Phosphorus depletion: an invisible crisis?
A.L. (Bert) Smit, Plant Research International, Wageningen UR, The Netherlands.
Phosphorus (P) is one of the major nutrients plants need to convert solar energy and carbon dioxide into the organic compounds necessary to sustain life on earth. The low concentration of P in most agricultural soils, alongside its low solubility, makes P a limiting factor for plant growth nearly everywhere in the world. In the past, agricultural ecosystems adapted to the low P availability by recycling P from organic waste streams (litter, (city) refuse and human excreta) and animal manure, as much as was possible. In addition, P input was raised by collecting manure produced by animals that had been grazing outside the farm. In many regions of the world this is still a common practice.
Since the second half of the 19th century, the mining of phosphate rock (PR) deposits has induced large ecological and agricultural changes in many parts of the world (Howarth et al., 1995). Availability of artificial P fertilizer reduced the need to recycle organic waste streams, including manure, and resulted in more specialization and intensification. Duncan Brown (2003) argued that a system with negative feedback (constrained by recycling nutrients from organic sources) was replaced by a system with positive feedback. Mineral fertilizers led to higher yields and removed the dependency on animal manure. The higher food productivity and security contributed to an increase in the global population, which in turn led to higher demands for food: an upward spiral was created. Phosphorus fertilizer and to a greater extent nitrogen (N) fertilizer enabled food production to keep pace with the ever-increasing world population.
In many parts of the world, agriculture became intensified and specialized (livestock and arable production became spatially separated). Not only did yields increase, it also became possible to make fields productive that would otherwise have remained marginal. A negative side-effect of the use of mineral fertilizer and specialisation was that manure was not used as efficiently as it used to be.
Nowadays, food and feed production cannot maintain its current level without mineral fertilizer. While N fertilizer gave the main boost to agriculture: at sites with a low P-nutrient status, it had a limited effect if not accompanied by P fertilization. Van Keulen and Breman (1990) made this clear for the West African Sahelian region.
The P cycle
In the pre-industrial period, production, processing and consumption of food, feed and fibre were closely related in space and time. Phosphorus that was removed from the soil through crop production was compensated for via regular flooding or shifting cultivation, or it was supplemented by P in manure from livestock grazing on surrounding rangeland. For current P fertilization with mineral fertilizer, it seems as if the P is used only once: a once-through process. Comparing the amount of P “consumed” in food by the global population with the annual amount of P used as fertilizer, it appears that only around 20% of the P used as fertilizer ends up in food consumed by humans (Cordell, 2010): a low efficiency. The losses that explain this low efficiency are occurring throughout the whole chain from mine to fork and include: mining losses, losses by soil erosion, and pre- and post-harvest losses. Erosion is probably the most important loss. Bennet et al. (2001) estimated that 22 million metric tons (Mt) of P ends up in the ocean annually, compared to 8 Mt of P in pre-industrial times. Other estimates of P losses by erosion range from 9 to even more than 22 Mt of P per year (Smil, 2000; Rockström et al., 2009). Global use of P fertilizer amounts nowadays to 17-20 Mt of P per year. This emphasizes the large amounts of P that end up at the bottom of the ocean, and are at present impossible to recover.
Next to these unrecoverable losses, accumulation of P in agricultural soil can also explain the above-mentioned low efficiency from mine to fork. Strictly speaking, accumulation (higher P input than P offtake with crops) is not a loss in the long term, but, given the strong binding of P to soil particles, it is not available for plant uptake in the short term.
Accumulation usually is the result of concentration of livestock, when manure production is in excess of the available area of surrounding arable soil. But accumulation occurs at various spatial scales: country, region and farm. Even within a farm, fields close to the farm centre have a higher P fertility than the fields further away because of the cost of transporting manure. Accumulation, however, also occurs for good reasons: to improve the P fertility of a field or to obtain the maximal economic yield. The special character of P in the soil (immobile, insoluble and bound to soil particles) makes it worthwhile to target a high P fertility of soil in intensive agriculture. To achieve this, relatively large amounts of P are needed. So, even farmers who follow the guidelines for P fertilization strictly in general still accumulate P in their soil.
