Does phosphorus kill plants?

[BrendanMays]

@syncretic

As with many chemical substances in the environment the dose makes the poison and over simplified yes/no choices do not help with understanding.

If considering using grey water on gardens I would look at the amount of P likely to be distributed and the needs and sensitivity of the plants. I would not stop there, there are many products that go down the drain, especially dishwasher products, that are high in sodium. Sodium is not a recognised plant nutrient in the sense that it is ubiquitous and any tiny amount that is required will almost certainly be there. However in excess it is a poison, the main reason for long term failure of irrigation systems is accumulation of sodium. Sodized soils are very hard to remediate.

Great, detailed answer @syncretic, and thanks to everyone for all the comments and thoughts so far. We’re planning to mark resolutions to answers as we go and once a question is solved, but we reserve the right to make changes if needed. Readers might find our Guide to greywater systems has some interesting info about phosphorus and other possible issues with grey water and your garden (and how to avoid problems).

@wrice, almost all seem to be phosphate free (Woolworths Essentials is one exception), but many fail the ‘GreySmart’ rating. Our free detergent report contains all the info on GreySmart ratings.