NH3 (Fertiliser)

No, I'm not a qualified scientist. I do like writing about science, though.

Ammonia is a colourless gas which can be found naturally. It's the gas that forms when you react nitrogen and hydrogen, as can be seen in the equation N₂ + 3H₂ → 2NH₃. Indeed, it is a very interesting gas because it's almost everywhere in all kinds of forms.

First of all, there's ammonia in the air, in water, inside us. It's also a very harmful gas, so to prevent ammonia from building up in organisms, it's deaminated into urea in the liver and kidney, which is a waste product that is removed from the body, with the help of some carbon dioxide as part of the urea cycle. This doesn't happen in some organisms - instead, ammonia is directly excreted.

Ammonia can also be found in the soil if used as a fertiliser. It's useful as a fertiliser because it has plenty of nitrogen, which is necessary for plant growth as it can help provide chlorophyll to plants which can thus increase the rate of photosynthesis. If the rate of photosynthesis is higher, more glucose is produced, so the plant has more energy and can thus respire, enabling the growth of the plant. You can tell I listened in my biology class.

In fact, there are three key elements enabling plant growth - nitrogen, phosphorus (P) - which is key in plant cell division - and potassium (K) - primarily in the form of potash salts (K that's soluble in water), and which helps enable growth for resisting pests. Preferably your fertiliser will contain all three, as part of an NPK fertiliser, but sometimes you don't have all three. In that case, NH3 is very suitable due to the vast amounts of nitrogen content.

For this to happen, you need to get the ammonia. This can be done on a large industrial scale in the Haber-Bosch process. Here, you react nitrogen from the air with hydrogen using an iron catalyst, high temperatures and high pressures. The equation from earlier is how the whole thing works.

But you can't pump the gas into the soil for it to work as a fertiliser, right?

Correct. This is where the ammonium ion NH₄⁺ comes in. The structure of an NH₃ molecule is such that there is a lone pair of electrons on the nitrogen atom's outer shell, waiting to be utilised. It is utilised in the form of protonation, where a proton is added to the NH₃ molecule, which forms the positively charged ion. Note that a proton has the same charge as an H⁺ ion, and it's clear to see why the ammonium ion looks as it does. So say you react ammonia with hydrochloric acid. The resulting product is ammonium chloride (NH₃ + HCl → NH₄Cl).

The best part about this is that ammonium chloride is very soluble, so you can use it as a fertiliser. In fact, 90% of all ammonium chloride is produced for this exact reason (the other 10% includes a myriad of reasons, including one as a food additive - E510 - which is used to flavour some types of liquorice). So apply the fertiliser into the soil, and all of a sudden, the plants will have nitrogen and growth can be stimulated.

This is just one of many fertilisers to use ammonia, such as ammonium phosphate.

That's not all bad, right?

I mentioned earlier that ammonia can be a harmful gas - this is due to it being rather corrosive. So if you're exposed to ammonia, you could experience irritation or burns if exposed to (for example) the eyes, lungs, throat or skin. Indeed, if too much ammonia is used as a fertiliser, it could be even more health-averse. It can also affect plants by damaging their leaves, as well as acidifying the soil itself. Bogs and peatlands can be damaged by ammonia exposure, and both play a key part as carbon stores (minimising the concentration of CO₂ in the atmosphere).

Ammonia also has many other uses, such as in cleaning products and in parts of industry, so exposure can also be a risk. Health guidance is here.

So should we phase out ammonia?

Note that most people's exposure to ammonia will have been rather minor - it doesn't pose as significant a threat as many other chemicals might. However, the threat is still significant - about 3,000 premature deaths (article from 2019) as a result of ammonia emissions in the UK. 

Emissions can be reduced through many ways, such as by reducing the amount of protein consumed by livestock (as this is then deaminated - indeed, this is often excreted as nitrogen compounds, such as ammonia). Keeping manure to be used as a fertiliser can also cut down on ammonia emissions, as you're producing less ammonia. This and many other methods can also be found in this report by the Royal Society - in it, it's clear that certain strategies (such as building housing for livestock where manure can also be stored) are rather expensive to carry out.

Perhaps I'm naïve, but I for one wonder whether you could simply reduce the amount of ammonia used in fertiliser, or the amount of fertiliser used in general. There are after all many consequences of fertiliser usage, such as algal blooms when it's spilled into lakes, which have been known for a while. Obviously this wouldn't be so simple as a growing human population requires more crops, which fertilisers accentuate the growth of - but as science develops, perhaps fertilisers which are better for the environment can be developed, minimising ammonia emissions.

This wasn't that bad a blogpost after all. I've learned about the role of ammonia as a fertiliser, as well as some basic chemistry facts. However, there were no footbridges, so that's a shame.

Perhaps there'll be some soon.