Rock bottom: can rocks in your dog’s water bowl protect your lawn?

fractal image, featuring the hashtag #272sci

Take a look at the Twitter hashtag #272sci

One quick thing before I dive into this month’s post: if you’re a Twitter user, check out my series of very tiny science tweets under the hashtag #272sci. The aim is to explain a science thing in one tweet – without using a thread – and it’s 272 because that’s the number of characters I have to use after including the hashtag and a space. So far I’ve covered leaf colours, frothy milk, caffeine and poisonous millipedes. There will be more to come!

Now, speaking of Twitter, a couple of weeks ago Prof Mark Lorch tweeted about Dog Rocks. Dog… what? I hear you ask (really quite understandably).

Well, it turns out that Dog Rocks are a product that you can buy, and that you put into your dog’s water bowl. Your dog then drinks the water that has been sloshing over the rocks, and, this is where we start to run into trouble, this is meant to have an effect on your dog’s urine. This, in turn, is supposed to protect any grass your dog might then pee on.

photo of a patch of dead grass

Dog urine damages grass

All right, so let’s start somewhere in the vague vicinity of some science: if you have a dog, or even if you’ve just spent some time with someone who has a dog, you’ve probably noticed that dog urine isn’t very kind to grass. Commonly, you see something like the photo here, that is, patches of yellow, dead grass, surrounded by quite luscious green growth.

Why is this? It’s because dog urine – like the urine of all mammals – contains urea, CO(NH2)2. Urea forms in the body when animals metabolise nitrogen-containing compounds, in particular, proteins. It’s essentially a way for the body to get rid of excess nitrogen.

People sometimes confuse urea with ammonia, for reasons that I’ll come to in a moment. But they’re not the same thing. Urea is odourless, forms a pH neutral solution and, if you extract it from the liquid in which it is dissolved, produces solid crystals at room temperature.

Pure ammonia, NH3, by contrast, is a gas at room temperature (boiling point -33.3 ℃), forms alkaline solutions (with pH values greater than 7) and has that pungent ‘ngggh get it away from me!’ smell with which we’re probably all familiar.

Sample pots full of pale yellow liquid

Fresh urine contains urea, but little ammonia

Although these two substances aren’t the same, they are linked: many living things convert ammonia (which is very toxic) to urea (which is much less so) as part of normal metabolism. And it also goes the other way, in a process called urea hydrolysis. This reaction happens in urine once it’s out of the body, too, which is the main reason why, after a little while, urine starts to smell really, really bad.

Okay, fine, but what has this got to do with grass, exactly? Well urea (and ammonia, for that matter) are excellent sources of nitrogen. Plants need nitrogen to grow, but dog urine contains too much, and too much nitrogen is bad – in the same way that too much of pretty much anything nice is bad for humans. It damages the blades of grass and a yellowish dead spot appears, often ringed by some particularly lush grass that, being slightly outside the immediate target zone, caught a whiff of extra nitrogen without being overwhelmed.

Back to Dog Rocks. Interestingly, the website includes an explanation not unlike the one I’ve just given on their fact sheet. What it doesn’t do is satisfactorily explain how Dog Rocks are supposed to change the nitrogen content of your dog’s urine.

photo of a dog drinking water

Dog Rocks are meant to be placed in your dog’s water bowl

The website says that Dog Rocks are “a coherent rock with a mechanically stable framework”. Okay… so… Dog Rocks won’t dissolve or break up in your dog’s water bowl. A good start. It goes on to say, “the rocks provide a stable matrix and a micro-porous medium in which active components are able to act as a water purifying agent through ion exchange” and “Dog Rocks will help purify the water by removing some nitrates, ammonia and harmful trace elements thereby giving your dog a cleaner source of water and lowering the amount of nitrates found in their diet.”

