The Impacts and Uncertain Future of Fertilizers
Synthetic fertilizers are widely used in intensive agriculture to add nutrients to the soil and increase crop yields. But they don't come without impacts. Since the industrial revolution, humans have dramatically modified nutrient cycles between the Earth's layers, and fertilizers are largely responsible.
Nitrogen
Since the invention of the industrial N fixation at the beginning of the 20th century by Haber and Bosch, the abundance of this element has dramatically increased (Graph 2). N in fertilizers is used to increase crop yields, but farmers apply excess amounts of fertilizer seeking maximum yields, while plants can only take up so much of it. The N remaining in the soil can leach and be washed out by runoff into water bodies. Soil also releases N into the atmosphere in the form of N2O, which is the main depleter of the ozone layer, and is deposited in distant freshwater bodies as well.
High N concentrations in the water results in eutrophication (increase in algae due to higher N inputs) and consequent hypoxia of the water (reduced O2 due to algal decomposition) that cause aquatic 'dead zones' where fish communities are destroyed. Eutrophic and hypoxic zones have alarmingly increased by 1800% in the last 60 years and are widespread worldwide (Figure 1, note: only shows coastal areas, not inland freshwater).
Graph 2 - Peak in N levels starting in the mid-20th century, mainly driven by the demand of synthetic N for agriculture (from Fields 2004).
Bringing this cycle back to its natural balance is therefore crucial in order to halt environmental degradation and secure global human and ecological health. The main way to stop further addition of N to lands is by reducing the use of synthetic fertilizers, by showing evidence to farmers about the lower amounts of fertilizers that are needed to maximize yields and reduce costs. Moreover, organic fertilizers (e.g. compost, manure) only take the N that was already in the soil and recycle it, and should therefore be preferably used. Last, cleaning polluted water would also be necessary in order to reverse this trend.
Figure 1 - World distribution of eutrophic and hypoxic in coastal waters (©WorldResourcesInstitute).
Phosphorous
Phosphorous (P) is another major nutrient cycle that has been altered by agriculture. Commercial P fertilizers are at the root of the fourfold increase in P in the environment in the last century. Excess levels are applied as well, resulting in residual levels remaining in the soil, where it flushes down to water bodies attached to soil particles. P has a similar effect on aquatic ecosystems than N: eutrophication and 'dead zones', as it is another limiting factor to algal growth.
P is necessary for crop growth and increasing yields, but in contrast to N, it cannot be synthesized and it is obtained entirely from limited reserves of phosphate rocks (Figure 2). These are predicted to peak by 2030, and be fully depleted before the end of the century. In addition, a few countries (mainly Morocco, China and the US) control most of the global P supply. The current agriculture is therefore dependent on a resource that is subject to international relationships and that will collapse in the near future.
Phosphorous (P) is another major nutrient cycle that has been altered by agriculture. Commercial P fertilizers are at the root of the fourfold increase in P in the environment in the last century. Excess levels are applied as well, resulting in residual levels remaining in the soil, where it flushes down to water bodies attached to soil particles. P has a similar effect on aquatic ecosystems than N: eutrophication and 'dead zones', as it is another limiting factor to algal growth.
P is necessary for crop growth and increasing yields, but in contrast to N, it cannot be synthesized and it is obtained entirely from limited reserves of phosphate rocks (Figure 2). These are predicted to peak by 2030, and be fully depleted before the end of the century. In addition, a few countries (mainly Morocco, China and the US) control most of the global P supply. The current agriculture is therefore dependent on a resource that is subject to international relationships and that will collapse in the near future.
Figure 2 - A phosphate mine in Morocco, which holds the vast majority of global P sources.
Almost half of the P produced is lost through soil leaching and erosion, thus making P application more efficient is paramount. Moreover, recycling P from where it is in excess, water bodies and soils, for further P production is also necessary. And finally, P can also be recycled back from manure and human excreta from sewage treatment plants. Actions to reduce P loss and to recycle it back into production are urgent to impede major agricultural drops and threats to global food security.
Diego Garcia-Vega - Novembre 11th 2017
Diego Garcia-Vega - Novembre 11th 2017