Getting to the bottom of plant nutrition

It is reasonably well understood that plants need a range of chemical elements to ensure that they grow and develop properly and can complete a full life cycle. Those elements, called nutrients in this plant-nourishing context, are broadly divided into two categories, those that are needed in relatively large amounts – macronutrients – and those required in lesser amounts – the so-called micronutrients. There is broad agreement about which 17 nutrients are essential for all plants (however, not having been examined for every one of the estimated 369,400 plant species (Conner Yearsley) that’s a rather sweeping generalisation), which are: C, H, O, P, K, N, S, Ca, Mg (macronutrients), and Cl, Fe, B, Mo, Zn, Cu, Mn, Ni (micronutrients). Apart from C – as CO₂ – and O – as O₂ – which plants acquire from the atmosphere, the great majority of their nutritional needs are met from what’s found in the soil (Arit Efretuei). One of the hazards therefore of being a soil-rooted plant is that those nutrients may be in insufficient supply in the immediate vicinity of the rhizosphere (Rebecca Lines-Kelly) to satisfy the plant’s requirements for healthy growth. Whilst that may present a selection pressure on the survival or otherwise of plants in that situation, it’s a major constraint and concern for those who are trying to grow crops to feed humankind. That is why considerable investment is poured into development of fertilisers to deliver additional nutrients to those all-important crop plants.

But that approach has its problems. Take, for example, phosphorus, an insufficiency of which nutrient restrains productivity of plants within terrestrial ecosystems globally (Enqing Hou et al.). Although this deficiency can be alleviated to some extent by addition of phosphorus fertiliser, there are serious concerns about the future availability of the mined phosphorus-rich rocks that are used to produce phosphorus-enriched fertilisers (Joséphine Demay et al.), and the alarming prospect of ‘phosphogeddon’* (Robin McKie; Olivia Allen). Furthermore, this is not just an issue for crop productivity and yield, but also for the impact phosphorus-impaired plant growth can have on vegetation’s uptake of atmospheric CO₂ by photosynthesis and hence its impact upon global warming (Jing Peng et al.).

With that background, it should come as no surprise that all available sources of additional plant nutrients should be considered in attempts to improve – or at least maintain – crop productivity. Which brings us to the matter of human faeces/feces (Vincent Ho) or excrement. Mindful of the much-repeated mantra to reuse, and/or recycle waste materials**, coupled with an increase in prices of fertilisers as one of the global consequences of the Russian invasion of Ukraine, use of human excrement as an alternative plant fertiliser is back on the radar – at least in Japan (Kyoko Hasegawa).

Rejoicing in the Japanese name “shimogoe”, which translates as “fertilizer from a person’s bottom”, this nutrient source is made from a combination of treated sewage sludge from septic tanks and human waste from cesspits. Compared to the more usual synthetic, or artificial fertilizers, this genuinely man-made alternative sells for about a tenth of the price. That cost saving isn’t the only advantage of shimogoe, its use reduces the carbon footprint associated with imported materials, and it diverts human ‘waste’ (Phoebe Braithwaite) to productive use rather than having to dispose of it at additional cost into the environment.

It should be noted that there’s nothing new in the use of human excrement as plant fertiliser (Lena Zeldovich), it’s something that’s been employed for centuries with varying degrees of acceptability in different countries (Lena Zeldovich). However, this source of nutrient enrichment is not without its dangers. What if the human provider of the material is infected? There is the danger that the faecal material will be contaminated and therefore a health risk to those applying the fertiliser to the fields and harvesting the crop, and to those who consume the produce produced in such an environment. One must trust that appropriate safeguards are in place to ensure that this doesn’t happen. But, it’s certainly something to be mindful of – as discussed by Tianyi Wang et al. in their intriguingly entitled research paper “Intestinal parasite infection in the Augustinian friars and general population of medieval Cambridge, UK” (Tianyi Wang et al.).