You’ll note they’re using the word nitrate. Nitrates are specifically compounds containing the NO3 ion, but I think they’re using the term in a more general way, to suggest any nitrogen-containing compound (including urea and ammonia). And by the way, nitrates are different from the similar-sounding nitrites, which contain the NO2 ion. Fresh urine from a healthy dog (or human, for that matter) shouldn’t contain nitrite. In fact, a dipstick test for nitrite in urine is commonly used to check for urinary tract infections, because it suggests bacteria are present.

Anyway, nitrates/nitrites aside, it’s the last bit of that claim which really makes no sense. Your dog is not ingesting anything like a significant quantity of nitrogen-containing compounds from its water bowl. Urea comes from the metabolic breakdown of proteins, and they come from your dog’s food.

Photo of puppies eating food that I totally picked because it's cute ;-)

The nitrogen-containing compounds in your dogs’ urine come from their food, not their water

It’s faintly possible, I suppose, that Dog Rocks might somehow filter out some urea/nitrates from urine. But then your dog would have to pee through the Dog Rocks and, honestly, if you can manage to arrange that, you might as well train your dog not to pee on your grass in the first place.

I suggest that there are three possible explanations for the positive testimonials for this product. 1) Owners who use it are inadvertently encouraging their dogs to drink more water, which could be diluting their urine, leading to less grass damage. 2) It’s all a sort of placebo effect: owners imagine it’s going to work, and they see what they’re expecting to see, or 3) they’re all made up.

You decide, but there is absolutely no scientifically-plausible way that putting any kind of rocks in your dog’s water bowl will do anything to stop dog pee damaging your grass. This is £15 you do not need to spend. But hey, you could avoid the money burning a hole in your pocket (see what I did there?) by buying me a coffee… 😉

Check out the Twitter hashtag #272sci here, and support the Great Explanations book project here!

Do you want something non-sciency to distract you from, well, everything? Why not take a look at my fiction blog: the fiction phial? You can also find me doing various flavours of editor-type-stuff at the horror podcast, – so head over there, too!

Like the Chronicle Flask’s Facebook page for regular updates, or follow @chronicleflask on Twitter. Content is © Kat Day 2021. You may share or link to anything here, but you must reference this site if you do. You can support my writing my buying a super-handy Pocket Chemist from Genius Lab Gear using the code FLASK15 at checkout (you’ll get a discount, too!) or by buying me a coffee – just hit this button:
Buy Me a Coffee at


After Waco: why are fertilisers so dangerous?

Yesterday there was news of a huge fertiliser explosion in the town of West, near Waco, Texas and as I write the search for survivors is ongoing.  It’s a dreadful tragedy:  the blast all but destroyed a school and a nursing home a few hundred metres away, and dozens of homes were also levelled.  More than 160 people have been injured and so far twelve have been found dead.

ammonium nitrateAt the moment the full details are still unknown.  Fertilisers have long been associated with explosives, and terrorists have been known to use fertiliser bombs (something I shall not be discussing in more detail for fear the men in dark suits might come knocking), although it seems that there’s no indication of malicious intent in this case.  Obviously factories make fertiliser all over the world, and they don’t all blow up on a regular basis, so clearly something went very wrong at 8pm local time on the 17th of April.

So why is fertiliser such potentially dangerous stuff?  Can we make it safer?

First of all, we should probably clarify what we mean by ‘fertiliser‘ (or fertilizer, for our American cousins).  Actually the clue is in the name; it’s something which makes the soil more fertile.  In essence, anything that’s added to the soil to supply one or more of the nutrients that plants need.  In particular, most fertilisers supply nitrogen.  If you were paying attention at school, you’ll remember that most of the solid stuff in plants actually comes from the air in the form of carbon dioxide (see that wooden table over there? A plant made most of that out of air. Air. How cool is that?)

However, just like us, plants also need to make protein for growth, and to do that they need nitrogen.  Unlike us, they can’t (with a few notable exceptions) get that protein from eating animals or other plants, on account of not having teeth, the ability to move and so on.  Except for triffids and that plant in Little Shop of Horrors obviously.  But good old air is about 80% nitrogen, so surely if they can get the carbon from carbon dioxide from air they can get nitrogen too?

