Introduction to Soil Elements

In this study, I analyzed the soil for the concentrations of ten elements.  Eight of these elements are essential to the growth of plants.  They can be divided into macro- and micro-nutrients, according to the amount of the element required by plants.  The other two elements are non-nutrients and are not essential to plant growth.  At high enough concentrations, other necessary nutrients can also become toxic to the ecosystem.  By category, the elements are:
 

General Overview

          The soil elements that were analyzed can all move back and forth between several chemical forms within the soil.  They may also be dissolved in soil solution as ions (molecules with a positive or negative charge).  They may be bound in insoluble forms, often through association with parent minerals.  The parent materials slowly release the elements over time as part of the natural weathering process.  As molecules of an element move between their various forms, they come to an dynamic equilibrium that shifts according to certain soil conditions, including: pH, texture, aeration of the soil, and the presence of other ions.
          The North Haven soil is acidic, sandy, and dry.  The elements we analyzed are usually present as cations (ions with a positive charge).  Cations are most likely to be found in their dissolved ionic form in acidic sandy soils, because the H+ ions in acid replace the positively charged ions on the parent material and other soil exchange sites.  In sandy acidic soils, these ions can leach out with groundwater or runoff, and for this reason often are deficient.
        Uptake by plants is a significant part of the cycles of some of these elements.  Plants absorb the molecules when they are in ionic form and then release them upon decomposition.  What follows is a description of these elements' roles in plants and slightly more detail on their general behavior in soils.  Except where noted, this information is from Brady and Weil, 1996.

The Macronutrients

          The macronutrients are essential elements that comprise between 0.1 - 1.0 % of a plant.  They generally are important to the structural molecules of a plant, including carbohydrates, proteins, chlorophyll, cell walls, DNA, RNA, sugar phosphates and phospholipids.
 

Potassium

        Potassium is crucial to most ionic functions of a plant, including stomatal control, the maintenance of turgor pressure, and charge balance during selective ion uptake across root membranes.  It is also a coenzyme in many biochemical reactions.  It comprises 1.0% of a plant.  Potassium is highly mobile and leaches easily from leaves and then is taken up in high quantities by microbes and plants.  The primary movement of potassium in the system is between plants and the soil solution, where it resides as an ion that easily leaches from the ecosystem.
        The primary source of potassium in soil solution is the weathering of parent rocks.  Within an acidic soil, potassium may be tightly bound in insoluble minerals (micas and feldspars), slowly available when associated with 2:1 type minerals, moderately available when associated with clay and humus colloids, and easily available when in soil solution.  The small amount of potassium dissolved in soil solution as an ion is highly leachable, although losses of potassium from runoff and erosion is not a significant problem in forests, compared to some elements.
 

Calcium

        Plants use calcium to build cell walls.  It also helps keep P available in the root zone by binding with other competitor ions.  It commonly comprises 0.5 % of a plant.  Because it is bound within cell walls, it does not leach from the leaves nor circulate within the plant.  However, it can easily leach through soil layers.  Its primary source is from weathering, and then it is stored as a cation (a positively charged ion) on soil exchange sites (negatively charged).

Magnesium

        Magnesium is the central atom of the chlorophyll molecule.  It also is an important co-enzyme.  It is very mobile in plants as a cation.  It generally makes up 0.2 % of plants.

Phosphorous

          Phosphorous is required for the formation of energy transfer and storage molecules (ADP, ATP).  Because it forms the backbone of DNA and RNA molecules, it also regulates cell division, root development, and protein formation.  On average, phosphorous comprises between 0.2% - 0.4 % of the dry matter in the leaves of a healthy plant.  It is crucial to many functions of plants, a key component of most fertilizers, and often deficient in non-fertilized soils.
        It is mobile as an anion in plants.  The primary source of potassium in soil solution is the weathering of parent rocks.  Within most soils are large amounts of potassium bound in unavailable forms.  In acidic soils, the largest proportion of potassium is bound in iron- and aluminum- bound insoluble minerals.  They may also bind with manganese.  In its ionic (available) form, phosphate strongly adsorbs to soil particles and does not quickly flush out of the system.  Still, losses in runoff are important.

The Micronutrients
 

              The micronutrients are essential elements that comprise less than .01 % (100 ppm) of a plant.  They generally are metabolically active in plants as important coenzymes.  Specific factors affecting their behavior in soils is mentioned below, but in general, these elements follow the behavior described in the general overview.

Iron

        Iron primarily originates from chemical weathering of the parent material and is not absorbed by plants in appreciable quantities; the amount found in plants is several orders of magnitude lower than the amount in mineral soil.  Its movement in soil horizons is due mainly to chemical processes within the soil, rather than association with organic matter or uptake by biomass.  Therefore, its distribution patterns exemplify the chemical redistribution occurring as the soil restratifies into horizons.  In fact, the distinctive color of the soil horizons are caused by iron.  Its presence gives the reddish tint to the Bhs and Bs horizons, and its absence leaves the E horizon a light gray color.
        Iron generally makes up 100 ppm (0.01 %) of plants.  It serves important roles, especially as an electron carrier in enzymes.  It also plays a role in nitrogen fixation and chlorophyll formation.

Manganese

        Manganese is generally plentiful in acid soil and may reach toxic levels below a pH of 6.5 (as in the pitch pine site).  It generally leaches out of acidic soils and deposits in alkaline soil layers.  Most plants contain around 50 ppm manganese.  It is key to many plant functions, including photosynthesis, respiration, and nitrogen metabolism, because it forms bridges between enzymes and their substrates.

Zinc

        In soils, zinc is tightly adsorbed to magnesium.  On average, plants contain around 20 ppm of zinc.  Zinc is a key component of growth control hormones and aids in protein synthesis.

Copper

        Copper is especially plentiful in acidic, sandy soils.  Though it only comprises 0.1 ppm of the plant, it is an important enzyme activator found mostly in the chloroplasts of leaves.
 

The Pollutants
 
Aluminum

       Aluminum is an element that is not used in significant amounts by plants. In soils, it immobilizes phosphorous and generally increases the acidity and concentration of cations (including the other elements analyzed in this study).  Like most elements, aluminum becomes toxic above certain concentrations; it is poisonous to some plants above 1 ppm and to most plants above 15 ppm.

Lead

        Lead complexes with organic matter in the soil and accumulates in certain organic tissues of plants.  In high enough concentrations, it can cause brain damage in humans.  Biomass is not a significant sink for lead and most is found in the forest floor and underlying mineral soil (Siccama and Smith 1978; Siccama et. al., 1980; Smith and Siccama, 1981; Heinrichs and Mayer 1980).  For more information about current scientific knowledge of lead's geochemical behavior, please see the background research in the introduction.
 
 

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