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Analytical Methods

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 Properties

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