Phosphorus balance of contrasting farming systems, past and present. Can food production be sustainable?
Phosphorus inputs and losses are calculated for contrasting farming systems: cereals were a major product in all systems. In typical modern British wheat farming, P input in fertilizer and removal in crop harvested exceed all natural inputs and losses by more than an order of magnitude. Net natural inputs (i.e. inputs - losses) to farms not receiving inorganic fertilizer would be expected to support grain harvest of the order of 0.1 ton ha-1 year-1. However, yields about an order of magnitude higher than this have been achieved in various farming systems in the past which did not use inorganic fertilizer. Phosphorus balances are calculated for four such farming systems: of these, only fields along the Nile in ancient Egypt, which received flood water annually, were probably in long-term P balance. In the US mid-west in the nineteenth century, decomposition of soil organic matter supplied P to crops, but at a declining rate. In a medieval English farm and in early twentieth century China, too, crop yield was probably dependent on soil P stores and so was not sustainable long-term. These case studies draw attention to the limits that phosphorus supply can place on sustainable crop yields because of slow rates of natural P input.