Considering that the world’s P resources are finite and that in the long term the world will have to rely on almost complete recycling of organic sources (including manure) for food production then local accumulation should be prevented as far as possible. In such a situation, accumulation in one region would imply that somewhere else the soil is depleted.
So despite the fact that gaseous losses of P are negligible, one hardly can speak of a nutrient cycle for this element. The reuse in agriculture of P-rich waste streams produced by society (sewage sludge and human excreta) has diminished for various reasons (Liu, 2005; Liu et al., 2008), ranging from urbanisation, “modern” sanitation systems, concern over contamination with heavy metals and pharmaceuticals to cultural barriers that prevent the reuse of human excreta. The question is, therefore, how long can we maintain this once-through use of mineral P fertilizer?
Duration of P deposits
The finiteness of the rock deposits from which P fertilizers originate has provoked a broad discussion within the last few years and has become a topic for international research groups (e.g. Cordell et al., 2010). Smit et al. (2009) estimated, based on the PR reserves as published at the time by the USGS (United States Geological Survey), that with current consumption, economically extractable reserves (“the reserve”) would be depleted within 125 years, and other reserves (currently not economically minable; the so-called “reserve base”) would be depleted within 340 years. Taking into account various developments such as: (i) an increase in the global population (of 50% by 2050), (ii) the change in diet preference towards more meat, and (iii) an increase in biofuel production, they estimated that within 175 years, all deposits (as published by the USGS) would be depleted.
Recently, based on a literature review by the IFDC (International Fertilizer Development Center (van Kauwenbergh, 2010), the USGS increased the estimated reserves considerably (from 15 billion tonnes of PR to 65 billion tonnes in 2011) and no longer made a distinction between reserves and reserve base. The IFDC report concluded that a “peak phosphorus” event would not occur in 20-25 years and that PR would be available far into the future. Based on the reviewed data, they estimated that with current rate of extraction there would be no problem for at least 300 years. However, this may be too optimistic, as there is every reason to believe that in the next decades the consumption rate of fertilizer will increase. Smit et al. (2009) estimated that the current use of 19 Mt of P per annum would increase by at least 3 Mt of P if only to compensate for the increase in global population towards 2020. If the global population were to adopt a western diet, another 10 Mt of P would be needed under the assumption of no improvement in the current 20% “efficiency from fertilizer to food” (Cordell, 2010).
However, the IFDC report also acknowledged that “whatever the estimate of the deposits is, they are a non-renewable resource and that further efforts should be taken to make more reliable estimates of available phosphate rock.” They also concluded that “there should be a global effort to more effectively mine and process phosphate rock and to utilize fertilizer and P-containing waste as efficiently as possible.” For more information, see the comment on the IFDC report of the GPRI platform (Global Phosphorus Research Initiative) at: http://phosphorusfutures.net/files/GPRI_Statement_responseIFDC_final.pdf.
In addition to the finiteness issue, geopolitical aspects are a reason for concern. Phosphorus reserves are found in a limited number of countries: more than 85% of the reserves are in just 5 countries: China, Morocco and the USA being the most important reserves. Also for geo political reasons, global agriculture might want to reduce the dependency on mineral P fertilizer. To realize this, fundamental changes in agriculture are needed. However, compared to a future shortage on fossil fuels, the awareness of a future P shortage is still minimal, despite the possible far-reaching consequences for food availability and security. In Table 1 a comparison is made between an “energy crisis” and a “P crisis”.
|Awareness of consequences||-||++|
|Governance on finiteness||-||+|
|Governance on environmental aspects||+||++|
Consumption and trade
Table 2 shows the amount of P used in P fertilizer for some regions of the world. In total, in 2005, the arable land and permanent crops in the world received 17.2 Mt of P mineral fertilizer. Taking into account the indicated area, this would translate to an average rate of 11 kg P per ha. This can be considered as relatively high, because a wheat crop of 3000 kg already contains more or less the same amount of P. However, the differences between continents are striking. Fertilization in Africa is only 2 kg P per ha on average, but with a skewed distribution: most countries in this continent have a fertilization of less than 1 kg of P per ha, whereas a few countries, namely South Africa, Egypt and Kenya, consume significantly more P fertilizer.
