Agriculture’s Critical And Complex N-Game

Critics of modern agriculture often cite its dependence on “synthetic nitrogen fertilizers.” They point to the carbon footprint of the natural gas used to make it and the fact that nitrogen from farms can end up as a water or air pollutant. Also there is the issue that under certain circumstances a fraction of farm applied nitrogen can be emitted from the soil as the very potent greenhouse gas – nitrous oxide. While these issues are real, the solution is not to somehow avoid using this fertilizer or to arbitrarily set limitations on the quantities that farmers can use to grow their crops. Unfortunately there are several such misguided approaches being pursued. A cautionary tale comes from the nation of Sri Lanka which recently banned fertilizer imports in its attempt to become the first 100% organic production region. That choice crippled their food supply and their important tea export industry. Canada recently mandated nitrogen fertilizer reduction for its farmers without tying that to a rational measure such as kg of fertilizer per ton of output. India is promoting “zero-budget natural farming which could undermine the food independence that nation has enjoyed since the “green revolution.” The European Union is promoting organic agriculture which excludes “synthetic fertilizers” as part of its controversial Farm to Fork Strategy. In fact, organic is often promoted as a solution to fertilizer issues without recognition that organic crop production is actually quite dependent on “synthetic” nitrogen that has ended up in the manure of conventionally raised animals. As one expert on the subject says, “follow the nitrogen.” And just to be clear, human produced nitrogen fertilizer starts as ammonia which is a naturally occurring form of that element and not something artificial as the term “synthetic” might imply.

Getting Fertilization “Just Right”

In the classic fairy tale Goldilocks wants her porridge “not too hot, not too cold, but just right.” The challenge for farmers is to apply nitrogen in a way that doesn’t represent either too much or too little, but what is “just right” for optimal crop growth. Fertilizers are one of the more significant operating costs of growing a crop, so growers have no incentive to over-apply. But conversely if a crop is short on nutrients during key growth stages, the farmer’s yield-based income will be compromised. Thus, the long-standing goal for optimal fertilization has been expressed as “The 4-Rs” –

1. the right amount

2. in the right form

3. in the right place

4. at the right time

This is a non-trivial challenge because of logistical limitations and the vagaries of weather, but the basic economics drive careful use.

Why Agriculture Needs to “up it’s N-game”

There are now two “game changing” factors driving more attention to nitrogen fertilizer issues -the war in Ukraine and Climate Change. The war has led to a dramatic increase in fertilizer prices and highlighted the desirability of a to shift to domestic sourcing. Rational concern about Climate Change is putting the spotlight on the greenhouse gas footprint of current nitrogen fertilizer production methods as well as on the emissions of nitrous oxide from fields.

In the face of these heightened concerns the agricultural sector is being called upon to “up its N-game.” The challenge is to meet increasing demand for food, feed, fiber, fuel and other biomaterials without driving land-use-change and without exacerbating nitrogen-related issues. Fortunately, the trend over the past three decades is encouraging. Consider the example the “I states” which account for around one third of US grain corn production. As shown in the graphs below, yield in 2021 was 35-51% higher than in the early 1990s but nitrogen use only increased between 8 and 18%. Thus “nitrogen use-efficiency” in those states (expressed as bushels produced per pound of nitrogen applied) has increased 29-35%.

That means that the use of nitrogen for corn in these three states in 2021 was 1.73 billion pounds lower than if it has been used at the rates it was in the early 1990s. That benefit is shown below for each year in which there was survey data.

Farmers use many different practices and technologies in order to optimize their use of nitrogen and other fertilizers. The following is a list of both existing and emerging n-game tactics. Some are well established but could be more widely employed. Those are highlighted with the symbol (>>>). Others that are relatively new, but which could make a significant contribution are highlighted with the symbol (+++). Those that are in the research phase are indicated by the symbol (***).

Nutrient Recovery

When animals (including humans) digest their food they fail to absorb all the nutrients it contains. That is why manure has always been used as a fertilizer as it continues to be today. Manure in its various forms (including after composting) is not an ideal fertilizer in that it requires the application of tons per acre and it isn’t amenable to some desirable farming practices such as no-till farming or precision application (described below). Even so, a new technology called a Varcor Processor (+++) is available today to do a much better job of recovering the fertilizer nutrients from manure in highly usable forms. There is also interest in setting up mechanisms to recycle human urine (***) as a fertilizer high in both nitrogen and phosphorus. However, since neither animals or humans actually make nitrogen fertilizer, these are limited potential sources.

Precision Fertilization

Farm field soils are not uniform in that they have different yield potential in different zones. It is common today for farm machinery to be equipped with GPS or other geo-referencing technologies in order to do “auto-steer” and to generate information like a yield map. To avoid wasting money on excess fertilizer the farmer can use “variable rate fertilization” (>>>) putting down more or less in each individual zone based on its growth potential. The application rates can also be guided by various imaging technologies that use “hyperspectral analysis” (>>>) to visualize the nutrient status of the growing crop and to adjust fertilizer rates on that even more precise zone basis. For crops that are irrigated it is possible to very closely link the supply of nitrogen and other nutrients to what the plants need at any given point in the growing season by “spoon feeding” (>>>)- delivering it through drip lines or other irrigation systems at levels that closely match what the plants will quickly absorb with their roots at each time point throughout the season. In non-irrigated agriculture that level of control is not possible, but fertilizer can be applied in a few “split applications” (>>>) to more closely match plant needs. Another option is a “controlled release formulation” (>>>) of the fertilizer in which a polymer coating slows the rate at which the nutrients move out into the soil.

