NUTRIENT CYCLES IN THE ECOSYSTEM
Olin C. Braids
Eder Associates, Inc.
The earth's terrestrial ecosystem depends on a thin mantle of soil from which agricultural
crops, forests, and wetlands derive nutrients to support growth. Although to the uninitiated, soil
appears to be an inert support medium, it is actually a medium active with life that is supported
by it and to which it imparts characteristics that allow the growth of plants. Microorganisms in
the form of bacteria, fungi, actinomycetes, and singular and multi-celled animals inhabit soil.
Usually these forms of life are limited to the first few inches to feet of depth. However, recent
investigations of deeper groundwater environments have shown the presence of bacteria up to
hundreds of feet in depth.
Soil exhibits unique chemical characteristics that also play a role in the support of the
microbial population and terrestrial plants. Soil is formed from the weathering of geologic
materials over the course of millennia. This process leads to the development of small geologic
particles and secondary minerals that provide chemically active surfaces. Clay mineral particles
are measured in microns, so have enormous surface area per cubic inch of volume. The
montmorillonitic clays have a multilayer crystalline structure with internal surface area in addition
to that of the outside surfaces. This characteristic allows the clays to react with positively
charged chemical ions in solution. The ions are attracted to the surfaces and leave the aqueous
soil solution. Chemically, this is known as adsorption. By this mechanism, ions such as calcium,
magnesium, hydrogen, potassium, and ammonium are held and concentrated on the particle
The combination of chemically active components, living organisms, and a physical
support matrix create conditions conducive to plant growth and biochemical cycling of nutrients
required for plant and animal nutrition. The critical elements required for natural and agricultural
crops are carbon (C), nitrogen (N), phosphorus (P), sulfur (S), and the micronutrients boron (B),
copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn). This paper will not
attempt to cover all of these nutrient elements, but will focus on the macronutrients.
Cycling of nutrients in soil is termed biochemical cycling because of the role of
microorganisms in various chemical transformations. When natural conditions prevail, plant
residues undergo decay, and their nutrients are returned to inorganic forms through the microbial
degradation process. Additions of nutrients in agricultural propagation are also subjected to
microbial transformation. The nutrients are released into the soil in plant-available forms where
they can be reabsorbed by plant roots.