Table 2: Use of mineral P fertilizer per continent (FAO statistics, 2005/611)
|Area2 (Mha)||P fertilizer applied (Mt P/a)||Average P rate (kg P/ha)|
2 Area of arable and permanent crops
It is striking that the continent with the largest P deposits in the world is not using P fertilizer for its own food production. Instead, in Africa, soil depletion of P is occurring (Smaling et al., 1993; Stoorvogel et al., 1993).
Table 2 also shows that mineral P fertilizer application in Europe is relatively low, but here one has to take into account that countries with an extended livestock industry usually have a surplus of manure, reducing the need for mineral fertilizer. In these countries, the dependency on mineral fertilizer is masked by the importing of P-rich feed from elsewhere. However, this feed could not be produced without P fertilization, e.g. soya production in Brazil usually is on P-deficient soils. Finally, P deposits are unequally distributed over the world, but the same holds for the access to inorganic fertilizer. For various reasons, a large number of farmers have no access to fertilizer. In Africa, for example, the price of fertilizer makes it unaffordable for many: it usually greatly exceeds the price paid in countries with a surplus of P.
Considering that P is an essential and non-renewable resource for global food production, it is remarkable that the use and governance of the remaining P reserves is far from sustainable. According to Cordell (2010), it is not only the (future) physical scarcity of P that is important: there is also a scarcity of P management (resulting in a low efficiency from mine to food), an economic scarcity (fertilizer not available to all farmers), an institutional scarcity (lack of governance structures at the international level) and a geopolitical scarcity (a limited number of countries controlling phosphate reserves, some of which are subject to political tensions).
It is probable that the world will have the time to realize some fundamental changes and reverse some recent trends before a physical scarcity of P becomes apparent. Some aspects would appear crucial:
- -The prevention of unrecoverable losses, which includes soil erosion but also, for example, sewage sludge. The latter is incinerated in some countries, but the resulting P-containing ashes are not recycled to agricultural sites.
- In general, the return flow from society to agriculture is decreasing, a trend stimulated by urbanisation and increased sanitation (Liu et al., 2008). However, it must also be realised that even a 100% reuse of human excreta cannot be the only solution (a global population of 9 billion people in 2050 would yearly excrete around 5 Mt of P, whereas the current use of P fertilizer is already 17-20 Mt of P).
- Fundamental changes should come from within agriculture, and better use of manure is a key element. The concentration of livestock in proportion to the surrounding area of arable land is important, but unfortunately in many places in the world it is difficult to integrate livestock and arable farming at appropriate levels.
- The human population finally is a key factor, with respect to overall numbers as well as the human consumption patterns. If no improvements are made in the current ‘conversion’ from fertilizer to food, the changing diet in developing regions (towards more meat) will increase dramatically the demand for mineral fertilizer, making all the above more urgent.
- To arrive globally at a more sustainable use of P, major research areas will need to include:
- monitoring of baseline data for global PR reserves and trade,
- a country-level analysis of P inputs and output to identify the most effective measures and policies,
- (agricultural) valorisation and reuse of societal waste products including human waste, sewage sludge, incineration ashes, etc.,
- plant breeding for rooting characteristics that would lead to a better use of the available P in the soil profile.
Finally, the concepts of “sustainability” and “the planet’s carrying capacity” are especially meaningful in relation to the finiteness of P resources. In the long term, this will require fundamental changes in the way our food is produced. Probably the world will have enough time for this transition (a full recycling of P), but awareness-raising will be necessary to achieve implementation of the policies and measures needed.
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