Bet you never thought fertilizer was magical but that changes today! In this episode, Steve discusses how scientific discovery brought us fertilizer, effectively changing the game for farmers around the world.
Who doesn’t love a fairy tale? There’s typically the promise of a make-over, at least one or two musical numbers and love prevails in the end! When we grow up we realize that fairy tales are just that…tales. Despite this realization, it seems like a few common fairy tale themes are still pervasive today - from believing only good things come from nature to clearly defined “good guys” and “villains.” Our fairy tale habit can be hard to break. Although there is no magic wand to instantly solve all our problems, our ability to continuously innovate and devise new methods for growing healthy crops is pretty magical when you think about it. Even more magical is our ingenuity in overcoming some of the limitations of nature to create fertilizer on the scale we need to grow our food.
Plants are the starting point for all of our food, whether that means the plants we eat or the plants that feed our farm animals. Even “lab grown meat” starts with nutrients from plants. All these foods that trace back to plants give us the carbohydrates and fats that provide us the energy to live, and also the proteins, vitamins and minerals we need to thrive and grow.
So, what do plants “eat”?
What makes plants so special is that to get their energy, they don’t have to “eat” like we do. That’s because they have the unique ability to get their energy from the sun. via “photosynthesis.” They can actually make the carbohydrates and fats that power the survival and growth of their cells. Those biological molecules are made from various combinations of carbon, hydrogen and oxygen.
But plants do need to be fed other non-energy nutrients. The top three nutrients plants need as “food” are nitrogen, phosphorus and potassium. They also need smaller amounts of other things like sulfur, calcium, magnesium, zinc and iron.
Figure 1 Picture by Hebi B. via Pixabay
Other than more unusual cases like insect-eating plants, the way these “foods” get to the plant is through their roots. Whether the plant is grown in a soil, an artificial potting mix or just in water via hydroponics, their roots have to absorb these “foods” we usually call “fertilizers.” In nature, most of these “mineral nutrients” for plants originate in rocks and become available through the process of weathering to generate soils. Within soils there are certain amounts of these minerals in forms that can be absorbed by the plant’s roots. But one important plant “food” has a more complicated back-story before it can become something a plant can pick up through its roots. That crucial plant food is the element, nitrogen.
Now nitrogen is extremely abundant in nature. Nitrogen gas, which is made of just two nitrogen atoms connected by a triple bond (N2), makes up around 78 percent of our atmosphere. But just like the old saying about ocean water (“water, water everywhere, but not a drop to drink”), that atmospheric nitrogen isn’t at all available to plants or to us or other animals. To be plant-available, nitrogen has to be in forms like ammonium (NH3-) or nitrate (N2O) or urea (NH4CO). There are two natural ways that the “inert” Nitrogen gas (N2) in the air gets converted into those plant-available available forms. One way is during a lightning strike. This mighty electrical discharge in the atmosphere turns some of the N2 into available forms and as it rains, and that nitrogen then effectively becomes a “fertilizer” for the soil. But there is an even more important way that nitrogen gas converts into plant food: and that is by the remarkable work of certain bacteria.
We tend to think about bacteria as organisms that can make us sick, but the vast majority of bacterial species are either neutral or beneficial for our survival. There is a particular kind of bacteria that has the ability to take the inert, N2 gas from the air and turn it into the forms that a plant can absorb via its roots – the nitrate, ammonium or urea.
We describe these particular bacteria as “nitrogen fixing.” There is an important family of plants we call “legumes” that have a special, cooperative relationship with a nitrogen fixing bacterium called Rhizobium.
Figure 2 The nodules on these roots house the nitrogen fixing, Rhizobium bacteria (Wikimedia commons)
Legumes provide “housing” for the beneficial bacteria in protective gall structures on their roots. The plant also provides a healthy dose of its sun-based energy as sugars that feed the bacteria. There are both wild and domesticated species of legumes including important crops that are great sources of the nitrogen-rich proteins we need in our diet. Examples of legume crops include soybeans, various dry beans, lentils, chickpeas and other “pulse crops.” Farmers will also sometimes grow non-food legume “cover crops” to leave some nitrogen in the soil that can be picked up later by the crops that don’t have that special relationship with “nitrogen-fixing” bacteria.
There are other bacteria out there that can fix nitrogen for crops that don’t have the specialized relationship seen with legumes. For instance, there are bacteria that live on the leaves of sugarcane that provide that crop with at least some of its needed nitrogen. There are also some bacteria that can fix nitrogen living inconspicuously inside various crops as “endophytes.” There is a start-up called Pivot Bio that is working to optimize that kind of relationship with a crop like corn, and could have bacteria supplying as much of 20 percent of the nitrogen that crop needs.
