Phosphorous the disappearing nutrient

Phosphorous (P) is an essential nutrient for life. It is a vital constitute of our DNA, ATP (the bodies “energy currency”), bones, teeth and cell membranes. Therefore, humans, like all other animals need to consume P with their diet. A problem is that most agricultural soils have been depleted, and store too little plant available P. This leads to lower yields and a decrease in food production. Thus, most farmers must apply mineral P fertiliser to their fields for optimal yields. 90 % of this P comes from non-renewable deposits in only 5 countries and it is predicted that these deposits will deplete within a few decades. The article by Childers et al. (2011) reviews and summarises the problems, what is known and possible solutions to the human phosphorus cycle.

Childers, DL, Corman, J, Edwards, M & Elser, JJ 2011, ‘Sustainability challenges of phosphorus and food: Solutions from closing the human phosphorus cycle’ BioScience, vol 61, no. 2, pp. 117-124. DOI: 10.1525/bio.2011.61.2.6

Traditionally, diverse, small farms would combine animal husbandry with crop production, where the crops were fertilised with the manure from the farm animals. Thus, the P in the crops was fed to animals, these in turn excreted P and with their manure P came back to the field. The only P that was lost from the system was that consumed by humans. Today most of the food in Europe and North America comes from large, specialised farms that either produce meat or crops and are often located far from each other, which makes transport of manure economically unfeasible. To still provide the animals and the humans with sufficient P in their diet mineral P has to be applied to the fields. Not only does mineral P come from non-renewable deposits but it is also inefficiently used, with only about 15 – 30 % of mineral P fertiliser is actually taken up by the crop. The rest is lost with most of it ending up in water bodies, in land fills or is fixed in the soil. P is transported from the field to streams and lakes via water runoff from the surface of soils or through soil erosion. While P is a nutrient on the fields and promotes plant growth, in fresh water bodies it can lead to toxic algae blooms and anoxic zones that are deadly for fish and other aquatic organisms and whole ecosystems can be destroyed that way (see Dead in the Green Zone by E.M.B. Doran).

Algae bloom in a lake in Lower Saxony, Germany. Credits: Christian Fischer (wikimedia.org).

On the farm P can be used more efficiently by regular testing of the availability of P in the soil. With knowledge of the availability, the fertilizer dose can be adjusted to only as much as the crop actually needs. Crops that need less or can use the P from the soil more efficiently can further reduce P fertilization. Agricultural scientists could focus more on breeding or bioengineering crops that use less or can take up P more efficiently than conventional breeds. Furthermore, consumers can contribute to P use efficiency mainly by two means: (i) reducing meat consumption and (ii) recycling their organic waste. A reduction in meat production would lead to a decrease in P use in agriculture since a lot more P needs to be used to produce one kilogram of meat than one kilogram of grain. Recycled organic waste can be applied to fields as compost to substitute parts of the mineral P fertilization, given a suitable infrastructure to collect, ferment, and provide the compost to farmers. In many countries this infrastructure does not exist or is functioning not optimally, so it is on politicians to set incentives for entrepreneurs to develop a functioning infrastructure. P can also be recycled on the farm, so that all crop residues are returned to the field or between farms so that the manure from a pig producer can act as fertilizer on neighbouring farms for their crops. Possibly the most controversial and psychologically challenging P recycling would be of human sewage. Human urine is P rich and essentially sterile. According to the World Health Organisation recycled urine would provide about half of the P that is necessary to grow crops. This would require a major step to rethink human waste as a resource.

Phosphorous, the disappearing nutrient is essential for food and for life in general. To reduce our dependence on the diminishing mineral P reserves and still be able to produce affordable food while preserving the environment, major infrastructural changes need to be made and stigmatic challenges overcome.

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Hannes Keck

Hannes Keck

I am a PhD student studying greenhouse gas fluxes from agricultural ecosystems at the Swedish University of Agricultural Sciences (SLU). Currently, my research focuses on developing a new measurement technique to make it easier to analyse all relevant greenhouse gas fluxes from terrestrial ecosystems. Before my PhD studies I was working in the field of soil science / physics at several European research institutes and completed my masters at University of Copenhagen, Denmark and SLU, Sweden in environmental sciences (EnvEuro programme).

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