Biosolids can supply appreciable
amounts of nitrogen and phosphorus but only low quantities of
potassium. Usually, additional nitrogen, phosphorus, and
potassium must be applied to soil-plant systems, if optimum
yields are to be attained for most crops. With good quality
biosolids, low quantities of other elements essential for plant
and/or animal and human nutrition will also be present.
The main concern with regards to nutrients in
biosolids is nitrogen and phosphorus contamination of water
resources. Mismanagement of biosolids can result in
nitrogen or nitrate leaching into groundwater and the
accumulation of phosphorus in surface soils which can increase
the risk of runoff/erosion losses to surface water, resulting in
eutrophication.
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Nitrogen
Nitrogen is the biggest factor in
determining the amount of biosolids that can be applied to a
given soil. A plant-soil systems nitrogen needs must be
known in order to properly apply biosolids. Since nitrogen
is very mobile in a soil environment, any excess will almost
certainly leach or runoff into
nearby surface or subsurface waters. Reactions in Soil.
Plant-available nitrogen added must be known
in addition to the total amount of nitrogen added. Since
the amounts of organic nitrogen mineralized in a soil environment
widely varies the amount of plant-available nitrogen also
varies. If a large amount of the nitrogen in the biosolids
is not mineralized while in the
root zone of the surface crops, then that nitrogen will not be
removed from the water moving through the system. This
nitrogen can later be mineralized in an aquatic environment
causing eutrophication. This result can be reduced by
applying smaller loadings more frequently, but even with proper
management nitrogen can still leach or runoff into aquatic
environments where it may result in eutrophication.
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Phosphorus
Phosporus is required in somewhat small
amounts by most plants, but exists mainly in a
non-plant-available form in most soil-plant systems, therefore it
usually is added to a sustainable crop system. An excess of
phosporus beyond a soil-plant system's needs is usually not a
problem. In fact, in order to meet the system's nitrogen
needs, excess phosphorus is almost always added. However,
continuous over-application of phosphorus can increase the soil
solution phosphorus concentration in surface and subsurface soils
to a level which can contribute significant amounts of phosphorus
to runoff water flowing into streams and lakes. This is
true for both soluble and insoluble phosphorus.
Phosphorus is also often times a lmiting
nutrient, so when it is intorduced into an aquatic environment it
will cause an explosion of growth and result in
eutrophication. Therefore, not only should the total amount
of phosphorus be controlled, but also the runoff/erosion must be
strictly controlled.
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Nutrient Deficiencies/Imbalances
Biosolids do not contain a
balanced amount of most nutrients and long-term application can
disrupt soil fertility and plant nutrition. For example,
lime-stabilized biosolids can decrease the amount of
plant-available micronutrients that are pH sensitive, such as
iron and manganese.
Also, bioasolids can greatly add to the
salinity of a soil. Many salts are present in most
biosolids, especially those from municipal systems that process
industrial waste.