It has long been a commonly held belief that zero tillage is a good way to keep phosphorus out of watersheds.
But the latest research at South Tobacco Creek, near Miami, shows that the soil conservation practice aimed at covering up bare dirt with crop residue may be leading to more of the nutrient leaking into Lake Winnipeg, not less.
“Our study at South Tobacco Creek shows that zero tillage, compared to conventional tillage, loses 12 per cent more phosphorus on a per-acre basis, and the average phosphorus concentration in the run-off is 42 per cent higher,” said Don Flaten, a soil science professor at the University of Manitoba.
“The reason for that is that most of the phosphorus loss in the Prairie landscape, including South Tobacco Creek, is in the form of dissolved phosphorus.”
That’s contrary to the conventional wisdom, which for years had been based on studies done in hilly, warmer, more humid areas such as Pennsylvania, where erosion of soil particulate matter was identified as the major culprit.
That belief, which formed the basis for advocating zero tillage as a Best Management Practice (BMP) for reducing phosphorus loading in waterways, doesn’t apply to the Prairies, where most of the run-off occurs during the spring thaw.
Here, the harsh Prairie winter cracks the cell walls in plant leaves and stems, and the composite nutrients are more easily carried away by water in a brownish phosphorus “tea.”
“Zero tillage is a very good way of reducing particulate phosphorus loss through soil erosion, but it is not a good way to reduce dissolved phosphorus,” said Flaten, at the MZTRA annual general meeting in Carberry last week.
“In fact in most studies in the world, you get an increase in dissolved phosphorus in zero tillage compared to conventional tillage because you have more residue on the surface that is leaking phosphorus, and the phosphorus fertility tends to become enriched at the surface as a sort of preferential accumulation due to stratification,” he added.
“The increase in dissolved phosphorus in zero tillage is greater than the decrease in particulate phosphorus that you get from the erosion control.”
That doesn’t mean that zero tillage is bad, said Flaten. It just has side-effects, just like many useful medicines used by humans.
It still offers many environmental and water quality benefits, from reduced sediment and nitrogen loading to improved wildlife habitat, as well as moisture conservation and reduced fuel costs for the farmer.
If the Assiniboine River at Headingley carries 600 tonnes of phosphorus out of a watershed of 38 million acres, the math shows that P losses amount to 0.08 pounds of P2O5 per acre per year, or 0.3 per cent of crop removal rates.
“The loss of P to water is actually very small on a per-acre basis,” he said.
“So 99.7 per cent of the P lost from our land is in the form of crops that we harvest, and ship to feed city people someplace.”
Even a little phosphorus in a body of still water – 20 to 50 parts per billion – can cause serious problems due to “blooms” of cyanobacteria, also known as blue-green algae.
Cyanobacteria are able to fix their own nitrogen, much like legumes, and with enough P to jump-start growth, they can turn lakes into a stinking green soup that may harbour nerve and liver toxins harmful to humans, livestock, wildlife and asphyxiate fish.
”If you take a look at overall environmental health, you are better off with zero tillage than conventional tillage, but we’ve got to be objective, and not depend on zero tillage to solve our phosphorus-loading problems in Lake Winnipeg,” he said.
“It’s not the BMP that’s going to save us.”
Vegetative buffer strips, also touted as a phosphorus-catching BMP, may not be performing as well on the Prairies as in other areas.
Recent studies indicate that they reduced P concentrations in run-off in only 50 per cent of cases, actually increased them “substantially” 18 per cent of time, and had no effect in 32 per cent of all cases, said Flaten.
“The overall reduction in phosphorus loading was only four per cent – 10 times lower than what we would expect based on literature values from other areas where they don’t have a snowmelt dominated run-off system. [email protected]