Does phosphorus kill plants?

Here’s a consumer question to sort out before using your grey water on the garden. Does phosphorus, which is often present in cleaning products, kill plants?

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No it doesn’t. Phosphorus is a vital nutrient for plant growth, and when discharged into rivers can lead to algal blooms. However, putting too much on a plant wont do it any good, it can lead to other nutrients being locked out, and will eventually kill a plant if the levels are way too high.

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Unless the plants are native to low Phosphorus § soils and are P sensitive (not all are P sensitive). In this case, yes if the grey water contains enough P it can kill those plants. Australia has some low P soils due to extensive weathering and some of our plants have adapted to those levels. Which is why when using fertilizer for Australian Natives they have very low levels of P compared to normal fertilizers used for cropping, and non native species.

P in large amounts can block in particular Iron (Fe).

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As others have pointed out the answer is ‘it depends’. I can give two examples from first hand observation.

I have an aerated waste treatment system. All my sewerage goes through it and since no volatile phosphorus § compound is involved all the P from cleaning products and elsewhere that goes into it comes out again in the transpiration area. The plants there, a mix of grasses and native trees all grow better than other places due to the extra water they get and the nutrients in the outflow. Nothing has died from excessive P. Pee is good, but that is another matter!

Some 18 years ago the previous owner of the property applied chicken litter to it. This is a mix of sawdust bedding and manure dredged up from the floor of local chook sheds. When fresh it is very strong! It is delivered by the truck load and generally the heap sits there for a while before being spread over the paddock. In my case there is a bald spot where the heap was. I have watched over the years as slowly the range of plants there has gone from only a few very hardy weeds to a wide range of weeds! Still the P content is too high for normal grasses to grow there. This is an extreme case due to the high clay content of the soil. Chemical ions (all your macro and micro-nutrients) will bind to the clay colloids and natural leaching due to rain water is slow. Simplified, this helps to make the soil fertile as, in contrast to sandy soils, NPK and trace elements are mostly retained and only released by plant activity. The downside is that a large excess, such as the P compounds from chook poo, binds to the exclusion of all else, thus coming close to sterilising the soil for lack of key nutrients.

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.

There are other issues with applying household grey water to gardens. The issue needs careful study in relation to your particular situation. Do not assume that it is “free” water and that you can just do as you wish without unwanted consequences.

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I think all detergents on the market are now phosphorous-free. I always check the label.

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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.

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hi @BrendanMays - I had a look at the link you had for GraySmart ratings and I can’t seem to find any ratings.

https://www.choice.com.au/compareall/home-and-living/laundry-and-cleaning/laundry-detergents/laundry-detergents

Which row should I look at in the laundry detergent test results to see Greywater ratings?.

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Hi Hans, welcome. You are not alone in needing good guidance on grey water. You may be looking at reusing grey water for the yard. Many non urban Aussies do not have access to reticulated sewage services. It’s a shared interest.

In the Choice review there is a line that references the manufacturers claims for “sceptic safe” and another for “low to no phosphate”. How well each product performs is also easy to see.

I use the review as a guide for which products to purchase. There are only a few options that tick both low phosphate and septic safe. Depending on where you shop not all of the choices may be readily available.

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Phosphorus in Australian Washing detergents has been largely removed or reduced due to the Accord Phosphorus Standard. If a product has a P symbol it has 7.8 g or less in a standard wash and if it has a NP symbol it has 0.5% phosphorus or less in it’s ingredients. This Voluntary Standard has been in place since 1994 and nearly all manufacturers comply.

The Standard can be read here:

https://accord.asn.au/sustainability/phosphorus-standard/

Want to know what’s in products then Accord has about 79% of manufacturers signed up to provide that info which can be accessed from the following links:

https://accord.asn.au/sustainability/whats/

https://accord.asn.au/sustainability/whats-question-answer/participants-ingredient-disclosure/

Unilever:

Colgate Palmolive links:

https://accord.asn.au/sustainability/whats-question-answer/colgate-palmolive/

Biozet:

http://www.biozet.com.au/product-ingredients.html

PZCussons:

Reckitts Benckiser:

Reckitt Benckiser provides ingredient information via their Consumer Toll Free Contact Line: 1800 022 046

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The main issue for using grey water in the garden isn’t necessarily phosphorus, but sodium.

Phosphorus

Phosphorus can be a concern if there is a potential for grey water to enter stormwater drains, streams and rivers. There are also a limited number of commercially available native plants which are highly sensitive to phosphorus, not the plant persay, but the mechanisms adapted by these plants growing in very low phosphorus soils (such as symbiotic relationship with mycorrhizal fungi).

Kevin Handreck as prepared a guide for phosphorus sensitive plants…

http://anpsa.org.au/APOL8/dec97-4.html

Sodium

Sodium in high concentrations can compete for other cations reducing plant uptake ability, as well as affecting soil properties. Sodium can make a soil dispersive when wet and set hard when dry.

Too much sodium can reduce plant growth, and if in the form of sodium chlorine (salt) can cause mortality (desiccation) in high concentrations.

High sodium can be overcome by using gypsum or incorporating organic matter. High salt is more difficult to overcome and might require improving draining and flushing salts from the soil.

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