Phosphorus is essential for plant growth, but soils in Virginia are naturally low in phosphorus. Virginia soils require supplemental phosphorus to maximize crop yields and so are a necessary agricultural fertilizer. Phosphorus enters to Chesapeake Bay watershed from the actions of man. Farming and agricultural activities are often seen as the only source of phosphorus, but that is not true. Phosphorus is a naturally occurring element that is naturally found in rock, soil, water and all living organisms. Of the estimated 17 million pounds of phosphorus entering the watershed, it was estimated in 2009 that about 8.3 million pound of phosphorus was from agriculture and 6.5 million pounds was from waste water treatment plants and urban runoff from predominately pervious surfaces (lawns). The growing suburban/urban population is a serious contributor to contamination. However, concentrated poultry feed operations are significant sources of excess phosphorus.
To give you some perspective on the quantities of phosphorus generated in sewage, according to the Delaware Department of Natural Resources and Environmental Control, the typical household generates 1-2 pounds of phosphorus per year and there are approximately 4.5 million households in the Chesapeake Bay watershed, representing 4.5-9 million pounds of potential phosphorus contamination (without the use of phosphorus removing technology at waste water treatment plants). According to a Maryland state study, each chicken generates approximately 0.35 pounds of phosphorus per year. Animal guano is a much bigger source of phosphorus than human waste, but the growing volume of households can create a problem that grows over the years.
The phosphate rock in its commercially available form is called apatite which is merely calcium phosphate. Other deposits may be from fossilized bone (animal or human) or guano (poultry waste). When plant materials and waste products decay through bacterial action, the phosphate is released and returned to the environment for reuse. Weathering and erosion of rocks and bones gradually releases phosphorus as phosphate ions which are soluble in water. A large percentage of the phosphate in water is precipitated from the water as iron phosphate which is insoluble. If the phosphate is in shallow sediments (wetlands), it may be readily recycled back into the water for further reuse. In deeper sediments in water, it is available for use only as part of a general uplifting of rock formations.
Most of the phosphate Chesapeake Bay watershed enters the water through the water run-off and release from waste treatment plants. Over application of phosphorus is caused by the use of manure from the concentrated poultry feed operations. Poultry typically has nearly equal concentrations of phosphorus and nitrogen, though crops typically require 2.4-4.5 times the nitrogen as phosphorus. Pig manure has slightly more nitrogen than phosphorus. Cow manure has typically twice the nitrogen as phosphorus and is much less of a problem. Historically, manure has been added to obtain the correct nitrogen content not the lower phosphorus needs. The excess phosphorus is released through leaching, runoff and erosion.
As for waste treatment plans without an expensive tertiary treatment, the phosphate in sewage is not removed during treatment. So what happens in the release areas from waste water treatment plants is that the phosphate sediment (iron phosphate) builds up over time. Lower the concentration in the waste stream and phosphorus is released from the sediment. As removal technology has improved for nitrates and phosphorus, waste treatment plants have had to exercise careful balancing act to control the algae blooms. I can find no studies of impact from release from single family septic systems and this may be due to the low concentration and plant uptake and mineralization.
Phosphorus can be found dissolved in the soil solutions in very low amounts or associated with soil minerals or organic materials. The relative amounts of each form of phosphorus vary greatly among soils, with the total amount of phosphorus in a clayey-textured soil being up to ten times greater than in a sandy soil. (University of Arkansas). Organic phosphorus in soils is made up of a large number of compounds, with the majority being of microbial origin. Organic phosphorus is held very tightly and is generally not available for plant uptake until the organic materials are decomposed and the phosphorus released via the mineralization process. Mineralization is carried out by microbes, and the rate of phosphorus release is affected by factors such as soil moisture, composition of the organic material, oxygen concentration and pH.
The reverse process, immobilization, refers to the tie-up of plant-available phosphorus by soil minerals and microbes that use phosphorus for their own nutritional needs. Microbes may compete with plants for phosphorus, if the decomposing organic materials are high in carbon and low in nitrogen and phosphorus. Mineralization and immobilization occur simultaneously in soil. If the phosphorus content of the organic material is high enough to fulfill the requirements of the microbial population, then mineralization will be the dominant process.
Most of the phosphorus added to soil as fertilizer and manure is rapidly bound by the soil minerals to the inorganic form and is not subject to rapid release. Thus, soil solution phosphorus concentrations typically remain very low, the concentration of inorganic phosphorus (orthophosphates) in the soil solution at any given time is very small, amounting to less than 1 lb/Acre. Phosphorus in the inorganic form occurs mostly as aluminum, iron or calcium compounds. This series of reactions is commonly referred to as sorption or fixation. Iron and aluminum compounds will fix (tie-up) phosphorus under acidic conditions (soil pH <= 7), phosphorus is preferentially fixed by calcium and magnesium compounds. Phosphorus availability to plants in most soils is greatest when soil pH is in the range of 6 to 7. Hard water is particularly high in calcium compounds, iron and magnesium which allows the mineralization and why septic systems may be able to handle the limited amount of phosphorus released by the typical human household each year.