On an overcast day, Roger Sylvester-Bradley walks along a hawthorn hedge, collecting a thick rind of mud on his leather boots, before stepping into a gently sloping field of barley.

He stoops to pluck an ankle-high seedling from the ground and examines its healthy mop of fine white roots. Turning them in his hands, he says, “when you see a plant that’s deficient in phosphorus, it doesn’t look like this.”

That’s something of a surprise to Sylvester-Bradley, a crop scientist at ADAS, an agricultural consulting company in Cambridge, England. Phosphorus occurs naturally in soil and is a critical nutrient for plant growth. For centuries, farmers have added extra to their fields to boost harvests, but Sylvester-Bradley and his colleagues are studying