Optimising crop yields without using fossil-fuel-manufactured fertiliser

Question

The green revolution produced a step change in crop yields. Although nitrogen-based fertilisers have been produced without fossil fuels, a very large proportion of global fertiliser production depends on fossil-fuel production: natural gas is burnt in the presence of steam, to create the ammonia at the heart of fertilisers: so it's not just the fossil-fuel inputs for energy, which are easily replaceable; it's the chemistry that creates the ammonia. Natural gas has provided a cheap, hugely-scalable means of creating ammonia.

What are the best alternatives to fossil-fuel-based fertilisers, for optimising crop yields? Can fertiliser production be decarbonised? Does organic farming offer a scalable opportunity to maintain nutrient yields - and if so, what are the industrial-scale routes to replenish the nutrients extracted in food?

Answer 1

(Draft answer till I get around to find some figures)

There's a few ways I see to go about this: Running a Haber Bosch process or similiar with a non-fossile source for energy and H₂, or finding other N sources. There's principally two routes, N fixing plants in crop rotation + organic fertilizer and reusing N-rich wastes and wastewater. The key difference beeing that the first is basically happening within one or a few farms, while the second version involves municipalities.

Nitrogen as hard limit
Plants need nitrogen to build protein, typically 1kg N for 6,25 kg protein. The amount of nitrogen available to the plant is a hard upper limit on the amount of protein we can get. While it's not enough to look at the nutrient balance of a field, the balance puts limits on the achievable yields. Nitrogen is the nutrient that is most strongly fossile fuel based, since production of N-fertilizers requires natural gas. So in this answer I will look only at nitrogen.
However, another important nutrient is phosphorus and here the world could face a shortage in the coming decades. Many of the ways to recycle nitrogen also recycle phosphorus.

Non-fossil Haber Bosch:
Haber Bosch (HB) needs power to drive turbocompressors and Methane for heat and hydrogen. For the latter, one could use Biogas. HB yields a bit less than one mole NH₃ per mole CH₄. One ha of corn yields about 47t/y, that amounts to roundabout 9400Nm³ Biogas at 52% methane. So we have ~220kmol methane, a slightly smaller amount of ammonia that amounts to maybe 2500kg/a of nitrogen fixed. This actually a lot. This approach is scalable if there's a gas grid in place: Biogas plants can feed the (upgraded) Biogas into the grid, where it is transported to the HB plant.
There are electrically powered approaches to HB that I have not yet investigated.

Crop rotation + Organic fertilizers:
needs a side by side comparision of conventional/intensive planting and extensive planting without. Even when fertilizers are used, the farmer wil have to follow some crop rotation to replenish topsoil, protect the ground from winter erosion etc. This means we cannot look at just plant yields with different fertilizer applications, but we must look at complete crop rotations at the farm level.
According to wikipedia, fixing 30-40kg N/ha is possible. Typical fertilizer application is in the range 100-170kg N/ha, for high yield crops.
This meta study states that on average organic farming practices (which in this context mainly means no fossils based nitrogen fertilizer) yield about 20% less than conventional agriculure - at the field level. The yield gap is assumed to be larger at the farm level, when nutrient limits come into play stronger. The authors themselves stress that whole crop rotations have to be looked at. Also there's a huge span, in some cases organic practices outperformed conventional agriculture. So far I found no farm level multi year comparison between organic and conventional agriculture.
While the question may still be open wether organic agriculture can feed the world (and this will depend on meat consumption, biofuels etc.), organic farming is scalable with one caveat: Manure is not always available in the quantities neccessary. Solutions that utilize waste and or wastewater will beimportant in the years to come.

Wastewater With activated sludge treatment, about 90% of the nitrogen content of the wastewater is sequestered in the sewage sludge, along with some phosphates and other interesting nutrients. Sludge from municipal wastewater is often contaminated with heavy metals and not usable for land application. These contaminants usually come from industrial wastewater. Banning untreated industrial wastewater from municipal sewer lines could help.
Most Wastewater treatment plants are designed for elemination of the remaining N (turning nitrate into athmospheric N), not for fixing nitrogen in a usable form. But apparently there are promising tests with MAP (Magnesium amoonium phosphate, can be used as a fertilizer) precipitation or duckweed for municipal wastewater. To my knowledge, technologies established for high strength wastewater (like ANA strip or Dekastrip) cannot be used economically for municipal wastewater.

