Searching for Ways to Add Phosphorus to Increase Soil Fertility

Organic Agriculture Centre of Canada

Researchers looking into effective ways of adding phosphorus (P) to increase soil fertility have saved organic farmers a lot of time and money.

In a recent paper in the Canadian Journal of Soil Science (2010) 90: 257-266, they showed that combining phosphate rock (PR) with the green manure crop buckwheat didn’t produce agronomically significant benefits.

While one of the three phosphate rocks tested increased P uptake of the buckwheat crop, the buckwheat residues did not enhance the yield of the next crop, says Melissa Arcand, lead author of the research study.

Based on the results, it wouldn’t be worthwhile for farmers to invest in this approach, she said. Yet using different green manure crops on different soils might produce more positive results.

For example, the soils in the study were generally alkaline. Acidic soils might work better. Other green manure crops, such as legumes, might also make a difference.

In synthetic fertilizers, the phosphorus is removed from the rock using acids, making the phosphorus more soluble and easier for plants to access.

The challenge for organic farmers is to improve plant uptake of phosphorus – an element and one of three critical macronutrients, along with nitrogen and potassium – without synthetic fertilizers.

There are no substitutes for macronutrients. “They’re essential,” said Arcand. “There’s no way around it.” A lack of phosphorus will usually hamper a crop’s growth.

Trying to increase phosphorus availability was one of the main purposes of the “Residues from a buckwheat (Fagopyrum esculentum) green manure crop grown with phosphate rock influence bioavailability of soil phosphorus” study.

In organic farming systems, phosphorus is supplied mainly through recycling of on-farm organic materials such as composts, green manures and animal manures. These organic materials contain phosphorus mineralized by soil organisms, making this macronutrient easier for plants to use.

Adding green manure residues to soils can increase soil mineralization rates of phosphorus, but low concentrations in residues often won’t meet crop demands. “Selecting green manure crop species that have high P uptake has the potential to overcome the limitations of organic materials to supply P in these systems,” states the study.

Buckwheat was chosen because previous studies have shown it can absorb concentrations of phosphorus beyond its own needs. So excess phosphorus could be left in the soil from buckwheat plant residue.

In the study, chemically untreated phosphate rock from sedimentary and igneous sources was used as replacements for synthetic fertilizers. Regardless of the source, phosphate rock has low solubility, particularly in alkaline soils characteristic of most soils under organic production in Ontario and the prairies.

Though the study results showed this approach didn’t produce significant benefits, the process itself was important. That’s because phosphorus from phosphate rock is a finite resource. The continued manufacture of synthetic fertilizers will eventually exhaust this source. When that happens, organic farmers may have an advantage because they will have learned how to increase phosphorus without relying on chemical fertilizers. 

“The depletion of rock P, which is a finite resource, for production of synthetic fertilizers means that at some point in the future, we can no longer depend on this source of P,” said Arcand.

“Instead, we must be able to redistribute the soluble, reactive forms of P from within the landscape and waterways where it is in excess to where it is deficient,” she said.  “In soils where plant available forms of P are deficient, but where total P is plentiful, we must also work on methods to improve its access to plants.”

In other words, the land and crop management practices of organic farmers will be needed when the existing supply of phosphorus from synthetic fertilizers is gone.

Phosphorus and oil are similar in having a finite supply. But unlike oil, which after being burned is converted into the gas carbon dioxide, phosphorus from synthetic fertilizer is not lost from the environment.

Instead, phosphorus is redistributed into the soil and water (often in excess concentrations) in different forms. “Phosphorus is adsorbed to soil particles or is taken up by plants, incorporated into animal tissues (from consuming plants), and is also incorporated into soil microorganisms,” said Arcand. 

The challenge for researchers and organic farmers alike is to find ways of recycling phosphorus in its many forms while avoiding the easy one-way path of synthetic fertilizers.

This article was written by Steve Harder on behalf of the OACC with funding provided by Canada’s Organic Science Cluster (a part of the Canadian Agri-Science Clusters Initiative of Agriculture and Agri-Food Canada's Growing Forward Policy Framework).  The Organic Science Cluster is a collaborative effort led jointly by the OACC, the Organic Federation of Canada and industry partners
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Posted September 2010