It is estimated that around 50% of the food produced today in conventional farming systems would not exist without fertiliser applications. Plants require nutrients to grow and to produce the seeds and fruits that we eat. The selective breeding of high yielding varieties of crops has meant that yields have dramatically increased since the 1960’s to keep pace with a rising global population. However, these productive varieties only have high yields when they are ‘fed’ with fertilisers containing nitrogen and phosphorus. The question is how can we increase the efficiency of use of fertilisers, thereby reducing their use, and decrease their side-effects on the environment?
Phosphorus fertilisers contain superphosphate, the major source of which comes from mining phosphate rock. Its production leads to hundreds of millions of tonnes of wate, although heavy metals and other impurities still sometimes make their way into the fertiliser and then the soil. When rain washes it from fields into freshwaters, the sudden influx of phosphorus fuels harmful population explosions of algae.
Nitrogen is usually in the form or ammonium or urea, but microbes in the soil soon convert any excess into other nitrogen-based molecules which behave very differently. What that means is that poor timing and over application of nitrogen fertilisers are not only inefficient but lead to water pollution (in the form of nitrates, where as much as 30% can end up in water courses), air pollution (in the form of ammonia gas) and climate change (from the gas nitrous oxide).
STRAIGHT FROM THE EXPERTS
Balancing act: keeping soil phosphorus at optimum is best all-round
“The large amounts of phosphorus fertiliser used in the past had led to a build-up in soil - the so-called ‘legacy P’ - and fertiliser inputs have decreased. Long-term field experiments in the UK show that it takes 5-16 years for the soil phosphorus available to crops to fall by half. They also showed that for soils near the optimum level, the efficiency of use is highest when the phosphorus applied and removed by the crop is nearly equal. Maintaining soils near the critical level should optimize both crop yields and the use of global phosphorus resources, while minimizing the risk of excessive phosphorus entering the aquatic environment.”
Can we keep taking the P?
“The majority of phosphate fertilisers are derived from finite rock phosphate supplies, and over 70% of known phosphate reserves occur in Morocco and Western Sahara. Therefore, not only are there issues around depletion of these reserves, but also potential geopolitical issues about security of supply. There are many other sources of phosphate that are simply disposed of currently, including food waste, animal bones from abattoirs and human sewage. Attempts to recover phosphate from these sources are currently few due to uncertainty over the efficacy of the novel products and the cost of these new technologies - but it is possible that in the near future, with an adequate amount of recovery and recycling, the UK could be almost completely phosphate fertiliser independent.”
When it comes to better ammonium fertilisers, biochemistry is king
“Nitrogen from fertiliser is more likely to stay in the soil if it is in the form of ammonium and synthetic inhibitors that stop ammonium turning to nitrate have been developed for use on farms - but they can lack stability and reliability. Moreover, their public acceptability has been questioned following traces being found in milk produced in New Zealand, likely via direct plant uptake by dairy cattle.
“Whilst they can’t provide enough nutrient to feed all our crops, legumes, such as peas and beans, have long been used by farmers to increase soil nitrogen, as they form a natural association with bacteria that capture nitrogen from the air and release it as ammonium in the soil. In a similar vein, utilising chemical inhibitors naturally excreted by plant roots could be a more effective strategy to manipulate the nitrogen cycle in soil. Some tropical grasses excrete compounds that stop ammonium becoming nitrate, an adaptation to conserve nitrogen in their nutrient-poor ecosystems. Several cereals also exhibit this ability, raising hopes it could one day be bred into all our staple crops.”
Inhibitors can help stop harmful air pollution
“Globally, urea is the most important nitrogen fertiliser. However, urea is associated with higher losses of nitrogen as ammonia gas, an air pollutant which forms fine particulates in the atmosphere and is implicated in human respiratory diseases. In addition, it leads to nitrogen being deposited on sensitive habitats, with potential loss of rare and specialised plant species. The nitrogen lost as ammonia from fertiliser also reduces the nitrogen use efficiency.
“To counteract this, urease inhibitors can be included with urea fertiliser. Urease inhibitors slow or delay urea becoming ammonia by blocking the activity of a microbial enzyme. This greatly reduces the emission of ammonia – by an average of 70% from our research results - enabling more of the nitrogen to move into the soil where it is available for plant uptake.
