Under-use of phosphorus-based fertilisers contributes to a growing yield gap
By Abdallah el-Kurebe
A study by the International Institute for Applied Systems Analysis, IIASA, has revealed that a growing imbalance between phosphorus and nitrogen fertiliser use in Africa could lead to crop yield reductions of nearly 30 per cent by 2050.
IIASA also stated that the under-use of phosphorus-based fertilisers in Africa currently contributes to a growing yield gap – the difference between how much crops could produce in ideal circumstances compared to actual yields.
“This phosphorus-specific yield gap currently lies at around 10 per cent for subsistence farmers, but will grow to 27 per cent by 2050 if current trends continue,” according to the study published in the journal, Global Change Biology.
Marijn van der Velde, a researcher now at the Joint Research Centre of the European Commission, who led the study while working at IIASA, said: “This research shows that the imbalance between nitrogen and phosphorus applications has the potential to further limit food production for a growing population in Africa.
“While nitrogen-based fertilisers could be produced by a process that extracts the element from the air, phosphorus must be mined from rock for which reserves are limited. That makes phosphorus fertilisers expensive, especially in the longer term.
“Farmers with limited money are more likely to buy and have access to cheaper nitrogen-based fertilisers, while this might work in the short term, in the longer term it has a negative effect on crop growth as soil nutrients become more imbalanced,” Mr. van der Velde further said.
According to him, as farmers use fertilisers for their crops, nutrients such as nitrogen and phosphorus build up in the soil, providing a reserve of nutrients that plants need to grow.
“But fertiliser use remains very low in Africa, and to increase crop production, it is widely recognised that farmers must increase their fertiliser use,” he said.
He observed that while nitrogen-based fertiliser usage has begun to increase in Africa in the last 10 years, “the application of phosphorus to crop-land has not kept pace, leading to a growing imbalance between nitrogen and phosphorus levels in soil.”
Using data from Food and Agriculture Organisation, FAO, crop trials as well as an established EPIC large-scale crop model to estimate how the imbalance affects current and future crop yields, the new study showed that increases in nitrogen and phosphorus inputs must happen in a way that provides crops with the balanced nutrient input they need.
“Previous research has looked at these effects on a field and local scale, but this is the first study to do so at the continental scale,” says another IIASA researcher, Christian Folberth, who also worked on the study.
In order to make optimal use of current nitrogen inputs, the researchers estimated that phosphorus applications would need to increase 2.3-fold.
“To close yield gaps nitrogen applications would have to increase 5-fold. Phosphorus applications would have to increase nearly 12-fold from 2.2 to 25.9 kg per hectare. But because of the cost of phosphorus, that remains a challenge.
“While much of the remaining phosphorus reserves are found in Morocco, on the African continent, we need to find better ways for African farmers to access this precious resource,” said Mr. van der Velde.
The global phosphorus cycle is, besides nitrogen, also increasingly growing out of balance with carbon, which is the subject of another recent paper by the same group of researchers and a new European Research Council grant for continued research by IIASA and an international team of scientists.
Mr. van der Velde further said, “The change in the stoichiometry of nitrogen and carbon from rising atmospheric CO2 concentrations relative to phosphorus has no equivalent in Earth’s history and the impacts will go beyond the agricultural sector.”