Well, there are a few plants that can do that, but most can’t.  The problem is that the nitrogen in air, N2, has one of the strongest bonds between its atoms.  It’s very difficult to break, which means it doesn’t get involved in chemical reactions very easily.  And since growing is basically one big complicated mix of chemical reactions, plants can’t easily use the nitrogen in the air.  Before we started chucking fertiliser on the soil plants managed of course, because useable forms of nitrogen do get into the soil from natural processes.  But if you want to grow large quantities of crops year after year, you need to provide a bit of a helping hand, and that’s what fertiliser does, whether it comes from a factory or, ahem, the back of a cow.

nitrogenBut, and here’s the thing, it’s that strong, triple, bond in N2 that makes fertilisers potentially explosive.  Because if it takes a lot of energy to break those bonds, then exactly the same amount of energy is released when they’re formed.  There is no way around this: energy cannot be created or destroyed, or made to disappear.  (Not in real life, anyway – Harry Potter and co follow different rules.  But they’re not real.  Sorry.)

Why do things explode?  Essentially an explosion occurs when a chemical reaction produces lots of hot gases, very quickly.  If these gases have nowhere to go, because they’re in an enclosed space, they put immense pressure on their immediate surroundings as they rapidly expand.  Ultimately those surrounding are apt to give way, with a bang. (High explosives, like dynamite and TNT, are a little different – but fertilisers aren’t high explosives, so we’ll save that topic for another day.)

ParticleTheoryCompounds that contain nitrogen have the potential to produce nitrogen gas.  Gases take up a lot more space than solids because their particles are further apart and, as I’ve already mentioned, when that hugely strong nitrogen triple bond forms lots of energy is released.  So there you are, hot (that’s the energy bit) gas.  Lots of it.  Surround it with walls – say in a container in a factory – and you have the potential for an explosion.

The fertiliser in this case appears to have been ammonium nitrate.  This is made by reacting ammonia (if you remember, Fritz Haber figured out how to produce that) with nitric acid.  Ammonium nitrate’s chemical formula is NH4NO3 – so plenty of nitrogen there.  In fact when ammonium nitrate decomposes it forms water vapour, nitrogen gas and oxygen gas (via some nitrous oxide, aka laughing gas, along the way).  Lots of gases.  Lots of heat.

The factory also contained lots of anhydrous ammonia.  Not especially surprising this, since you need ammonia to make ammonium nitrate – this was a fertiliser factory.  Anhydrous just means ‘no water’, in other words pure ammonia, NH3.  The boiling point of pure ammonia is -33 oC, so you have a bit of a problem right there if your cooling systems fail; it will quickly turn into vapour at room temperature.  This vapour is pretty nasty.  You know that smell when you use hair dye or perming solution (if you’re still in the 80s)?  That.  Times a hundred.  It’s toxic and corrosive (it poisons you while damaging your lungs), and environmentally damaging.  Oh yes, and flammable.  Not as flammable as say, petrol, but flammable enough.

Reports are that there was a fire at the plant before the explosion, so it looks as though the ammonia might have caught fire.  Ammonium nitrate isn’t easy to ignite, but if the fire is contained and it’s exposed to sustained heat it’ll start reacting.  It decomposes at about 210 oC and once it’s started it’s very difficult to stop, because the reaction gives out a lot of heat which causes the surrounding material to react, and so on in a catastrophic spiral – something chemists call a runaway reaction – ultimately leading to detonation.

So fertilisers are potentially dangerous because they contain nitrogen in a more reactive form, which plants can use.  There’s nothing you can do to make fertilisers explosion-proof.  You can’t say, put additives in to make them less explosive.  It’s in their nature.  Take away their explosiveness and you take away their ability to act as fertilisers.

Factories, though, should be following detailed safety procedures and have numerous protective backup systems to prevent disasters like this.  We don’t yet know what went wrong here, but let’s hope some serious lessons are learned.