Clover is an amazing plant. It can start growing as soon as the snow melts, and it can make its own fertilizer.
Plants, we all learn in grade school, work by sending roots into the ground to harvest water, minerals, and nutrients, while leaves spread out above to harvest energy in the form of sunlight. While broadly correct, this is an oversimplification. First, the vast bulk of the physical matter that makes up plants is carbon removed from the air as a part of the work leaves do when making energy. Strange though it may seem, if you look at a towering oak tree most of what you see has been extracted from the air, not the ground.
The roots certainly do take up both critical minerals and nutrients. Some, like phosphorus, potassium, calcium, magnesium, and various trace minerals, occur in soils in widely varying amounts, which are dictated by the rock from which the soil derives, the climatic conditions, and probably a whole lot of other factors I’m not smart enough to understand. Farmers often apply concentrated forms of these minerals to address growth-limiting deficiencies in the soil.
A Most Curious Nutrient
Nitrogen is a special case. It is more abundant than carbon in the atmosphere, but the leaves of plants cannot capture it directly from the air. On the molecular level nitrogen is quite inert in its gas form, and converting it into something that can be used by plants requires a complex, specialized process. In nature this is overwhelmingly done by bacteria. Even nitrogen fixing plants like clover and beans rely on symbiotic bacteria to do the actual chemistry.
The reason legumes and other nitrogen fixers haven’t crowded out all other forms of plant life is that fixing nitrogen is energetically expensive. A clover plant effectively trades energy to the bacteria in its roots in exchange for nitrogen. Most other plants would rather take advantage of the nitrogen left behind when a clover plant dies than pay for it themselves.
A lack of usable nitrogen often limits the growth potential of the crops humans grow for food, either for ourselves or to feed livestock. For most of our history we relied on nitrogen fixing plants to provide it; we would let a piece of ground fallow or use it as pasture for several years, allowing nitrogen to accumulate in the soil as clover and other nitrogen fixing plants lived and died. Then we would plow it and plant it with crops, which would draw down the nutrient bank. This worked, but it required quite a lot of land. Every year a significant chunk of acreage would have to be fallow, and the amount of nitrogen that would be captured by the fallowing was limited by biological processes.
But in the early twentieth century chemists figured out how to directly convert atmospheric nitrogen into a fertilizer that could be applied in any amount a farmer wished. Along with advances in plant breeding and farm machinery nitrogen fertilizer made the modern food system, a system that provides us with an abundance of food beyond anything our ancestors could have imagined.
Methods of Nitrogen Capture
One early commercialized form of nitrogen production was the Birkeland-Eyde process, which used electricity to make nitric acid. It worked, but within a couple decades the more energy efficient Haber-Bosch process, which uses natural gas and high pressure to capture atmospheric nitrogen in the form of ammonia, had replaced it.
I’m not going to attempt to explain the technicalities of how each process works. If you’re interested there are lots of excellent articles and videos that do just that, and if you’re not interested then spotting eight inexpert paragraphs on various chemical reactions coming over the horizon would likely convince you to flee this article, for which I wouldn’t blame you.
The important thing to understand is that getting nitrogen from the air into a form that plants can use is always energy intensive, whether it’s bacteria or chemists overseeing the process. One early method used electricity to do the job, but soon a better method was developed.
Though the scale and efficiency of fertilizer plants has increased in the past century that better method, the Haber-Bosch process, still underlies most commercial nitrogen production. It is one of the most enduring and consequential scientific discoveries, one that has reshaped human society along with millions upon millions of acres of farmland.
The Nitrogen Fertilizer Problem
You might think I’m going to launch into a discussion of hypoxic dead zones and nitrates in the water table and all the other issues attendant to overfertilization, but I want to put those aside. Right now I am only thinking of the problem that concerns even the most cynical industrial farm growing monocultures in the smoldering remnants of the Amazon rainforest, namely, that nitrogen costs money.
If you’ve made it this far with me you’ve hopefully grasped the underlying theme that nitrogen requires lots of energy to produce. In the Haber-Bosch process this comes in the form of natural gas, trillions of cubic feet of which are used to produce fertilizer every year. This means that the price of fertilizer rises and falls with the price of energy, and the price of energy is something farmers cannot control.
For a grain farmer managing thousands of acres of corn or soy, profit per acre often barely exceeds cost per acre. The margin between the value of the final crop and all the inputs that go into it, costs like seed, machinery time, pesticides, labor, and so on, is often shockingly small. Fertilizer is a wildcard. At best it is an input cost like any other. But because energy prices are volatile, and because nitrogen fertilizer is basically energy converted to another form, a spike in energy costs can blow up even the most conservative budget.
If someone could develop a machine that let farmers make their own nitrogen fertilizer for a fraction of the cost it is no exaggeration to say it would be a Nobel prize worthy achievement. But anyone claiming to have invented such a machine should be prepared to provide remarkably strong evidence to back it up.