Analysing sediment excavated around the pelvises of skeletons from the 13^th/14^th century, Wang et al. concluded that almost two-thirds of the friars had roundworm (Ascaris lumbricoides) infections at the time of their death, compared with one third of ordinary residents. Although it can’t be known for certain, the last-named author of the study – Piers Mitchell – is reported as saying that “the friars’ higher infection rate could be down to them having used their own faeces as manure in friary vegetable and herb gardens, or purchasing fertiliser containing human or pig excrement” [cited in Mark Bridge].***

Readers concerned at how safe it is to use human excrement – and urine**** – as a plant food nowadays will hopefully be somewhat reassured by work of Franziska Häfner et al. (2023). Although that work didn’t involve looking at roundworm infection, the team assessed the human waste for over 300 chemicals – including rubber additives, insect repellents and pharmaceuticals – which people sometimes empty into their toilet. More than 93% of these compounds were not detected in the crop – white cabbage (Brassica oleracea var. capitata f. alba (Ionna Maria Alexandra Ștefan & Andreea Daniela Ona)– that they investigated; the remainder were present only at very low concentrations in the plants (Phoebe Weston). Overall, Franziska Hafner said that products made from human urine and faeces “are viable and safe nitrogen fertilisers” and “did not show any risk regarding transmission of pathogens or pharmaceuticals“. Whilst that means that there are only another 369,399 species to test for a similar assessment of the safety of this faecal fertiliser option, this approach may be just the job.

* Fear not, apparently this imminent disaster has been averted – for a while at least – with welcome news of substantial reserves of phosphate-rich rock beneath Norway (Katie Hobbins). For an appropriately nuanced assessment of this announcement, and the likelihood of phosphogeddon, see Ed Conway.

** Although usually shown as “reduce, reuse, recycle”, reducing the amount of faeces a human produces is probably not an option, nor is it recommended.

*** For a consideration of this work, and, in particular, important context regarding sanitation in Mediaeval times, see the article by Mark Bridge.

**** Although the focus of this item is human faeces, human urine is the more nutritious in terms of plant nutrients (C Rose et al.). So much so, that it’s been calculated that the nutrients present in the urine of the 12 million inhabitants of the Paris area could deliver all of the nitrogen and half of the phosphorus currently spread in the fields of that region (Tristan Martin et al.). The fertilising capacity of urine for plants has long been known and its addition to compost heaps is a long-standing and time-honoured – if maybe nocturnally-performed? – practice amongst gardeners (Jonathon Engels; Jessica Lane). This method of nutrient-recovery even has its own name – peecycling (Drew Swainston)(!).

READ THE ARTICLES

Häfner, F., Monzon Diaz, O.R., Tietjen, S., Schröder, C. and Krause, A. (2023) “Recycling fertilizers from human excreta exhibit high nitrogen fertilizer value and result in low uptake of pharmaceutical compounds,” Frontiers in Environmental Science, 10. Available at: https://doi.org/10.3389/fenvs.2022.1038175.

Hou, E., Luo, Y., Kuang, Y., Chen, C., Lu, X., Jiang, L., Luo, X. and Wen, D. (2020) “Global meta-analysis shows pervasive phosphorus limitation of aboveground plant production in natural terrestrial ecosystems,” Nature Communications, 11(1), p. 637. Available at: https://doi.org/10.1038/s41467-020-14492-w.

Martin, T.M.P., Esculier, F., Levavasseur, F. and Houot, S. (2022) “Human urine-based fertilizers: A review,” Critical Reviews in Environmental Science and Technology, 52(6), pp. 890–936. Available at: https://doi.org/10.1080/10643389.2020.1838214.

Peng, J., Dan, L. and Tang, X. (2023) “Phosphorus limitation on carbon sequestration in China under RCP8.5,” Advances in Atmospheric Sciences, 40(7), pp. 1187–1198. Available at: https://doi.org/10.1007/s00376-022-2195-y.

Rose, C., Parker, A., Jefferson, B. and Cartmell, E. (2015) “The characterization of feces and urine: A review of the literature to inform advanced treatment technology,” Critical reviews in environmental science and technology, 45(17), pp. 1827–1879. Available at: https://doi.org/10.1080/10643389.2014.1000761.

Stefan, I M A, and Ona A.D (2020) “Cabbage (Brassica oleracea l.). Overview of the health benefits and therapeutical uses,” Hop and Medicinal Plants. Available at: https://journals.usamvcluj.ro/index.php/hamei/article/view/13994 (Accessed: August 4, 2023).

Wang, T., Cessford, C., Dittmar, J.M., Inskip, S., Jones, P.M. and Mitchell, P.D. (2022) “Intestinal parasite infection in the Augustinian friars and general population of medieval Cambridge, UK,” International Journal of Paleopathology, 39, pp. 115–121. Available at: https://doi.org/10.1016/j.ijpp.2022.06.001.