Phosphorus denotes the chemical element (with the symbol P and the atomic number 15) present in a few types of minerals found on Earth. It comes from a Latin word for morning star, and it is so named because it glows when exposed to oxygen.1
Phosphorous (note the o in the last syllable) is an adjective meaning (1) of, relating to, or containing phosphorus, and (2) of or relating to phosphor (phosphor being a general term for various luminescent substances).2 In its first sense, phosphorous is synonymous with phosphoric.
We don’t have the scientific knowledge necessary to sort through the many words beginning with phosph- (some related to phosphorus and some unrelated). The point of this post is that phosphorus is the noun and phosphorous is its corresponding adjective.
The adjective is often used in place of the noun—for example:
The less polluting pig, called the “enviropig,” has a gene that allows it to better digest phosphorous in its food. [New York Times]
Fertilizers contain three primary plant nutrients: mostly nitrogen and phosphorous with smaller amounts of potassium. [National Geographic]
The document said one of the main reasons for the murky colour of the Manawatu River and high levels of phosphorous was the large amount of sediment finding its way into the water. [Stuff.co.nz]
In fact, this happens so often that we should probably just consider phosphorous an alternative spelling of phosphorus. Among the dozen or so dictionaries we checked, though, not one has noted this.
Phosphorous is so often used as a noun that examples of its use as an adjective are hard to find. We gave up after two:
And they’d used phosphorous grenades which burn a man alive. [Telegraph]
A study at the University of Michigan … found that Ann Arbor’s ban on phosphorous fertilizers for grass led to a 28 percent drop in the pollutant’s levels in nearby Huron River. [Scientific American]