There are many benefits to zero and minimum tillage practices, but phosphorus loss can be a side effect
Conservation tillage, including zero and minimum tillage, reduces both soil erosion and the transportation of soil-bound nutrients to surface water. While one of the purposes of the practice is to minimize the negative impact of farming operations on the environment, recent studies have shown that even conservation tillage can have environmental trade-offs in some regions.
“There’s a broad range of efforts in Manitoba to try to minimize tillage — in all of the Prairie provinces,” says University of Manitoba soil scientist Don Flaten, “But minimizing tillage means something different, depending on what area you are in.”
In particular, Flaten points to southeastern Manitoba, where heavy and wet, clay soils leave farmers struggling to make zero tillage work as well as it does in drier areas with sandier soils. From an environmental standpoint, what works well in a wetter climate or on a hillier landscape is not necessarily going to improve the phosphorus situation in the Prairies.
Flaten references recent studies, conducted as part of the Watershed Evaluation of Beneficial Management Practices program (WEBs). WEBs is an Agriculture and Agri-Food Canada (AAFC) national initiative; its purpose is to evaluate the performance of best management practices (BMPs) on a small watershed scale. The program’s findings help researchers understand how management affects water and land. They also help producers farmers choose the most effective practices for their operation, given their location.
“What we found in our experiments at the twin watersheds site near Miami, Manitoba, is that conservation tillage reduced the losses of sediment, or eroded soil losses. It also reduced the amount of nitrogen losses off of this watershed site,” says Flaten. “But it actually increased the loss of phosphorus.”
Even before conservation tillage was introduced to the site, erosion wasn’t a major problem. Most of the runoff losses of phosphorus were lost in the dissolved form. What came as a surprise, though, was that after introducing conservation tillage to one of the two watersheds, the total amount of phosphorus loss due to runoff was actually higher on the conservation tillage site than it was on the conventionally tilled site.
“When we introduced conservation tillage — probably a combination of the crop residues on the surface themselves and some stratification with enrichment of phosphorus concentrations at the very surface of the soil — it increased the susceptibility of the conservation tillage watershed to more phosphorus losses,” says Flaten.
Certainly, conservation tillage reduces phosphorus losses in humid environments with sloping landscapes where phosphorus loss occurs as a result of rainfall runoff. In these environments, erosion can be an important source of phosphorus loading to surface water. Many farmers in Western Canada see the benefits of soil and water conservation practices as well — better crop yields, reduced fuel costs, as well as the overall improvement of farm efficiency — and know that they more than outweigh the small amount of phosphorus that is lost through a conservation tillage system.
“It’s not a big deal as far as most farmers are concerned in the Prairies,” says Flaten. “There are so many other benefits to conservation tillage that they’re not going to be too concerned about this one problem that might be associated with it.”
While the agronomic losses of phosphorus in both the conventionally tilled and the conservation tillage watershed sites are so small that most farmers wouldn’t notice, the environmental impact from those losses can be profound. It takes only a very small amount of phosphorus to cause major problems in water quality. “There are blue-green algae that fix nitrogen from the air just like a legume crop such as peas or alfalfa and all they need is a little bit of phosphorus in the water — 20 to 50 parts per billion — and they can flourish,” says Flaten. It’s for this reason that Flaten is careful to point out that a beneficial management practice that works well in one situation should not be used as a universal remedy.
Instead, researchers suggest that farmers implement additional management strategies — ones that better fit their practice, but continue to further reduce the accumulation of phosphorus on or near the soil surface.
“We need to move away from looking at beneficial management practices as cure-alls,” says Flaten. “There are so many other benefits to zero tillage. We don’t want to make that phosphorus issue overly important compared to the many other benefits we see from the practice.”
“Even more universal,” he continues, “is the importance of keeping our eyes open for the pros and the cons, the benefits and the side effects of our beneficial management practices. Because it isn’t just conservation tillage that has these potential side effects — it’s almost everything we do in agriculture.” †