When humans began to grow their own food about 10,000 years ago, the supply of nitrogen was typically a limiting factor for the productivity of farms. So, for the many important crops that were not legumes, people found other sources of fertilizer. If they were farming near the ocean they could use seaweed and “fish meal.” Another source was the big deposits of “guano” (or bird poop) that can be found in major nesting sites.
Figure 1 Guano (image by MonikaP, Pixabay)
Even human waste (otherwise known as “night soil”) can and has been used as a fertilizer. But the biggest source was typically the manure that came from the animals that people had domesticated. These were the main nitrogen fertilizers for non-legume crops for millennia.
Now, there was a real limit to how much crop growth could ever be supported from animal poop and the various other natural sources. What I’d like to point out is that unlike nitrogen fixing bacteria, “cows don’t make fertilizer.”
For instance when a cow or pig or chicken or wild bird eats a plant, it absorbs and uses most of the “fertilizer” nutrients like nitrogen, potassium and phosphorus that were present in its feed. But animals fail to absorb all the nutrients, so some are left in the manure. If that manure is put back on a crop, it can act as a fertilizer, but it is a limited supply. I once interviewed a USDA expert on the utilization of manures and he estimated that only 5 percent of our current crops could be adequately fertilized with just the available manure or other “natural” sources.
So, until the early 20th century, total farm productivity was limited to the amount of nitrogen that came from lightening, legume crops, manures or other plant- or animal-based sources. It has been estimated that this original nitrogen supply could never have fed more than around 1.5 billion people worldwide, and certainly not our current population of 7 billion.
It wasn’t until the 19th century that humans even understood the chemical makeup of our world. But when chemistry first began to emerge as a science, there was a widespread belief that substances coming from the living part of nature were somehow different from specific man-made chemicals.. It was thought that the chemicals from nature had a “vital force” that couldn’t be imitated with “synthetic chemicals” made by chemists.
The debunking of this “vitalism” idea began in 1828 when the German chemist Fredrick Woehler made urea from inorganic compounds and showed that it had exactly the same properties as the urea in the urine of animals. Once people realized that the atmosphere contained essentially an unlimited supply of nitrogen, the question then became, how to convert it into a form that could feed crops.
In the early 20th century, two German scientists worked out a way to turn nitrogen from the air into ammonia and subsequently to other plant-available forms of nitrogen. Fritz Haber worked out the basic chemistry and Carl Bosch developed a high-pressure catalyst method that increased the scale of its production.
Both scientists received Nobel Prizes for developing the combined Haber-Bosch process, forever revolutionizing farming and crop productivity beyond the limits of legumes and salvageable nutrients left in animal waste.
This new source of nitrogen “food” for plants is now called “synthetic nitrogen”, a term that is actually quite misleading. The forms of nitrogen in something like animal manure are diverse and complex, including molecules like proteins or nucleic acids. But once the manure is in a wet soil environment, the bacteria living there “mineralize” those fertilizers into the simpler forms that the plants can absorb: ammonia, nitrate and urea.
Ammonia is also the form of nitrogen that those nitrogen-fixing bacteria provide for their legume host plants. The “synthetic” nitrogen fertilizer made through the Haber-Bosch process also starts with ammonia, which can then be turned into nitrate, urea and other available forms just as occurs with the mineralization or “organic” fertilizers. Once nitrogen is in these forms that can be used by a plant, there is no difference to the plant whether that “N” started out as something we call “natural” or “synthetic.” To the plant, these “foods” are identical.
Today when you read an organic product label or hear the organic rules they will typically say that the crop was grown without “synthetic” or “artificial” fertilizers. That makes it sound like there is some real difference, but that concept is a throwback to the debunked concept of “vitalism” – that idea that the chemical from “nature” has some magic property that can’t be synthesized.
This actually gets a bit absurd in the rules for organic. Because, as I mentioned, “cows don’t make fertilizer,” organic farmers wouldn’t be able to grow very much in the way of crops if they could only use manures from animals that were raised on organic feed crops. Thus, the rules also allow organic farmers to use the manures from conventionally fed animals.
So, nitrogen atoms captured from the atmosphere by the Haber-Bosch process are taken up by a crop and then later fed to animals. Those same atoms then “magically” become okay for organic once they have gone through the digestive system of an animal and come out as manure. Organic farmers are also allowed to use “blood meal” and “bone meal” from those “conventionally fed” animals. Those later sources are perhaps safer than manure or composted manure from a microbial safety point of view, but I’m not sure that sounds so great to a consumer.
Now there are some functional differences between “organic” and “conventional” fertilizers. The “synthetic” fertilizers that are already in plant-usable forms can rapidly “feed” the plant, but particularly when the nitrogen is in nitrate form, it can move into the water that runs off a field during a heavy rain, or leach down into the zone below the roots and even into the groundwater below. The nitrogen in manure or other organic forms of fertilizer is released more slowly by the mineralization process. That can be a good thing to avoid losses and water pollution during rain events, but it can also mean that the nitrogen isn’t sufficiently available to support the plant during periods of peak growth. Then the organic fertilizer might continue to be mineralized at a time when the crop is no longer in the mode to take it up and so then it can become a potential water pollution issue. Preventing water pollution is a challenge whether or not the fertilizer is “organic.”