Using municipal wastes and wastewater
Source separated organic wastes can be composted (or fermented) and applied to land. This depends heavily on the behaviour of the households doing the separation. I've seen compost from SSO that looked pretty good, and I've seen some with lots of visible plastic contaminants.
I think one key to close nutrient cycles between city and countryside is cleaner waste, meaning better separation, separation between industrial and household wastewaters (and maybe between gray and black and yellow wastewaters).

There is no simple comprehensive plan for a future agriculture without fossile fule based fertilizer. The trick will be to employ closed nutrient cycles at many scales - on the farm, betweeen farm and food processing, between city and countryside. Closed loops are only possilbe with good waste and wastewater management and are aided by source separation.
Where these loops are insufficient, green fertilization with legumes etc. and non-fossile HB are needed to supplant fertilizer.

Answer 2

I wanted to comment but since i can't...

Are you talking about the energy generation to produce fertilizer ? Then of course you can go non-fossil sources...you can use the sun the wind the water i think that if it is a closed environment you can even use the methane you generate to generate your own power

But what you should ask yourself is do you really need industrialized fertilizer at all? Look to the nature plants have been there in abundance for a long time you can get all you need free from the nature.

The overall fertility and viability of the soil, rather than the application of fertilizers as quick fixes, is at the very heart of organic gardening.

Many organic materials serve as both fertilizers and soil conditioners—they feed both soils and plants. This is one of the most important differences between a chemical approach and an organic approach toward soil care and fertilizing. Soluble chemical fertilizers contain mineral salts that plant roots can absorb quickly. However, these salts do not provide a food source for soil microorganisms and earthworms, and will even repel earthworms because they acidify the soil. Over time, soils treated only with synthetic chemical fertilizers lose organic matter and the all-important living organisms that help to build a quality soil. As soil structure declines and water-holding capacity diminishes, more and more of the chemical fertilizer applied will leach through the soil. In turn, it will take ever-increasing amounts of chemicals to stimulate plant growth. When you use organic fertilizers, you avoid throwing your soil into this kind of crisis condition.

Use earthworms they are really good for the health of any soil and they are a free infinite fertilizer productor.

Also you can try to plant a few spare leguminous plants on your farm because they take the nitrogen from air and put it on the ground avaible for other plant's to use if did right you won't need to add nitrogen never again just le the nature do the job!

I will stop here you can find much more about organics fertilizer and how to organic care not only your plants but your soil too on google

Answer 3

Note: the plants need nitrogen (for protein synthesis), not necessarily ammonia. Ammonia however is only one source of nitrogen. So I'm using the general term nitrogen below:

In practice this has already been done through various methods:

1. no-till: crop leftovers (e.g. stalks and husks from maize) are left on the field. Plowing is minimized or abandoned completely, with specialized sowing machines inserting the seed into the soil in a small disturbance. Over time this provides mulch and, on breakdown, organic nutrients. This starts to increase humus content, which traps more nutrients (prevents against leaching) and provides a beneficial environment for soil microbes (some of which fix atmospheric nitrogen). Moisture is retained better, soil structure is improved and soil compaction is minimized. Yields have been found to be comparable or even better than the conventional method. (I like this method since the waste is turned from an additional liability to an asset.)
2. Biochar: (partially carbonized but not burned biomass). Much the same benefits as humus/compost as in the previous point. Please do a web search on this.
3. Companion or rotation planting: Some legumes (beans, alfalfa, etc.) and other plants have a symbiosis with nitrogen-fixing bacteria, which colonize their roots, and these nitrogen-rich deposits remain in the soil for some time. Obviously not good for large single-crop farms, but has been used in gardening and traditional farming for millennia. Some mixtures are available commercially today to inoculate beans and other seeds with the relevant bacteria to ensure the maximum benefit is available.

I feel the need to point out that farming should not be a balance statement about what you put into the soil and what you take out of it. These methods take the long-term view (years or even centuries) to create a beneficial environment in the soil. This environment is able to attract more beneficial factors than just what one uses to fertilize the soil (e.g.: carbon from atmospheric CO_2, atmospheric nitrogen, more complex organic molecules that originally were synthesized in a plant via sunlight energy).