“The problem is every time we’re removing forage, there goes our phosphorus and we’re never replacing it.”
– Joanne Thiessen Martens, Organic Agriculture Centre Of Canada Research And Extension Associate
Truly sustainable organic production systems recycle nutrients that are removed from the soil by crops.
That is why farmers use legumes as cover crops and in rotations to restore essential nutrients such as nitrogen (N), which is the only nutrient that can be fixed from the atmosphere.
But what happens when other nutrients such as phosphorus (P) are depleted? Restoring them is much less straightforward, says an organic extension worker with the University of Manitoba.
“I get phone calls and e-mails all the time from farmers who have been organic for a while, say 10 or 12 years, saying ‘I think I’m running out of phosphorus,’ or ‘I think my phosphorus is depleted,” said Organic Agriculture Centre of Canada’s (OACC) research and extension associate at University of Manitoba Joanne Thiessen Martens speaking during a presentation on organic production techniques for Organic Week in February.
With soils deficient in P, the only alternatives are external nutrient sources such as manure or rock phosphate, or using practices that boost available P.
It’s a problem often observed after several years of forage-based rotations.
It’s been documented at the University of Manitoba’s Glenlea Long-Term Rotation Study, where they are comparing the productivity and sustainability of annual and forage-based crop rotations under organic as well as conventional management.
Available P was lowest in a forage-based rotation (wheat -alfalfa -alfalfa -flax). It was highest in an annual grain rotation (wheat -pea -wheat -flax).
“The problem is every time we’re removing forage, there goes our phosphorus and we’re never replacing it,” said Thiessen Martens.
One potential solution lies with using more composted manure. Tests at Glenlea have shown it to be an effective method of adding phosphorus to the system, or, more accurately, recycling P back in, she said.
In a posting on the OACC website last fall, Thiessen Martens wrote that after P depletion was observed at their plot sites, applying composted manure (at a rate of 4.5 tons per acre) replaced almost half of the total P which had been removed from that organic forage system between 1992 and 2005.
“If we would add manure a
little more often, we’d probably be doing okay,” she said.
The challenge right now is that manure is often viewed as a disposal problem and not the valuable resource it is for recycling P back into soils, she added.
There are also phosphates now polluting lakes which could be captured. That’s something to potentially look at in future, she added. “That phosphorus is a valuable resource. We could be collecting that and getting it back into the soil where we need it.”
Thiessen Martens said another approach lies with figuring out how to get more P out of the residual pool of phosphorus in soil, or the stuff that’s not available to the plant because it’s tied up by strong chemical bonds to the soil’s organic matter. “There’s a lot of it there,” she said. “If we can find a way to get it out and use it, that can be helpful too.”
Other options for farmers trying to manage for long-term P are to plant P solubilizing crops such as buckwheat and legumes in rotation. Worked back into the soil, those crops can help boost available phosphorus.
The good news for organic farmers, meanwhile, is that the presence of a fungi that helps plants take up nutrients appears much higher in soil that is managed organically. Thiessen Martens showed slides comparing the presence of mycorrhizal fungi in both conventionally managed and organically managed soil.
Organically managed soil shows both higher volume of mycorrhizal fungi spores as well as more diversity in spore sizes, shapes and colours.