Question
Hello, I am in 8th grade, and for my science fair project, my
question is - How does the amount of phosphorus in fertilizer
affect plant growth? I am hoping that you might beable to help me
out with some information, maybe some facts about phosphate or
phosphorus, and the effect it has on plants. Thank you, Megan

Answer
early on man realized decaying organic matter(manures)helped plants grow. the american indians suggested early new world settlers bury a dead fish when planting their corn crop. in manure(or other organic composted material-carbon based) you will find nitrates, phosphates, and potassium compounds produced as bi-products of decomposition.
phosphorous was always associated with proper rootgrowth on plants. it still is. a well balanced fertilizer always had macronutrients(Nitrogen, Phosphorous, & Potassium) in a 3-1-1 ratio. these were always the big 3. with more knowledge now we know in a good fertilizer many micronutrients are present also called trace elements. phosphates and nitrates also have an effect on the soil pH(Acid or base level of the soil). soil pH is often the determining factor in whether a plant can absorb chemical  elements(N,P,K,Ca,Mg, etc.)needed for healthy growth of the plant.
times changing and humans getting smarter we now know that certain compounds containing phosphorous(phosphates) can also be harmful to humans. w/the use of many fertilizers(especially non-organic sources) our ground water(streams, rivers, lakes too) were becoming comtaminated from  the improper introduction of phosophorous into farming and other horticultural practices. the phosophorous was running off the land and polluting our water sources.
in short - phosophorous is a needed chemical in fertilizers for proper plant growth. however the amounts and forms of phosophorous are also important. numerous states have introduced new laws establishing limits of phophorous used in fertilizer and how they are formulated.
this makes the phosphorous available in fertilizers safer for us as humans and less likely to harm our environment. to make a full circle of this discussion we now realize that properly composted organic matter produces the right combination of chemicals needed by plants to grow properly. phosophorous is a main chemical needed by plants for proper healthy growth. it does aid in good root development and does effect the soil Ph. this is all very important for our ecosystem.
hope this helps
rick