What's with the P in the Great Lakes?

Monday, April 6, 2015

There are lots of questions, but the answer is often that 'we don't know' or 'it depends'

by PAT LYNCH

When beaches are closed in the Great Lakes due to algae bloom, questions are asked. If this happens during the heat of the summer, a lot of blame gets thrown around. Blame has been attributed to mussels that are somehow releasing phosphorus (P). And then there is blame given to global warming. The lakes are warming up, making the P in the lakes more available to plant growth.

But a significant amount of blame has been laid at the door of agriculture. It is true that Ontario soils probably put 0.3 to 0.4 pounds per acre per year of available P into the Great Lakes. On a per acre basis, this is nothing. But, collectively, it is a significant amount.

Here are some of the questions being asked are. Has no-till added to the extra P in the water? Are livestock producers putting a lot of P into the water? Do fields that are tile-drained add more P than untiled farms? If we quit using all P fertilizers, that will reduce the amount of P going into the Great Lakes? The answer to that last question is that it won't make a significant difference.

I have spent a lot of time trying to get answers. For most of these questions, the answer is: "We don't know." There are, however, some professional opinions.

On the question of whether tile drainage increases the amount of P leaving a field, Ontario Ministry of Agriculture Food and Rural Affairs water quality engineer Kevin McKague  says: "The answer to that (as with most questions in agriculture) is 'it depends.'" It is site-specific, as always, and dependent on the weather patterns and the crop in the field at the time these weather patterns occur.

At the sites we have been observing year-round for a couple years now (silt loam soils under a corn, beans, wheat rotation with soil tests in the mid-teens), 30 per cent to 50 per cent of the precipitation that fell left the field either as overland runoff or as tile discharge. Of that runoff, 60 per cent to 90 per cent of that water leaving the field edge left through the tiles.

The average concentrations of total P and soluble P were generally lower in the tile water, but there was more of it. In the end, about 50 per cent of the total P came through the tile and the other half overland. For the soluble P, about 25 per cent came through the tile and 75 per cent from the overland flow.

So, tile drains do contribute to P loss. But do they contribute more than if they were not there? That is a tough science question to answer as I can't go back and monitor the same field under a no-tile situation. If we plugged up the tile, my guess is we would see more overland flow. My summation of his answer is that whether a field is tiled or not will not greatly change the amount of P leaving a farm.

On the question whether no-till increases the amount of P loss from the field? Probably. But, to me, the difference is so small as to be negligible when you consider the benefits of no-till. I put conservation tillage of corn stalks in the highest category of reducing P losses.

There are other things "we believe" will reduce the amount of P leaving the farm. First is to incorporate broadcast P. This is the same rule that applies to manure. It should be mixed into the soil. This is not a big concern for Ontario since most broadcast P is applied in the spring before corn and incorporated. It will mean a change of procedures for some growers. If you are going to use conservation tillage on corn stalks, you can broadcast P on corn stalks before tillage.

The other big thing you can do is plant cover crops. Cover crops will slow the overland flow of water. This forces more dissolved P into the soil, where it can be held.

There are lots of reasons why we must take P in the Great Lakes seriously. Society is watching us and demanding changes. People will not be swayed by science as much as by emotion. We can get ahead on this one by managing on-farm P. Besides, you paid for this P so you want to make use of it. BF

Consulting agronomist Pat Lynch, CCA (ON), formerly worked with the Ontario agriculture ministry and with Cargill.