Beans and other legumes deplete the soil of phosphorus

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[NAIROBI] Global food security could be seriously affected by diminishing levels of phosphorus in agricultural soils around the world, said scientists who have studied the flow of the mineral through soils and crops.

Researchers analysed nine years of data on phosphorus flows in cereals and legumes worldwide. They found "significant imbalances" between the phosphorus going in and coming out of the soil — in many places, the amount of phosphorus available to plants is decreasing.

Phosphorus is essential for plants to grow, but when crops are harvested it leaves the soil with them.

The scientists found that agriculture in Asia, in particular, consumes much more mineral phosphorus fertiliser in proportion to crop production than any other region. Eventually this could cause environmental, economic and social problems, they said.

"This is a particularly relevant and important topic in the light of the increasing global population," said the scientists, led by John Lott, biologist from Canada-based McMaster University. "High quality phosphorus reserves are diminishing and the cost of fertilisers is escalating rapidly."

Writing in the latest available online issue of International Journal of Agricultural Resources, Governance and Ecology they call for more efficient use of phosphorus fertilisers, and for more selective breeding and genetic modification to produce crops that require less of the mineral.

In Nairobi, scientists at the World Agroforestry Centre and the Africa Soil Information Service started a four year partnership project in 2008 to monitor and map changes in soil phosphorus across Sub-Saharan Africa and make suggestions for improving levels of the mineral in the soil.

"Although we do not yet have consistent data on the prevalence of phosphorus deficiency we do know that it is widespread and is considered the main biophysical constraint to food production in large areas of farmland in sub-humid and semi-arid Africa," Keith Shepherd, chief soil scientist at the World Agroforestry Centre, Nairobi, told SciDev.Net.

Many soils in Africa are either naturally low in phosphorus due to poor parent rocks, or phosphorus is not readily available to plants because iron oxides common in African soils hold it too tightly.

But unsustainable cropping practices further deplete soils, so that the amount removed through harvesting crops, soil erosion and other factors, exceeds the amounts put in through fertilisers.

"Phosphorus is vital for stable food productions systems and for buffering against climate change impacts on soil," he said. "This is important for both crop and livestock production."

Link to abstract in International Journal of Agricultural Resources, Governance and Ecology