THE PHOSPHORUS CYCLE
The P cycle differs from the other two nutrient cycles as P does not occur in the
atmosphere, nor is it released as a gaseous product to the atmosphere (Fig. 3). The source of P
is the apatite family of minerals. These are calcium phosphates with a calcium chloride, fluoride,
hydroxide, or carbonate component. Weathering of the minerals releases P which is absorbed
into plant tissue. Upon death and decomposition of the plant, the P is released into soil to be
Agricultural crops are dependent on adequate supplies of P for optimum yield. Soil P
reserves are generally lower than the amount required for optimum yield. Phosphate rock is
mined from deposits laid down by ancient seas. The rock phosphate is treated with sulfuric acid
to convert the complex mineral to calcium dihydrogen phosphate and gypsum. The product is
the superphosphate purchased as fertilizer. This chemical form of phosphate is much more
soluble than the original mineral form.
A complication with P fertilization is that a significant percentage of added P will form
insoluble complexes or become fixed on colloidal surfaces. This means that an excess of P over
that required by a given crop must be added to provide an adequate amount of available P. Over
a period of several growing seasons, some of the fixed P will become available, but in the
meantime, more P must be added each year to compensate for that removed in crops.
Phosphorus is required by all living organisms as it is a component of nucleic acids,
phospholipids, and ATP which functions in energy storage and transfer. The high-yield
production of row crops presently required to sustain world food and fiber production is
dependent on a supply of P fertilizer to augment the naturally occurring P.
Organic P is another form of P found in soil. From 3 to about 50 percent of soil P occurs
as organic P in U.S. soils. The majority of the organic P is present as sugar phosphates and the
next most abundant form is phopholipids. This form of P represents the P incorporated into the
biomass in the soil.
Fresh water ecosystems are usually P deficient. When P is introduced from runoff of
agricultural land or discharge of groundwater (frequently contaminated with sewage or septic
effluent), the increased fertility leads to aquatic plant or algal blooms followed by die off
accompanied by reduction in dissolved oxygen. This condition is termed eutrophication. It leads
to poor water quality and eventual evolution from a lake to a bog as vegetation accumulates.
Phosphorus in groundwater does not normally pose a contamination problem because
phosphate is so completely removed from percolating water in soil by interaction with the soil
minerals. An exemption to the rule is organic P which is not ionic and may not interact with the
Sender associates consultin ginesr p.c.
Fiue3'D HSPOU YL ate ihy 93
CYCLE (after Richey, 1983)
mineral matter in soil. Organic soils pose different conditions for P leaching as mineral matter
and iron hydrous oxides are present in minor amounts if at all. These soils also contain the
higher percentages of organic P.
Nutrient cycling has been a part of the biosphere ever since the first organisms were
present on earth. The proportions of C, N, and P in fresh and marine waters and soil over the
geologic history of the earth has varied as weathering reactions proceeded and life developed the
capacity to incorporate the nutrients and to transform the nutrient chemicals.
As civilization developed, agriculture became more intensive and reused the same parcels
of land. This practice depleted the accumulation of nutrients in the soil requiring replenishment
from outside sources. Animal and green (tilled in plants) manures were undoubtedly the first
nutrient sources artificially added to soil. Modem day agriculture has required the development
of multi-billion dollar industries supplying fertilizers. It has also brought concerns about the
entry of those fertilizer elements into waters that cannot remain pristine in a nutrient-rich state,
or remain drinkable with excessive nitrate concentrations. An understanding of the nutrient cycles
will assist in the control of nutrients, allowing them to be useful where required and remain
isolated from ecosystems where they impair their biological balance.
Bolin, B., The Carbon Cycle in SCOPE 21, The Major Biogeochemical Cycles and Their
Interactions, edited by B. Bolin and R.B. Cook, John Wiley & Sons Ltd., 1983.
Richey, J.E., The Phosphorus Cycle, in SCOPE 21, The Major Biogeochemical Cycles and Their
Interactions, edited by B. Bolin and R.B. Cook, John Wiley & Sons, Ltd., 1983.
Stevenson, F.J., Origin and Distribution of Nitrogen in Soils, in F.J. Stevenson, Ed., Nitrogen in
Agricultural Soils, American Society of Agronomy, Madison, WI, 1982.
Stevenson, F.J., Cycles of Soil, John Wiley & Sons, Inc., 1986.