What limits plant growth? When we learn about plants in grade school, we learn that they “produce their own food” from sunlight — but in reality, they’re also limited by the availability of certain nutrients.
The three most important nutrients, in decreasing order of importance, for a plant to have are:
Nitrogen
Phosphorous
Potassium
Let’s focus on nitrogen, the first and most limiting factor for plant growth.
Nitrogen, Nitrogen Everywhere, and Not a Drop to Feed Plants
Nitrogen is one of the most abundant elements in the atmosphere; our air that we breathe is about 78% nitrogen (and only about 20–21% oxygen).
The problem, however, is that plants can’t make use of the elemental nitrogen (made of two nitrogen atoms, often written as N2). Instead, plants need nitrogen in the form of nitrate, or NO3.
And one of the biggest sources of nitrate for European civilizations was… concentrated bat poop, or guano.
That’s right. In the 1800s, Europeans were importing tons of bat poop, to fertilize their crops, because they needed the nitrate for their crops.
Bat guano, going by its official term, contains a lot of nitrogen, but more importantly, it contains it in the form of nitrate! The bats eat insects and pass most of their shells, or chitin. Bacteria and fungi break down the crushed insect shells, converting them into ammonium (NH4), then into nitrite (NO2), then finally into nitrate (NO3).
In the 1800s, European explorers and imperialists traveled to islands, like those off the coast of Peru, where birds and bats nested in huge numbers. They harvested vast amounts of guano, exporting thousands of tons of the stuff.
Another reason they wanted nitrate? It’s a great fertilizer — but it’s also a key component in making gunpowder. And when your crops grow well, it gives you more time to think about conquering your neighbors.
Eventually, and thankfully for the poor bats and birds that were being displaced by miners marching in to harvest their droppings, we discovered a more efficient method to produce nitrate. The Haber-Bosch Process, named after the German chemists Fritz Haber and Carl Bosch who discovered it, converts atmospheric nitrogen (N2) and hydrogen gas (H2) directly into ammonia (NH3), which can then be further converted into nitrate.
Haber and Bosch discovered this process, but Nature also has its own method of converting elemental nitrogen into nitrates.
Nitrogen-fixing maize – with or without biotech?
Lightning-Fast Nitrate Formation
In order to crack apart elemental N2 so that the nitrogen atoms can bond to oxygen, it takes a lot of power — but lightning has that in spades.
Whenever a lightning strike occurs, some of the energy is absorbed by nitrogen in the atmosphere, splitting apart the two nitrogen atoms. Once free, they quickly latch on to partners — and instead of going back to each other, will bond to nearby oxygen instead.
The nitric acid, the result of nitrogen and oxygen bonding, gets absorbed by the falling raindrops that accompany lightning. These raindrops both shift the nitric acid into nitrate, and also carry the newly formed nitrate down into the soil — the prime spot for it to then be absorbed by plants.
The more lightning, the more nitrates, which means the more fertilizer in the soil below — so after a big storm, don’t be surprised if plants seem to be doing especially well! They got water from the rain, but they also got a surprise boost of nutrients from the lightning-created nitrate fertilizer.
Soil Nitrates, No Lightning Needed
Of course, there’s not nearly enough storms or lightning to create all the nitrates that plants need. Most of us aren’t shoveling bat guano out onto our yards. So where does most of the nitrate in the soil come from?
The answer, like so many other answers that I write about, is microbes! In the end, it almost always comes down to the microscopic bacteria and fungi that are found practically everywhere.
Soil, like all other environments, is filled with bacteria. Many of these bacteria spend a lot of energy on converting atmospheric N2 into ammonia, NH3, which can be taken up by plant roots. These bacteria grow in symbiosis with the plants, providing their plant hosts with ammonia as a source of nitrogen, and in exchange receiving other nutrients and molecules from the plant that become food sources for the bacteria.
Some plants, such as legumes, even grow special lumpy nodules on their roots, to serve as a living and breeding area for these bacteria to thrive! Without the bacteria, these plants wouldn’t be able to obtain their nitrogen; they provide a safe and food-rich habitat to the bacteria in exchange for their nitrogen fixing work.
We look at lightning as destructive, because of its large-scale effects of starting fires and causing potential harm to anyone who gets hit. However, the massive energy discharge of lightning can also help plants, by splitting atmospheric nitrogen apart and forming nitrate, a vital ingredient for plant growth and one of the largest components of fertilizer.
Lightning is one way that nature makes nitrates, but they’re also found in the guano, or fecal deposits, from bats and seabirds, and produced in large quantities by bacteria that live in the soil.
For most fertilizer, we use a synthetic process, called the Haber-Bosch Process, to produce our own fixed nitrogen from the air. Thank goodness that we don’t need to rely on lightning strikes in order to provide food to our crops!
