, improved winter hardiness, better resistance to certain
fungal diseases, and greater tolerance to insect pests. Potassium
deficiency is relatively easy to detect compared to deficiencies
in phosphorous. The tips and edges of the oldest leaves begin to
yellow (chlorosis) and die (necrosis), so that the leaves appear
to have been burned on the edges (Brady
and Weil, 1999).
The original sources of potassium are the primary minerals, such
as micas (biotite and muscovite) and potassium feldspar (orthoclase
and microcline). As these minerals weather, the potassium becomes
more available as readily exchangeable and soluble potassium which
can be adsorbed by plants roots. At any one time, most soil potassium
is in primary minerals and nonexchangeable forms. In relatively
fertile soils, the release of potassium from these forms to the
exchangeable and soil solution forms that plants can use directly,
may be sufficiently rapid to keep plants supplied with enough potassium
for optimum growth. Conversely, in relatively nonfertile soils,
the levels of exchangeable and solution potassium may have to be
supplemented by outside sources, such as chemical fertilizers, poultry
manure, or wood ashes. Without these additions, the supply of available
potassium will likely be depleted over a period of years and the
productivity of the soil will likewise decline (Brady
and Weil, 1999). Methods for measuring total, exchangeable,
and soluble potassium are given in Page
et al., 1982.
Chemical
Analysis | Physical
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