The farming systems we are putting in place now will need to feed the 9-10 billion people that will inhabit our planet in 2050. This is a huge challenge.

Many people speak about “sustainable farming,” which is a legitimate and important concept. Truly sustainable means that we need to track all of the inputs and outputs in the global food system and see that we can extrapolate that system indefinitely into the future.

One of the most, if not the most, important factor in sustainability is nitrogen. Plants need a lot of nitrogen to grow, and this is often the limiting factor in large-scale food production.

Thinking about where nitrogen ultimately comes from – the entire nitrogen cycle – is like thinking about where energy ultimately comes. It’s a very useful question to ask. For example, when people claim that they can run their car on water they are failing to ask this basic question. When you do you realize that the energy is not coming from burning the hydrogen and oxygen, but from whatever energy source you used the split the water into hydrogen and oxygen.

The same thought process applied to nitrogen is also illuminating.

Chemical Fertilizer vs Manure

About half of the nitrogen that ends up in our food comes from manure, with the other half coming from chemical fertilizers. Ultimately the nitrogen in manure or fertilizer comes from the atmosphere, which is 78% nitrogen. A small amount of nitrogen in the soil comes from other biological sources, such as nitrogen fixing crops and bacteria.

Plants will deplete soil of nitrogen, which has to be replaced in order for production to be sustainable. Let’s consider the two main sources of this nitrogen – manure and chemical fertilizer.

The green revolution was largely due to developing techniques for mass production of nitrogen fertilizer. The advantages of chemical fertilizer are that it can be mass produced, that production has a relatively small footprint, and the fertilizer is easy to store and apply and is cost effective.

There are, however, two main downsides to chemical fertilizer. The first is that it is often produced with petroleum, which serves as the source of hydrogen that binds with the nitrogen to make the fertilizer. Without getting into the “peak oil” controversy, I think it’s safe to say that using a fossil fuel as a source of fertilizer is not indefinitely sustainable.

The second big problem with chemical fertilizer is that it is very soluble (which is actually good for plant absorption), which means any nitrogen not absorbed by the plants will easily run off into ground water. This then gets into streams and rivers, and then into lakes and oceans. Once there it will cause a bloom of algae that feeds on nitrogen, dropping oxygen levels in the water and creating a dead zone.

Manure as a source of nitrogen has the advantage that it is essentially recycled nitrogen. Animals eat grass or crops which are the source of nitrogen, which then ends up in the manure, and is then put back in the fields to grow more crops. Of course, some of the nitrogen from crops also goes to humans (that’s kind of the point), it does not all go to feeding manure-producing animals. That nitrogen needs to be replaced.

Some of that nitrogen comes from grazing (essentially transferring nutrients from grazing fields to crop fields), but then we have to consider the sustainability of grass fields. In many parts of the world fields for grazing are losing nitrogen and phosphorous, and are not sustainable without further inputs.

There is also only so much manure to go around. If we want to maintain current production with manure only, we would need to essentially double the manure supply in the world, which is simply not possible.

Also we have to consider the overall footprint of manure farming. If we compare productivity per amount of land, we can’t just consider the land used directly for growing crops. We also have to consider the land used to feed the animals that were the source of the manure.

Output per unit of land is 20% less for manure-based farming than chemical fertilizer-based farming. That is not even considering the land necessary to produce the manure, however. Nitrogen-fixing cover crops or crop rotation may help, but then you also have to consider that land as well.

Using nitrogen-fixing plants also requires that those plants are tilled back into the soil, which releases CO2 into the atmosphere. Manure also releases nitrogen into the atmosphere through volatilization of the ammonia. It contributes to environmental nitrogen as well, although not nearly as much as chemical fertilizer.

The Solution

As is often the case with any complex problem, there is rarely a simple answer. If we want to optimize sustainable farming we need to consider the entire system, all inputs and outputs, all the land use, and all of the environmental impacts.

Most of the sources I have read acknowledge that we will need chemical fertilizer in order to maintain food production. Some pro-organic sites claim that we can feed the world with manure-fertilized organic farming, but the numbers just don’t add up. Where will that manure come from? Right now much of the manure used for organic farming comes from animals fed with grain from chemically fertilized crops – so the ultimate source of that nitrogen is also chemical fertilizer.

Keep in mind manure is not really a source of nitrogen but is rather a way of recycling some of the nitrogen back to the farmland or transferring nitrogen from land used for grazing to the farm.

Making optimal use of manure is important, but I find most compelling the analyses which argue that we cannot feed the world with nitrogen from manure alone.

We can also use nitrogen-fixing crops to take some nitrogen from the air and put it back into the soil. The main problem with this approach is that it increases land use. Some studies, however, have shown that using a winter cover crop can put some nitrogen back in the soil and reduce the need for chemical fertilizer without reducing yields.

Many sites agree that we need to optimize our use of chemical fertilizer. Farmers sometimes over-apply fertilizer, because they want to maximize yield. The cost of under-fertilizing is lost yield and therefore lost money. The cost of over-fertilizing is the extra cost of the fertilizer and the increased runoff into the environment. Apparently many farmers worry more about the former loss than the latter, and so tend to over-fertilize.

Timing is also important. If fertilizer is applied when plants are soaking up nitrogen quickly there will be less left over to get into the environment. There are also other timing issues, like avoiding applying fertilizer before a big rain.

It seems that a combined strategy using as much recycled nitrogen as possible, nitrogen-fixing crops, and optimally applied chemical fertilizer, can maximize yield per unit of land while minimizing environmental impact. This can be extra work for farmers, however, so they need an incentive to do this.

As the human population grows, however, it will likely become necessary to add new options. One intriguing possibility is genetically engineered crops that are able to fix their own nitrogen. Imagine a wheat variety that can fix nitrogen from the air – no need for nitrogen fertilizer.

Legumes are plants that can do this now. Actually it is bacteria that live on their roots that fix the nitrogen from the air. There is ongoing research to engineer cereals that are able to develop the same symbiotic relationship with these bacteria. This is a complex task, however, and estimates are that we are at least 20 years away from this goal.

Still, this is the kind of technology we need to be working on so that we do have more options in 2050 when there will be more than 9 billion human mouths to feed.

Conclusion

A thousand years ago we were essentially mining our world for nutrients, and the reservoir was so vast that we did not have to worry about the entire system. That changes with the industrial revolution and the explosion of the human population. This was followed by the green revolution made possible by synthetic fertilizer, which caused further population increase.

As we start to push up against the limits of the natural resources of our planet, we have to think in terms of entire systems – what are the inputs and outputs. Farming has lots of inputs and outputs, but one of the biggest is nitrogen.

When we consider the entire nitrogen system we can figure out strategies for optimizing the flow of nitrogen to maximize food production with the smallest environmental impact.

One of those environmental impacts is total land use. Every acre of wild field or forest we convert to farmland has a huge negative impact on the environment – much more than the differences in how we farm that land. Without synthetic fertilizer as a source of nitrogen the need for land use would increase dramatically.

But synthetic fertilizer is causing significant runoff which is having a negative impact also. What we need is smart land and nitrogen use that combines the best of all available approaches.

This is partly why I think the false dichotomy of organic vs conventional is harmful. An integrated evidence-based approach to best practices, without an appeal to an arbitrary ideology, is what we need.