Somebody just asked me a question that I had a hard time answering, so this is more or less their question which I am repeating.
When it comes to gardening, plants need phosphorus (P). And they asked if it was bad since there are products (say toilet bowl cleaner) that are P free. They figured that the cleaner does not have P because it harms the environment. For me that shouldn't be true since plants need P to reproduce and grow. Maybe it something else?
How should I answer this?
Plants absolutely have a need for phosphorus. Phosphorus is important for plant growth, in particular it's part of the component elements that DNA is made of. Plants short of phosphorus tend to display discolored leaves and stunted development.
Too much phosphorus in the environment is potentially a bad thing though. In particular excessive phosphorus in waterways tends to contribute to harmful algal blooms such as this one in Toledo Ohio that rendered the public water system undrinkable for weeks. Because of this it's a good idea to only use Phosphorus when you need to (ie. when a soil sample indicates it would help), and to limit the amount of your fertilizer that makes its way to a waterway.
The problem is scale. We fertilize our gardens to a specific effect. When phosphorus and nitrogen from laundry detergent and other types of cleaners is let go into the environment in massive quantities it has a tendency to encourage algae growth in to the detriment of nearly everything else, including animals.
It chronic, it can then lead to low levels of oxygen in the water, further impacting other species.
Everything in moderation! Don't water your garden with 1000 gallons of fertilized water.
Yes, phosphorus is essential to plants. It's important for humans, too (in moderation), but it's one of the three major components of fertilizer for plants (the P in N-P-K). Phosphorus helps with flowering, roots, photosynthesis, cold-tolerance, plant maturity, leaf size, etc.
The major concerns I've heard about phosphorus are with supplemental phosphorus (as opposed to phosphorus naturally in the ground), and are environmental, and ethical, but there are some health concerns, too. It all depends on what kind of phosphorus you're talking about. It's pretty difficult to find supplemental phosphorus that everyone can agree is healthy, fine for the environment, etc., including when it comes to organic forms of supplemental phosphorus.
Sources of phosphorus (including organic and non-organic forms):
Water pollution is a concern with water soluble phosphorus; so, that may be what people are concerned about with the toilet cleaner.
Anyway, my source is just research all over the Internet, which I've done the last few years. It's not all indisputable, but this is what I recall.
In reading the claims against forms of phosphorus in this answer, be careful not to assume that I take any particular stance on any particular thing I was talking about (with the exception of wood ash, which I think is great stuff, despite claims against it).
Great answers, I just add a note:
Phosphor pollutes the environment (water systems) because it is a fertilizer: Phosphor fertilize the algae (which is good), the algae eventually die (this is natural), algae decompose (bacteria like it), decomposition take oxygen (bad bacteria), fishes and other animals in water will suffer or die (bad phosphor!).
But as fertilizer, it should not be a problem, but if you have ground water: every movement of phosphor will fertilize plants (and not algea), so when fertilize reach water, it should have no more phosphor.