“Urease inhibitors may also have application in reducing ammonia emissions from livestock housing, a major emission source, although further research is needed to assess the potential and develop cost-effective delivery mechanisms.”
Water under pressure
“Degradation of water quality by farming practices is a national problem and contributes to the high proportion of our rivers not meeting ‘good ecological status’. Both inorganic fertilisers and manures/slurries cause water quality issues. Tackling it involves forming a robust picture of the magnitude of the pollution and the key sources responsible - including those on (and not on) farms.
“A combination of regulation (such as Action Programme rules in Nitrate Vulnerable Zones), incentivization (such as agri-environment schemes) and on-farm advice is used to encourage uptake of best management practices. Given the often limited and uncertain efficacy of individual best management practices (such as riparian buffer strips), it is advisable to think of the land-water continuum as representing a pollution cascade from source, through transport and delivery, to impact in the aquatic environment.
“Ideally, ‘treatment-trains’ of best practice should be implemented across this continuum to increase water quality protection, but the growing occurrence of extreme weather events, challenges even this strategy. In addition to conventional pollutants associated with fertilising and crop protection strategies, emerging contaminants are associated with new farming practices, including veterinary products and micro-plastics. This means the unintended consequences of agriculture on water quality continue to evolve.”
Manure: going back to our roots?
“Nutrient ratios in manures and slurries are not always in balance with what is required by crops, so yields can be compromised by not enough nutrients (typically nitrogen), or pollution caused by too many nutrients (typically phosphorus). Getting the nutrient balance right is easier with chemical fertilisers than organic manures.
“The quantity of organic carbon in manure treated soil is consistently more than in inorganic fertiliser treated soils. This not only has positive implications for climate change, but also the longer-term sustainability of crop production systems reliant solely on inorganic fertilisers. The most efficient solution probably involves a balance of both types.
“While inorganic fertilisers can be tailored to crops and easier and cleaner to handle than manures, there is also an issue of supply. UK agriculture has largely become divided into either arable in the east, or livestock farms in the west. One solution to this issue is for farms to rear livestock and crops, as was the norm in the 19th century – but the current trend to more plant-based diets could also impact.”
“The microbes present in soil and associated with plant roots can benefit the growth of host plants by assisting with nutrient acquisition, pathogen suppression and environmental stress. Inorganic fertiliser application reduces both the species richness and diversity of bacteria in the immediate vicinity of wheat roots. We’ve shown the amount of growth promoting bacteria living around the roots was 91% for unfertilized plants whereas for fertilized plants only 19% were found to show beneficial traits.
“These results clearly indicate that inorganic fertilizer application affects both the structure and function of the plant root microbiome. Future work will aim to understand the genetic mechanisms of bacterial plant growth promotion as well as the optimisation of microbiome assisted agriculture with better, tailored fertilizer usage.”
“Whilst much of the attention has been focused on the environmental impacts of the three macronutrients — nitrogen, phosphorus, and potassium —in agricultural systems, it is important not to overlook that crops actually need at least 11 other essential elements to fulfil their yield potential. For humans and livestock which consume these crops, we need more than 20 elements. Fertilisers are essential for supplying these micronutrients in many agricultural systems, including for crops growing on temperate and tropical soils.
“For instance, the micronutrient selenium is often added as a trace element to fertilisers used on grazing pastures in the UK. In many parts of South Asia, for example on wheat-rice cropping systems in Pakistan, nitrogen fertiliser use efficiency of crops is less efficient than it could be, because yields are otherwise limited by micronutrient elements such as zinc and boron”.
Alternative approaches are being sought to the ‘high input’ approach to farming because of the negative environmental consequences of fertilisers, such as aquatic pollution, loss of soil biodiversity, and increased greenhouse gas emissions. However, getting rid of such chemicals altogether is, for the foreseeable future at least, unrealistic. By identifying situations where they are being applied unnecessarily, or finding non-chemical alternatives to inorganic fertilisers (combined with interventions to mitigate the negative environmental impacts of those we do use) the aim should be to develop integrated, sustainable systems that reduce and optimize inputs but also maintain food supply