Phosphates in our Ditches

I have previously written about the phosphate cycle. In nature, plants take up phosphate from the soil, and it becomes a part of plant tissue. The nutrient is returned to the soil when the plant dies. If it is ingested by animals or people, the phosphate is returned to the soil when the animal defecates. The cycle is complete.

In modern food production systems phosphate is a scarce resource. It is mined thousands of miles from here, is transported to where it is needed and applied to fields and gardens. The phosphate works its way up the food chain, and ultimately ends up in a livestock barn or human stomachs.

We flush our toilets and that phosphate is on its way to Lake Winnipeg. Animal manure is applied to agricultural fields. If the same amount of manure phosphate is applied as what the plants will take up, the natural phosphate cycle is intact. If surplus manure phosphate is applied, the extra is on its way to Lake Winnipeg. No matter how the phosphate is treated, it does not simply disappear.

As the phosphates get to Lake Winnipeg, they encourage algal growth in the lake, which in turn consumes oxygen resulting in a sterile lake unable to support fish or anything else. 

There are currently projects underway to see whether excess phosphate can be removed from Lake Winnipeg. Experimentally, cattails in the lake are being harvested and removed to see if the lake could benefit from such a removal. This may offer possibilities, but in my mind, the biggest problem is not addressed: the recovered phosphate is now a long ways from where it is needed, namely the farm fields.

Recently, David Dawson pointed out to me that the Highways Dept and Municipalities cut the grass and cattails in our ditches regularly. The lush growth in the ditches is the result of nutrients coming off the adjacent fields. In spring, many of these ditches become raging torrents. The rotting mass of cut grass is flushed down into the rivers and into Lake Winnipeg where it releases its phosphates.

Suppose, David says, an enterprising farmer cut the grass in the ditch, baled it up, took it to his farm and fed it to his cattle. Then the farmer collected the manure from his cattle and dumped it back in the ditch. There would be an outcry and rightly so. But, in fact, the farmer would be returning less to the ditch than he had taken out. The cattle would have utilized a good part of it. So why is it OK to leave all that grass in the ditch but not OK to dump the manure back in the ditch?

If we are seeking ways of removing phosphate from Lake Winnipeg, surely it makes more sense to prevent the phosphate from getting there in the first place. A relatively simple solution would be that the Highways Dept include in its grass cutting contracts a clause that the cut grass be removed. The material could be composted and recycled for public use. It’s not rocket science.

By Eric Rempel