The cost of nitrogen fertiliser

Artificial nitrogen fertiliser’s role in farming

Plants require nitrogen in order to grow and function. In the natural cycle, atmospheric nitrogen is taken up by bacteria in both the soil, and on some plant’s roots, and converted into nitrates and nitrites, which are then absorbed by plants. As the plant decomposes, denitrifying bacteria convert these chemicals back into nitrogen gas, which can re-enter the atmosphere. Modern agriculture seeks to significantly increase the productivity of land and so increases the nutrients available for growth. Nitrogen is one of the primary fertilisers spread on agricultural lands to increase yield and is one of the major input costs on farms.

Synthetic fertiliser has been used for about 200 years, though there was a rapid increase in use through 1960-1990, strongly contributing to increased global food production. The synthetic nitrogen fertilisers used today are produced using the Haber–Bosch process, where heat and pressure are used to combine nitrogen from the atmosphere with hydrogen to create ammonia. This is a highly energy intensive process, with the carbon released amounting to 1% of global greenhouse gas emissions.

As we have seen in recent months, the price of artificial fertiliser is highly exposed to international politics, gas prices and export bans. However, even at the best of times there are many costs to using artificial fertiliser that should be taken into account.

Finding the optimal fertiliser rate

The quantity and type of fertiliser spread should vary by which nutrients are lacking, to what extent, the crop types, and other limiting factors (i.e., rainfall). Each cultivar will have its own needs and preferences, and these should be considered with care. Soil tests can be used to understand what is available, and as the plants become established tissue tests can be used to find out what nutrients are being successfully taken up into the plants and used to decide on later fertiliser applications.

The aim with fertiliser should not be to produce the greatest yield, but to achieve the greatest profit. One good way of thinking about how much fertiliser to apply is to consider the Variable Cost Function. This helps to visualise how much applying a given quantity of fertiliser is expected to increase crop revenue, and to what extent that revenue is eaten into by the cost of the fertiliser.

Crop yield can increase strongly with the introduction of fertiliser; however, the rate of increase will flatten off with increasing fertiliser rate. It is important to consider diminishing marginal returns - achieving maximum yield may not generate the greatest profit, when the cost of additional fertiliser is considered. Financially, it may be better to spend a lot less on fertiliser and produce a little less grain. The greatest challenge here is knowing the relationship between fertiliser use and yield for a given site.

Notably, the profit line (black) is relatively flat and near its maximum value for a range of the graph. This suggests that at least 95% of the maximum profit could be achieved with a range of fertiliser rates. Generally, this is a very wide range, which encouragingly means that farmers do not need to be very exact in fertiliser rate selection and can often reduce the rate for the environment's benefit without significantly impacting profits. However, this does question the economic value of some high-precision farming techniques, as they will be making small adjustments within the flat part of the curve.

So far, we have only been considering the cost of purchasing fertiliser, however, including other running costs such as machine wear and tear, fuel usage, soil compaction from heavy machinery, and labour costs should also be included in the calculation.

Damage to soils and crops

Naturally, the first cost of applying excessive nitrogen fertiliser is that profits are reduced by overspending on inputs. However, there are a number of other consequences that are not as immediately apparent.

Applying nitrogen fertiliser can lead to subsurface soil acidification, which impacts root health and therefore grain production. This can be managed by applying lime to the fields, which can increase yields by 10-40% though it can often take 4-5 years to be effective. Investing in improving the soil pH can reduce the quantity of phosphate fertiliser required. Increased soil acidity increases the available aluminium in soil. Aluminium is toxic to plants, leading to restricted root growth. As a result, the roots cannot seek out phosphate present in the soil around them and the farmer needs to apply more phosphate fertiliser to the soil to increase growth. Once lime is applied and the available aluminium reduces, the roots will be able to reach out to the already present phosphate.

If too much nitrogen fertiliser is added to soil, it not only costs money but can damage the crop. In some cases, at excessive fertiliser rates the grain yield may actually start to decline. This can occur as nitrogen fertiliser drives chlorophyll production, producing bigger leaf structures with larger surface areas for the photosynthesising pigment. This leads to the overfed plant overinvesting in leaf, using up its energy and then not producing as much grain. Likewise, this overinvestment in leaf growth can result in underdeveloped roots, making the plants more vulnerable to wind damage and soil pathogens.

Spreading too much nitrogen can also increase the soil and mineral salts; reducing the water availability while leaving the salts behind. This results in the leaves looking burnt from dehydration, with yellow / brown edges and wilt.

The externalised costs

Some of the costs of excess fertiliser use are externalised, meaning that while that farm doesn’t directly pay them the wider community does. An important example here is water pollution requiring expensive water treatment. Once spread onto the land, any fertiliser that isn’t taken up by plants sinks into the soil. Synthetic fertiliser can leach into rivers and aquifers, adding to the nutrient leaching already seen from the slurry produced by livestock, leading to water contamination. Alternatively, bacteria can convert it into nitrous oxide – a greenhouse gas around 265 times more potent than carbon dioxide, adding a further 2% to global emissions.

Nitrogen run-off from farmland causes more than three-quarters of global eutrophication, where excess nutrients cause extreme plant and algal blooms. These blooms block out the light, plunging the ecosystems below into darkness, resulting in die-offs. As the algal bloom dies, the mass decomposition consumes oxygen, creating dead zones in lakes and oceans. In England, just 16% of surface and ground waters meet the criteria for “good ecological status”, and none of our lakes or rivers meet the criteria for “good chemical status”. We have some of the most polluted waters in Europe. [Ref: The Plan]

Should the relationship between fertiliser rate and water pollution be known, the pollution cost can be included in the cost calculation. This would then lead to a lower recommended fertiliser rate. When farmers are not held accountable for the cost of pollution, they will be economically motivated to use higher levels of fertiliser than they otherwise would. This suggests that Government intervention (including financial incentives as seen in the new Environmental Land Management schemes) could be required to reduce pollution levels. It is usually cheaper to prevent pollution than to clean it up and compensating farmers for using less fertiliser can be a cost-effective tool here.

Some plants and fungi are better adapted to growing in soils with high levels of nitrogen. Nitrogen-tolerant species such as nettles and hemlock thrive in these environments, to the detriment of other more nutrient sensitive species, leading to reduced wildlife diversity.

Excessive nitrogen fertiliser also increases emissions of nitrous oxide (N2O) from the field, a potent greenhouse gas that contributes strongly to global warming. Nitrogen-fixing cover crops, and some new biological fertilisers, can be used to draw some of these emissions back down.

To learn more or to develop a customized analysis, connect with the climate science team at Climate Spheres today.

References:

https://www.intechopen.com/chapters/67454

https://www.soilassociation.org/media/21286/fixing_nitrogen_soil_association_report.pdf