Preventing Nitrogen Loss

After a nitrogen fertilizer has been applied in a field it can be a while before it is taken up by the growing crop and in the meantime, it can be converted to forms that allow it to move into the air or water so that it is no longer available for the crop and can cause problems in the environment. There are several technologies that act as “Nitrogen Loss Inhibitors.” For instance urea is a very practical form of nitrogen to use as a fertilizer, _but there are enzymes present in soils called ureases that convert it to ammonia (NH₄) which is volatile so that it moves away in the atmosphere only to be washed down later and cause a form of water pollution known as “eutrophication.” There are products called “urease inhibitors” (>>>) that prevent that potentially major form of nitrogen loss. When fertilizer nitrogen is in the positively charged ammonium form (NH₄+) it is in an available but non-mobile form. There are microbes in the soils that convert the ammonium to nitrate (NO₃-) which is very mobile in water so that it can leach into ground water or be washed into streams. If the soil is waterlogged or compacted so that it doesn’t have air available, the nitrate can also be lost to the crop if it is “denitrified” meaning that it is converted to N₂ gas that goes back into the air as a harmless gas. Unfortunately, in that process some of the nitrogen is turned into nitrous oxide (N₂O) which is an extremely potent greenhouse gas. There are products called “nitrifications inhibitors” (>>>) which reduce these nitrogen loss and pollution issues. The GPS-based autosteer technology also allows the grower to employ “controlled wheel trafficking” (>>>) so that only a small percent of the field is ever compacted by the wheels of heavy equipment. If no nitrogen fertilizer is applied to those potential soil compaction wheel tracks, the risk of nitrous oxide emissions is greatly diminished.

Using “Green” Nitrogen

In the early 20^thcentury, the German scientists Fritz Haber and Karl Bosch invented the process through which the inert nitrogen gas that makes up 78% of the atmosphere could be converted into ammonia and from the converted to other forms that can fertilize plants. Up until that time the world had been tapped out of nitrogen from natural sources including the mining of deposits of bird guano. Hydrogen is also needed for that reaction and natural gas (CH₄) has has always been used in the Haber-Bosch process because it was the cheapest source. Hydrogen can also be produced from water using wind or solar generated electricity and there are technologies now available to make much lower carbon footprint nitrogen (+++) and they are getting more cost competitive. Another advantage of using renewable energy to generate nitrogen fertilizer is that it can help with the reduction of import dependence.

Nitrogen Fixers

The term “fixer” has some negative connotations, but in nature there are certain beneficial bacteria that can “fix” nitrogen meaning they have a unique ability to take some of the nearly inert N2 gas that makes up 78% of the atmosphere and convert it into ammonia (NH₄) which is the starting point for all the biologically important forms of that element. There is a family of plants known as legumes that have a special relationship with one of these bacterial species called Rhizobium. The plant supplies the microbe with the sugars that then provide the considerable amount of energy required for that process. The plants also “house” these bacteria in specialized structures along their roots called “nodules” that created a very low oxygen environment which is also important for the fixing process. Several major and minor legume crops have this capability and require little to no nitrogen fertilizer (soybeans, dry edible beans, peas, peanuts, lentils, chickpeas, alfalfa…). When legumes are part of the crop rotation (>>>) they leave a fair amount of nitrogen for the next non-legume crop (e.g. corn, wheat, canola…). There are also legumes that can be used as cover crops (>>>) between seasons to increase the supply of nitrogen in the soil.

There has been long term interest in finding ways to enable non-legume crops to also benefit from nitrogen fixing bacteria. There are quite a few different soil dwelling bacteria that are able to fix nitrogen and some grow on the roots or even the leaves of plants. However when nitrogen fertilizers are present in the soil these bacteria don’t turn on their own, energy intensive fixation capabilities since they can simply use what is available. One strategy has been to develop novel bacterial strains that will keep their fixing capabilities running either by identifying mutants or more recently by using gene editing technology. Two such products (+++) have been commercialized – one developed by Azotic Technologies called Envita™ and one from Pivot Bio called PROVEN® 40. These bacteria can supply around one quarter of the nitrogen needed for a crop like corn. At scale, that could represent a very reduction in the energy required to make fertilizer.

There is also promising research looking at ways to modify grain crops (***) so that they facilitate nitrogen production by bacterial species that grow on their roots. Scientists from Oxford, Sainsbury Laboratory, North Dakota State University and MIT have developed lines of barley modified to produce a signal compound called rhizopine which induces an associated bacterium (***) called Azorhizobium caulinodans to fix nitrogen. They then pair that with a modified version of that bacterium which will only fix nitrogen when associated with the modified crop lines which will only do so when associated with the desired crop line. Researchers at the University of California Davis are working with signaling compounds from rice which influence nitrogen fixing bacteria in biofilms on their roots (***).

Nitrogen fertilizer will always be a critical input for crop production but with a significant carbon footprint. The agricultural sector is on a track to address these issues through gains in use-efficiency, alternative means of production, and potential breakthroughs for expanded biological fixation. These paths are far better for humanity than restrictions that would compromise productivity and drive land-use-change to make up the shortfall.