Spotlight on macronutrients: Phosphorus… and Potassium

It’s a little naughty to consider these two elements together, I know, because this may unintentionally add to the confusion that often ensues in class when you ask students to tell you the full chemical names for elements with the symbol P – phosphorus – and K – potassium. Oh, the near-Pavlovian, knee-jerk temptation for them to say potassium for P (but not nearly as annoying as those who spell phosphorus with an additional ‘o’ – phosphorous…). Anyway, and since there is so much out there in the cybersphere dealing with phosphorus, I will only flag up Professor John Raven’s typically thoughtful review of the evolution of autotrophy (‘self-feeding’, e.g. photosynthesis, but not restricted to that plant-like process) in relation to requirement for P, ‘the ultimate elemental resource limiting biological productivity through Earth’s history’. Written to redress a perceived imbalance in emphasis that has hitherto concentrated on the roles of C, N and Fe in the evolution of autotrophy and right that historical wrong, Raven’s review ranges widely, from the origins of life, to the roles of P in organisms, PUE (phosphorus use efficiency), to growth-limitation via an effect on water use efficiency (WUE) from P insufficiency.

The K contribution is a consideration of the so-called potassium paradox. For many years K has been added – in the form of KCl – to soils as a fertiliser in efforts to improve agricultural productivity of corn and other grain crops (despite K being one of the most abundant elements in the earth’s crust and being more readily available than N, P or S…). Indeed, so universal is the presumption that K is needed that artificial fertilisers are typically defined by their NPK rating, because K is usually added along with the major plant-growth-limiting nutrients N and P. A study by Saeed Khan et al. has questioned the basis of traditional tests to detect K soil levels – and hence the justification for additional inputs thereof – and even the need for K fertilisation at all. Indeed, their work showed instances of an increase in soil K level in the absence of artificial inputs – ascribed to return of K from plant residues to the soil. Furthermore, their extensive survey of more than 2100 yield-response trials confirmed that not only is KCl addition unlikely to increase crop yield, but – in more than 1400 instances – such K fertilisation actually led to a ‘detrimental effect… on the quality of major food, feed and fiber crops, with serious implications for soil productivity and human health’. As the authors explained, ‘Potassium depresses calcium and magnesium, which are beneficial minerals for any living system’; for example, diets low in Ca can also trigger human diseases such as osteoporosis, rickets and colon cancer. Another major human health concern arises from the chloride in the KCl, which mobilises Cd (cadmium) in the soil and promotes accumulation of this heavy metal in cereals. A paradoxical situation, indeed!