How to grow more food on the same amount of land: the challenge has been a constant in human history. New answers have allowed growth in population and in living standards, but with today’s surge in food prices the need to raise agricultural productivity is once again pressing. To grow more food is possible – but dogmatism about how or where to do so would be unwise.

The spectacular increase of the 1960s in wheat and rice grown per acre of crop land has slowed to as little as 1 per cent per annum. That is not because innovation has stopped, although because of apparent abundance it has taken a lower priority, but because marginal land has been brought into production while some land has been degraded. The rising price of energy inputs – oil and fertiliser – will inevitably reduce the productivity of a complementary input: land.