Together with colleagues from, among others, the Environmental Assessment Agency (PBL), Van Ittersum is sounding the alarm bell about the increasing phosphorus shortage in worldwide grasslands in Nature Communications (16 February). “Grasslands in North-western Europe are often well fertilised, but this is rarely if ever the case elsewhere,” says Van Ittersum. “As a result the soil is becoming increasingly exhausted.” He also states that mowing and grazing is not sufficiently planned. “And it involves over three billion hectares of grassland; two thirds of the global agricultural area consists of grass.”
Fertiliser is disappearing
Like other plants, grass plants require phosphorus from the soil for growth, for example as a building block of the DNA that is present in every cell. The phosphorus from the grass is then used by grazing livestock. A part of the phosphorus is then removed from the land via milk, meat, wool and skins, while a much larger percentage ends up in the manure of the grazers. Approximately half of the manure is removed from the grassland to fertilise arable fields or be used for other purposes. This way, the grasslands are experiencing an increasing shortage of phosphorus. In time, this will lead to less grass and roughage for livestock, and less milk and meat for farmers – all at a time when the production should actually be increasing.
Using a large-scale global simulation model called the Dynamic Phosphorus Pool Simulator (DPPS), the scientists managed to map the phosphorus cycle for all countries from 1970 to 2005 for the first time. They then calculated the scope of the global phosphorus demand in 2050.
Grain as animal feed
Van Ittersum: “Better grassland management is crucial for the global food supply as we have to produce 80 per cent more grass and hay worldwide by 2050. Firstly due to the global population growth, and secondly because wealth is increasing, and in many poor countries we see a linear relationship between increasing incomes and a growing demand for meat and dairy. In addition, there is a risk that more and more grain, which is currently used to bake bread, will be used as animal feed. The increasing cultivation of bio-fuels is also competing with the cultivation of food crops. If we don’t improve the management and fertilisation of grasslands while livestock continues to increase, we are at risk of over-grazing, soil erosion and loss of soil fertility. Problematic weeds are advancing and the deserts are expanding. We have an urgent need for fieldwork and empirical data to test our model calculations locally to provide the market with specific management recommendations.”