Of course, fossil fuels are a feedstock in the manufacture of many useful substances. But usually, what they contribute to those substances is carbon (and specifically, carbon in a form that’s easy and economical to work with). But the key ingredient in fertilizers is nitrogen compounds, and sometimes various other elements like phosphorus. But not carbon: Plants get the carbon they need from the atmosphere, not from fertilizer.
And sure, making nitrogen compounds is energy-intensive, and right now, that energy mostly comes from fossil fuels. But that’s the same story as all the rest of our considerable energy usage: It’s mostly fossil fuels because they’re cheap, but it could be any other source of energy.
So just what are fossil fuels used for, in the manufacture of fertilizers, and how easy would they be to replace in that role?
It appears that the main fossil-fuel based synthetic fertilizers are ammonia-based, using atmospheric nitrogen but requiring hydrogen from natural gas (mostly methane, CH4). But that’s a lot of fertilizer.
Undoubtedly, there are a lot of potential alternatives to natural gas to source hydrogen, but if they were economically competitive they’d be in use instead of natural gas.
As of 2018, the Haber process produced 230 million tonnes of anhydrous ammonia per year.[72] The ammonia is used mainly as a nitrogen fertilizer as ammonia itself, in the form of ammonium nitrate, and as urea. The Haber process consumes 3–5% of the world’s natural gas production (around 1–2% of the world’s energy supply).
You are correct that “green hydrogen” is more expensive to produce.
As of 2024, low-emission hydrogen, including both blue and green hydrogen, accounted for less than 1% of global hydrogen production, with green hydrogen only making up around 12% of all low-emission hydrogen production.[5][6] Green hydrogen is more costly to produce compared to all other methods of hydrogen production, however, with new technological developments as well as the increasing cost of natural gas, the cost gap is expected to narrow by 2030.
As far as I know, the primary elements in fertilisers are nitrogen, phosphorus and potassium.
Nitrogen is largely extracted from the atmosphere by the Haber process, which requires energy: but that doesn’t have to involve carbon.
Then as you say, the other two elements are probably limited by available sources. The elements themselves are not especially rare, but perhaps not readily available in useful form. In fact I have seen suggestions that phosphorus might be a limiting factor?
What’s a limiting factor depends on your particular soils. Some soils are naturally higher in phosphorus than others. Some have had so much phosphorus piled on to them in fertilizer over the years that the runoff has become a serious pollution problem.
The economic feasibility of making fertilizer boils down to breaking chemical bonds. Methane and feedstock hydrocarbons are all very stable molecules, so it takes a significant amount of energy to break them and add nitrogen and/or phosphorus to create carbon-based fertilizers. Ideally, solar power could be used as the energy source to create fertilizers, but politically and mechanically, hydrocarbon based energy is easier to use.
Yep, phosphate is mined from limited deposits, and there were serious concerns that we would shortly start running short until a massive new deposit was found a couple of years back in Norway, which should last at least the next 50 years.
I was in the middle of writing a review on the importance of an organism that can improve phosphate availability to plants at the time, and had to re-write the whole thing due to the find, so I wasn’t completely thrilled. They could have waited.
The natural gas (methane, CH4) is used for the hydrogen that’s a feedstock to the Haber process. And the carbon either ends up as CO2, or gets cycled back into urea fertilizer.
So the carbon is a byproduct, not a goal of the Haber process for producing ammonia with the nitrogen from the air.
Still seems important. Why ship the stuff all the way from Norway (unless of course you’re in Norway) if there’s an organism that means you need less of it?
Got a link to the paper, if it’s publicly available?
Solar power can be used to grow nitrogen-fixing plants.
It’s not published, it should be part of my PhD thesis which I’m still working on; arbuscular mycorrhizal fungi. There’s quite a lot of research out there, largely on phosphorus uptake, but they’re associated with all kinds of benefits and everyone gets all excited but unfortunately it’s one of those things that typically behave beautifully in a controlled lab environment, but in a field or glasshouse situation adding them has much less predictable effects - there’s often no impact and sometimes a negative impact.
They are pretty much ubiquitous anyway, in fact trying to keep them out the negative controls is actually a major issue of working with 'em, there’s probably already some there in a reasonably healthy soil.
Ah. Thanks for info. Yeah, the mycorrhizals are doing all sorts of interesting stuff. I think to a large extent we need to get better at getting out of their way so they can do it.
To an extent, yes, but there’s a couple of thousand species and they don’t all have the same impact, or the same impact on different plants. Or the same impacts on the same plants in different soils.
Changing tillage practices can favour some over others, so we may be inadvertently selecting for ones that aren’t all that helpful, but we don’t really understand what’s going on enough to know that for sure.
So there’s some/enough phosphorous floating around in the environment (much like nitrogen but nowhere near as common)? It’s just a matter of fixing it into a form/molecule plants can use?
I know with the current political issues, it’s been mentioned that Canada supplies a lot of potash as fertilizer for the USA. Apparently they mine it in Saskatchewan but the USA had imposed an import tariff recently - for a while. The USA also removed some sanctions on Belarus which is another source of potash.
Kind of- but phosphorus can be in the soil, not the air, in forms plants can’t access. Some bacteria can help change it to a form which can be taken up by plant roots, and some mycorrhizae can trigger increases in the populations of those bacteria. Meanwhile, other mycorrhizae have a massive network of fine hyphae, so they can collect enough phosphates from an area for plant health even when soil concentration is low. They basically exchange phosphates for carbohydrates supplied by plants. Well, that’s one of the things they do, anyway.
There kind of has to be. All living things need phosphorus (it’s part of both DNA and ATP), and have it in our bodies. Whatever phosphorus there ever is, there always is. When we die, it doesn’t just disappear. It could potentially be diluted in soil or water, or converted into a less-accessible chemical form, but those are things that can be undone by living things.
Rephrasing, the heat required is a function of the difference in bond strengths between the products and reactants, not just the feedstocks/reactants. My gas stove shows that methane is plenty reactive in the presence of oxygen. The heat input for steam methane reforming is because using water instead of O2 and going to CO instead of fully oxidizing to CO2 is net endothermic.
SMR can be replaced with water electrolysis powered by your generation of choice. This is generally less economical than SMR, although YMMV if you’re remote and paying high prices for ammonia transportation and have a lot of wind/solar with not much other demand for it.
The synloop where very stable N2 is reduced to ammonia is exothermic and provides heat for other parts of the process.
Is that what you meant to type? Converting very stable N2 to almost anything else will almost always require energy input. Converting very reactive O2 and a carbon-containing compound to CO2 is what provides the energy.
I am not sure there is any contradiction, here. The reaction is exothermic, at 92 kJ/mol; Wikipedia gives typical parameters for running the ammonia reactor as 450 °C (kept cool by heat exchangers) and 300 bar. And ammonia production requires energy input as in 1–2% of global energy consumption.
That 300 bar is doing a lot of the heavy lifting getting over the entropy hump.
It has been something of a disappointment to me that that 1-2% energy use and its concomitant CO2 emission has not been clawed back by solar driven hydrogen production. It seems one of the low hanging fruit of greening to supplant methane and coal based ammonia production. Unlike the ridiculous attempts to create a hydrogen economy for ICE cars, ammonia already has an entrenched distribution chain and end use.
(As an aside, one reason hydrogen continues to be touted as a viable energy transport technology, is that it is a way of sneaking methane back into domestic energy distribution conversation. Hydrogen won’t work, but look here, gosh, natural gas does, and it isn’t as bad as coal. So it’s all OK isn’t it?)