Plant Cuttings

Life’s little certainties…

Nigel Chaffey discovers Nature has its own taxes.
Death and Taxes
Jesse Kunerth / 123RF

When Benjamin Franklin (diplomat, scientist, inventor, writer, founding father of the USA, etc.) penned the phrase “In this world nothing can be said to be certain, except death and taxes” in 1798, he – probably! – didn’t have the present precarious state of the world’s phosphorus supply in mind. Yet, if we are to avoid deaths of millions of humans – many of which are as yet unborn – we might have to bite the bullet and pay an appropriate – additional – ‘tax’. To whom do we owe this particular doom-and-gloom-laden prophesy? Eric Roy et al., and their assessment of the consequences of increasing crop productivity in tropical regions as part of a global attempt to boost world food production by addition of inorganic phosphorus (Pi) fertilisers.

Recognising concerns about feeding an increasing human population – that’s predicted to exceed 9.7 billion by 2050, from its present value of nearly 7.5 billion – it’s acknowledged that food production needs to increase. Whilst all manner of solutions are actively being sought to achieve that most worthy of endeavours, an attractive option is to increase current productivity of tropical regions to match that of existing yields elsewhere on the planet. Whilst increases in yield can be achieved by addition of such essential plant nutrients as Pi, this comes with a cost – whose magnitude I was previously unaware of.

A large proportion of the added Pi is fixed – rendered unavailable to plants – by the soil itself*. This additional amount – for which no benefit in terms of extra yield is seen – is what’s been called a phosphorus tax by Roy et al. Whilst we might not like paying human-imposed taxes, we can usually do so out of earnings. Unfortunately, that isn’t an option with nature’s imposed Pi tax because the amounts of inorganic phosphorus available for its payment are finite.

So, in the long term, we can’t just fertilise our way out of this particular problem. Although the research was conducted in a comparatively small area of Brazil, the global relevance – where 23 % of tropical soil area is considered to be P-fixing** – is clear. Such a situation underlines the need to consider all available options and technologies – and those we’ve yet to develop – if we are to avoid starvation of apocalyptic proportions ***. However, if anybody is looking to make an investment in the stock market, they could do worse than consider shares in a phosphate mine…

* An interesting question for plant nutrition students comes to mind, nitrogen- and phosphorus-fixation: Compare (and contrast…).

** Intriguingly, such P-fixing soils have the useful attribute of holding nitrate ions against leaching, which therefore makes that essential nutrient more available to plants. Infamously, when applying nitrates to the soil to boost crop production inappropriately, it’s long been known that > 50% of what’s applied may never be taken up by the plants (e.g. this PDF) – a ‘nitrogen tax’ if you will. In this instance, much of the nitrate leaches from the soil into freshwater bodies where it may contribute to environmentally-damaging eutrophication. So, P-fixing soils, not all bad news [discuss…]?

*** And is particularly unwelcome news coming on top of dire predictions about a four-fold increase in Pi inputs if grasslands are to achieve the expected 80 % increase in growth of grasses which provides fodder to sustain ruminant meat and milk production. If we persist in eating animal products – that are more resource-depleting than plant products – that is. An argument in favour of becoming vegetarian to avoid paying the P tax..?

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