The goal for sustainable crop fertilization is to strive for the “Four R’s.” Nutrients in the right place at the right time in the right amounts and the right forms for optimal crop utilization and minimal off-site movement.
That is a non-trivial challenge given the wild card of the weather and variable crop needs during the growing season. For some situations, a “slow release” fertilizer is a good thing and there are conventional fertilizers with that kind of property so they can act more like a manure.
For some of the more sophisticated, modern means of precision fertilization, the highly available “synthetic” forms are best. Those fertilizers are well suited to something like “variable rate fertilization,” a method in which the fertilizer is applied at different rates in different parts of the field based on true, site-specific crop demand based on geo-referenced aerial imaging and past crop yield data.
For crops that are irrigated, the “synthetic” fertilizer can be “spoon fed” in the water to fully meet crop needs with minimal potential for having the fertilizer end up someplace where it is not wanted. These more precise and sustainable strategies are far less feasible using only the “natural” fertilizers allowed for organic farming.
So, getting the nitrogen to the plant and not into off-site water is one major challenge when it comes to feeding plants. There is another important environmental issue associated with fertilization of crops that has to do with the “carbon footprint” of the fertilizer. “Synthetic” nitrogen is made by starting with the relatively cheap energy source of natural gas. That means its production is based on the use of a “fossil fuel” which has a greenhouse gas “carbon footprint”. So the natural gas that it takes to make, say, 150 pounds of synthetic nitrogen to grow an acre of corn has a carbon footprint equivalent to driving a passenger car 194 to 421 miles (if it is a car that gets 20 mpg)
When that nitrogen feeds a crop which then feeds an animal, the portion that remains in the manure, blood or bones in an “organic approved fertilizer,” is nitrogen that can be used again without a new round of fossil fuel usage. However, that does not avoid the whole greenhouse gas issue.
When something like manure is stored, and particularly when it is composted for the sake of food safety, the nitrogen is still there, but some of the carbon in the manure is converted into methane by bacteria that grow during the part of the composting process when the oxygen supply runs short.
Methane is more than 20 times as potent as carbon dioxide as a greenhouse gas. In a big pile of manure or any organic waste stream, it isn’t really possible to prevent there being micro-sites where there isn’t oxygen, particularly during the peak demand for oxygen during the composting process. That means that quite a bit of methane is emitted during the hot part of the composting process so that the carbon footprint of that kind of fertilizer per pound of nitrogen is 7-14 times as big as that for fertilizers made via the Haber-Bosch process. Getting back to the 20mpg car example, the extra carbon footprint of getting the 150 lbs of nitrogen from the compost instead of the “synthetic” nitrogen is equivalent to driving an extra 2400 to 2600 miles.
On the one hand it might seem that we are stuck with a carbon footprint issue whether farmers are using Haber-Bosch nitrogen or getting it from composted manure, but there is a potential solution out there that I stumbled across a few years ago. People who were developing wind or solar generated electricity in remote areas off the grid ran into issues with timing of the sun or wind lining up when there was demand for the power. They needed a way to store the energy, and one possibility was to do “electrolysis” of water and generate hydrogen.
Hydrogen is kind of a dangerous thing to store, so some people thought, “what if we turned that hydrogen into ammonia using the Haber-Bosch process.” The natural gas used in large-scale fertilizer production is really just a source of hydrogen, and hydrogen generated from water by electricity could serve the same goal. Now, commercial fertilizer production is done on a large-scale for efficiency purposes, but a few different groups have now come up with small-scale processes that could work for a small wind farm or solar installation.
Also, ammonia can be used to power modified diesel engines, so the U.S. military has been doing some work on this with the idea so that they could have a way to generate fuel for something like a military convoy in a remote place.
Figure 5 Could a windmill like this make the fertilizer for this farm? The answer is blowin in the wind (Image by Benjamin Combs b3njamin)
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What would be really cool is if this process could be refined so that a single farm with a windmill or a remote village in Africa with a windmill or a solar panel could make nitrogen fertilizer for the local crop.
The ammonia could be converted to even more stable forms of fertilizer like nitrate or urea. The other big plus would be that the nitrogen in this case would have been made using renewable energy sources and thus it would have “zero carbon footprint.” Ironically, because a “chemical process” would be used in this case, this local, environmentally ideal fertilizer would probably not qualify as organic, but as with all the forms of nitrogen, by the time the plant is absorbing them they are all the same. The plant wouldn’t care.
So maybe this was a bit more about fertilizer than you ever wanted to know, but Nitrogen is an extremely important element in feeding humanity and experts continue to find precise and innovative methods for the least environmental impact possible.