Opinion: Fertilisers and farming are a hot issue at the moment. I’ve spent 30 odd years working in organic and now regenerative farming. These have quite different attitudes to fertilisers than intensive (mainstream) farming. The latter has mostly viewed them as a tool to maximise yield with little attention paid to negative side effects such as nutrient pollution of waterways. The former, as part of the toolkit to build healthy soils that then support healthy crops and pasture, are based on a more holistic and ecological view of what a farm is. 

The term ‘fertiliser’ is something of a mare’s nest. It’s such a generic term as to be nearly meaningless if you’re getting into the details of nutrients in agriculture. Even ‘nutrients’ is vague – chemical elements nails it down. The primary issue is nitrogen fertilisers are totally different to all the other fertilisers, for example phosphorus and potassium, which I term ‘lithospheric nutrients’. This is because all lithospheric fertilisers are mined from the ground. They are also fossil nutrients, the same as coal is a fossil fuel (fossil energy), so they will run out, at which point agriculture is over.

Nitrogen is totally different. Virtually all the nitrogen on the planet is in the atmosphere as gas – it is where we ‘mine’ our nitrogen fertilisers from. The form of nitrogen in the atmosphere is dinitrogen. It is almost completely chemically unreactive. Dinitrogen does not form chemical compounds, such as rocks, hence why nearly all the nitrogen on the planet is above our heads.

To make nitrogen fertilisers, dinitrogen has to be ripped apart which takes a lot of energy to break the triple chemical bonds that hold it together. Nitrogen fertilisers are made by taking dinitrogen from the atmosphere and combining it with hydrogen to make ammonia.

The Haber–Bosch process has been the main way to make ammonia for the past 100 years or so. This is a horrendously energy inefficient process that uses about 1.5 percent of the global energy supply and 6 percent of the global natural gas (methane) supply. As most of that energy comes from fossil fuels, the production of ammonia has a massive impact on the climate emergency. Only in the past decade have better ways of making ammonia been invented, but they are still highly energy inefficient and still use lots of energy.

Ammonia is then the starting point for the creation of all other ‘reactive’ nitrogen compounds, ie all forms of nitrogen that are not dinitrogen, which includes all forms of nitrogen used by biology. All nitrogen fertilisers are therefore made by the Haber–Bosch process or its newer replacements. While the new fertiliser factory outside Taupō, which will open next year, may be less dirty than alternatives, it is not totally clean.

It is possible to make nitrogen fertilisers from renewable energy. Indeed the first nitrogen fertilisers were made by hydro electricity in Norway by Norsk Hydro. But this is much more expensive that making it with fossil fuels, so virtually no one uses this approach. It is not clear if the new fertiliser factory will be using renewable or fossil fuels. Even if it does use renewables the problems with nitrogen fertilisers do not end at the factory. Using nitrogen fertilisers is as big a problem, if not bigger, than their production.

First, there is far too much nitrogen in the environment. Half the reactive nitrogen in our environment was made by humanity – and it is poisoning our natural life-support systems. Excess reactive nitrogen in our environment is identified by the Planetary Boundaries framework to be a far bigger risk to our safety than climate change, synthetic chemicals and loss of natural lands to farming and urbanisation. Indeed, it is a bigger risk than all the other environmental problems humanity has created apart from biodiversity loss. We must therefore dramatically reduce the amount of reactive nitrogen in the environment if humanity is to survive. That means less not more nitrogen fertilisers, regardless of how they are made.

Nitrogen fertilisers also have a big climate impact when they are used – which lithospheric fertilisers don’t. Depending on the type of nitrogen fertiliser and how it is applied, several percent of it is turned into nitrous oxide – a very potent greenhouse gas. If the soil is very wet, a range of other noxious compounds are produced such as nitrogen dioxide and nitric oxide, which are toxic, deplete ozone and cause acid rain.

The core problem is that by the fundamental physics, chemistry and biology, nitrogen fertiliser cannot be green. Some are less dirty than others, but they are all dirty. Innovations such as slow-release membranes can reduce the negatives but can’t eliminate them.

In comparison, the lithospheric nutrients are dug out of the ground, typically refined to purify them, then shipped around the world and applied to the land to replace the nutrients that are removed in the harvest. This takes quite a bit of energy but nothing compared with the amount used to make nitrogen fertilisers.

Most importantly if the lithospheric nutrients are not replaced, their levels in the soil fall which results in plant yields decreasing to the point that trying to grow crops is useless. It is therefore utterly essential to replace lithospheric nutrients. 

Nitrogen is also the only nutrient that does not have to be applied as fertiliser by farmers. This is because of biological nitrogen fixation, where a small range of plants, such as legumes, have a symbiosis with bacteria that can extract dinitrogen from the atmosphere and turn it into reactive nitrogen – the exact biological equivalent of the Haber–Bosch process but powered by sunlight, so totally natural, clean and renewable. Aotearoa New Zealand’s pastoral agriculture has been entirely based on this through the use of grass and clover pastures. Indeed until the expansion of dairying since the 1990s, pastoral and arable farming here used no nitrogen fertilisers – all the nitrogen came from clovers biologically fixing it. This is how organic agriculture that prohibits the use of nitrogen fertilisers works. It is therefore possible to farm without using any nitrogen fertiliser at all, in contrast to lithospheric fertilisers. Yes, compromises have to be made in the production system and there can be profit implications, but, Aotearoa New Zealand farmed for nearly its entire history without nitrogen fertilisers so it is possible to do so again.

Many of the problems in farming have come from simplifying both farm systems and how we understand them. We need to view and manage farms as the complex adaptive systems they are. This is what increasing numbers of the international political and science community are calling for, for example in the Stockholm+50 report. When talking about fertilisers in farming, the details are really important and also the big picture of how they fit into our farms and the wider environment.

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