Phosphorus is a chemical element essential for life. Without adequate phosphorus, plant growth and crop yield are reduced. As plants grow, they remove phosphorus from the soil, which is subsequently removed from fields when crops are harvested. As a result, fertilizers containing phosphorus compounds are often added to soil.

However, fertilizer phosphorus added beyond the crop’s yearly needs can remain in the soil, and can become more tightly bound to soil than recently added fertilizer. Not much is known about the availability of “residual phosphorus” or “legacy phosphorus” to plants.

Because it is more tightly bound, it may not show up in “soil test phosphorus” analyses that measure readily available soil inorganic phosphorus to determine fertilizer requirements. So researchers at Agriculture and Agri-Food Canada (AAFC) are investigating legacy phosphorus in studies at various locations across Canada, through a phosphorus project led by Dr. Noura Ziadi at AAFC-Ste. Foy.

“The availability of long-term plots is key to studying new problems as they develop, such as the potential over-application of phosphorus fertilizers, says Barbara Cade-Menun, research scientist for Agriculture and Agri-Food Canada. “While it’s well known that not all the fertilizer given to plants will be used, what we don’t know is how fertilizer compounds in the soil interact with the other soil elements, microbes and plants over time.”

One study in this phosphorus project is led by Dr. Barbara Cade-Menun at the Semiarid Prairie Agricultural Research Centre (SPARC) in Swift Current, Saskatchewan. Her team is studying a set of agricultural plots that were established in 1967.

 Until 1995 these plots received both phosphorus and nitrogen fertilizer, but then each was split in two so that phosphorus treatments could be stopped on one half of each plot. The result is a long-term, controlled study of legacy phosphorus.

What has surprised the researchers is the growth of the wheat plants on the plots not fertilized by phosphorus: although tests showed very low concentrations of soil test phosphorus on the no-phosphorus plots, there was no difference in yield, or in grain phosphorus concentration as compared to the fertilized plot.

The plants that did not receive additional fertilizer were able to use both the legacy fertilizer phosphorus, as well as other pools of the phosphorus that had accumulated in the soil, such as organic phosphorus from decaying plant material.

While this research will need to be replicated with other crops in other soil and environmental conditions, it suggests that farmers might be able to change the way they think about phosphorus fertilizer. Accessing existing phosphorous from the soil would be a cost saving for the farmer and potentially reduce the risk of phosphorus loss into nearby water bodies.

Dr. Cade-Menun’s work was aided by the Saskatchewan Structural Sciences Centre at the University of Saskatchewan, as well as the Canadian Light Source Synchrotron.